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.

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.

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,

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

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

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

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 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

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