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lower-than-normal idle....like 250 RPM

R

RaccoonJoe

NAXJA Forum User
Location
Fort Wayne, IN
The title says it all....well, most of it, anyways. Extremely low idle, warm or cold, in gear or not, it doesn't matter. When sitting at a stoplight, my idle is roughly 250 (last mark before 0 on my tach). Sometimes, it will dip slightly below that mark....that's when she starts to rattle/shake/try to stall.

On a side note....when coming off a freeway off-ramp, sometimes it will idle close to 1K for a while, then slowly drop back down. This is only after cruising for 10+ miles @ ~65 (2K RPM). Also...I *always* see a spike in RPM upon starting....usually up to ~ 1700 RPM. Sometimes, when the motor is hot (90+ F and running errands through town), she'll spike up to 2500 RPM or better, then drop back down to ~ 250

Relevant info....89 XJ with 4.0L, 216K miles, AW4 auto, new TPS
 
Search this forum for "ReniX Files Ecomike" and read that thread. Sounds like a ground and or TPS calibration issue.
 
I've looked through that thread....and had a TPS calibration thread of my own, after the dealer screwed up the installation. During my troubleshooting of the TPS issues, I re-ran all of my ground lines.

Any other ideas??
 
my 89 does the same thing and I think the cps is on the way out. About 3 months ago I had starting issues and reset the cps and it still starts now, but I know the cps is bound to go soon. Other than the low idle I runs great.
 
Start by rechecking all the ground wires with an ohm meter from the battery ground (- post) to the ground wire at the sensor. It shoould read less that 1 ohm. If higher then there is still a ground problem. Then recheck the TPS calibration just to be sure these (ground and TPS Idle calibration) have not changed, drifted, etc.

Also check the input voltage to the TPS. Do this for both sides of the TPS!!!!! Both the ECU & TCU sides.

If you have not done so, pull, clean amd oil the IAC. Do not move or force the Pintle shaft to move, or you may damage it. I use carburator cleaner aerosol spray while running the throttle at about 2000 rpm (by ear since I can not see the gauge while under the hood), to clean the throttle body.

IF all the grounds, power supply feed voltages (ECU & TCU) are good and the TPS is at, at least 0.80 or better 0.83 volts (I use 0.82 to 0.83 and get a 750 rpm idle in park, 600 rpm in drive) then I would see about testing the IAC. I have not tried this, but you might pull the IAC from the throttle body, and turn on the power (do not start it!!!), and cycle the engine off, power on, on and off to see if the IAC is resetting (Motor turning at all) when the power is cycled on and off? Other than that you might need to borrow or buy a good IAC to see if that fixes the problem. IF it is not the IAC, then it might be a temperature sensor giving bad data to the ECU. Lastly, and you do this as a last resort, is the prior owner may have manually reset the throttle body idle stop or idle air screw settings, which is a big No-No, but if they did, you will need to reset one of them. Try the other stuff first!!!!
 
Ecomike said:
Start by rechecking all the ground wires with an ohm meter from the battery ground (- post) to the ground wire at the sensor. It shoould read less that 1 ohm. If higher then there is still a ground problem. Then recheck the TPS calibration just to be sure these (ground and TPS Idle calibration) have not changed, drifted, etc.

Also check the input voltage to the TPS. Do this for both sides of the TPS!!!!! Both the ECU & TCU sides.

If you have not done so, pull, clean amd oil the IAC. Do not move or force the Pintle shaft to move, or you may damage it. I use carburator cleaner aerosol spray while running the throttle at about 2000 rpm (by ear since I can not see the gauge while under the hood), to clean the throttle body.

IF all the grounds, power supply feed voltages (ECU & TCU) are good and the TPS is at, at least 0.80 or better 0.83 volts (I use 0.82 to 0.83 and get a 750 rpm idle in park, 600 rpm in drive) then I would see about testing the IAC. I have not tried this, but you might pull the IAC from the throttle body, and turn on the power (do not start it!!!), and cycle the engine off, power on, on and off to see if the IAC is resetting (Motor turning at all) when the power is cycled on and off? Other than that you might need to borrow or buy a good IAC to see if that fixes the problem. IF it is not the IAC, then it might be a temperature sensor giving bad data to the ECU. Lastly, and you do this as a last resort, is the prior owner may have manually reset the throttle body idle stop or idle air screw settings, which is a big No-No, but if they did, you will need to reset one of them. Try the other stuff first!!!!
Click to expand...


don't take the ground thought lightly!!!!!!!
my - battery cable looked just fine but when it got hot the Resistance when way up!!!!
and so did the idle!

I KNOW, your is a low idle problem...............Don't take the ground issue light lightly.

Flash.
 
Ahhh! I have pretty much the same problem.

When I start cold, it's 500 RPM's. After driving, it's 1000-1200 RPM's. And upon every start it surges to 2000 RPM's.

If you find the solution, please tell me! I will do likewise! I also checked all my grounds when it was hot and they were fine!
 
My 90 idles at 300 rpm and has since it was new, my dad was the original owner. I have had to do a rebuild on the engine at 140k due to it being run for 50 miles with a blown head gasket and no water(it wasn't me). When I rebuilt the engine I also replaced all sensors and output devices which included injectors, pump, distributer, and everything else that you can think of. I have put in a high output altenator, Optima battery, new and extra grounds.

No change!!

I think it has more to do with the computer than anything else.

When it is cold or the AC is on, it idles as normal. Otherwise this Jeep is the best running anything I've ever owned and even though it idles like a John Deer tractor it never stalls or falters. I'm leaving well enough alone.
 
Just to clarify for future readers, it is possible to calibrate a TPS and have a spec idle spped, only to have the ground resistance change a little bit (say only +/-5 to 10 ohms) later on (minutes, hours, days, weeks or months later) and have the idle go up or down with the change in the ground resistance. Increased resistance since the TPS calibration will increase the idle rpm, and a decrease in ground resistance after TPS calibration will lower the idle rpm. The TPS itself can also drift, or worse (and even harder to verify) the TPS idle volatge itself can drift (making it non-repeatable!).
 
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