Puzzled with high idle only when rolling and starting

[///Manuel]

FIX! Hose clamps on PCV hose!

I checked my TPS at .97V when closed. Replaced the IAC. I also cleaned the MAF sensor.

The engine revs to 2.2k rpm when starting and drops slowly after a few seconds (3-4) whether cold or hot.

Then, when rolling if I shift to neutral (manual tranny) the idle goes to 1,500 and stays there until I stop. Very annoying when running the engine between 1k and 2k since the idle wants to throttle pretty much.

EDIT: Disconnecting the IAC prevents the car from starting when cold, the car barely starts when hot. When disconnected the engine idles fine and revs fine as well although sometimes on the verge of boging.

As soon as I plug the IAC back in, the idle jumps up some and slowly drops to normal. I couldn't find an important vacuum leak or any leak for that matter, however the throttle body has a hole in it of about 3/16" diameter so I believe it could get enough air through that to keep the car running when idling with IAC disconnected.

EDIT2: I disconnected the air intake going into the throttle body as well as the IAC and then plugged the throttle body plate hole with my finger. The car nearly stalled but sucked enough air to keep cranking slowly. I believe there are slight leaks near the PCV valve as well as the brake booster so I will be replacing parts in there and see if that fixes my issue.

FIX! Hose clamps on PCV hose!

 

[///Manuel]

Anybody any clue of what to look for? I disconnected the MAF got the same exact behavior only, when the idle went back down, it ran very rough. So the MAF is not causing it.

What are some of the sensors used during idling?
Engine coolant temp (ECT) sensor?

 

[///Manuel]

Anybody? I have no error codes.

 

[///Manuel]

Please please please???

 

[budwich]

As you indicated, the ECT is used to adjust things a bit. If the sensor is indicating to the computer that the engine is still cold, then the computer will attempt to run at a higher idle (to prevent stalling). Additionally, when you checked your TPS, the resistance should be "smooth" when the sensor is adjusted and the voltage should also move similarly. Your static measurement is somewhat high (I think according to chilton book). You could try and replace it especially if it has never been replaced on your 92.

 

[Mic_]

did you check/clean the IAC

also how about the engine coolant temo sensor to pcm (not the one to the gauge)?

 

[little x]

I have this same problem of idling high when rolling, but it drops back down when I stop. And I've done almost exactly what you have: cleaned MAF, adjusted idle with MAF unplugged, adjusted TPS, replaced IAC, and I still have a high idle when rolling. Keep us posted if anything works in fixing it.

 

[///Manuel]

Interesting little x, thanks you guys, I'll keep investigating. I definitely have an idle of 1500rpms at 15mph+, and it comes down as I slow down under 15mph and come to a stop.

Note: I replaced the IAC.

 

[gijoecam]

Could be an IAC issue, could be a simple vacuum leak. At any rate, the engine is getting more air than it should. If you unplug the IAC, it should stall. If it doesn't, it's definately a vacuum leak somewhere.

-Joe

 

[///Manuel]

Ah ha, unplugged the brand new IAC and the car kept running and running well! Idles perfect and responds well to throttle input by immediately going back to idle after accelerating. So definitely vacuum leak. Not sure where from though, gotta keep looking...

EDIT: Searched for a leak, couldn't find rapidly one. Unplugged a hose, the idle would drop close to the point of stalling, plugged the hole where the hose came from and the engine idled nicely again, that's with IAC unplugged.
Couldn't hear any engine variations or hissing sound from anywhere otherwise.

I voluntarily sprayed some carb cleaner into one of the vacuum lines and the engine varied but not by much at all and that is with spraying directly into the line! So my take is that wherever it's leaking from it wouldn't leak enough for me to notice any change...

 

[///Manuel]

Disconnecting the IAC prevents the car from starting when cold, the car barely starts when hot. When disconnected the engine idles fine and revs fine as well, although sometimes on the verge of boging when throttle is applied.

As soon as I plug the IAC back in, the idle jumps up some and slowly drops to normal. I couldn't find an important vacuum leak or any leak for that matter, however the throttle body has a hole in it of about 3/16" diameter so I believe it could get enough air through that to keep the car running when idling with IAC disconnected.

Could I have a bad new IAC? It seems slow to adjust or is it getting bad data from a sensor?

 

[dkkne]

*sigh* if you are going over 5 MPH, even if you are in neutral, you engine RPM will be raised slightly by the ECM, this is 100% normal.

 

[///Manuel]

Remember it's a manual. The throttle response when released is not right. It's just too damn slow to adjust and I should be able to drive between 1k rpms and 2k rpms with no problem like I used to. I checked the ECT voltage, it's fine.
Still no codes.
Again, the car runs perfect without the IAC except when cold! Both my old one and the new unit show the same symptoms. How annoying...
Also, I'm almost positive that this is affecting my gas mileage.

 

[///Manuel]

UPDATE: I found this rec.autos.makes.ford.explorer post which seems to show the same symptoms. I disconnected the air intake going into the throttle body as well as the IAC and then plugged the throttle body plate hole with my finger. The car nearly stalled but sucked enough air to keep cranking slowly. I believe there are slight leaks near the PCV valve as well as the brake booster so I will be replacing parts in there and see if that fixes my issue.

 

[///Manuel]

By adding small hose clamps to the PCV hose on both sides, I fixed my problem.

 

[gijoecam]

So, it was a vacuum leak after all, huh? Guess I called it back in post #9. Please make the check out to......

Glad she's fixed!

-Joe

 

[///Manuel]

Yeah I went off what you said about the stalling. When I put my finger on the throttle plate hole and unplugged the IAC off, the motor was about to stall but didn't and the leaks could be heard clearly then.

Thank you all for your help and suggestions.