This was happening last week, so I replaced the IAC valve with a brand new premium one. It is still having this issue. The truck seems to run fine, and the idle lowers itself ok when I slow down to traffic lights, and stop signs. But if I am at a complete stop, and the truck is in park or neutral, and I rev up the engine and let my foot off of the gas, the idle wont catch. It will dip down to around 500 or 400, I will see the dash lights slightly dim and come back, then the idle will come up to where it should be. Sometimes it doesn't catch at all, sometimes it will just stall. Its a 1998 5.2.....


What can cause the idle to dip below normal, then return to normal besides the IAC valve?


BTW the very first sign which started this off, was when I was going to accelerate from a stop sign last week, and the truck kind of sputtered before it accelerated, then it died. It hasn't done that since, but now the truck dips below idle, and sometimes stalls when I rev it while in park.


Thanks

 

wish I knew what this is as well mine is doing the same thing but it always catches ,someone told me that the front O2 sensor is for this but I wonder how to test if that is whats wrong .its 50 bucks for this and some people have two ,but someone told me if its not running right alot of the time this is bad and it will not post a code so how do I know ?

 

I just went through this same battle with my 01 last week. Turned out to be the upstream 02 sensor....which I should have tested before I replaced the tps and iac. The way to test whether the sensor is bad or not is to simply unplug it from its harness located above the transmission. If the idle is fixed while the sensor is unplugged, then the sensor is bad. You don't want to run the truck with that sensor unplugged for long tho, it'll make it run rich and can screw up your cat if its not already gone.

 

alright, thanks guys....02 sensor next. I'll keep ya updated.

 

Just picked my 98 5.2L 4x4 up at the dealer for a similar problem 2 days ago. Ran fine, then all of a sudden, when I lifted my foot off the gas, it completely died. Started up fine, but wouldn't idle.

The shop said the only thing that was wrong was a dirty TB and Idle control motor. They cleaned it and now it runs fine.

Could that be a possibility?

 

Could be.... I just replaced the IAC. I'll take the throttle body off and clean it.....


Thanks

 

I had this same problem and 2 different shops replaced the IAC (total of 4 times), this would work for a little while (3 months to 3 days) and then it would start up again. Finally took it to the dealer and they replaced the IAC and cleaned the TB. Since then, no issues and it has been well over a year now.

 

I think some of the aftermarket IAC motors react to slowly...... My throttle body is clean as a whistle, the IAC is new, and I still sometimes have trouble with it.

 

is it possible for the 02 sensor to go bad without throwing a code?....or at least start to go bad?


No code is being thrown.

 

Yep. There are actually relatively few failure modes for the O2 that will actually set a code...... but, O2 sensor have a LOT of different failure modes. By the time the PCM actually figgers it out, it's probably been bad for quite some time. They should be changed every 60-80,000 miles anyway.