1990 3.1 L Misfire when cold
tomj76
12-12-2014, 02:32 PM
I have a 1990 3.1L, multiport fuel injection (NOT sequential FI), which runs very well most of the time. However, when I cold start it runs rough (misfire) for about 1 minute (no throttle) or until acclerated. The is no misfire when running normal, but it might be running a little rough when stopped at a traffic light (in Drive, foot on brake).
I put new plugs and wires on, but it did not affect this problem.
I am not using coolant, or oil, the plugs that were removed were somewhat worn, enough that I expected to notice a difference after they were changed.
I have not yet (1) cleaned the MAF, (2) checked carefully for vacuum leaks, (3) checked for leaking injectors.
Since this is a 1990, OBDI, I can't say which cylinder is misfiring or how the fuel trims look etc.
I put new plugs and wires on, but it did not affect this problem.
I am not using coolant, or oil, the plugs that were removed were somewhat worn, enough that I expected to notice a difference after they were changed.
I have not yet (1) cleaned the MAF, (2) checked carefully for vacuum leaks, (3) checked for leaking injectors.
Since this is a 1990, OBDI, I can't say which cylinder is misfiring or how the fuel trims look etc.
Blue Bowtie
12-13-2014, 11:06 AM
Did you inspect the coils for damage, corrosion, or contamination which could cause flashover? Are the plug gaps too wide?
Have you tested the solenoid resistance of each of the injectors? This series is infamous for having shorting/low resistance injectors which affect all of them (batch fire).
Have you tested the solenoid resistance of each of the injectors? This series is infamous for having shorting/low resistance injectors which affect all of them (batch fire).
Schurkey
12-14-2014, 04:06 PM
Since this is a 1990, OBDI, I can't say which cylinder is misfiring or how the fuel trims look etc.
You won't get a readout of cylinder misfire, but you should be able to access short- and long-term fuel trim, and of course O2 sensor voltage and O2 cross-counts.
I'd also be looking at the coolant temp sensor and wiring--is the CTS showing ambient temperature before you start the engine after the engine has been off for a long time?
Yes, vacuum leaks and the idle air system are also important.
First Guess: If this were ignition-related, it'd be worse on acceleration rather than at cold idle--although anything is possible.
You won't get a readout of cylinder misfire, but you should be able to access short- and long-term fuel trim, and of course O2 sensor voltage and O2 cross-counts.
I'd also be looking at the coolant temp sensor and wiring--is the CTS showing ambient temperature before you start the engine after the engine has been off for a long time?
Yes, vacuum leaks and the idle air system are also important.
First Guess: If this were ignition-related, it'd be worse on acceleration rather than at cold idle--although anything is possible.
tomj76
12-15-2014, 12:12 PM
@Bowtie, I checked plug gaps when I replaced the plugs, and coils look OK as far as I can tell, but removing them is no small job so I have not gone to that step yet.
@Schurkey, I can only get the SES light "blink" mode data, so there is a read out of fuel trim through the blink rate, but it isn't numeric. Also, Fuel Trim data isn't reported during the first two minutes because the O2 sensor hasn't heated up to the point where its signal is used to adjust FI timing. However, it still would be worthwhile to check to see if the Fuel Trim is right after everything "smooths out". I can only see the coolant temperature reported by the sensor that goes to the instrument panel. I suppose I can check the resistance of the other one that it used by the PCM. I once had a problem with an intermittent open circuit in the PCM temperature sensor circuit (it was on the PCM board), and the engine would not run at all (other than idle) while the circuit was broken. If you even touched the accelerator the engine would stall. After the PCM warmed up (heater running during warm up), everything ran fine. This is a different symptom, so the CTS could be the problem but only if it is in a different way.
Since my post I noticed a symptom that might be pointing to a low FI coil resistance (I've previously replaced three due to coil shorts). The symptom is a slight "hiccup" or hesitation just as you press the accelerator pedal, before the engine responds, almost as if the engine is about to stall, then it goes as normal. It is a symptom that I noticed on the two occasions I had injectors problems, just before they failed to a short circuit. Checking the resistance of the two injector banks is not difficult.
@Schurkey, I can only get the SES light "blink" mode data, so there is a read out of fuel trim through the blink rate, but it isn't numeric. Also, Fuel Trim data isn't reported during the first two minutes because the O2 sensor hasn't heated up to the point where its signal is used to adjust FI timing. However, it still would be worthwhile to check to see if the Fuel Trim is right after everything "smooths out". I can only see the coolant temperature reported by the sensor that goes to the instrument panel. I suppose I can check the resistance of the other one that it used by the PCM. I once had a problem with an intermittent open circuit in the PCM temperature sensor circuit (it was on the PCM board), and the engine would not run at all (other than idle) while the circuit was broken. If you even touched the accelerator the engine would stall. After the PCM warmed up (heater running during warm up), everything ran fine. This is a different symptom, so the CTS could be the problem but only if it is in a different way.
Since my post I noticed a symptom that might be pointing to a low FI coil resistance (I've previously replaced three due to coil shorts). The symptom is a slight "hiccup" or hesitation just as you press the accelerator pedal, before the engine responds, almost as if the engine is about to stall, then it goes as normal. It is a symptom that I noticed on the two occasions I had injectors problems, just before they failed to a short circuit. Checking the resistance of the two injector banks is not difficult.
Schurkey
12-15-2014, 03:14 PM
@Schurkey, I can only get the SES light "blink" mode data
Time to acquire a real scan tool.
Time to acquire a real scan tool.
Blue Bowtie
12-15-2014, 06:34 PM
Actually, the SES lamp will indcate whether the BLM is in lean or enrichment mode at any given time. Search for "Field Service Mode" and you may find some useful information, a little like this (from my other site):
On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.
WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.
If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.
A 3.9K resistor installed in ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel Mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is commonly referred to as the "Limp Home" mode of operation.
If that isn't enough information, clip and save this:
GM OBD I System Operation
This generation of engine control systems is a refinement of the first engine control computers and systems. First generation engine controls have some control over spark timing, a few output devices like the EFE system and EGR, and a minor role in fuel mixture adjustment. The OBD I system is designed to control and sense a wider range of functions than the earlier systems, including complete control of fuel delivery, and has proven to be much more reliable and effective.
The system has two basic modes of operation when the engine is running - Open Loop Mode and Closed Loop Mode. In a nutshell, Open Loop mode means the ECM is controlling the engine with essentially no feedback from the engine sensors - the control loop is said to be "open". Closed Loop mode means that the ECM sends control signals to the engine, scans the sensors for feedback to determine further corrections, then adjusts accordingly. The process is continuous, and the control loop is said to be "closed" since the sensors effectively communicate back to the ECM regarding the effectiveness of any adjustments made.
Open Loop Mode
When the starter is engaged, if the coolant temperature indicates less than 100°F and the intake air temperature is within 20°F of the coolant temperature, the cold start injector provides a spray of fuel or the cold enrichment subroutine allows longer fuel injector pulses on the service injectors. The cold start or cold enrichment subroutine is of eight seconds duration at a maximum. On a cold-start injector system the extra fuel is distributed to each cylinder via a air distribution system built into the bottom of the intake manifold. If the engine temperature is greater than 100°F, the cold start injector/subroutine is disabled by the cold start switch or ECM. Upon startup the ECM utilizes information in the PROM to establish the initial pulse rate for the injectors and the engine starts. At this time the engine is operating in Open Loop mode and will continue to do so until the engine warms up sufficiently to enable the oxygen sensor.
Closed Loop Mode
After a warm up period the ECM scans the sensors. If all sensors are operating and within their proper ranges, the ECM will enter Closed Loop operation. This means that the sensors are dynamically controlling the engine fuel and spark. The ECM receives information on intake air flow, coolant temperature, intake air temperature, exhaust gas oxygen content, barometric pressure, manifold pressure or vacuum, air conditioner status, power steering pump load, EGR operation, evaporative canister valve operation, engine RPM, vehicle speed, transmission gear selection, fuel pump status, combustion detonation, and throttle position. This information is used to calculate the proper ignition timing advance and pulse width for the injectors, and fires the injectors for the calculated period. This procedure is repeated continuously in very rapid sequence to maintain the optimum fuel air ratio. The electronic spark control components provide maximum advance, if engine knock is detected the spark is automatically retarded. This too, is a continuous process. It should be noted that the following components are matched for optimum performance - distributor EST module, ESC module, knock sensor and ECM PROM. These components are not interchangeable between 5.0L - 5.7L engines. In the event the information received is higher or lower than the normal range, a code will set in the ECM, and the Check Engine or Service Engine Soon lamp will light.
Block Learn / Integrator
The factory ECM/PCM has a learning capability which allows it to make corrections for minor variations in the fuel system to improve performance and driveability. There are two learning features. The Integrator and Block Learn (INT and BLM) and Block Learn Memory (BLM) cell. The INT and BLM feature is normal with a value of around 128 (the possible range is 0-255). If this value is higher than 128, it indicates that the ECM is adding fuel to the base fuel calculations because the system is running lean. A value lower than 128 indicates that the ECM is taking out fuel because the system is running rich. The integrator is a short term corrective action while the BLM is along term correction. The BLM value will change if the integrator has seen a condition which lasts for a longer period of time. There are from two to sixteen different cells which the ECM modifies, depending on RPM, airflow or manifold air pressure and other conditions such as AC on or off, etc. The ECM learns how much adjustment is required in each cell, retains it in memory, and applies these adjustments when the engine operates in that cell or RPM/Load Range. These features of the ECM allows the system to adjust itself constantly to your engine and assure peak performance for stock and other than stock engines.
NOTE: When the vehicle power is disconnected for repair or to clear diagnostic codes, the learning process has to begin all over again. To reinitialize the ECM, drive the vehicle at operating temperature with moderate acceleration and idle conditions for at least twenty minutes.
Performance PROMs typically change the parameters for fuel flow, fuel cut-off and spark advance-timing and will allow increased fuel flow and modify the spark advance curves during rapid acceleration. They can be programmed to tolerate differences in idle quality and manifold vacuum due to altered cam profiles and fuel systems.
Field Service Mode
On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.
WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.
If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.
Backup Fuel Mode
A 3.9K resistor installed between ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is more commonly referred to as the "Limp Home" mode of operation. Backup fuel is usually around 12:1 or richer from the factory.
Miscellaneous
An easy way to tell whether the PROM has been installed correctly in the PCM, and contains valid code set, is to watch the SERVICE ENGINE SOON light as you key on. Make sure the ignition is off for at least twenty seconds. Turn the key to the run position without starting the engine and watch the SES light. It should illuminate, then go out, then illuminate again and stay illuminated. This is called a SES light bounce. If you do not get a bounce then something is wrong and damage could result. If you try to start/run the engine when a proper SES light bounce is not present, then the computer will default to backup fuel and spark mode because it cannot execute a valid computer program. This can happen if the PROM is in backwards, or not seated, or contains a faulty program, or if the PCM has been damaged electrically.
(By the way, yes, I AM writing a book...)
On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.
WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.
If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.
A 3.9K resistor installed in ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel Mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is commonly referred to as the "Limp Home" mode of operation.
If that isn't enough information, clip and save this:
GM OBD I System Operation
This generation of engine control systems is a refinement of the first engine control computers and systems. First generation engine controls have some control over spark timing, a few output devices like the EFE system and EGR, and a minor role in fuel mixture adjustment. The OBD I system is designed to control and sense a wider range of functions than the earlier systems, including complete control of fuel delivery, and has proven to be much more reliable and effective.
The system has two basic modes of operation when the engine is running - Open Loop Mode and Closed Loop Mode. In a nutshell, Open Loop mode means the ECM is controlling the engine with essentially no feedback from the engine sensors - the control loop is said to be "open". Closed Loop mode means that the ECM sends control signals to the engine, scans the sensors for feedback to determine further corrections, then adjusts accordingly. The process is continuous, and the control loop is said to be "closed" since the sensors effectively communicate back to the ECM regarding the effectiveness of any adjustments made.
Open Loop Mode
When the starter is engaged, if the coolant temperature indicates less than 100°F and the intake air temperature is within 20°F of the coolant temperature, the cold start injector provides a spray of fuel or the cold enrichment subroutine allows longer fuel injector pulses on the service injectors. The cold start or cold enrichment subroutine is of eight seconds duration at a maximum. On a cold-start injector system the extra fuel is distributed to each cylinder via a air distribution system built into the bottom of the intake manifold. If the engine temperature is greater than 100°F, the cold start injector/subroutine is disabled by the cold start switch or ECM. Upon startup the ECM utilizes information in the PROM to establish the initial pulse rate for the injectors and the engine starts. At this time the engine is operating in Open Loop mode and will continue to do so until the engine warms up sufficiently to enable the oxygen sensor.
Closed Loop Mode
After a warm up period the ECM scans the sensors. If all sensors are operating and within their proper ranges, the ECM will enter Closed Loop operation. This means that the sensors are dynamically controlling the engine fuel and spark. The ECM receives information on intake air flow, coolant temperature, intake air temperature, exhaust gas oxygen content, barometric pressure, manifold pressure or vacuum, air conditioner status, power steering pump load, EGR operation, evaporative canister valve operation, engine RPM, vehicle speed, transmission gear selection, fuel pump status, combustion detonation, and throttle position. This information is used to calculate the proper ignition timing advance and pulse width for the injectors, and fires the injectors for the calculated period. This procedure is repeated continuously in very rapid sequence to maintain the optimum fuel air ratio. The electronic spark control components provide maximum advance, if engine knock is detected the spark is automatically retarded. This too, is a continuous process. It should be noted that the following components are matched for optimum performance - distributor EST module, ESC module, knock sensor and ECM PROM. These components are not interchangeable between 5.0L - 5.7L engines. In the event the information received is higher or lower than the normal range, a code will set in the ECM, and the Check Engine or Service Engine Soon lamp will light.
Block Learn / Integrator
The factory ECM/PCM has a learning capability which allows it to make corrections for minor variations in the fuel system to improve performance and driveability. There are two learning features. The Integrator and Block Learn (INT and BLM) and Block Learn Memory (BLM) cell. The INT and BLM feature is normal with a value of around 128 (the possible range is 0-255). If this value is higher than 128, it indicates that the ECM is adding fuel to the base fuel calculations because the system is running lean. A value lower than 128 indicates that the ECM is taking out fuel because the system is running rich. The integrator is a short term corrective action while the BLM is along term correction. The BLM value will change if the integrator has seen a condition which lasts for a longer period of time. There are from two to sixteen different cells which the ECM modifies, depending on RPM, airflow or manifold air pressure and other conditions such as AC on or off, etc. The ECM learns how much adjustment is required in each cell, retains it in memory, and applies these adjustments when the engine operates in that cell or RPM/Load Range. These features of the ECM allows the system to adjust itself constantly to your engine and assure peak performance for stock and other than stock engines.
NOTE: When the vehicle power is disconnected for repair or to clear diagnostic codes, the learning process has to begin all over again. To reinitialize the ECM, drive the vehicle at operating temperature with moderate acceleration and idle conditions for at least twenty minutes.
Performance PROMs typically change the parameters for fuel flow, fuel cut-off and spark advance-timing and will allow increased fuel flow and modify the spark advance curves during rapid acceleration. They can be programmed to tolerate differences in idle quality and manifold vacuum due to altered cam profiles and fuel systems.
Field Service Mode
On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.
WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.
If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.
Backup Fuel Mode
A 3.9K resistor installed between ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is more commonly referred to as the "Limp Home" mode of operation. Backup fuel is usually around 12:1 or richer from the factory.
Miscellaneous
An easy way to tell whether the PROM has been installed correctly in the PCM, and contains valid code set, is to watch the SERVICE ENGINE SOON light as you key on. Make sure the ignition is off for at least twenty seconds. Turn the key to the run position without starting the engine and watch the SES light. It should illuminate, then go out, then illuminate again and stay illuminated. This is called a SES light bounce. If you do not get a bounce then something is wrong and damage could result. If you try to start/run the engine when a proper SES light bounce is not present, then the computer will default to backup fuel and spark mode because it cannot execute a valid computer program. This can happen if the PROM is in backwards, or not seated, or contains a faulty program, or if the PCM has been damaged electrically.
(By the way, yes, I AM writing a book...)
tomj76
12-16-2014, 04:36 PM
Thanks for the replies.
I've used the A-B terminals in the past with a short circuit for the Field Service Mode, but I was not aware that a 3.9k ohm resistor would initiate another mode. Very helpful.
However, I don't think the fuel trim data is valid at all when the PCM is operating in open loop mode, while O2 sensor is cold (it doesn't even generate a voltage when it is cold). It usually takes about 2-3 minutes for the O2 sensor to get hot enough and the PCM to switch to closed loop mode. Time to get some real world data.
I've used the A-B terminals in the past with a short circuit for the Field Service Mode, but I was not aware that a 3.9k ohm resistor would initiate another mode. Very helpful.
However, I don't think the fuel trim data is valid at all when the PCM is operating in open loop mode, while O2 sensor is cold (it doesn't even generate a voltage when it is cold). It usually takes about 2-3 minutes for the O2 sensor to get hot enough and the PCM to switch to closed loop mode. Time to get some real world data.
Schurkey
12-16-2014, 10:55 PM
Actually, the SES lamp will indcate whether the BLM is in lean or enrichment mode at any given time...
I appreciate the time 'n' effort required to remind us of '80's paper-clip technology. Truthfully, I had forgotten most of this. I mean, it's not that I'd forgotten HOW to do this...I'd forgotten that it was POSSIBLE to do this.
That said, the reason I happily forgot all this was because Dealership techs never needed to know it at all, they had scan tools provided for them.
Independent techs (like me) needed to know it for a few years, and then dropped it like a broken rubber when aftermarket scan tools showed up and proved their worth.
At this point in the March of Technology, even hobbyists can afford a scan tool. My choice is for a "used but functional" professional-level tool rather than a brand-new consumer-grade unit. As long as the tool has software suitable for the VIN number of the vehicle(s) you're working on, you're set.
Time to get some real world data.
Yup.
Time to acquire a real scan tool.
I have a Snap-On "red brick" MTG-2500. This is the enhanced "graphing" version of the MT-2500. I have software and adapter cables suitable for 1980 1/2--2006 domestic, (and some Asian capability, which I've never used.)
This level of scan tool can be had on eBay or Craigslist for $400 and up depending on software version and included adapters/keys. There are other choices--an OTC Genysis has similar or greater capabilities, a Snap-On Solus would be better than my MTG.
I sold an MT-2500 with software to 2003 and a reasonable assortment of adapters and keys for $300 about two years ago. Would probably go for less than that, now.
There are "laptop" style scan tool software solutions--others on this site suggest them, I've never tried to use that style scan tool.
The least-likely tool to help would be a consumer-grade scan tool. Even that would be an upgrade from a consumer-grade CODE READER (not a "scan tool") which I consider to be a complete waste of time, money, effort, and enthusiasm.
I appreciate the time 'n' effort required to remind us of '80's paper-clip technology. Truthfully, I had forgotten most of this. I mean, it's not that I'd forgotten HOW to do this...I'd forgotten that it was POSSIBLE to do this.
That said, the reason I happily forgot all this was because Dealership techs never needed to know it at all, they had scan tools provided for them.
Independent techs (like me) needed to know it for a few years, and then dropped it like a broken rubber when aftermarket scan tools showed up and proved their worth.
At this point in the March of Technology, even hobbyists can afford a scan tool. My choice is for a "used but functional" professional-level tool rather than a brand-new consumer-grade unit. As long as the tool has software suitable for the VIN number of the vehicle(s) you're working on, you're set.
Time to get some real world data.
Yup.
Time to acquire a real scan tool.
I have a Snap-On "red brick" MTG-2500. This is the enhanced "graphing" version of the MT-2500. I have software and adapter cables suitable for 1980 1/2--2006 domestic, (and some Asian capability, which I've never used.)
This level of scan tool can be had on eBay or Craigslist for $400 and up depending on software version and included adapters/keys. There are other choices--an OTC Genysis has similar or greater capabilities, a Snap-On Solus would be better than my MTG.
I sold an MT-2500 with software to 2003 and a reasonable assortment of adapters and keys for $300 about two years ago. Would probably go for less than that, now.
There are "laptop" style scan tool software solutions--others on this site suggest them, I've never tried to use that style scan tool.
The least-likely tool to help would be a consumer-grade scan tool. Even that would be an upgrade from a consumer-grade CODE READER (not a "scan tool") which I consider to be a complete waste of time, money, effort, and enthusiasm.
Blue Bowtie
12-17-2014, 04:18 AM
So if you have the MT2500 you should be able to read the short- and long-term fuel trims, which could be valuable data.
tomj76
12-17-2014, 11:32 AM
My scan tool is an Actron CP9185 which I suppose is considered a consumer grade tool (although I've seen shops that use them too). It can support OBDI through an optional adapter. I don't know how much real time data it provides from an OBDI vehicle.
Schurkey
12-17-2014, 09:57 PM
My scan tool is an Actron CP9185 which I suppose is considered a consumer grade tool (although I've seen shops that use them too). It can support OBDI through an optional adapter. I don't know how much real time data it provides from an OBDI vehicle.
http://www.amazon.com/Actron-CP9185-AutoScanner-Diagnostic-Scanner/dp/B000T8UG52/ref=sr_1_1?ie=UTF8&qid=1418874586&sr=8-1&keywords=Actron+CP9185
According to the comparison chart, "live data" on OBD-I is "optional". I suppose you pay Actron some additional money, and download a software upgrade, along with buying the appropriate cable set.
http://g-ecx.images-amazon.com/images/G/01/automotive/detail-page/actron_comp_313_rest.gif
http://www.amazon.com/Actron-CP9185-AutoScanner-Diagnostic-Scanner/dp/B000T8UG52/ref=sr_1_1?ie=UTF8&qid=1418874586&sr=8-1&keywords=Actron+CP9185
According to the comparison chart, "live data" on OBD-I is "optional". I suppose you pay Actron some additional money, and download a software upgrade, along with buying the appropriate cable set.
http://g-ecx.images-amazon.com/images/G/01/automotive/detail-page/actron_comp_313_rest.gif
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