CChevy

This is a 1990 Chevy K1500 with only 100k on the dash. Very clean truck. So, a few days after purchasing it earlier this year it started a surge at idle. When I tried to drive it, it would buck when going into gear and die when I was coming to a stop. When cold starting the truck, it will go through the normal warm up cycle and then the idle comes down and the truck almost dies, then it surges back up to 1000 or so RPM. After about 10 min. of this, the truck will assume a steady very high 1400 RPM idle and stay there. When I shut the engine off and start it again, it goes back to surging. 4 mechanics have been through this truck and nobody can figure it out! The problem seems to be narrowed down to moisture. Its parked curbside in my neighborhood, and after heavy rainstorms and sitting overnight is when it really surges. Also, it has never thrown a check engine light. Now for the list of things that have been checked or replaced.

Intake manifold gasket replaced
Idle air control replaced
Distributor replaced
Distributor cap replaced
Crank position sensor replaced
Spark plugs replaced
Spark plug wires checked
Coolant temp sensor replaced
Throttle position sensor replaced
Oil pressure sensor replaced
MAP sensor replaced
ECM checked, cleaned, and connections re-greased
Engine harness connectors checked and re-greased
Chassis and engine grounds cleaned and checked
Alternator replaced
EGR checked for sticking
Any water leakage into the ECM area and cab have been examined, nothing was found.

The problem was always seemingly fixed after replacing each of these items. It ran great for a few days, and then I park it in my spot, we get a rain storm, and it doesn't idle again the next day. If it sits in the sun long enough, it goes back to smooth running. The most recent trip to the shop resulted in the alternator catching fire while the truck was dry and turned off. They figured the issue could have been bad alternator diodes burning up sensors the entire time. It ran great after the new alternator, until another rain storm came through. I'm about ready to call an exorcist at this point! I figured I would try a good Chevy forum, try to get some more brains on this one. Any ideas are welcome, please help and Thank you.

Sabino56

I didn't see checking fuel pressure on your list. If it hasn't been checked, I'd recommend checking. Fuel pressure can be marginal and cause all sorts of drivability problems and not cause a CEL or throw a code. The ECM has no feedback from fuel pressure - just assumes it's OK.

I have a 93 C1500 5.7L suburban. I had a surging at idle + other drivability issues. Turns out my problem was marginal fuel pressure. I had about 10-11 psi and it would start OK but sometimes would surge and almost die. I finally tracked it down logging data and noted that idle enrichment would kick in as it would about to die - RPM would run up well above 1k and then after a bit, idle enrichment would cut out, RPM would gradually drop until cycle repeated. I replaced the fuel pump with an ACDelco+ new fuel filter and had solid 13PSI - problem went away plus my other drivability problems went away.
No idea if this is your problem - no idea why moisture would be a factor if it's fuel pressure.

But regardless, if you haven't checked and verified fuel pressure - that would be the next step in my book. You're looking for a solid 12-13 psi. Here's a link which might help https://www.chevytalk.org/fusionbb/s...php?tid/253427
Good luck

kevinkpk

In my mind it is fuel related, not ignition. Fuel pressure as stated, first thing to check. Has any of the fuel lines (etc.) been replaced, also have you put gas line dry in it, and was there a different result?

CChevy

Wow, very interesting. I don't think fuel pressure has been checked yet. Those symptoms sound very similar to mine though. I will have to check this out. Thanks so much. I do not think I noted the engine size in the original post, but mine is a 5.7L 350 as well.

CChevy

No I haven't. Someone put a K&N fuel filter on it it looks like. No appearance of line replacement. What is gas line dry? Fuel additive or something?

kevinkpk

Yes, you say this problem is after a rain storm. This is an additive that dry's moisture in the gas tank, water in gas tank is heavier therefore if present it will set at the bottom of the tank. I assume you have a gas cap on the fill spout.

CChevy

yes, I do. Ok, I will look into this. Thanks.

Sabino56

I like your suggestion to look for a leak into the tank. That would explain the rain storm relationship much better than marginal fuel pressure all the time.

CChevy

Fuel filter looks clean and fuel pressure checked out 12-13 psi. It just rained pretty hard, I looked everywhere for water leaking in while it was raining, pulled the glove box out and found nothing. Even under the hood there is hardly any moisture at all after it stopped raining. It ran decently when I tested it just now. It honestly feels like it misses a cylinder every now and then, its not a perfect idle even when it isn't surging. I'm going to let it sit overnight, I'm positive it will surge for the morning cold start after the storm.

Irish_alley

i know you said you clean and check the grounds. but this to me all points to poor connection somewhere. i would disconnect the ecm connection and clean then put back together. as far as the engine grounds i might even replace them as they can appear in good connection but be broken internally or have corrosion at the connection.