dies randomly at idle in gear

[Boog]

My truck sometimes just dies while idleing in gear, even with the A/C off. It starts right back up easily. There are no stored codes. No obvious vacuum leaks.
This 91 sportside I recently bought is 350/auto and I have been replacing some parts I am sure are at least 25 years old. A little tuneup at the actual 82,300 miles.
I rebuilt the tb (nice conical spray pattern now), replaced the AC plugs and AC plug wires, fuel filter, ecm's coolant temp sensor with an AC one also, new thermostat.
About those plugs. The underhood label calls for plug gap of 35 thousandths. I gapped the new ones at that and it did not want to run, at all. I have never gapped plugs at 35 on an HEI vehicle, always 45. But I opened them up on this truck to 45 and it runs better. And the random dieing was happening before I began this tuneup.
The truck has been in the local chevy dealer's shop numerous times before I bought it for this and a trans leak they had trouble stopping.
I don't do business with that dealership.

[Nick_R_23]

Likely a bad pickup coil or ignition control module in the distributor. Napa sells a complete, brand new distributor with a cap and rotor for about $150.

[Just call me Sean]

Did you use AC Delco original type replacement plugs?

[Boog]

Yes.

[Just call me Sean]

Strange. My trucks run fine with the plugs at the proper gap.

[Nick_R_23]

Originally Posted by Nick_R_23

Likely a bad pickup coil or ignition control module in the distributor. Napa sells a complete, brand new distributor with a cap and rotor for about $150.

Super common issue on these trucks.

[Boog]

Originally Posted by Just call me Sean

Strange. My trucks run fine with the plugs at the proper gap.

The AC plugs that were replaced were also gapped at 45 so Dad must have been through it too. I wonder if increasing the plug gap makes it fire stronger.

Originally Posted by Nick_R_23

Super common issue on these trucks.

I am pretty sure it has never had a new distributor. The shaft has no play and on first inspection inside looks good. That doesn't mean it is without fault though.

With the few parts I have replaced it starts easier now where before it spun over a good bit before starting each time, now it starts very quickly. So I am making headway with it.

This is an original, all stock '91 Chevy truck that has never been driven hard. My Dad more or less babied this truck. It has 82,300 miles on it and uses absolutely no oil and seems to be real good on gas mileage though I haven't put the pencil to it. I am reasoning that age is taking it's toll on electrical parts. It seemed to run better by replacing the plug wires alone and they were AC Delco wires but 30 years old. I bought new AC Delco wires to go back on it. My Dad was a stickler for keeping a GM vehicle all GM.

[Just call me Sean]

There isn't much to the engine control system on these trucks and since you did most of it I also would suspect something with the distributor. I've had bad ICMs and pickup coils do strange things. Other than that, it might be doing what my truck was. Mine was dumping the maximum amount of fuel it could into the motor due to improper oxygen sensor readings. The oxygen sensor was good though, I had to add grounds everywhere to fix it.

[Nick_R_23]

Just replace the whole thing, regardless of shaft play. Ignition modules can fail with age, pickup coils can crack, wear, or have issues with the magnetic portion. You can buy pickup coils and ignition modules, but by the time you add in a new cap and rotor (which you haven’t said if they were replaced or not), you’re at the cost of an entire new unit with a warranty. Aside from the ECM, the distributor provides the most control over the engine on these trucks.

[Boog]

Well looking at some dists the Napa one looks to be the best value @ $126 and I trust Napa brand. There are 21 stores around me. I'll check a couple more things yet.

[ky-donzi]

When my sons went out it was a similar situation but it continued to get worse....more often ..and harder to restart

Btw the pickup also pulses the tbi injectors, that was the tail tale sign. Eventually it wouldn’t restart and upon taking the air cleaner lid off I saw no fuel, did some reading and then replaced the whole distributor

[Just call me Sean]

I changed the pickup coil only. It was like $20.

[Nick_R_23]

Originally Posted by Boog

Well looking at some dists the Napa one looks to be the best value @ $126 and I trust Napa brand. There are 21 stores around me. I'll check a couple more things yet.

Those are the ones I go with, and to be honest, I now consider it a normal wear part and replace them on every new TBI truck I buy as part of the service/tune up. I think I’ve installed 4 just this year.

[Getter-Done]

Those Random problems are the worst

My suspect would be the ground on the ECM.

Had a Baby Caddy that would die at the stop sign crank back and go.

That was what that issue was.

Hope you find the problem

[61_FL_Apache]

Throttle position sensor? I had to replace mine for similar issues.

[Just call me Sean]

Originally Posted by Getter-Done

Those Random problems are the worst

My suspect would be the ground on the ECM.

Had a Baby Caddy that would die at the stop sign crank back and go.

That was what that issue was.

Hope you find the problem

I had to ground my computer to the battery, and I had to ground the computer case to the battery.

[Boog]

The ecm on these models is located behind the passenger side kick panel isn't it?

[Just call me Sean]

Mine is behind the glove box.

[Boog]

Ok thanks.

[Getter-Done]

If you put your Coffee cup in the Left cup holder it could disturb things

JK
Hope you find the issue soon

[Boog]

Well I finally got my fuel line adapter to test the pressure and it is a solid 11psi so that is ruled out.
So today I yanked the distributor out and I believe I found the problem as you guys suggested.
These dists are super simple with only a few parts. I chose to replace the pickup and module in mine. I don't have any pics but the retainer clip for the pickup was broken and half of it gone. The magnet part has bits missing. The shroud over the pickup was bent where those loose pieces got jammed up in there. So I spent considerable time straightening that shroud and cleaning up in there then replaced the pickup and module. I dropped it back in there and got it timed up. At this time it appears to idle a bit smoother in park and in gear. I haven't driven it yet. But I feel good about the repair.
I'll post back if the issue returns once it's driven a bit. Thanks guys.

[Getter-Done]

Great News

[Boog]

Update:
The problem is still there fellas.
The dist repair definitely helped it but the low warm idle speed and dieing when idling in gear is still there.
I guess the iac is next. It may be sticking. Yesterday while idling the compressor cycled on and the idle speed shot up way more than normal for 30 seconds or so then dropped back down. Only once.
I sure would like to see the live running data but I only have an obd2 scanner.

[Boog]

Ahah!
I did not replace the IAC.
I found, on these forums and another, directions for resetting the "minimum idle speed" on these TBIs. I did not find this info in my Chiltons manual.
First you have to drill out the plug covering the screw on the front of the TB unit. I didn't have to remove it from the engine to do so. Instructions are to turn on key, cross A and B pins of the ALDL then unplug the IAC. Start engine then adjust the idle speed screw to around 400 rpm or so. Turn off the engine and plug the IAC back in and restart. A relearn drive fine tunes the idle control.
Mine would not even idle, period, with the IAC unplugged. I had to turn the screw in 3 turns just to get a slow idle then almost a 4th full turn to get to what I feel is around 400 rpm and I stopped there. I have no tach but my earometer feels it's close. Once IAC is plugged back in the idle speed comes up more. I drove it all around town for an hour or so and it never once died at a stop as it did before.
That's just one drive but it feels promising.

[Boog]

I replaced the dist today with a new Napa unit.

One little run to the gas station. Short trip but it felt better.
We'll see.