high idle - 90 Cherokee - RENIX

I am trying to diagnose a high idle situation on my 90 XJ Cherokee. When ambient air temps are above about 80 degree, it idles fine (around 700-800 rmp) after warm up. When ambient air temps go below 80, the idle speed increases with cooler temps. The idle speed can be upwards of 1700 rpm when it is below say 40 degree ambient air temp. I have rulled out the IAC valve because I have started the engine and let the idle settle down then unplugged the the connector to the IAC and then engine slowly starts to idle faster and faster as the engine warms up (so the IAC valve is not opening further, which would be letting more air through the IAC port). Could unmetered air be somehow be entering the engine after the warm up period to cause the increased idle speed. What is throwing me a curve is the the idle speed is direcly realated to air temp. Thanks for any suggestions.

Andy

You may have a sensor ground circuit issue. Check the resistance between your negative battery post and the brown with white wire at your Intake Air Temp sensor harness. Must be less than 1 ohm.

First test the IAT, Intake Air temperature sensor, hot, and cold and compare the ohms reading to the data table in the Renix manual (or search the old threads for the data table here in the OEM forum).

Disconnect the O2 sensor, start it and run the cold to hot operations, repeat the conditions that led to the high idle, to see if the O2 sensor is involved in the high idle. If it is involved, you must then determine why it is involved, is it bad, or is there an intake manifold, or an exhaust manifold leak near the O2 sensor confusing the O2 sensor...

Third, test for intake manifold leaks at the head to manifold gasket area that open or close as the engine warms up, from thermal expansion. You may have a bad gasket, or loose intake manifold bolts!

OP,
Your method of unplugging the IAC during warmup as a diagnostic is not a good one. It is normal and expected for the engine idle speed to increase as the engine reaches operating temperature, IF the ECU has no way to adjust the idle speed down. The ECU does this during normal operation by closing the IAC some. If you haven't done it yet, just pull the IAC out, clean it and the port it fits into. It will only take you about 10 minutes.

I agree, unplugging the IAC while running is probably not the best way to test it, but I already swapped it out for a junkyard unit and cleaned the throttle body very well recently with the same issues. I also have recently replaced the O2 sensor. I guess next step is the IAT sensor and the grounding test. Thanks so far!!

With ignition off/engine off I have got 7.5 ohms from brown with white wire on IAT harness sensor to negative terminal.

OK. Send me your email address and I'll send you the fix. It's for the models with the C101 connector which you don't have. Pretty much the same fix anyways. You should have no trouble finding and fixing the splices with these instructions. If you have any issues after receiving the instructions, post them up.

NCSU90XJ said:

With ignition off/engine off I have got 7.5 ohms from brown with white wire on IAT harness sensor to negative terminal.

Click to expand...

Hmm, while that ground needs to be fixed anyway (they are all piggy backed on a common wire IIRC), I sort of doubt an extra 7.5 ohms on the IAT resistance would be noticeable. The IAT OEM spec is 7500 ohms at 40*F, and 185 ohms at 212*F

Ecomike said:

Hmm, while that ground needs to be fixed anyway (they are all piggy backed on a common wire IIRC), I sort of doubt an extra 7.5 ohms on the IAT resistance would be noticeable. The IAT OEM spec is 7500 ohms at 40*F, and 185 ohms at 212*F

Click to expand...

If all the sensors which share that ground circuit are experiencing at least 7.5 ohms or more of resistance there could be a problem. Keep in mind that circuit is the ground for the CTS, IAT, TPS and MAP and ground through the ECU. The ECU then grounds at the engine dipstick tube stud.

cruiser54 said:

If all the sensors which share that ground circuit are experiencing at least 7.5 ohms or more of resistance there could be a problem. Keep in mind that circuit is the ground for the CTS, IAT, TPS and MAP and ground through the ECU. The ECU then grounds at the engine dipstick tube stud.

Click to expand...

That is why I said it needs to be fixed anyway.

CTS and IAT would be insensitive to 7 extra ohms, but the TPS would add several hundred rpm to the idle for the additional 7 ohms (I actually checked that once, LOL).

I suspect the MAP sensor is also over sensitive to an extra 7 ohms, as it has voltage span similar to the TPS!

Does the O2 sensor use that same ground? If it does it would cause a slight lean or rich bias!

O2 grounds at the dipstick tube stud.