Engine RPM's won't decrease when clutch is disengaged

Iceman203 · Jul 22, 2014

I have a 1998 Ford Ranger 3.0L XLT 4x4 the engine RPM's won't decrease when clutch is disengaged. For example if I shift from 2nd to 3rd @ 2500rpm's, I lift my foot completely off the gas and disengage the clutch (depress it) the engine rpm's will maintain 2500rpm's as long as I keep the clutch disengaged. A few times the rpm's will actually increase. Engaging the clutch and shifting into gear drops the rpm's and everything goes back to normal. The rpm's never fluctuate unless the clutch is disengaged.

I have heard it could be possibly three things the IAC, TPS or a vacuum leak.

I recently had the truck at the dealership for a vacuum leak. While under load typically 4th to 5th gear shift the AC would go from blowing out of the vents to the defroster. They replaced a vacuum canister and a low pressure line going to the 4 wheel drive.

Any suggestions would be greatly appreciated. Thanks

Spott · Jul 22, 2014

Definitely sounds like a vacuum leak or the IAC is stuck open, which amounts to the same thing.

You can "sniff out" a vacuum leak by spraying carb cleaner or starting fluid around the various hoses, fittings, and gaskets in the engine bay, with the engine idling. If there's a leak, it will draw the fluid in and the engine will rev up for a moment. Carefully directing the spray around will allow you to locate the leak.

Another possibility would be that the throttle isn't closing as all the way. You can take off the air intake hose and look at the butterfly valve; it should touch the sides of the throttle body when the gas pedal is not pressed.

Campbell5149 · Jul 22, 2014

sounds to me like a tps or throttle plate could be gummed up and hanging. It is possible the IAC is sticking and causing a high idle as well. take off the IAC and inspect the pintle. make sure its not full of carbon and returns to a good seal. make sure the throttle plate moves freely and spring pulls it closed. Do you have a way to put a scanner on and monitor TPS voltage? It's possible its a vacuum leak if it always revs high with no load regardless of how much you previously depressed the throttle.

Iceman203 · Jul 22, 2014

It very rarely revs high. It typically only sustains the rpm I shifted at. Sometimes after a couple seconds it will drop the rpms but rarely, and again it only happens when the clutch is disengaged.

black_demon69 · Jul 23, 2014

check throttle plate and clean sometimes there is a little buildup around edges of plate and it wont close all the way. also inspect throttle cable make sure it moves freely clean and lube it with silicone or teflon spray lubricant like tri flow. clean and lube throttle return spring on outside of throttle body. had the same problem and this what i did to fix..

Iceman203 · Jul 27, 2014

Sorry guys for not replying back. I had a window open that I didn't realize logged into the forums and when I tried to log in from another window it kept saying "thank you for logging in Iceman203" then it went straight back to the log in screen. I ended up resetting my password and almost emailed the admin before I realized what was going on. Lol.

I cleaned the throttle plate with carb cleaner and the IAC, the truck seemed to be working (rpms dropping when foot off gas and clutch disengaged). Then after about 15 miles it started sticking again. I replaced the IAC with a new one and cleaned the throttle plate thoroughly and it hasn't happened since. So I have to assume one of the two was the problem. Thanks for all your help.

backporch · Mar 5, 2020

I have a similar issue. The idle will not usually rise when I put the truck in neutral, but it sometimes takes a while for it to drop. Even after driving 10 miles, it might still be around 1500-1800 rpm when coasting in neutral. It typically drops down to 7-800 when at a light. I checked the TPS with Torque and the reading is consistently (19%if I remember correctly) when I take my foot off the accelerator pedal.

I believe I plugged all the vacuum lines that I cut for the 4wd on the front hubs. The previous owner installed manual locking hubs. IAC valve?

Engine RPM's won't decrease when clutch is disengaged

Iceman203

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