Random shutdown/IAC goofiness

[Jatkat]

Howdy fellers. I'm about to drag up a very tired subject, so be prepared

My K2500 (1995, TBI, 7.4) Has been giving me some very frustrating issues as of late. Seemingly without warning, it'll shut itself off. Can happen at an idle, or when I'm coasting to a stop, or even under light throttle. I slapped a jumper into the ALDL connector, threw a code 54. Typically a 54 would indicate a bad relay, or a bad oil pressure sensor. Replaced both, no change. Cleaned up and redid the fuel pump ground, same story. I thought I had it figured after cleaning up the pump ground, but it went back to doing it today. No code 54 this time. At the same time, I've been chasing a high idle.

I borrowed one of those swanky scanners from a local mechanic that can actually talk to these old trucks. Everything seemed fairly normal, temp sender was communicating properly, and the IAC was adjusting the idle right. (I had earlier done a re-learn along with turning down the forbidden screw a little bit). Today, I still had the scanner with me, and after one of it's shutdowns, I plugged it in and looked at the data. The truck was calling for an idle of 975, but the actual was waay up at 1300. I noticed the IAC count had dropped to zero (all the way open). I limped it down the road, and let it idle for a while. Gave it a hoof full of gas, and the idle re-adjusted, and the IAC count jumped up to 65 or so.

Am I looking at a bad IAC here? Would that be enough to kill the truck if it snapped shut when it wasn't supposed to?

As far as I'm aware, the ECM is acting as it should, though I know they can be hard to track when they do start to malfunction.

Sorry for the extremely wordy post, but I've been pulling my hair out at this one. I just want my truck to be safe enough for us to drive without dying in the middle of an intersection!

 

[PlayingWithTBI]

I noticed the IAC count had dropped to zero (all the way open).

Zero is all the way closed so, either your min idle adjustment screw is holding the throttle blades open or you may have a vacuum leak.

 

[Jatkat]

If it was all the way closed, how on earth did it rev up to 1300 or so? The IAC seemed to be controlling idle to around 700-800 RPMS before, with a vac leak I would assume it couldn't drop it any lower than with the IAC closed. My truck does have that little vacuum solenoid that shoves the throttle forward when its cold, but the truck was at around 150 degrees by then.

 

[Jatkat]

I might be travelling down the wrong path focusing on IAC.

 

[Jatkat]

Is it possible that the "0" count was possibly a false value? Will an IAC report as 0 if it is not communicating properly?

 

[Schurkey]

What is the fuel pressure?

Does the pump prime when you first turn the key on?

Code 54, according to quick internet research, does NOT signify a failed relay or oil pressure sending unit, except in the abstract. It signals lack of voltage to the pump. Could be a poor connection, loose wire, popped fuse...ANYTHING that would reduce or prevent voltage in that circuit.

Fix that first. Then chase your goofy idle.

 

[Jatkat]

What is the fuel pressure?

Does the pump prime when you first turn the key on?

Unsure of fuel pressure, but truck runs perfectly fine 99% of the time. It already has a new filter in it. Primes as it should, except for immediately after an unplanned shutoff. Then it'll take a bit of cranking before it starts to prime again. Unsure if this is because of residual pressure or because the pump isn't responding. Wiring to the pump seems fine, except I haven't dropped the tank or pulled the bed to inspect the top of pump harness. I cleaned up a bunch of existing wiring that the previous owner had tapped into.

 

[Schurkey]

Unsure of fuel pressure,

Test it. Preferably with the gauge connected long enough to show whether or not the stalling is caused by lack of fuel pressure, or if the lack of fuel pressure is the end result of the stalling.

Primes as it should, except for immediately after an unplanned shutoff.

That sounds like evidence that the pump isn't running, causing the stall and still isn't capable of running immediately afterwards.

Then it'll take a bit of cranking before it starts to prime again.

That's not "priming". That's most likely "waiting for oil pressure to build" before the pump gets power. More testing needed, but it's pointing towards a failure somewhere in the fuel pump relay circuit--from ECM to harness to relay, including the power supply to the relay.

 

[Jatkat]

Sorry for the late update! I'm waiting for the silly inline TBI fuel test fitting. I will provide a short update though. The truck seemed to be improving. I went about 4 days without a shut off. Let it sit for around a week, and started doing it again. An interesting note, right after it has one of it's "episodes" my base idle will rocket up to around 1500 RPM's for a few minutes. The scanner will indicate the ECM is still calling for a lower RPM, but the truck will simply run at the higher RPM until it decides not to. Weird.