I have a '94 4.0L with auto trans, 4x4, and (only) about 109K. I've had it a few months now and all has been fine until today. I was stopped at a traffic light and the Check Engine light came on. It runs real quiet so first thing was to check the tach. Nope, not stalled. Normal resting rpm's. Traffic started moving and so did I. Nothing seemed wrong. About one minute after this - as I'm driving slowly - the rpm's surge upwards causing a slight panic. It soon returned to normal. I see the C.E. lite is no longer lit. Time to check pants. OK. (Whew!)

Can't say which happened first; the light going out or the rpm's surging. The light came on again before arriving home and went off while I was in the driveway - along with another quick burst of rpm's. I checked thru the Forums a bit and found nothing similar. I'll take it to Advance Auto or Auto Zone tomorrow for a free Diag Code reading. Meantime, I thought I'd scribble this note and see if anyone could shed some light on this.

Thanks

 

Yes, get the codes first.

Guessing would be a waste of time.

The computer uses the IAC(idle air control) valve to set the idle, this is a step motor that the computer can set at over 100 positions, the motor opens and closes a valve to let more or less air into the intake, more air = higher RPM, less air = lower RPM.
The valve on this can get "sticky" so it doesn't always go where computer wants it, so computer has to readjust it.
If this happens computer will turn on CEL.

EVAP(gas tank vent) system has a valve controlled by computer, computer uses engine vacuum to suck gas fumes from fuel tank, if this valve is acting up CEL will come on, and if there is a vacuum leak on the EVAP system idle can go up suddenly then computer uses IAC valve to lower it.

Vacuum leaks on other hoses can do the same thing.

And there is more.....................get the codes and write them down, no "I think they said 123456............", lol.

 

Got this fixed today. Mechanic says he could not find any record of OBD code but, based on his past experience, felt the Idle Air Control Motor was the problem. He said it's common with Ranger of this age. Also got the fuel filter replaced based on Ron D's earlier reply to another posting I wrote. Took a round-a-bout way home and it ran fine - maybe a little peppier than it had been just prior.

 

Check the hose clamps as well. Make sure they are tight. This can cause vacuum problems to