GMC truck stalling and idle issues

Maintenance/Repairs
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1

Hi,

I have a 90 GMC Sierra K2500 5.7 that is giving me trouble.

Initially, with the AC on, the truck would idle low and shake at a stop while in gear. I tried replacing the IAC, EGR valve, TPS, coolant temp sensor, and one more that I don’t remember. Nothing worked and I eventually took it into a shop.

They did some diagnostics and all they could find was a dirty cap and replaced it. That fixed the shaking problem and the truck idles a bit better.

About a month later with only my fan on (no AC), while driving to work the truck died suddenly while moving. It started up fine with no codes, but I took it into a different shop for another look.

Again, they couldn’t find anything wrong with it. Fuel pressure looked good at 13 psi and he couldn’t get it to act up.

A week later, while steering with the AC on, the truck died again, but started up fine.

It still seems to idle low once it’s warmed up, and the exhaust sputters at lower RPMs. My AC growls but blows cold air. It does seem to load the engine quite a bit.

I’m just not sure where to look to try and solve this issue. The truck seems to run fine, but I don’t really trust it.

Thanks for any insight

2

Which cap was it? Could be a clue to the remaining problem. Your symptoms sort of sound like the truck is intermittently running lean. Do you get any diagnostic codes saying that? Vacuum leak? Have the brake power booster’s ability to hold vacuum assessed, and replace the PCV valve on a flyer. See if that has any effect. Could also be a spark problem, so having the ignition system tested would probably be next. An intermittent fuel pump or clogged fuel line/clogged injector is a possibility too. If I had that problem I’d measure the compression in all of the cylinders to get a baseline.

3

Distributor cap and rotor.

It also lost the ignition coil earlier this year, but when that went out it wouldn’t start.

Vacuum feels good at all the lines (just using my finger though), and the PCV valve has vacuum.

My injectors look good, I know that’s another thing one of the shops checked. No one has checked the vacuum booster, but I’ve never lost braking power.

The only codes I ever get happen when I unplug a sensor or something like that. Never while driving.

Thanks!

4

If replacing the distributor cap provided some help with the problem, then replace all the secondary ignition components.

After all, your vehicle is OBDI. And OBDI doesn’t detect for misfires.

Tester

5

Theclast shop that looked at the truck mentioned the pickup in the distributor could also cause intermittent stalling.

Would it be worth it to install an entire distributor along with plugs, wires, and coil? Or is that pickup something relatively easy to replace?

6

Let me ask you this simple question.

Can you remember the last time the secondary ignition components were replaced?

If not, it’s probably due.

Tester

7

Cap, rotor, and ignition coil all this year. Plugs and wires are about 18 months old.

8

Maybe a stretched timing chain?

https://www.rockauto.com/en/catalog/gmc,1990,k1500+pickup,5.7l+350cid+v8,1157655,engine,timing+set,5756

Tester

9

The number of times you mention AC, and AC growls, start there and after that is fixed see what is next.

10

Bad idle air control (IAC) valve maybe? It seems like the truck isn’t keeping the throttle open wide enough to keep it idling.