91 c1500 stutter on acceleration

[badass66ss]

First off I want thank all of the folks who contribute to this forum as there is a wealth of knowledge here. Although this is the first time I am posting as I just finally created a login, I have been browsing the forums for the past 4 years. I especially want to thank ChevyTech as he has inadvertantly answered many of my questions while helping others. Now, my problem child. I recently bought a new (old) 91 c1500, 350, TBI, automatic. The truck has had a stutter on acceleration since day one. Once the truck warms up, when accelerating, it acts like air is retarded. What I have done: plugs, wires, cap, rotor, distributor, CTS, MAP, new injectors, coil, new ECM, new federal PROM, fuel filter, timing control module, and knock sensor. We bypassed fuel filter to check fuel pressure and I get a steady 12 psi during idle and acceleration. I checked all ground straps and all are secure and the wiring is not damaged. My uncle is a retired mechanic of 45 years and he is stumped as to what could be causing this. Also, when unplugging O2 sensor or other components, I do not receive a cel. PLEASE HELP.
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[badass66ss]

Sorry, also replaced TPS, muffler and cat. Thanks!
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[mountie]

Originally Posted by badass66ss

Sorry, also replaced TPS, muffler and cat. Thanks!
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This might be in left field, but I had a sort of stutter on a light acceleration when I had 25,000 miles on my '88 350 TBI. It was in the torque converter. The Lock Up was bad.

[ChevyTech]

You’re welcome and thank you for mentioning my posts to other member’s questions.

I agree it could be a transmission issue, if the shutter you refer to is not a misfire, but the first thing I would work on is the no cel light. The system should turn the service engine light on and store a trouble code if the oxygen senor is unplugged and the vehicle is driven.

The service engine light should also come on with the key turned on and engine not running.

If the light never comes on the bulb could be burnt or may have been removed by someone. Another possibility is that some of the power supply circuits to the ECM are not supplying power. Sometimes the ignition switch on the lower part of the steering column will fail and only supply power to some circuits coming out of it.

Here is a pin-out chart for testing the power leads to the ECM. This chart should be accurate for your 1991 350TBI truck.
http://www.wedgeparts.com/images/747ecm1.jpg

The chart above is from this site which has a lot of helpful information:
http://www.wedgeparts.com/t8tbi-4.html

Here is another chart that may help:
http://www.chevythunder.com/1227747_ecm_198892.htm

I would definitely get the service engine light working first.
If the light does work but the system will not set a trouble code I would pursue that.

[badass66ss]

Thank you both for your replies. I really don't believe this is a tranny issue as it is more of a misfire/fuel problem. The exhaust sort of pops and the new tail pipe is completely black inside after only 100 or so miles. I feel that it is possibly over fueling. The cel does come on when the ignition is turned on so the bulb is intact. I have a bad feeling ChevyTech is on to something, when I began looking through the wiring under the dash, they have a ton of wires spliced for an aftermarket alarm... Thanks for the chart, hopefully I will find time after work tonight to pull out my meter and start checking.
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[TennesseeZ]

Originally Posted by badass66ss

First off I want thank all of the folks who contribute to this forum as there is a wealth of knowledge here. Although this is the first time I am posting as I just finally created a login, I have been browsing the forums for the past 4 years. I especially want to thank ChevyTech as he has inadvertantly answered many of my questions while helping others. Now, my problem child. I recently bought a new (old) 91 c1500, 350, TBI, automatic. The truck has had a stutter on acceleration since day one. Once the truck warms up, when accelerating, it acts like air is retarded. What I have done: plugs, wires, cap, rotor, distributor, CTS, MAP, new injectors, coil, new ECM, new federal PROM, fuel filter, timing control module, and knock sensor. We bypassed fuel filter to check fuel pressure and I get a steady 12 psi during idle and acceleration. I checked all ground straps and all are secure and the wiring is not damaged. My uncle is a retired mechanic of 45 years and he is stumped as to what could be causing this. Also, when unplugging O2 sensor or other components, I do not receive a cel. PLEASE HELP.
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Please explain the benifit of bypassing the filter, it seems to me you'd want to know the pressure that the TBI sees with the filter in line, as that's the way the engine normally operates? Subscribing to your thread, as I have 2 vehicles that do the same thing, and I've already tried everything I know.

[badass66ss]

TennesseeZ, the fuel filter is new and the only reason I bypassed it was for ease of checking fuel pressure. It ran the same without the filter thus that isn't causing the problem. I have a feeling that I have some Wiring problems to hunt down... Stay tuned
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[ChevyTech]

Anytime a TBI truck seems to be running rich you should be suspicious of the coolant sensor that the ECM monitors. You can find information on this in the bottom two threads below.

At the top of this forum you can find helpful information in the post that looks like this:

88-98 FAQ's "Is your truck running weird? Check here it could help"

Here is a link to this thread at the top of this forum where you can find helpful information.
http://67-72chevytrucks.com/vboard/s...d.php?t=445487



Running Rich, or acts like no choke = Coolant sensor testing:
http://67-72chevytrucks.com/vboard/s...d.php?t=322811

http://67-72chevytrucks.com/vboard/s...d.php?t=379142

[badass66ss]

I actually replaced the cts and it was not the fix. Did some testing last night and I had two "sensor grounds" that did not seem to have a ground. Is this normal? I also finished the wiring nightmare under the dash and removed all the wiring that was spliced into my harness for the aftermarket alarm. I found a set of relays that look factory but have been cut up behind the driver side kick panel. I will post a picture when I get to my computer. Not sure what they are or where the wiring is going...
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[ChevyTech]

I see you said you replaced the coolant sensor in your first post – sorry my bad!

You are onto something. If the senor ground is bad it will run rich. All the grounds should test to be good.

The ECM and engine sensors ground at the front of the intake manifold under the front intake bolts/studs or by the thermostat.

It is common for the grounds at the front of the intake to be bad.

Check the grounds and move them to a better spot if needed.

[badass66ss]

Ok so the 2 grounds that were not lighting my test light were A11 and D2. I actually put a new thermostat in and a new end on the ground wire. The stud in the intake was clean as well. Should I try jumping a ground from the battery to that connector for testing purposes?

[ChevyTech]

A11 & D2 should be connected to ground WITH THE ECM PLUGGED IN.

If you unplugged the connector from the ECM and then tested for ground there would be NO CONECTION TO GROUND.

A12, D1 & D6 are the ECM grounds.

Putting a jumper from the ground wires on the intake to a known good ground is a good test.

[badass66ss]

Ok, so I checked all grounds and voltage readings at the ECM... Everything seems to be in the operating ranges listed on the pin chart. So I again am at a loss. Any ideas???
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[mountie]

Originally Posted by badass66ss

Ok, so I checked all grounds and voltage readings at the ECM... Everything seems to be in the operating ranges listed on the pin chart. So I again am at a loss. Any ideas???
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Last year, when I was frustrated, solving a sluggish ignition/acceleration, I went to a friend's shop that had a Snap-On analyzer, it showed what was good, so I could focus on the glitch, which was a failing ECM & coil.

(The coil was the OEM style MSD replacement, and it wasn't old.)

[ChevyTech]

Originally Posted by badass66ss

Ok, so I checked all grounds and voltage readings at the ECM... Everything seems to be in the operating ranges listed on the pin chart. So I again am at a loss. Any ideas???
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Have you tried disconnecting and plugging the vacuum hose to the EGR and then taking it for a test drive?

Another thing you may want to do is test the compression. As these TBI truck get old it is becoming common to see the head gaskets fail between the center cylinders. When they first start leaking, often the engine will ping a little, and not run as smoothly.

[badass66ss]

Yes I tried the EGR test to no avail, the truck has good compression and the gaskets only have a few thousand miles on them.
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[ChevyTech]

Can you give us a better description of what is happening?

Does it misfire? Is it dropping a cylinder or two on acceleration?

Is it only on hard acceleration, or any acceleration?

Does it feel like the truck is trying to pull a train? No power?

Are the spark plugs Black with soot?

You replaced the ignition parts so you must have thought it was an ignition problem, is this correct.

Have you tried buying your fuel somewhere else?

Have you taken a fuel sample to see if there is water in the fuel?

[mountie]

The fuel........

Today's additives / Ethanol damages fuel systems, especially carburetors on motorcycles but also cars / trucks.

The jetting / injectors get corroded. If you let an engine sit without being started for a couple weeks, ..even a new tune-up will seem to go bad after an engine not running for a short while.

I have 2 expert tuners (one NHRA T/F & the other high performance motorcycles) told me to put either Sta-Bil or a similar product in the gas tank..... EVERY MONTH.!!

If you own a motorcycle, get Sta-Bil now, & put it in your tank, before it's too late if you have carbs.

I put it in my truck. It runs & idles great.

Something to consider......

[Weasel Diesel]

Ok, you might wanna focus some attention to this. The IAT Intake Air Temp sensor. Now these little pukes will lie if they go bad. Ethanol in today's fuels eat old ones up by eroding the sensors coating. Todays replacements are made with a better coating than those of old. Remove the IAT, and visually inspect it. If it is black, gooked up in any way, or looks eroded, replace it... usually about 10-15 bucks at box store. Just recently, I had a scanner on one that read correct info, but truck ran like too much fuel, not enough air. I took a shot in the dark, inspected it, It didn't look right, so I replaced it. Bingo, that was it. The little POS was lying the whole time.

[badass66ss]

Can you give us a better description of what is happening? the truck seems flooded once it is warm, I would compare it to a choke staying open on a carb when it is warm and you are giving it full throttle

Does it misfire? Is it dropping a cylinder or two on acceleration?It doesn't misfire

Is it only on hard acceleration, or any acceleration?Mostly hard acceleration, but the warmer the truck gets the worse it gets and even moderate acceleration causes the problem, it idles perfect

Does it feel like the truck is trying to pull a train? No power?The truck feels powerful, but the stutter is not allowing it to show its true power

Are the spark plugs Black with soot?I haven't checked them again since i replaced them

You replaced the ignition parts so you must have thought it was an ignition problem, is this correct.No, i was just doing my due diligence and replacing a lot of ollllllld parts

Have you tried buying your fuel somewhere else? Sam's Club, Chevron, Shell.... all the same problem

Have you taken a fuel sample to see if there is water in the fuel?

[badass66ss]

Mountie, Stabil is a great product and I use it often, especially in my 66 chevelle that doesn't get driven as much as it should.

Weasel Diesel, 91 c1500's do not have IAT sensor, but that is good info as i have seen them go bad.

I feel like the truck is not going in to closed loop operation. From what I understand, the only items that tell the ECM to go to closed loop is the O2 sensor and CTS sensor. Is this correct? I am going to check the voltage on the O2 tomorrow when the truck is at temperature. I have a 195F thermostat, and the guage seems to be reading about that temp.

[ChevyTech]

The system will not go into closed loop without seeing voltage readings on the oxygen sensor that shows the oxygen sensor is warm enough to use to control the mixture.

My guess is that the system is going into closed loop operation.

Try leaving the oxygen sensor unplugged and see how the truck drives after it warms up. This should set a trouble code for the O2 sensor and turn the service engine soon light on. This will hold the system in open loop and it not modify the mixture but will instead run off the program. This will also test if the ECM can recognize a problem and set a trouble code.

If the oxygen sensor is giving the ECM bad data it could be causing a rich mixture when it goes into closed loop operation.

Air getting in the exhaust system can affect the o2 reading. In open loop operation (cold) the ECM ignores the o2 sensor reading, but in closed loop (warm) the ECM uses the o2 reading to determine if the engine is running lean or rich. If the exhaust system leaks ahead of the o2 sensor, (or just behind it), the pulses in the exhaust will draw in air and cause the o2 sensor to get an incorrect reading. A vehicle with an A.I.R. will have the same problem if the air management valve allows air to be injected in the exhaust manifolds during closed loop operation.
Exhaust flow moves forward and backward in the exhaust system more then many people realize.

You can monitor open or closed loop status without a scan tool on OBD1 vehicles.

You can go into "Field Service" mode by grounding the A&B pins and starting the engine. In this mode, the SES light will indicate whether the system is in Open or Closed loop. In Open Loop the SES light flashes two and one-half times per second. In Closed Loop the SES light flashes once per second. Also in Closed loop the light will stay OFF most of the time if the system is running Lean. It will stay ON most of the time if the system is running Rich. While in the Field service mode new codes cannot be stored and the closed loop timer is bypassed.

[badass66ss]

Update: so I checked closed loop operation and is indeed going into closed loop. I also read the voltage on the O2 and it varied between 0-.70. Not sure if this is normal. I then unplugged the O2 and drove for about 20 minutes. Still no cel and the truck ran the same. I double checked ALL grounds based on the wiring diagram as the pin chart was missing a few. Again, all checked out ok. Thoughts?
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[ChevyTech]

It should have set a trouble and turned the light on.

Just because parts are new does not mean they are any good.

If the ECM was a rebuilt unit, it does not suprise me if it is bad.

It looks like the ECM is bad.

You should see O2 readings swinging from close to .1 up to .9 of a volt if everything is working well. The little lower voltage indicates it is running a little rich. Don't expect a perfect voltage reading on a vehicle that is not running right.

[Mark S]

Originally Posted by ChevyTech

The system will not go into closed loop without seeing voltage readings on the oxygen sensor that shows the oxygen sensor is warm enough to use to control the mixture.

My guess is that the system is going into closed loop operation.

Try leaving the oxygen sensor unplugged and see how the truck drives after it warms up. This should set a trouble code for the O2 sensor and turn the service engine soon light on. This will hold the system in open loop and it not modify the mixture but will instead run off the program. This will also test if the ECM can recognize a problem and set a trouble code.

If the oxygen sensor is giving the ECM bad data it could be causing a rich mixture when it goes into closed loop operation.

Air getting in the exhaust system can affect the o2 reading. In open loop operation (cold) the ECM ignores the o2 sensor reading, but in closed loop (warm) the ECM uses the o2 reading to determine if the engine is running lean or rich. If the exhaust system leaks ahead of the o2 sensor, (or just behind it), the pulses in the exhaust will draw in air and cause the o2 sensor to get an incorrect reading. A vehicle with an A.I.R. will have the same problem if the air management valve allows air to be injected in the exhaust manifolds during closed loop operation.
Exhaust flow moves forward and backward in the exhaust system more then many people realize.

You can monitor open or closed loop status without a scan tool on OBD1 vehicles.

You can go into "Field Service" mode by grounding the A&B pins and starting the engine. In this mode, the SES light will indicate whether the system is in Open or Closed loop. In Open Loop the SES light flashes two and one-half times per second. In Closed Loop the SES light flashes once per second. Also in Closed loop the light will stay OFF most of the time if the system is running Lean. It will stay ON most of the time if the system is running Rich. While in the Field service mode new codes cannot be stored and the closed loop timer is bypassed.

Chevytech, Once again your expertise AND your ability to explain that expertise comes out. I'm new to computer controlled engines (my last hotrod had points and a condenser) and now I'm getting up to speed on them since I've been driving my 89 C1500 SCSW fleetside, 5.7 5speed. Helpfull info like this makes what would be an exercise frustration, doable. Many thanks, Mark