1991 ford ranger 4.0 high idle

[91 blue ranger]

I have I problem with my Ranger. It has a high idle, like at 1300 but will go down after a few seconds, but if I unplug the iac it idles perfectly. Replaced the iac and cleaned the maf sensor. Any ideas?

 

[91 blue ranger]

If i unplug the maf sensor it idle a little rough. I looked for vac leaks but did not find any.

 

[91 blue ranger]

With the engine warm and the iac plugged in, if I tap the fuel petal a little bit it will rev up to 1500ish than hold at 1300 than drop to normal. Can not find the problem. Could it be a temp sensor or some thing like that?
Also I replaced the transmission (5 speed) and the neutral sensor is not on the transmission. Did not know if this would make a huge difference in the idle.
Thanks for any help!!!!

 

[RangerDanger2]

Bad idle control valve, or you have a vacuum line leak - again most likely at the ICV. They're like $40.00, I would replace it first.

Is the motor warm or cold when the symptom occurs? I know the 4.0L has a few high idle settings, when the engine is cold and first started, it definitely will idle momentarily at or around the 1,300 rpm mark, then drops to slightly under 1,000 rpm, the drops to 650 rpm when the motor is warmed up.

Hope this helps and good luck.

 

[91 blue ranger]

Thanks for responding!!
I already replaced the idle control valve, but no luck. IF I unplug the Idle control valve it idles at 800 to 900. And looked for vacuum leaks, did not find any. If the engine is warm it will idle at 800 to 900 rpms, but as soon as I tap the gas petal it shoots up to 1500 and holds there for a couple seconds. It is not nice for shifting as the rpms do not drop fast enough. Any more ideas??

 

[alwaysFlOoReD]

When you unplug the iac the idle should drop to ~500, or even stall. Recheck for vacuum leaks.

 

[91 blue ranger]

Will do
Thanks

 

[97ranger xlt]

a bad PCV valve can cause a high idle and not throw a code. they are cheap.

 

[Shran]

If your clutch switch is working properly, the neutral switch on the trans is irrelevant (should idle down when stopped with clutch pushed in.)

I agree with a vacuum leak. I have often found them on the vacuum tree attached to the upper intake or the various small vacuum lines that run off of it... these attach to the HVAC plenum area, vapor canister purge solenoid (another often broken item...) and brake booster. Check over all these, I bet you'll find a cracked rubber piece or something disconnected.

Also check the intake tube where it goes across the radiator, specifically the bottom of it. About 90% of 4.0's I've had have at least one crack somewhere. A glob of RTV will fix it. It causes a weird unmetered air issue since it's not pulling everything through the MAF sensor.

Could also be a leaking seal between the upper and lower intake and fuel rail... or even the seal at the throttle body.

 

[91 blue ranger]

Thanks for the responds Shran.
I went out to the truck and found only one leak, the line that goes to the air box. I plugged it it did not help. What do you guys use to look for leaks? I used starting fluid and carb cleaner.

If your clutch switch is working properly, the neutral switch on the trans is irrelevant (should idle down when stopped with clutch pushed in.)

With the neutral sensor wire uncrossed and when I rev the engine the rpms will not go down farther than 1500 rpm until I press in the clutch. So I crossed the wires. now it will go to an idle but it takes awhile.

 

[91 blue ranger]

a bad PCV valve can cause a high idle and not throw a code. they are cheap.

I checked the pcv valve and. (it rattles) Does that mean it is good?

 

[sgtsandman]

I checked the pcv valve and. (it rattles) Does that mean it is good?

Yes. You want it to rattle. That means it's working properly.

 

[91 blue ranger]

Yes. You want ti to rattle. That means it's working properly.

thanks

 

[97ranger xlt]

it may still be leaking air though, or its plumbing is leaking... it would be worth rechecking to eliminate
that as the source of the vac leak.

 

[91 blue ranger]

Checked the vac line going to the pcv looks good no cracks