When attempting to shift transmission into gear the engine shuts off, check engine light on

MIKE MCMANUS

MEMBER
1996 GMC VANDURA
5.7L
V8
2WD
AUTOMATIC
150,000 MILES

This car listed as a 1996 but it's probably a 1995 since it does not have an OBD II port. The check engine light is on, but when I stick a jumper between the boxes, nothing flashes at all. If we disconnect the battery, the check engine light is on before we even start it. Is the computer (ECM) shot, and that's why it doesn't flash any codes? The van will start, but won't idle without a lot of gas, and shuts off when you try to put it in gear.

7 Replies

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

Hi,

The idea that the CEL is on before the engine is started isn't uncommon. It should turn on. However, which jumper pins are you using?

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Also, the idea that it is hard to keep running leads me to believe it could be a fuel issue. Are there any smells from the exhaust when you keep it running? Is there is any smoke from the exhaust?

Do me a favor. Let's start by first checking fuel pressure. Here is a link to help. It may be you have too much fuel resulting from a bad fuel pressure regulator. Also, confirm there are no engine vacuum leaks.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Do this and let me know the results.

Joe

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Wednesday, January 20th, 2021 AT 7:38 PM

MIKE MCMANUS

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Thanks for the response! Two clarifications: First, after we disconnect and reconnect the battery, the CEL is on right away when we start it. I am putting the paper clip in A and B (upper row, two slots farthest to the right), and there is no reaction at all--the CEL stays on, doesn't flash or blink or anything. Also, the van has a TBI, not the more modern fuel injection, so there is no rail with a Schrader valve, so I'm not sure how to check the fuel pressure given that. I have wondered if it is a failing fuel pump, though. I will look for a vacuum leak. Also, the engine timing is likely off--we have been rotating the distributor slightly, trying to find a sweet spot. I could get a timing light, but the front of the van under the hood is so crowded it is very very hard to see, and we can't keep it idling enough to even check the timing.

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Thursday, January 21st, 2021 AT 2:21 PM

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

Can you record it cranking, starting and running for me to hear? As far s the fuel pressure, there is a T that goes between the TBI and fuel line. The gauge attaches to the T.

Let me know.

Joe

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Thursday, January 21st, 2021 AT 9:15 PM

MIKE MCMANUS

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4 POSTS

Hey Joe,

I did record it starting and running and almost idling, but my phone can't/won't upload it. It starts right up, even when cold. If you keep your foot on the gas for a couple minutes, it gets to where it barely idles, but conks out soon. If you try to put it in gear (auto trans), it dies immediately. White smoke first thing in the morning, but tapers down to a white mist. (I allow for the chance that a head gasket went bad--we just put in a water pump, but it would barely hold water before this.) He was too frazzled to let me take off the doghouse and see if there was a T on the fuel line. I had looked once, long ago, but didn't find one, though I didn't look that hard.

Tried again with the paper clip in A and B (top row, way to the right). No luck. I do have an OBD II that will read OBD I with the right cable, but I can't find that cable in Denver, and shipping is ten days out. This is my initial question--does the fact that it won't blink out any codes indicate that the computer is shot? Also, because it starts I don't think it's the IAC, Because the IAC runs it for a few minutes, then sort of hands it off to the computer, right? Seems to shut off on the handoff.

I might try again to upload. We do appreciate your input. Do you have a "tip jar"?

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Monday, January 25th, 2021 AT 12:15 PM

JACOBANDNICKOLAS

MECHANIC
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No tip jar. LOL Hay, pull the IAC and see if it is in good condition and carbon isn't an issue.

The idea that it won't flash could mean something other than the computer. It could be a wiring problem such as a ground. See if there is continuity to ground as well as power at the DLC.

You could have some carbon or other issue with the IAC. Here is a link that explains how to service one. Take a look through it.

https://www.2carpros.com/articles/idle-air-control-valve-service

Also, please confirm the engine coolant temp sensor's signal to the computer is accurate. If it is saying the outside temp is extremely hot or cold, the air-fuel mixture will be off and could be the cause.

I'm not sure why you can't remove the dog box for testing, but it would be a good idea to confirm fuel pressure.

Let me know if I can help in any way.

Joe

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Monday, January 25th, 2021 AT 6:45 PM

MIKE MCMANUS

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Hey Joe, Sorry for the long delay! So yes, I took out the IAC and yes that appeared to be at least part of the problem. The little "plunger" that extends out of the unit was all the way extended. I tried to fix it, but I wasn't confident in my cleaning and fixing, so I went a bought a new one from NAPA ($60.00) and that fixed it temporarily anyway. It started up and idled and he even drove it a mile or so, but once we parked it wouldn't start again (turned over but didn't fire up). The instructions on the new IAC talk about driving at 40 MPH for a while to acclimate the new set up, but we didn't quite get to that. It ran well, but rich (dark smoke that smelled like gas), and even idled as it should.

The bigger picture going on here is that the fellow lives in his van, and has only lived in vehicles for years. He is 66 years old with huge anxiety issues, like I suggest taking off the doghouse and he has a panic attack and starts yelling and screaming. So all this adds a significant degree of difficulty. I am an outreach worker who is a decent mechanic, and more and more people are living in their cars so I wind up doing stuff like this often.

So, it has new IAC, relatively new plugs, new rotor, cap and coil, and I think we are ballpark on timing. Check engine light still on, all the time, even after a battery disconnect. I can get a computer for $100.00 if that's all it takes. I have an OBDII (Actron) that will read OBD I if you have the right cord, which I don't, and I don't want to pay $45.00 for a cable just for one usage, that won't even tell me anything if the computer is in fact fried (I sniffed the computer, and it doesn't smell fried, but that also doesn't confirm that it is functioning).

I looked and looked again and found no port on the fuel line to check fuel pressure.

Thanks again for all your help!

Mike

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Monday, February 22nd, 2021 AT 9:38 AM

JACOBANDNICKOLAS

MECHANIC
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Mike,

To check fuel pressure, you need to get a T and put it between the supply line and the fuel rail. If there is no schrader, that is the only way to do it.

Do me a favor. Here are the directions for the IAC replacement. Take a look through them. At the end, it explains what to do for the relearn. The attached pics correlate with the directions.

_____________________________________

Change Vehicle Bookmarks Library Request Conversion Calculator Technician's Reference - Collision
IAC
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1996 GMC Truck G 1500 1/2 Ton Van V8-5.7L VIN R
Idle Air Control (IAC) Valve Replacement
Vehicle Powertrain Management Fuel Delivery and Air Induction Idle Speed/Throttle Actuator - Electronic Service and Repair Procedures Idle Air Control (IAC) Valve Replacement
IDLE AIR CONTROL (IAC) VALVE REPLACEMENT
Throttle Position Sensor Electrical Connector

pic 1

pic 2

pic 3

pic 4

REMOVAL PROCEDURE
1. Remove the electrical connector from the Idle Air Control (IAC) valve
2. Remove the IAC valve attaching screws.

CAUTION: If the IAC valve has been in service: DO NOT push or pull on the IAC valve pintle. The force required to move the pintle may damage the threads on the worm drive. Also, DO NOT soak the IAC valve in any liquid cleaner or solvent, as damage may result.

3. Remove the IAC valve assembly.
4. Remove the O-ring.

CLEANING AND INSPECTION PROCEDURE
1. Clean the IAC valve O-ring sealing surface, pintle valve seat, and air passage.

Use the carburetor cleaner and a parts cleaning brush in order to remove carbon deposits. Follow instructions on the container.
Do not use a cleaner that contains methyl ethyl ketone, an extremely strong solvent and not necessary for this type of deposit.
Shiny spots on the pintle or seat are normal. They do not indicate misalignment or a bent pintle shaft.
If air passage has heavy deposits, remove the throttle body for complete cleaning.

2. Inspect the IAC valve O-ring for cuts, cracks, or distortion. Replace if damaged.

NOTE: If installing a new IAC valve, replace with an identical part. The IAC valve pintle shape and diameter are designed for the specific application.

3. Measure the distance A between tip of IAC valve pintle and mounting flange. If greater than 28 mm, use finger pressure in order to slowly retract the pintle. The force required to retract the pintle of a new valve will not cause damage to the valve.

MEASUREMENT PROCEDURE

NOTE: If installing a new IAC valve, replace with an identical part. The AC valve pintle shape and diameter are designed for the specific application.

Measure the distance A between tip of IAC valve pintle and mounting flange. If greater than 28 mm, use finger pressure in order to slowly retract the pintle. The force required to retract the pintle of a new valve will not cause damage to the valve.

INSTALLATION PROCEDURE
2. Lubricate the IAC valve O-ring with clean engine oil.
2. Install the IAC valve assembly.
3. Install the attaching screws.

Tighten
Tighten the IAC valve attaching screws to 3.0 Nm (26 lb. In.).

4. Install the electrical connector.
5. Reset the IAC valve pintle position.

5.1. Turn the ignition switch to the ON position for 5 seconds.
5.2. Turn the ignition switch to the OFF position for 10 seconds.
5.3. Start the engine.
5.4. Check for the proper idle operation.

_______________________________

The black smoke indicates it's dumping fuel. We need to check pressure.

Let me know if the directions helps.

Take care and God Bless,

Joe

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