Intermittent High Idle Issue 3.4

Hey guys, I have an 01 S-Runner 3.4 5 speed.

When warm, usually on its second start (say I’ve parked it and left it for a short time, not long enough to let it cool down completely), my truck will occasionally start having a strange high idle issue.

For example, say I’m going 45 MPH. If I put it in neutral to slow to a stop, the idle will hover around 1500 to 2000 RPM. The idle will go closer to normal (850-1000) when I slow below 15-20 MPH and come to a stop. Before this starts happening, it will idle perfectly at 750 at all times when warm.

I replace the TPS, ECT sensor, swapped out the IAC valve for a cleaned used one, changed any rough looking vacuum hoses and it still does it.

I can’t think of anything else that would affect the idle so randomly and be dependent on speed, anybody got any ideas?

 

Seems like you’ve tried all the common causes, last basic thing I would try is to do a vacuum leak test. I used starting fluid and sprayed it on all vacuum connections and listened for a change in the idle. If it’s leaking it’ll suck the starting fluid and raise your idle.

If not, it might be time to try with more niche high idle causes.
Do you have a obd reader which can give readings live while driving? You might be able to see what is reading different on a hot-start vs cold start and determine the issue from there.

I had a similar, but opposite symptoms. It was high at all times except after it was parked for a short period. After a short period sitting it would idle normally. For me it ended up being the thermostat, it was stuck open and the coolant temp never got hot enough for the idle to drop fully during normal driving. I only noticed after watching coolant temp through obd reader during a hot start vs cold start.
I also had the idle fluctuations when coming to a stop, though mine was a fairly quick bouncing between 1200-2000. It would steady out when below 15-20 as well. I still don’t really understand what system is affecting the idle while in neutral decelerating. Though it sounds like yours is more of a steady high idle when decelerating?

I’d check to make sure the high idle while decelerating isn’t correlated to the brake pedal. If it is it could be a vacuum leak in the brake booster.

 

Thanks for your response!

I think next thing I’ll do is remove the ECM and partially disassemble to check for any corrosion or obvious damage. If I find nothing there, I’ll go get a live OBD2 reader.

Sucks to have such an intermittent issue, especially since throwing parts at it doesn’t seem to work! Lol.