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Working on a High Idle

acschilling

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92 XLT
Hi everyone! 92 4.0 Standard. So I'm chasing down a high Idle. After replacing the fuel pump, TPS, ECT and CKP sensor there isn't much change. It starts fine and wants to drop to normal idle, but then it jumps up to 1300-1500 RPMs. I decided to try pulling sensors and started with the IAC valve and it went to normal, consistent idle. I warmed up the truck to pull the codes this morning and got KOEO 122; KOER 411 & 412. 122 is TP low (possibly grounded or open circuit). Sensor replaced two days ago before pulling codes.
411 and 412 (Idle speed system not controlling idle properly); 411 too high; 412 too low.
Wiring at the connectors appear good; have not probed anything.
Before probing, mostly because trying a new IAC sounds easier than probing wires, I was wondering if an idle that fixes itself with unplugged IAC points to a toasted IAC? Anyone with experience dealing with this?!
I don't get a CEL when unplugging the IAC, but unplugging the TPS I do.
Thanks for any help! Pardon me of it's a bit long, I just wanted to be thorough.
 


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RangerX

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Just for hahas, try removing the plastic air intake tube and inspect the bottom side for holes. They are notorious for disintergrating at the accordion pleats on the bottom side, can’t be seen from just opening the hood.
This was my experience with a high idle was chasing down.
Unmetered air, and lots of it, will cause a high idle.
 




Gadget X

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Also, check your PCV valve. With the engine running, if you block the opening on the bottom of the valve and your idle drops to normal, then it's letting air through all the time, which would equate to a vacuum leak=high idle. Had this happen to me recently. Wouldn't have found it, except that it was "buzzing" when the truck was running.
 




acschilling

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Just for hahas, try removing the plastic air intake tube and inspect the bottom side for holes. They are notorious for disintergrating at the accordion pleats on the bottom side, can’t be seen from just opening the hood.
This was my experience with a high idle was chasing down.
Unmetered air, and lots of it, will cause a high idle.
So my tube looked good. But the bottom section of the rubber piece that fits around the end, under the hose clamp, up to the TB is pretty gone. I need a new regardless. Any idea what those are called?
Tightened it up as best I could. Still bouncing.
No IAC connected and it's a smooth idle.
 




acschilling

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Also, check your PCV valve. With the engine running, if you block the opening on the bottom of the valve and your idle drops to normal, then it's letting air through all the time, which would equate to a vacuum leak=high idle. Had this happen to me recently. Wouldn't have found it, except that it was "buzzing" when the truck was running.
I shook the PCV and it rattled. I didn't run it with it disconnected though.

Cleaned the PCM harness under the alternator, tightened the BB vacuum hose (pushed it further up on the intake).

Still bouncing. Then...

No IAC connected and it's a smooth idle.
 




Gadget X

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I shook the PCV and it rattled. I didn't run it with it disconnected though.

Cleaned the PCM harness under the alternator, tightened the BB vacuum hose (pushed it further up on the intake).

Still bouncing. Then...

No IAC connected and it's a smooth idle.
Mine rattled as well, it was just letting air through all the time, which equated to a vacuum leak. I think the spring inside can weaken over time.
 




acschilling

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Mine rattled as well, it was just letting air through all the time, which equated to a vacuum leak. I think the spring inside can weaken over time.
Understood. I'm going to try picking one up tomorrow. Good maintenance at the very least. There could be something to this since cold starts are rough (IAC UNPLUGGED) but once it's warmed, idle is normal. I drove to and from a meeting, at least an hour, with IAC UNPLUGGED, and idle was steady entire time.
 




1991 sploder

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Yea look up inside the old iac valve the shaft may be rusted u can possibly clean it the idle air control valve needs a chance to take over maybe try loosening the back plate screw on the throttle that adjusts the idle down and see if it stays up on its own like around 750rpm also check to see if the shaft on the inside of the idle air valve is rusted in place... Sounds like a run away idle situation.
 




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