My engine stalls at idle why?

see if the gasket below the tbi unit has blown use choke cleaner then check the hose leading to map sensor for being soft ifit is replace it. as it may be sucking together I am assuming all timing stuff is correct. then make sure tbi unit is spraying correctly sometimes they lose contact on top.

The gasket below the tbi is brand new and all bolts are tight. No leaking. Both injectors are making good contact.

ok a couple thing I forgot, check canister pruge vavle it may be open at idle and shouldn't be. it should have vacuum on one side only at idle if it h as vacuum going through valve then that may be your problem also if fuel filter hasn't been changed try that as well. also check )2 sensor for silicone on it as that screws it up so it gives a crappy reading to the pcm and it leans mix out. same with purge valve being open. also check egr for blockages and egr valve for being stuck open. the canister purge valve is red.

I have the truck listed above K2500 with a 454 and 4l80e. I recently replaced the torque converter and the truck is stalling at stop lights or if I’m sitting in the driveway the truck will start and idle great, put it in gear and it will idle with my foot on the brake for 1 minute to 1:30 and then will just shut off. not sure of the cause or what to check. It has a new TPS, IACV, plugs, wires, cap, rotor, and distributor. Any help or tests to do would be greatly appreciated, thank you.

Hi,

The first thing I would do is check to see if there are any diagnostic trouble codes stored in the system. This vehicle has an OBD1 system that doesn't require a scan tool to check. All that's needed is a short jumper wire or even a paper clip.

Here is a link that explains how it's done:

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

Next, when this happens, does the engine just shut off or is there anything that happens that seems to cause it to stall?

Let me know.

Take care,
Joe

Hi there, thanks for the reply.

I have no check engine light so I didn’t think to check for codes, but I’ll check it out. Also to answer your question no nothing happens or seems to be a cause it just idles normal like it would be sitting at a stop light and then like clockwork it just stops running no chugging or anything just turns off. I’ll pull codes and check back with you, thanks.

Hi,

Let me know. It could be a bad ignition control module. Does it start right back up for you every time?

Let me know as well as if you find codes.

Take care,
Joe

Yes, it does start right back up, no problem. Also If I didn’t mention it, the issue didn’t start until I installed the torque converter I never had this problem before.

Pulled stored codes and got:
42- Electronic spark timing (EST) circuit grounded
86- Analog/digital PCM error
87- Eprom error
Let me know what you think I can do with this information because I have no idea! Haha! Thanks

I fixed the timing recently and the codes could be from unplugging the advance. Thanks. let me know what you think.

Hi,

Two of the codes deal with the transmission. The EST is the one that is of most interest at this point. I suspect this has the 4L80 transmission. If that is wrong, let me know.

Here are diagnostics related to the EST code. Take a look through them and let me know what you find or if you have other questions.

_________________________________

1991 Chevy Truck K 2500 Truck 4WD V8-454 7.4L
With 4L80 Automatic Transmission
Vehicle ALL Diagnostic Trouble Codes ( DTC ) Testing and Inspection Manufacturer Code Charts DTC 42 Powertrain Controls - ECM/PCM With 4L80 Automatic Transmission
WITH 4L80 AUTOMATIC TRANSMISSION
Code 42 - Electronic Spark Timing (EST)

pic 1

Wiring For Diagram For Code 42 Electronic Spark Timing

pic 2


CODE 42 - ELECTRONIC SPARK TIMING (EST)


CIRCUIT DESCRIPTION:

When the system is running on the Ignition Module, that is, no voltage on the by-pass line, the Ignition Module grounds the EST signal. The PCM expects to see no voltage on the EST line during this condition. If is sees a voltage, it sets Code 42 and will not go into the EST mode.

When the rpm for EST is reached (about 400 rpm), and by-pass voltage applied, the EST should no longer be grounded in the ignition module so the EST voltage should be varying.

If the by-pass line is open or grounded, the Ignition Module will not switch to EST mode so the EST voltage will be low and Code 42 will be set.

If the EST line is grounded, the Ignition Module will switch to EST, but because the line is grounded there will be no EST signal. A Code 42 will be set.

Code 42 sets if there is an open or a short to ground in the EST or by-pass circuit.


TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 42 means the PCM has seen an open or short to ground in the EST or bypass circuits. This test confirms Code 42 and that the fault causing the code is present.

2. Checks for a normal EST ground path through the ignition module. An EST CKT 423 shorted to ground will also read less than 500 ohms; however, this will be checked later.

3. As the test light voltage touches CKT 424, the module should switch causing the ohmmeter to "over range" if the meter is in the 100-200 ohm position.


Selecting the 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched".

4. The module did not switch and this step checks for:

^ EST CKT 423 shorted to ground.
^ Bypass CKT 424 open.
^ Faulty ignition module connection or module.

5. Confirms that Code 42 is a faulty PCM and not an intermittent in CKTs 423 or 424.

DIAGNOSTIC AIDS:

The "Scan" tool does not have any ability to help diagnose a Code 42 problem. SEE DIAGNOSIS BY SYMPTOM - NO TROUBLE CODES STORED.


___________________

Let me know.
Joe

Hi there,

I was pretty confident that the stored codes were from me unplugging CKT 424 as described in the diagrams above to set my initial timing, since the previous owner replaced the distributor I wanted to be sure the timing was correct. I decided to clear the stored codes and got the truck to shut off 3 times before checking again, after doing that I checked for codes and there is nothing just code 12 continuously for “system good”.

Any other ideas? Thanks

Are you sure it isn't something simple like a vacuum leak? Does the engine run smooth? Does it always stall at a stop?

Joe

I have checked all vacuum lines that I can find everything is plugged into the transmission. The truck always shuts off at a stop while in gear after 1 minute to 1:30 of idling every time without fail. I cannot figure it out.

When you replaced the distributor, was the ignition control module new as well? Also, does it just stall or is there a jerking first?

Joe

I did not replace the distributor the previous owner did. I have not checked to see if the ICM is new as well, and no jerking present when it quits running, it’s almost as if the computer just tells the truck to shut off.

I have a feeling it's related to it or the coil. Here are directions for testing. Let me know what you find or if you have other questions.

The first directions are for the ICM. The attached pics are the flow charts.
______________________________________

1991 Chevy Truck K 2500 Truck 4WD V8-454 7.4L
Ignition System Check
Vehicle Powertrain Management Relays and Modules - Powertrain Management Relays and Modules - Computers and Control Systems Engine Control Module Testing and Inspection Component Tests and General Diagnostics Electronic Control Module (ECM) Tests - TBI Ignition System Check
IGNITION SYSTEM CHECK
Ignition System Check (Remote Coil/Sealed Module Conn. Dist.)

pic 1

Wiring Diagram For Ignition System (Remote Coil/Sealed Module Conn. Dist.)

pic 2


IGNITION SYS. CHECK (SEALED MODULE CONN. DIST.)

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Two wires are checked, to ensure that an open is not present in a spark plug wire.

1A.If spark occurs with EST connector disconnected, pick-up coil output is too low for EST operation.

2. A spark indicates the problem must be the distributor cap or rotor.

3. Normally, there should be battery voltage at the "C", and "+" terminals. Low voltage would indicate an open or a high resistance circuit from the distributor to the coil or ignition switch. If "C" terminal voltage was low, but "+" terminal voltage is 10 volts or more, circuit from "C" terminal to ignition coil or ignition coil primary winding is open.

4. Checks for a shorted module or grounded circuit from the ignition coil to the module. The distributor module should be turned "OFF," so normal voltage should be about 12 volts. If the module is turned "ON," the voltage would be low, but above 1 volt. This could cause the ignition coil to fail from excessive heat. With an open ignition coil primary winding, a small amount of voltage will leak through the module from the "Bat." to the "tach" terminal.


5. Applying a voltage (1.5 to 8 volts) to module terminal "P" should turn the module "ON" and the "tach" terminal voltage should drop to about 7-9 volts. This test will determine whether the module or coil is faulty or if the pick-up coil is not generating the proper signal to turn the module "ON." This test can be performed by using a DC battery with a rating of 1.5 to 8 volts. The use of the test light is mainly to allow the "P" terminal to be probed more easily. Some digital multi-meters can also be used to trigger the module by selecting ohms, usually the diode position. In this position the meter may have a voltage across it's terminals which can be used to trigger the module. The voltage in the ohm's position can be checked by using a second meter or by checking the manufacturer's specification of the tool being used.

6. This should turn "OFF" the module and cause a spark. If no spark occurs, the fault is most likely in the ignition coil because most module problems would have been found before this point in the procedure. A module tester could determine which is at fault.


______________________________________

Coil testing.

1991 Chevy Truck K 2500 Truck 4WD V8-454 7.4L
Ignition Coil Test
Vehicle Powertrain Management Ignition System Ignition Coil Testing and Inspection Component Tests and General Diagnostics Ignition Coil Test
IGNITION COIL TEST
Ignition Coil Testing

pic 3


PROCEDURE:

1. Disconnect the distributor lead and wiring from the coil.

2. Connect an ohmmeter as shown in step 1 of the image above. Use the high scale. The reading should be infinite. If not replace coil.

3. Connect an ohmmeter as shown in step 2 of the image above. Use the low scale. The reading should be very low or zero. If not, replace coil.

4. Connect an ohmmeter as shown in step 3 of the image above. Use the high scale. The meter should NOT read infinite. If it does, replace the coil.

5. Reconnect the distributor lead and wiring to the coil.

__________________

Let me know.

Joe

So I checked the system and did the coil test and everything seems good the coil gave me the correct reading with an ohm meter. I did learn today that if I try to get the truck to die while the engine is still cold before warming up it still does the same thing but instead of shutting off it just drops the idle low around 300-400 rpm's. Thanks

Hi,

Then the ignition system doesn't sound like the issue. Tell me, are there any exhaust leaks near the exhaust manifolds? Also, have you confirmed fuel pressure is within spec? And last, are you certain there are no engine vacuum leaks?

Let me know. Also, let me know if when this happens if it feels like the engine is loading and trying to move the vehicle. Almost like leaving a clutch out on a manual transmission.

Let me know.
Joe

There is exhaust leaks at the manifolds where they connect to the pipes, I broke 2 studs on one side and 1 on the other when I had to remove the exhaust to get the transmission out. could that really cause this issue? An exhaust leak? Also no vacuum leaks and have not checked fuel pressure yet.

Hello,

I just went through this thread. Yes, it seems to me that something involved with the converter repair has caused this. Sound like it was a bit a a hassle if you broke 2 studs you must have been up in and around the whole engine compartment. You either accidentally hit something or didn't hook something back up. A ground wire somewhere would be common for this. Maybe one going from the engine to the frame or the engine to the transmission or transmission to the frame. The grounds that run under there get highly weathered and easily broken off. If I may ask what was the situation that made you change out the torque convertor?
Tom

I changed it because the converter lockup wasn’t working and the converter wouldn’t build pressure very fast so if you let off the throttle while cruising once you got back to the gas it would rev up like slipping before it grabbed but it was the TC not actual transmission slipping. This issue was fixed after the new converter but then came my new problem and so far I can’t figure it out?

Also after the install I noticed a broken ground strap but couldn’t see where it came from I assumed it was an engine to frame (because the firewall to frame was still good) so that’s where I hooked it up, is it possible that ground should go to a bell housing bolt instead? Would it make a difference?

As far as I know every plug is plugged in, no vacuum lines damaged or unhooked no broken wires anywhere I can find. Any help you may have is appreciated, thank you.

Hi,

An exhaust leak before the O2 sensor can allow unmetered air to enter the exhaust. As a result, the sensors tell the computer the mixture is too lean. Then, the computer compensates by making the fuel mixture richer. It could be too rich. If you have a live data scan tool, check the short term fuel trims and let me know what they are.

Joe

No, I don’t have a live data scanner. I’ll fix my leaks because it needs to be done anyways, and then see if it changes anything.

Hi,

I hope that helps. Are the leaks before the oxygen sensors? I'm questioning it from past experience. I can't guarantee that is the cause, but it's possible. Ford V8's are known for it.

Let me know if you need anything or have other questions.

Take care,
Joe

Yes, the leaks are before. there is only one oxygen sensor if I’m correct and it’s at the Y pipe underneath the passenger door. I’ll be sure to let you know if I have any other questions, thanks for your help!

Hi,

There should be two. One should be in the left side rear area of the exhaust manifold. The other is in the exhaust under the truck.

See attached pic.

Let me know.
Joe

Okay, thank you!

Hi,

You are very welcome. Let me know if you have other questions or need help.

Take care,
Joe

Hey there, I fixed my exhaust leaks and the problem still persists. You have any other ideas?? Haha! Thanks

Hi,

I read back through all our posts. When you checked for a vacuum leak, was it only a visual inspection or did you use something like carburetor cleaner to check?

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

When it does stall, does it happen quickly or does it slowly stall?

Joe

Only visual and feeling with my hands/listening I was nervous to spray something to test, I was scared of it catching on fire. Also it just shuts off almost like turning the key off unless I give it some gas pedal right as soon as I can hear it shut off then it will barely stay running for maybe like 3 seconds then levels out and runs fine.

I understand your concerns. Your description sounds like it is more related to a fuel supply issue than it does electrical. In most cases, an electrical component just shuts down regardless if you hit the throttle. Have you confirmed fuel pressure? Heck, it could even be too high.

Joe

No, I haven’t tested fuel pressure yet. just another test I’ll have get around to doing!

The idea that you are able to keep it running is making me think it could be related. I still question a vacuum related issue as well. If you have a vacuum gauge, connected it to a port on the intake and start the engine. It should read between 18 and 20 inches. Try that and let me know what you find.

Joe

Sounds good! Will do.

Hi,

This post has resurfaced under new. I was wondering if you had any luck finding the issue.

Let me know.
Joe

No, I haven’t done any further test yet, just haven’t had much time at the moment. Hopefully I can get to it soon and check again for vacuum leaks and fuel pressure. Thanks

Hi,

Not a problem. Whenever you get to it, let me know. I was just curious why it listed as a new post. It confused me.

Take care of yourself,

Joe

I have replaced the throttle position sensor and the idle air control valve both to no avail. When idling the truck will idle real low and most of the time die. Just tonight after replacing the IAC valve it wants to die even if it is moving if you are not accelerating.