My 4.0L idles at about 3500 rpm intermittent. Is there anything I can do to make it idle normal again?

 

Just a thought, and this is a long shot, but the price is right. Disconnect the - bat cable, leave it off for 30 minutes, reconnect. Then start the engine and let it idle for 5-10 minutes (ie, don't go driving around). I recall reading in my owners manual that this is required for the computer to "relearn" idle. By not doing this, my understanding is that it will eventually "re-learn", but it takes more time that way.

 

Your IAC (idle air control valve) might be dirty, you could try cleaning or replacing it.

 

I cleaned my IAC and disconnected by battery for 30 minutes. So far, so good. It idles normal again. I am not sure which one fixed it, but Thanks.

 

The problem is back. Anyone has any other suggestions?

 

sounds like your idle control sensor that sends a signal to your computer command control box (computer brain) may be malfunctioning and may need replacing. It may be telling the computer to run the engine at a high idle speed needlessly. This may not be the answer but might be worth checking and at least, you can rule that possiblity out.

 

The IAC keeps your idle the same with load changes like AC, heater,etc. but if you have other problem like a vacuum leak, it'll cause youe idle speed to increase too.

What year is it? There are several DTC on OBDII vehicles for the IAC. My 97 Ranger 4.0L was showing lean system both banks and was idling just over 1000rpm but got worse. Just before I fixed it, idle was 1800rpm all the time.

A general test for a IAC is to disconnect it and the idle should drop but may also stall. The only thing is better do it quickly or your check engine light will come on because it sees an open circuit.

You need a volt meter to check the IAC when your idling at normal speed and when it goes up. Higher voltage equals higher speed.

Don't touch that throttle stop screw if your vehicle is OBDII. There is no known procedure to adjust it. Make sure that you are on that stop too That would change your TPS voltage at idle. When you turn your key to ON, the PCM reads that TPS voltage and uses it a base idle, not sure what'll happen if it was different later!

 

Does your '91 have a TPS (throttle position sensor)?? I don't know if ALL model years incorporated this feature. I'm still learning alot about the '90's decade models. If it DOES have a TPS, check that out too. Make sure it is clean & functional, all the connections are good, and the servo that controls the throttle linkage is working OK.

 

Okay, it's a 91 4.0L. The TPS voltage on pre-96 is adjustable and needs to be between certain limits. Somewhere in the archives there is a post of mine outlining the procedure for that.

If the voltage is above the high limit, it could cause your problem but probably the TPS is bad in that case anyway. When installing a new TPS, that procedure should be followed too.

Basically, you read the TPS output voltage, pull the IAC connector, loosen the TPS mounting screws and move it around to get the proper reading. Disconnect the negative battery terminal, connect the IAC and wait a while. You may have to open up the clearance holes inthe TPS also. The voltage is something like .84-.96v.

I forgot to add that you adjust your idle speed with the throttle stop screw then do the above adjusing.