high idle after warming up

[brucecooner]

At a cold start, the 4.0 (1988 Renix) will settle in at about 750 rpm, with an occasional dip to four or five hundred every couple of seconds.

 

Here lately once it's warmed up, it will idle at about 1250 rpm. 

Turning on the AC drags it back down to about 1000 even, but it still wants to idle higher.

 

It's very very consistent in this behavior, normal-ish idle when cold, high when warm. 

 

It had a few incidents in the past where I'd start it and it would immediately race to about 3000 and stay there.

I replaced the IAC valve last year, but I have seen that race condition once since. So it got better at least.

Because this new behavior only happens after warmup, and the race happened at startup, I don't think they're related.

 

So is there a sensor or system that affects the idle speed upon reaching a certain temperature?

 

It's so consistent (normal at startup, high at warmup) that I don't feel like it's a vacuum leak, but it could be an unexpected leak when some valve or something opens up to a passage missing a diaphragm or something like that. 

Could it be related to an emissions system maybe?

 

[Eagle_SX4]

Check your manifold bolts. They like to loosen up. It is probably the most common cause of high idle issues.

[MiNi Beast]

Tps sensor sounds like what you might be after. Next to the iac on the throttle body. 

[MiNi Beast]

32 minutes ago, MiNi Beast said:

Tps sensor sounds like what you might be after. Next to the iac on the throttle body. 

 

[brucecooner]

5 hours ago, MiNi Beast said:

 

I cleaned the throttle body some time ago, it wasn't thickly coated but did have some gunk so needed it.

And I tested the TPS before I replaced the IAC (which included cleaning the relevant connectors on the harness).

The throttle response has always been normal, and wouldn't the idle speed be controlled by adjusting the IAC valve?

 

I did "clean" the IAC by running a little throttle body cleaner through it, and did flush out a bunch of dark stuff.

It still ran the same, but I was still getting the race condition and I wasn't sure if I had harmed the IAC by cleaning it, hence its replacement.

I measured the new IAC against the old one and they were set the same so it didn't need calibration that I could tell.

 

I've heard about the intake bolts loosening.  Is it possible for heat expansion of everything once warm affect the seal negatively?

The bolts I can reach on the intake seem nice and tight though.

 

 

 

[MiNi Beast]

Yeah behind the collector those lower bolts tend to lossen up. Check all for good measure, it has been that simple before with jeeps.