Just recently I've noticed my truck will idle high while I'm driving. I'll come up to a stop light, take the truck out of gear and my foot off the clutch and
the revs will jump to between 1300 and 1600 rpm. Once I push in the clutch the revs will start to gradually drop until I put it and gear and hit the gas
to move through the intersection.

I pulled into my driveway this afternoon and the revs stayed around 1500 so I put the parking break on, jumped out and looked under the hood. I checked to
make sure the throttle wasn't stuck open (it wasn't) and I messed the with TPS to verify it was connecting properly. The revs stayed high. I even actuated
the throttle valve by hand to see if I could shake it loose and that didn't help either.

Finally I jumped in the cab and pressed the clutch down, without touching anything else, and the revs dropped back to normal. Does anyone have any idea
what might cause this? I just replaced the clutch, slave and master cylinder, and bled the system so all that is new.

I know there is that sensor on the clutch pedal that verifies the clutch is pressed before letting you start the truck. Does that sensor have anything to do with
regulating the engine RPM?

 

IAC(idle air control) valve is sticking open a bit

It has two wires connected
After engine is fully warmed up let it idle
Unplug those two wires from IAC Valve
RPMs should drop to about 500 or so, or engine may even stall, either is good it means valve closed and no vacuum leaks.
If RPMs stay high then could be bad IAC Valve or vacuum leak

Shut off engine and remove IAC Valve, two bolts, you can wait until engine is cold
Turn on the key and plug IAC Valve's wires back in
IAC Valve should open all the way
Unplug the wires and it should close all the way
Repeat as many times as you like to make sure it is working and not sticking 1/2 way or ??
Open and close fully

If IAC Valve working then you have a vacuum leak

Clutch pedal switch does indicate "neutral" to the computer(PCM), but this not really used for idle RPMs.

Is the speedometer working as it should?
speed above 5mph will hold RPMs higher, its an emissions thing

 

Thanks Ron.

I read a few other posts where you said it would be the IAC so that's on my list of things to check. I just wasn't sure about the clutch thing. The RPMs drop
off immediately when I press the clutch down. I know correlation is not causation but its starting to look that way lol.

The warm up choke loop (I don't know if that's the proper term for it) also works perfectly. I'll start the truck cold and it'll high idle for a few minutes then come out
of choke and drop down to normal RPMs like clockwork which I imagine wouldn't be the case if the IAC was jamming.

I'll run through the troubleshooting process you outlined above just to be safe and report back.

The Spedo is working fine. I've also got an ultragauge so I can pinpoint the engine RPM pretty accurately.

 

On the back of the head there should be a Ground strap to the Firewall, this is the main ground for all cab electrics.
Make sure strap is there and making good clean metal contact on both ends.

If you have Cruise Control disconnect its cable from throttle and see if problem goes away.
Clutch pedal switch release vacuum from Cruise module, but so should brake pedal

But since you did check that throttle was close when idle was high I doubt thats the problem.

It is looking like the PCM(computer) is holding open the IAC Valve for some reason, a software glitch that "neutral"(clutch pedal down) resets
Unfortunately you can just simulate "neutral" to PCM, because then it would set REV Limiter at 3,000rpm

 

No cruise control on this truck. I'll try cleaning the IAC and I've got a can on throttle body cleaner I've been meaning to put through it.

The only other thing, and this is a long shot, is that this started happening about the same time I refilled the gas tank and added a bottle of
STP fuel system cleaner. Not sure if that would have any sort of influence over anything but its worth mentioning.

 

Update: Unplugged the IAC and the revs dropped down to the high 500s and the whole truck shook like crazy but didn't stall. Plugged it back in while it was
still running (not sure how smart that was) and they jumped back up and then settled back down around 750-800 RPM. Took it for a drive and still the revs would
randomly sit around 1700 with no throttle input. It's very sporadic. I'll spend some time this weekend cleaning things up and see if that does anything.

 

So IAC Valve is working fine, you can unplug IAC and plug it back in while engine is running, it is not "electronic" just a 12volt solenoid, so no voltage surge issues

Then you are down to the PCM doing the high idle thing.

On manual trans vehicles software in PCM should hold RPMs high, between shifts, i.e. when clutch pedal is in only, and then only for a few seconds.
And idle should stay higher, 1,200-,1,500rpm until speedometer is below 5MPH, thats an emissions thing
Reads like this software has failed in some way

 

Now I'm a computer guy by trade, so I know the chances of any kind of software just quitting is pretty low. Especially if that software is hard coded into the firmware/PCM.

Part of me still thinks this is a hardware issue. A computer is only as good as the data is receives.

When I replaced the master cylinder, I had to remove the clutch position sensor. After reinstalling it moves up and down around the skinnier part of the plunger. Could that
be causing it to falsely read as the clutch being engaged? What I might try doing is waiting till the problem presents itself again, shutting the truck off while idle is high, make sure
the truck out of gear, and try and start it without pushing in the clutch to see if that sensor is telling the PCM that the clutch is pressed in.

It sounds like revs are being held high because the PCM thinks clutch is depressed. That might explain why pressing the clutch sometimes fixes it.

 

Cleaned the MAF and the IAC, still getting the same problems, but not as bad or for as long so I maybe onto something.

Also, I took the truck out on the highway and noticed a new symptom that might be related. I'll accelerate up to around 55 mph and shift from 4th to 5th
and the truck will start bucking like crazy when I start to give it anything more that 50% throttle. I'm not sure if that's a missfire or what. It'll sometimes do it in 4th
gear but only when under a heavy load. And it almost always does it in 5th when gas is applied. Thoughts?

 

Reads like you are running out of gas at higher demand
1996 should have 30-35psi fuel pressure at the engine when its running

There is an FPR(fuel pressure regulator) on the fuel rail, it has the Return Fuel line attached to it and a Vacuum line
FPR has a spring and valve inside, spring is set for about 42psi
When engine is idling Vacuum assist pulls on spring so 30-35psi is set.

Get a fuel pressure gauge and attach it to the fuel rail, there is a Schrader Valve(looks like tires air valve) on the fuel rail for testing fuel pressure.

Turn key on and off a few times and check pressure, should get above 40psi, with no vacuum assist, engine off.
If pressure drops right away use vise grips with cardboard to GENTLY squeeze Return Fuel line coming out of the FPR, blocking flow back to gas tank
repeat test
If pressure holds then replace FPR it is leaking fuel/pressure
If pressure still drops then Check Valve inside Fuel pump is leaking, which is not a "big deal" at this time, it is only used when key is off, it holds pressure in system for easier restart.
It shouldn't effect fuel pressure when pump is running, but "shouldn't" is the key word there, lol, it could be stuck 1/2way open, so you aren't getting full flow from pump, this wouldn't effect lower RPM pressure but would be a problem at higher RPMs/load.


Start engine
Watch pressure, should be stable between 30-35psi
Raise RPMs above 2,500 or so and hold it high, watch pressure, if it starts to drop even slightly then there is a problem, it should actually go UP a bit as you raise RPMs because vacuum goes down as you open throttle plate


Change fuel filter first and test again

 

Update: Replaced the fuel filter just because its a $10 part and it took me 5 minutes. That helped out with the high idle problem a little bit. It seems to return to normal RPMs
quicker than it did before. As for the bucking, its still an issue. I imagine its because I have oversized tires which have essentially geared the entire transmission down a step.
1st is now 2nd and so on. I can cruise at 60mph in 4th gear and only hit 2000 RMP. Once I put it into 5th it'll drop to around 1200 and I don't think that little motor at that RMP
has the guts to move the truck very fast.

Not gonna worry about it. Moving onto the AC compressor, but that's for another thread I suppose.

Thanks for the help Ron.

 

Check your coil pack for a crack.

 

I've noticed now that it's cooler and the AC I'd off the ranger likes to idle higher than it did with the AC on. No tac to tell what rpms are but noticeably louder and higher idle.
cleaned icm and mass air.
It stays higher idle until I'm stopped for aproz 4 seconds.
Any help is appreciated.