94 k2500 no low end power and horrible fuel mileage

[hunter4ever12]

I have a 94 k2500 (true 3/4 ton not light duty) with a 350 and auto trans in it. It has little to no low end power at all I'm not sure it would even peel out in dirt, literally. It's also getting terrible fuel mileage about 8mpg. The idle on the truck once warmed up is a bit low as well. In gear it idles about 700 rpms. Once warmed up I do get a code 35 (IAC code) while idling. Previous owner replaced the IAC 3 months ago though. It also doesn't want to downshift when you get on the gas. I mean it will but you have to press the pedal to the floor for it to do so.

My initial thought was the kickdown cable was not adjusted right, but I looked and realized it doesn't have one cuz it's a 94 and that's the year they switched to an electronically controlled transmission, so the downshift is controlled by the tps. So far I have checked the base timing it's spot on at 0 deg before tdc. I haven't really dug much more into it yet. I know it could be several things like: tps, egr, fuel filter, intake leak, temp sensor ect ect.... Just wondering where you all would start first? I do have a parts truck with known good parts so "throwing parts at it" won't cost me a dime. But in all reality I would like to spend as little time as possible in diagnoses and repair.

Thanks in advance

[j cAT]

Originally Posted by hunter4ever12

I have a 94 k2500 (true 3/4 ton not light duty) with a 350 and auto trans in it. It has little to no low end power at all I'm not sure it would even peel out in dirt, literally. It's also getting terrible fuel mileage about 8mpg. The idle on the truck once warmed up is a bit low as well. In gear it idles about 700 rpms. Once warmed up I do get a code 35 (IAC code) while idling. Previous owner replaced the IAC 3 months ago though. It also doesn't want to downshift when you get on the gas. I mean it will but you have to press the pedal to the floor for it to do so.

My initial thought was the kickdown cable was not adjusted right, but I looked and realized it doesn't have one cuz it's a 94 and that's the year they switched to an electronically controlled transmission, so the downshift is controlled by the tps. So far I have checked the base timing it's spot on at 0 deg before tdc. I haven't really dug much more into it yet. I know it could be several things like: tps, egr, fuel filter, intake leak, temp sensor ect ect.... Just wondering where you all would start first? I do have a parts truck with known good parts so "throwing parts at it" won't cost me a dime. But in all reality I would like to spend as little time as possible in diagnoses and repair.

Thanks in advance

low power and poor mileage could be restricted exhaust.

[hunter4ever12]

The exhaust seems to flow pretty good and I just did install a glass pack type muffler replacing the stock one. How would I check the flow of the exhaust to see if I have a restriction? Could I use a vacuum/pressure gauge and remove the O2 sensor and screw the fitting for the gauge in there to get a back pressure reading?

I do know the exhaust donut gaskets leak I added sea foam and after doing so and driving it there was a considerable amount of smoke coming from under the hood I opened it up and that's where it was coming from. Could that be an indication that my cat is possibly plugged (ie the smoke "backed up" and couldn't flow out the back so it came from the small exhaust leaks)

[j cAT]

exhaust leaks do cause the O2 sensors to report a lean condition so you will burn more fuel this can cause the converters to run too hot. using a vacuum gauge see if there is a problem with the engine/restricted exhaust. if the engine is restricted on the exhaust then the vacuum will drop with higher RPM this is because the engine is a pump if the output is restricted the the input cannot suck . follow instructions on how to use a vacuum gauge to troubleshoot engine problems .. since this is OBD1 need to use the old testing stuff for finding problems.

[hunter4ever12]

Gotcha I will see what I can come up with. As far as the exhaust leaks go I did know that about changing the fuel/air delivery ratio. I just bought the truck a couple weeks ago so I'm trying to get caught up on the little things wrong with it that the previous owner didn't address. I've got the donuts for the exhaust flanges just haven't put them on. I have actually had the donuts in my "parts stock" for a long time haha. I bought them to temporarily fix leaking up pipes on my 97 7.3 Powerstroke but ended up getting bellowed up pipes for a good deal so never used the exhaust donuts.

[Schurkey]

No substitute for a proper scan tool. You (or someone else) could look at the data stream to verify that the fuel, timing, sensor inputs, etc. are all OK. Just because the IAC was replaced, does not mean the wiring to it is in suitable condition.

You did not mention that you disconnected the wire as specified in the instructions when setting the timing. Did you?

Don't forget the "other" converter--if the torque converter fails, (one-way clutch for the stator slips) then low-speed acceleration will be terrible, although high-speed could be unaffected.

"I" would fix ALL engine and exhaust issues first. If the problem remains, consider the torque converter.

[j cAT]

this is OBD1 . a PITA !

[Schurkey]

Originally Posted by j cAT

this is OBD1 . a PITA !

Why?

[hunter4ever12]

Originally Posted by Schurkey

No substitute for a proper scan tool. You (or someone else) could look at the data stream to verify that the fuel, timing, sensor inputs, etc. are all OK. Just because the IAC was replaced, does not mean the wiring to it is in suitable condition.

You did not mention that you disconnected the wire as specified in the instructions when setting the timing. Did you?

Don't forget the "other" converter--if the torque converter fails, (one-way clutch for the stator slips) then low-speed acceleration will be terrible, although high-speed could be unaffected.

"I" would fix ALL engine and exhaust issues first. If the problem remains, consider the torque converter.

When checking timing I did disconnect the advance wire as you're supposed to yes....the kicker is though after hooking the wire back up my timing didn't advance at all, what could cause this? Bad ecm? As far as wiring on the IAC I looked it over pretty good and it seems to be in good shape. I did give some thought about it being the torque converter but the fluid neither smells or looks burnt and there are no " black specs" or metal in it (or indicators of burnt up clutch/bands) I can't rule out it being the torque converter for sure but I don't think that's my issue.

So far here is what I found/did: removed the oxygen sensor before the cat and drove it around made no difference in driveability or acceleration, so I don't think my exhaust is plugged. I did find out the actuator for the front differential was malfunctioning and the front diff was locked in 4x4 all the time(that issue is corrected now.) Next while test driving it this weekend it almost died on me. I drove it around for about 20 min and then pulled back into the driveway and put it in park. Rpms were really low like 500, and it sputtered and dieseled but didn't die, rpms jumped back up a little on its own and then it repeated the process over and over. No codes came on during this time, and my oil pressure always stayed in an acceptable range. So not really sure what caused that or why.

I think the next thing ima do is see about borrowing my buddies scan tool and go from there. I also plan on rebuilding the TBI and HEI system in the very near future. I'll track down this issue one way or another.

[j cAT]

the scan tool is for OBD11. a 1994 is OBD1. OBD1 code reader. 500 rpm is good. should be 500-600RPM and stay steady in and out of drive. IAC does this. the vacuum must be checked and do the testing. the MAP can be bad reading wrong. the voltage out at idle should be 1.2 volts. if its wrong the the mixture will be off causing it to run rich fouling plugs. you say dieseling this is from high amounts of carbon in the combustion chamber. this reduces the size of it increasing comp ratio . this then causes it to get so hot the engine runs because the glowing carbon is causing the fuel vapor to continue to fire.

[Schurkey]

Originally Posted by j cAT

this is OBD1 . a PITA !

Originally Posted by Schurkey

Why?

Originally Posted by j cAT

the scan tool is for OBD11. a 1994 is OBD1. OBD1 code reader.

Ummm...so why not use an OBD 1 SCAN TOOL? It's not like they're scarce, and they read the data stream just as you'd expect from a SCAN TOOL.

Code-readers are consumer-grade trash whether they're for OBD 1 or for OBD 2, or both.

My scan tool is about a thousand years old, but it reads OBD 1 for Domestic and Asian vehicles as far back as '81 for GM applications, including some ABS. What it won't do is go newer (VIN-specific) than 2006; and software newer than 2009 doesn't exist as that's when Snap-On ended support for the tool I have. This does not bother me particularly as I don't own a vehicle newer than '03. At whatever point I upgrade to a more-modern scan tool, I'll about have to "give" this one away on Craigslist, as the eBay pricing is so low as to be impractical to auction.

[Schurkey]

Originally Posted by hunter4ever12

When checking timing I did disconnect the advance wire as you're supposed to yes.

Excellent.

Originally Posted by hunter4ever12

...the kicker is though after hooking the wire back up my timing didn't advance at all, what could cause this? Bad ecm?

Or a broken wire between the molded-plastic disconnect and the ECM connector.

I'd figure out which terminal in the ECM connector is for the timing plug, pop the connector off the ECM, and run an ohmmeter between that pin on the ECM plug, and the ECM-side of the disconnect plug. If you have continuity, but it's not grounded...I think you're looking for an ECM. If you don't have continuity, or it's grounded, you need some wire repair.

The service manual for the vehicle will have more-detailed diagnostics.

A scan-tool will tell you how much advance the computer is providing. IF (big IF) the computer is providing NO advance, you can be reasonably sure the lack of advance is a major cause of the poor performance.

[j cAT]

so you have a OBD1 scan tool ? so you can chart out O2 sensor voltages / fuel trims/ map/ and timing on a OBD1 Gm vehicle ? what is the tools model number ?

[Schurkey]

Originally Posted by j cAT

so you have a OBD1 scan tool ?

What, you've never seen a real, live scan tool for OBD1?

Originally Posted by j cAT

so you can chart out O2 sensor voltages / fuel trims/ map/ and timing on a OBD1 Gm vehicle ?

Yes, I have an OBD 1 (and 2) Scan tool. What do you think cars got diagnosed with when OBD 1 was new? Mind you, I'm not sure what you mean by "chart out" O2 voltage, etc. There is no chart or graphing function on most OBD-1 scan tools. There is a simple digital readout of current conditions, and some have the ability to datalog for short periods of time. My tool does do rudimentary graphing, but most similar tools won't. The data available to an OBD-1 scan tool is dependent on the computer; I have a fairly-extensive list of data parameters on my '92 Lumina, less on my '88 K1500, and things are fairly prehistoric on the '81--85 computers--but ECT, O2, MAP, TPS, fuel trim, knock-sensor/timing retard...yeah, all that and more is available. Holy crap, you're making me feel old. I haven't dicked with a pre-'86 computer since at least 1990..

Originally Posted by j cAT

what is the tools model number ?

Snap-On MTG2500. Extremely similar to this photo of an MT2500 that I sold a couple of years ago. As alluded to, the "G" in MTG signifies the graphing function. An MT2500 with pre-1999 software goes for about $250 on eBay, maybe less. People wonder why I keep harping on the advantage of "used" PROFESSIONAL-GRADE scan tools in preference to "new" consumer-grade code-readers.



OBD 2 adapter, plus a fistful of OBD 1 adapters.


I'm gonna have to take a photo of the OBD 1 software screen and upload. With luck, I'll have a chance to do that in a couple of hours. Gotta fix the damn taillights on my K1500 first. How can a light bulb kick my ass so thoroughly?

Any decent professional-grade scan tool will do OBD-1, if the tool's software allows, and if you have the appropriate vehicle interface adapter.

[Schurkey]

Had an extensive reply typed...then lost it somehow. I hate when that happens.


CONTENT FOR THE ORIGINAL POSTER:
I connected my scan tool to my '88 K1500; it does NOT provide data on spark advance. OTOH, when I set-up the scan tool to put the vehicle computer into "Limp-Home" mode, the idle gets slower and rougher, timing is retarded to initial, and the fueling goes full-rich. I don't know if your vehicle will display ignition timing. I think my '92 Lumina does. If you put YOUR vehicle into "Limp-Home" mode, and it doesn't run any different...it's probably because it's already IN "Limp-Home" mode, with retarded timing, rich fuel, poor power, poor economy. You don't need a scan tool to put the vehicle into "Limp-Home" mode. You can jumper a couple of connectors at the ALDL. Problem is, it's been so long since I've done it that way that I've forgotten the details.



REGARDING THE CAPABILITIES OF AN OBD-1 SCAN TOOL:

[EDIT August, 2024] I have since moved from this scan tool--Snap-On MTG2500--to a somewhat newer unit, (but still so old as to be no longer supported by Snappy) a Snap-On Solus Pro. The Solus Pro does everything the MTG2500 does, but with a bigger screen, more memory, FAR better graphing, and the availability of newer software--vehicles up to 2015 or maybe even a couple of years newer. However, the following description is for the older and more-primative '2500 series scan tools which Snappy made from 1988 to 1992 or thereabouts, and with software updates to 2009. ("Ancient!) Also, photos in Posts 14 and 15 were not showing up, and I fixed that. [/EDIT]

The data provided by my "Scanner" when connected to an '88 K1500 is

RPM, O2 mv, Integrator number
Open/Closed loop, Rich/Lean
TPS voltage, Block Learn number
IAC counts, O2 crosscounts
Coolant Temp, Prom ID
MAP "hg, MAP Volts
ESC counter, Knock Y/N
A/F Learned, Battery Volts
Batt volts High Y/N, Batt volts low Y/N
AIR diverter solenoid, AIR switch solenoid port/converter
Vehicle speed, High Gear Y/N
TCC Command Y/N, TCC apply circuit active Y/N
P/N switch (In gear or in neutral), A/C requested

The scanner scrolls four lines displayed to get all the info. I can only see four lines at a time. Newer scan tools have larger screens.

It will record 100 "frames", which is to say 100 updates to the data stream, (all the above data points at approx. 1 second intervals) and then view them later to look for anomalies. 100 frames is probably laughable today.

Beyond "seeing and recording" the data stream, I can use the scan tool's "Functional Test" setup to put the vehicle computer into:
Field Service mode,
TPS check/adjust mode
AIR solenoid functional test (Force air to exhaust)
Backup spark/fuel (Limp-home mode)
So this is the beginnings of "bi-directional control" which becomes much more sophisticated in newer vehicles--and newer scan-tools.

Dicking with OBD-1 is bad enough, doing it without a scan tool is two steps away from hopeless.

Sorry for blurry photos--I couldn't use the flash because it wouldn't capture what was on the screen. So I had to hold the camera VERY still!

My early software is Domestic-only. Asian available at extra cost. European software is moderately rare and expensive. I have a later cartridge that provides Domestic and Asian from '96 to 2006.




VIN-specific.


Functional Tests are described above. Custom Setup is for programming the scanner for metric vs. standard units of measure, and for choosing what the red LEDs represent.


Interfaces with ABS and body/transmission computers if equipped.


The graphing that the MT2500 doesn't do. In this example, I'm comparing manifold vacuum to O2 sensor crosscounts. When I stab the gas pedal, the vacuum drops and the crosscounts go to zero.
The way I have the LED lights set up, the first LED is telling me that the vehicle is in closed-loop operation. The second LED is showing that at that moment, the O2 sensor is detecting a rich mixture. The other two LEDs are for TCC "locked" and for "Knock sensor active", neither of which are happening as the trans in in neutral.