HIGH idle .randomly

[88AMComanche]

Last night I started my truck & it revved to around 2700 rpms so I quikly put it in reverse to get moving,not thinking its gonna rev like that the whole way home.

I THOUGHt it wold idle down once it warmed up maybe ......Its stayed at almost 3000 all the way home. Needless to say it ran a bit hot once i stopped. Its done this a few times in the past but never stayed high like that & never quite that high.

Would a Map sensor cause this? Is there a shortcut way of finding out what causes it? I don't have an Ohm meter/tester. Lil lost on where to start even. Will check messages tonight if anyone has any solutions.

[87Chief]

I'm not totally sure, maybe someone might chime in, but maybe a throttle position sensor?

[KYMJ]

Idle Air Control valve. I'm fighting that right now. Stop the truck and let sit for 30 seconds, restart and see if it stops. I replace my IAC last year with a $25 ebay unit and it lasted about 6 months. Bought another from OReilly's , now 6 months later, same thing. But, like I said, stop, shut down for 30 seconds and normal. Pain in the butt, I guess one of the quirks of driving and old vehicle.

[kyleag89]

Sounds more like the TPS to me.. This exact thing happened to me when I cleaned my throttle body and didn't adjust the TPS correctly. A simple adjustment might fix it but it could be junk.

Scroll down for a basic TPS adjustment

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

[cruiser54]

Good chance your TPS is on the way out. Get a new one and adjust as outlined below.

 

RENIX TPS ADJUSTMENT

 

Before attempting to adjust your TPS be sure the throttle body has been recently cleaned.

It's especially important that the edges of the throttle butterfly are free of any carbon build-up.

 

IMPORTANT NOTE: Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector

of the TPS . The letters are embossed on the connector itself.

Touch the black lead of your meter to the negative battery post.

If you see more than 1 ohm of resistance some modifications to the sensor ground harness will be

necessary. The harness repair must be performed before proceeding.

I can provide an instruction sheet for that if needed.

 

MANUAL TRANSMISSION:

 

RENIX manual transmission equipped XJs have a three-wire TPS mounted on the throttle body.

This manual transmission vehicle TPS provides data input to the ECU. The manual transmission

TPS has three wires in the connector and they're clearly embossed with the letters A,B, and C.

 

Wire "A" is positive.

Wire "B" is ground.

 

Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors..

Note the voltage reading--this is your REFERENCE voltage.

 

Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your

OUTPUT voltage.

Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For

example: 4.82 volts X .17=.82 volts. Adjust the TPS until you have achieved this percentage. If

you can't achieve the correct output voltage replace the TPS and start over.

 

AUTOMATIC TRANSMISSION:

 

RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-

wire connector, same as the manual transmission vehicles have, and it is tested the same as the

manual transmission equipped vehicles--FOR ENGINE MANAGEMENT RELATED ISSUES.

 

However, the automatic TPS also has a square four-wire connector clearly embossed with the letters

A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module.

 

Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your

REFERENCE voltage.

 

Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT

voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage.

For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage.

If you can't, replace the TPS and start over.

 

So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and

the other side feeds the TCU. If you have TRANSMISSION issues check the four-wire

connector side of the TPS. If you have ENGINE issues check the three-wire connector side of

the TPS.

 

For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is

stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.

 

Revised 11-28-2011

[smithe1811]

I hadthe same issue on my 88, found loose bolts on my exhaist manifold to be the culprit.

Erik

[PiginaMJ]

I have had the same issue recently however.

 

It just started after I replaced all the vacuum box/connections with new ones. Any thoughts? It idles fine until it warms up on a drive then idles high??

[cruiser54]

I have had the same issue recently however.

 

It just started after I replaced all the vacuum box/connections with new ones. Any thoughts? It idles fine until it warms up on a drive then idles high??

 

Check the TPS on the engine side just to be sure. Only takes 3 minutes.