99 Chevy Blazer Rough Idle

99 Blazer Idles Rough and the Check Engine light is on. The mechanic showed number 6 valve misfiring. Replaced plugs/wires and distributor cap. Cleaned fuel injection. Problem still persists.

Could be lots of things....if the check engine light is on, a trouble code is stored, go to Autozone and they will read the codes for free. Write down the code and come back and tell us what it was.

Could be lots of things....if the check engine light is on, a trouble code is stored, go to Autozone and they will read the codes for free. Write down the code and come back and tell us what it was.

Autozone generated a code of PO300 or maybe P0300. It gave the following possible causes:

1st - Ignition System Fault - spark plugs, ignition wires, coil

2nd - Vacuum Leak

3rd - Fuel injection fault

4th - High or low fuel pressure

Could it possibly have something to do with the exaust system?

"Replaced plugs/wires and distributor cap." Did you replace the rotor too? That should have been done as well. How were the injectors cleaned?

p0300 is one of the hardest to fix, because of all the possible causes. It could be fuel or electrically related.

Test your fuel pressure, it should be:

KEY ON ENGINE OFF;

60 - 66psi while pump is running
55 - 60psi after pump shuts off, this pressure should hold steady and not drop.

KEY ON ENGINE RUNNING;

The pressure should drop 3 - 10psi



The coil is a common p0300 cause, to test:

http://www.image-hosting.ws/direct/1343.jpg

The Specifications it refers to at the begining of the chapter are:
Primary resistance..........0.2 to 0.5 ohms
Secondary resistance......5,000 to 25,000 ohms


TonyMazz posted this in an earlier thread, it describes the P0300:

This may help ... It is some insight to a P0300 Engine Misfire code...it's what the dealer would do...

Circuit Description

The Crankshaft Position (CKP) Sensor is the primary input to determine if misfire is occurring. Engine misfire is detected by monitoring crankshaft speed variations between cylinders. If a crankshaft deceleration occurs during a combustion or power stroke, the Control Module (PCM/VCM) will compare this change in crankshaft speed to the previous cylinder. If the crankshaft speed change is more than a maximum allowable speed, the misfire is detected. Misfire may occur in a specific cylinder or all cylinders randomly.

When an engine is misfiring, brief decelerations in crankshaft rotational speed will be detected by the CKP. The Control Module determines which cylinder has misfired based upon the Camshaft Position CMP Sensor input. Misfire data is stored for each cylinder in separate accumulators. After 100 combustion events, the misfire totals are compared to a calibrated maximum number. If the misfire is excessive, a this DTC will set.

Conditions for Running the DTC

Important: If start-up ECT is below -7°C (20°F), misfire detection is delayed until ECT is more than 21°C (70°F). If start-up ECT is more than -7°C (20°F), misfire detection begins after a 5 second delay.



No active VS sensor DTCs
No active TP sensor DTCs
No active MAF sensor DTCs
No active CKP sensor DTCs
No active CMP sensor DTCs
The fuel level is more than 10%.
The engine speed is between 450-5000 RPM.
The system voltage is between 11-16 volts.
The throttle position is steady within 2% for 100 ms.

Conditions for Setting the DTC

The VCM detects a deceleration in the crankshaft speed characteristic of either an emission type misfire or a catalyst damaging type misfire.

Action Taken When the DTC Sets

If the VCM determines that the engine misfire is significant enough to have a negative impact on emissions, the VCM turns ON the MIL after the misfire has been detected on 2 non-consecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.

Conditions for Clearing the MIL/DTC


The Control Module turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.

A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature is more than 70°C (158°F) during the same ignition cycle).
Use the scan tool Clear Information function.

Diagnostic Aids

The Misfire Index counts the number of misfires. The scan tool can monitor the Misfire Index. There is a current and history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring or use the history misfire counter for misfires that are not currently present.

Many different condition could cause an intermittent misfire.
Check for the following conditions:


Check the IC control circuit for a intermittent short to ground.
Check the spark plug wires and the coil wire for the following conditions:
Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
Check the wire routing in order to ensure that cross-firing is not occurring.
If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
Check for contaminated fuel or a low fuel level and the following conditions:
Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.
If the fuel level is low, the fuel pump may draw air into the fuel rail, causing a stumble and possible misfire condition. Check the fuel trim numbers in the freeze frame to determine if this has occurred. It would be likely if the short term fuel number was above + 20.
Check HO2S for abnormal voltage readings.
Check for a vacuum leak as a possible cause of the engine misfire.
Sticking intake or exhaust valves can cause a misfire when the engine is cold.
An intermittent may be caused by any of the following conditions:


A poor connection
Rubbed through wire insulation
A broken wire inside the insulation
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis. See: Diagnosis By Symptom

Test Description
The numbers below refer to the step numbers on the diagnostic table.


The misfire is considered random on all cylinders if, while viewing the misfire counters in the misfire data list, the misfire seems to move to different cylinders.
The misfire is considered consistent if the misfire is occurring on the same cylinder(s) consistently.
When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.

Check your fuel pressure.

Check your fuel pressure.

I tried a new ignition coil and the problem still persists.

The injectors were cleaned by that three step process. Solution poured into the tank, sprayed and swapped inside the housing under the air intake, and fed dolution through air intake hose. The distributor cap was replaced too.

I am thinking it is most likely a vacuum issue.

Let me know what you think. Thanks

I tried a new ignition coil and the problem still persists.

Did I ask you to check about the ignition coil?....No

Check the fuel pressure.

I swear to god lately people have been getting a little thick on the boards.

You should have checked the fuel pressure first and if it was within the specs I gave, then you should have moved on to the coil. I hope you tested the coil before replacing it...sometimes it takes a while to locate the cause of a P0300, if you just replace everything we tell you to check, it will get expensive and it doesn't have to.

I am not as advanced as some others with auto mechanics. Right now I am trying to be as educated on the problem as possible. Alot of the procedures are going to be done by the Chevy dealer because of the warranty. But this has got me up to speed as to what should happen and how. I agree with you on the fact that some mechanics start replacing parts before doing proper diagnostics. I'll get back when I get some more answers. -- Thanks

I am not as advanced as some others with auto mechanics. Right now I am trying to be as educated on the problem as possible. Alot of the procedures are going to be done by the Chevy dealer because of the warranty. But this has got me up to speed as to what should happen and how. I agree with you on the fact that some mechanics start replacing parts before doing proper diagnostics. I'll get back when I get some more answers. -- Thanks

First think you will need to stress them to do is check the fuel pressure.

Fuel injectors 1 and 6 were not working. Replaced them both. -- Thanks