Iac question

[Mike1919]

Okay. I am aware that there are a thousand other threads about high idle fir a 4.0 but I'm working on fixing it and after reading through all the other high idle threads I want to clear something up. If i unplug the Iac valve while running and tge idle drops to normal, instead of almost stalling, wouldn't that mean it HAS to be a vacuum leak and not a sensor. And I'm positive the throttle plate is closing all the way. Am iI on the right track here?

 

[RonD]

Yes, that would be the right track, with warmed up engine.

There is an anti-dieseling/TPS(throttle position sensor) adjustment screw on the throttle linkage, some mistake this for an idle screw, which fuel injected engines don't have(no jets), since I assume you are not the first owner of the vehicle........someone may have adjust this screw and it is holding the throttle plate open a bit more than it should, with IAC unplugged and engine warmed up, you would adjust it so engine would idle at about 500 RPMs.

Also when looking for high idle reasons, unhook throttle and cruise control cables, they can also hold throttle plate open a bit more than it should be.

 

[Mike1919]

I did replace tge throttle body with one from a junkyard because the previous owner drilled out the butterfly valve and i checked to make sure its closing by shining a light through it. I really don't want to pull the intake due to a vacuum leak.

 

[Mike1919]

I think thats the only place left it can leak. I unplugged all the lines from the vacuum tree and capped it off and still nothing. I also sprayed ether liberally around everything and didn't find anything.

 

[RonD]

And the anti-dieseling/TPS(throttle position sensor) adjustment screw on the throttle linkage?

Throttle body leak is not uncommon, thin gasket there.

 

[hank857]

I unplugged my IAC about 200000 miles ago as it was hi idling at stops. Shop said there was a problem with the wire harness. So I just unplugged and have no issues with AC on or heater. Have passed emissions tests annually no problems. I did run the throttle screw under the idle stop up to compensate.

 

[Mike1919]

I think the tb gasket was part of the cause. When i first got it it was idling at 1500. Now i replaced the tps, iac, and throttle body with a new gasket and it idles at around 900-1000. Much better but I like things to be on point.

 

[grumpyguy]

This was my problem...
http://www.therangerstation.com/tech_library/4_0-loose-intake-bolts.shtml

BTW, I've owned this truck for over twenty years and this occurred about ten years ago. When my mechanic fixed it he showed me the gasket and it was very obvious what happened.

 

[RonD]

I think the tb gasket was part of the cause. When i first got it it was idling at 1500. Now i replaced the tps, iac, and throttle body with a new gasket and it idles at around 900-1000. Much better but I like things to be on point.

900-1,000 with engine warmed up and IAC unplugged?

If so then you do have a leak somewhere, lower intake as grumpyguy suggested is common on 4.0l OHV.

 

[enjr44]

900-1,000 with engine warmed up and IAC unplugged?

If so then you do have a leak somewhere, lower intake as grumpyguy suggested is common on 4.0l OHV.

Or he never adjusted the throttle body so it idles at 500 (with IAC unplugged) when he changed it. This thread sort jumps all over the place. I think he needs to start over with everything connected and really troubleshoot the problem.

Sort of like, with everything connected, and the engine warm, where does it idle (should be around 750)? Disconnect IAC, where does it idle (should be 500 or less, maybe stall)? If not, does adjusting TB stop change anything (if so, adjust to 500 or less)? IF not, find vacuum leak.

Ron, This may not be exactly right. But, it is in the ballpark. I know you have detailed this in previous threads with great detail.

 

[Mike1919]

How much did he charge to replace the gasket?

 

[Mike1919]

When warmed up it idles 750 with ac on and 900 with ac off

 

[RonD]

800-900 is almost normal for 4 cyl automatic, manual trans would be lower
V6 would be 700-800 for auto, 600-700 for manual

 

[Mike1919]

Yeah. My sisters beetle with a 4 cyl auto idles at about 1000 so i can understand that but mine is a 4.0 5 speed

 

[Mike1919]

Okay here's where it idles with the ac on and engine warmed up

And with ac off and engine warmed up