oldtruckguy

98 Silverado, 5.7L. Recently started idling high after warm up. After driving for a few minutes the idle goes up to around 1500 rpm. After shutting the engine off and restarting it idles normally, then may or may not go back to high idling. Also, sometimes when restarting after engine has warmed up, idles too low. Engine nearly stalls when put into gear but recovers after acceleration. Recently cleaned throttle body thinking that may clear up the problem but didn't make any difference at all. Any ideas?

Gumby22

What you’re describing sounds like a malfunctioning IAC valve.

Here’s more information about it - https://chevroletforum.com/forum/198...urious-101819/

oldtruckguy

Thanks, after reading a post from someone having the same issues I read your response listing the possibie causes, one being bad battery or alternator I had both tested and they checked out good. Read the info on the link you provided and I agree it helps to see and try to understand the inner workings of parts like the IAC. Very ingenious design. I'm going to replace the IAC and hopefully the problem is no more. What are your thoughts on using an aftermarket part? I The price of an IAC ranges from around $13 (not going to use that one) to over $100.

Gumby22

Generally speaking, there’s nothing wrong with aftermarket parts. The IAC valve in particular is a very simple component with only a few moving parts, so there’s not much that can go wrong with them.

That said, when it comes to aftermarket parts, you tend to get what you pay for. The brands I trust and install on customer vehicles are SMP, Delphi and Walker.

Make sure to clean the IAC bore prior to installing the replacement.

exflatfoot

I had similar issues. I found a vacuum hose was cracked at the rear of the TBI

oldtruckguy

Replaced IAC and high idle problem solved. Still having low idle or fuel issue. After shutting off the engine and restarting idle is low, around 400rpms, when I put the truck in gear the engine sputters and dies and is hard to start or I just sputter along until engine recovers. After recovery runs ok. Doesn't happen every time but seems to be more often than not. Any thoughts?

Gumby22

It’s possible the torque converter clutch/clutch solenoid is sticking.

How does it respond if you shift to neutral instead of give it throttle?

oldtruckguy

If I shift to neutral it continues to idle low but I am able to give it throttle and the engine responds normally, rpms go up. Never thought that that the torque converter could cause such a problem, what is the connection if you don't mind explaining to a driveway mechanic .

Gumby22

Since I’m not sure how much you know, here’s a crash course. Feel free to ask for clarification.

Under normal conditions, the torque converter slips during idle and acceleration. This is possible because the internal components of the TC rotate independently of each other.

At highway speeds, there’s a hydraulic circuit within the TC that allows the internal components to be “locked” together and eliminate the slipping - that’s what’s called the torque converter clutch (TCC).

The TCC is controlled by a solenoid which allows, or stops, fluid pressure to the TCC hydraulic circuit. As I’m sure you can deduce, fluid pressure means the TCC is activated and the TC is in a “locked” state; and no fluid pressure means the TC is “unlocked” and slipping can occur.

If the TCC solenoid gets stuck on, or a problem with the TC prevents unlocking, the TC can no longer slip.

With the TC locked at idle, the transmission is trying to move the truck forward and it puts a heavy load on the engine.

The effect is the same as if you had a manual transmission and didn’t release the clutch when stopping.

oldtruckguy

Thanks, the TC has always been a mystery to me how it works, I have a much clearer understanding now. Beginning to think I have a fuel problem though. I couldn't get out of my driveway this morning because the engine kept dying every time I put the truck in gear, then it wouldn't idle at all even in Park or Neutral and was difficult to start. Now the Service Engine Soon light is coming on intermittently. I ordered a code reader, should be in tomorrow so maybe that will help diagnose the problem.