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High idle 1991 Ranger 4.0 OHV, 1200 rpm

crs.wdp

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First time posting here but I've been reading all the posts on here as well as other sites trying to solve my high idle. I'm running out of posts to read so I figured I'd try posting it and see if anyone has any more tips.

It idles at 1200/1300 rpm. Its a 1991 Ranger 4.0 OHV Auto 4x4 STX reg cab 6ft bed.

This is where I'm at:

-I swapped the IAC with a made in USA Hitachi, as per one of the threads I read which mentioned only Motorcraft and Hitachi worked well on the rangers. It didn't seem to affect anything.

-If I unplug the IAC it drops to 1000.

-I did a smoke test with a cigar but couldn't find anything before I felt like I was gonna pass out, ha. All vacuum lines are in good shape. In addition to the smoke test I checked with starter fluid and propane. I've unhooked and plugged the vacuum tree and no change. I individually checked all vacuum lines with a vacuum pump and they all hold vacuum. I'm pulling 18 inches of vacuum at idle.

-New PCV and has tight seal.

-New air cleaner

-I've checked the voltage of the MAF, ICS, BMAP (Hz) and the IAC, all of which seem to be in line with specs found on the interwebs.

-The MAF sensor wires were cleaned and that certainly helped with the performance of the engine but not much of a difference in the idle.

-TB was cleaned and doesn't seem to have much gap around the buttlerfly.

-I don't see an EGR valve or a DPFE sensor. Not sure where the DPFE is if I have one.

-The A/C coolant lines are disconnected and the electrical wires don't have any voltage but I do get a light on the control switches in the cab if I press in for A/C. I can apply voltage to the compressor and see the clutch engage so I know it's current state is disengaged. Could it be possible the ECM is bumping the idle thinking that the A/C compressor is continuously engaged?

-The vacuum line that runs from just in front of the TB to the evap canisters has a solenoid. I can apply voltage and get it to funtion but does not have a perfect seal, not sure if it needs a 100% seal. Also, the connection is not complete (no voltage) see attached photos. Anyone know where / what this plug above the driver side valve cover plugs into?

-Is it possible the coil pack or another part of the ignition system is faulty and causing a high idle?

-The engine runs pretty smooth. At idle and when stopped at a light in gear it rumbles / shakes just a bit, but doesn't seem to misfire. Acceleration seems ok and there is no knocking or pinging.

Thanks is advance for help / advise.
 

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Dirtman

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The hose to the evap purge solenoid valve looks worn through. Go check on that...

20200511_171616.jpg
 

franklin2

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1000 rpm with the idle air valve unplugged is too high. I don't have the specs for your year, but the earlier trucks idled at 750 for automatic or 675 for a manual with the idle air control unplugged.

Are you positive the idle stop screw has never been touched? Or did you just buy this thing and are working through problems one at a time?
 

crs.wdp

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The hose to the evap purge solenoid valve looks worn through. Go check on that...

View attachment 42056
Thanks for the reply. Good eye, can’t believe you saw that in the photo. I saw that when I bought the truck and thought it was a smoking gun but the line is a hard plastic with a soft rubber jacket. I tested all the lines with a vacuum pump disconnecting from their source and plugging. It holds vacuum with no leaks.
 

Dirtman

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While your in that area, unplug that line entirely and cap it right at the intake manifold. It wont hurt anything but will rule out the purge valve leaking. If the purge valve leaks neither the smoke or spray test would work since the smoke would end up going through the charcoal canister and back to the fuel tank.

Other than that, I'm pretty stumped. Franklin made a point, if you bought the truck like this it is possible someone touched the "NEVER EFFING TOUCH THIS" screw on the throttle body. You need to test that with a volt meter on the TPS but I don't know the exact procedure.
 

crs.wdp

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1000 rpm with the idle air valve unplugged is too high. I don't have the specs for your year, but the earlier trucks idled at 750 for automatic or 675 for a manual with the idle air control unplugged.

Are you positive the idle stop screw has never been touched? Or did you just buy this thing and are working through problems one at a time?
Thanks for the reply. I just bought the truck and am going through it. I’ve not messed with the idle stop screw. It’s unscrewed all the way and doesn’t even touch the linkage arm. It seems if I screw it in though it will move the throttle open and increase the idle, right?
 

crs.wdp

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While your in that area, unplug that line entirely and cap it right at the intake manifold. It wont hurt anything but will rule out the purge valve leaking. If the purge valve leaks neither the smoke or spray test would work since the smoke would end up going through the charcoal canister and back to the fuel tank.
Ah yes, that makes sense. I will plug it and redo a smoke test. I’ll report back. Thanks!
 

Dirtman

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Just plug it and start the truck lol. The smoke test wont do anything at this point. If the idle drops, you found the leak. And just so were clear i mean remove the hose from the intake manifold and plug the port. Just let the hose hang it wont bother anything being unplugged for a bit. If the idle is down, go get yourself a new purge valve and hook it back up.

Long shot... but worth trying.
 

crs.wdp

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Just plug it and start the truck lol. The smoke test wont do anything at this point. If the idle drops, you found the leak. And just so were clear i mean remove the hose from the intake manifold and plug the port. Just let the hose hang it wont bother anything being unplugged for a bit. If the idle is down, go get yourself a new purge valve and hook it back up.

Long shot... but worth trying.
I disconnected and plugged the vacuum port on the front of the TB that runs to the evap canister, no change in idle.

Any idea what that plug is above the driver side valve cover? One of the two wires has continuity with the evap line solenoid. I can't find it's mate or anything that looks like it is disconnected.
 

Dirtman

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Can you get a pic?
 

Dirtman

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Also wouldnt hurt to get a shot of the passengers side of the engine. Maybe I can see something...
 

crs.wdp

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Can you get a pic?
Here are a bunch of pics. The one circled in red is the mystery plug that has continuity with one side of the evap solenoid plug. I can trace the wires along the yellow line and into the gray plug then one of them to the evap solenoid. Also, can you identify the sensor circled in green?

There are two other "mystery" plugs on the driver side of the engine compartment. So far I've not found any on the passenger but I included a pic of the passenger side also.

Thanks Dirtman!
 

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crs.wdp

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HIGH IDLE SOLVED!

These ranger / explorer forums have been such an awesome resource I thought I should post my experience in hopes it may help someone else. It does seems that high idling is fairly common on the 4.0 OHV so here is what my problem was.

After running out of the more common / obvious solutions to the high idle, i.e. air leak and or faulty sensors, I started looking to the ignition system. Using a timing light I discovered that #6 cyl wasn't firing consistently. Turns out the plug wire was hanging on by a thread and the plug was gapped about .005 instead of .054. Once I fixed these the engine purred beautifully but then the idle climbed even higher to 1600! The engine just sounded so good and seemed pretty normal so I borrowed a friends photo tach and put it on the bottom crank and turns out it was idling right around 800 after a 5 minute warm up. So looks like my high idle was really just that my tachometer was way off. After some research I discovered that some of the Ford tachs have a built in potentiometer so you can calibrate / adjust it. Mine was easily adjusted and all is right in my driveway.
 

Dirtman

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It's up there.
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It's down there.
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I poop in the furnace.
Lol... well at least its fixed.

The tachs are adjustable and can be set for 4 or 6 cylinders, they use the same tach for both and just swap a pin or wire on the back (Im not exactly sure how you do it).
 

Eddo Rogue

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its a temp fix but try covering the hole on the throttle plate with a piece of tape. I had this same problem after replacing/cleaning everything from the intake up, and it did the trick! I went from 1500rpm all day to 1250 cold and 750 warm. Now I have make it do that without the tape...
 

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