Renix High Idle

I hope you can pass along some new insights into why a 89 4.0 automatic has no high idle at start up in cold weather, but will not idle below 1500 to 1800 when warmwn up? At times after you shut it off for a moment or two it will idle in the 700 to 800 range but soon returns to 1500? I have replaced the entire vacume line with factory replacement kit. Pulled valve cover off and tanked it along with baffels inside, cleaned IAC and throtle-body.This is a 1 owner 1989 Laredo 4dr Metallic Gray looks new all stock and runs great except the high idle.
Thanks

sounds like a classic tps problem. Checked it?

1989cherokee said:

I hope you can pass along some new insights into why a 89 4.0 automatic has no high idle at start up in cold weather, but will not idle below 1500 to 1800 when warmwn up? At times after you shut it off for a moment or two it will idle in the 700 to 800 range but soon returns to 1500? I have replaced the entire vacume line with factory replacement kit. Pulled valve cover off and tanked it along with baffels inside, cleaned IAC and throtle-body.This is a 1 owner 1989 Laredo 4dr Metallic Gray looks new all stock and runs great except the high idle.
Thanks

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Two things come to mind right off, one is a loose ground at the dip stick engine ground that is changing resistance as the engine/ground heats up, the second is a bad intake manifold air temperature sensor (MAT), especially since the idle is increasing as the engine heats up. Does not sound like a TPS to me.

If I recall correctly I had the same problem with mine last year and it was the intake manifold air temperature sensor that time. As I recall, mine was also hard to start but only on cold rainy mornings on the first start, as well as idling high only after the engine warmed up. But the temp sensor can go bad in different ways, and cause different problems or symptoms so just the high hot idle only is good indication that temp sensor is not feeding the ECU the correct temperature readings.

It is real easy to test with a multi meter! Test it cold and hot, Resitance should drop to about 450 ohms at 165 F to 185 ohms at 210 F, and start at 3,400 ohms at 70 F. or 7,500 at 40 F.

Vacuum kit???

Did you replace the vacuum line that runs under the battery?

Check you intake/exhaust manifold. Make sure its tight. My 88 had the same problem. After a warmup the manifold would separate from the engine. Check the back bolts.

1989cherokee said:

IThis is a 1 owner 1989 Laredo 4dr Metallic Gray looks new all stock and runs great except the high idle.
Thanks

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Theres the problem. Its metallic gray. The metallic grEy ones dont have the issue but the grAy oens do. its a classic problem of color to engine influx mismanagement.....

Thanks you guy's for all the help, i will try the manifold temp sensor next, i have checked just about all the known problems that cause high idle but no luck. Also having no high idle at cold start gives hint something else is to blame.Replaced battery cables with 4g and checked all grounds.Did not check vacume line under battey?:dunno:

X2 on the TPS and/or exhaust manifold.