4.0 Engine RPM Sensor

e21pilot · Sep 23, 2014

One of my 1992 4.0L Rangers seems to have an issue with rpm. When in neutral. if I rev the engine to 3000 rpm and let my foot off the gas quickly, I notice the rpm does not smoothly go down to idle. Instead it drops down in steps kind of holding its rpm for a little bit before dropping some more until it gets to idle.

I'm guessing this is a problem with the control system as the engine is clean and tuned and generally runs very well.

Is there some rpm sensor that my be operating a little out of tolerance given its age? Just wondering what could cause this type of problem?

Destroyer000 · Sep 23, 2014

idle air control maybe. i know ive had a few problems with this. you could unplug it and see what it does. usually if they die its working.

RonD · Sep 23, 2014

The only RPM sensor on the '92 4.0l is the CKP(crank position) sensor, and if it has a problem engine won't run.

Yes, reads like dirty IAC valve, or throttle plate spring is too loose, could even be throttle cable sticking a bit or cruise control cable, if so equipped.

I doubt the TPS(throttle position sensor) would cause that, but a simple test would be to manually move the throttle slowly to raise RPMs and see if RPMs jump a bit.
TPS sends Computer(EEC-IV) 1volt when throttle is closed and the 4.5volts when throttle is open all the way, so as throttle opens voltage increases, this is how the computer knows to increase fuel quickly, before MAF or O2 sensors tell it to, kinda like the accelerator pump worked on a carb engine.
If voltage jumps around RPMs will as well.

EEC-V(1995 and up) kept RPMs high and slowly reduced them for shifting and emissions, don't think EEC-IV does that, at least never seen it on my '94

e21pilot · Sep 15, 2015

Problem Solved

After doing numerous things to resolve the RPM issues I had, I simply replaced the throttle body with a new OEM one and every RPM issue I had was resolved. RPM is now stable and predicable. It almost seems like I have a new engine these days. Wish I would have replaced this part first!

jeremysdad · Sep 15, 2015

Ermergerd! Thanks for posting your resolution of the issue. You seriously just helped someone who searched your issue with Google!

You, Sir, are an internebz hero. I loathe people that post 'can you help me?' and never follow up. This is a genuine 'Thank you!'.

Rep for you, if you care about such silliness.

OIG's do. We're a silly lot. lol

4.0 Engine RPM Sensor

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