low idle warm 90 jeep 4.0

Your symptoms can be caused by a dirty connection on the TPS. I recommend spray contact cleaner on them to clean them up. WD40 will work in a pinch.

Our idle goes something like yours every couple years and the clean fixes it right up.

Mike

Mike Romain did pass the time by typing:

That and probably a gummed up/sticky IAC valve or butterfly. Amazing what a teeny bit of crud in the wrong place can do.

How long has it been since you've cleaned the throttle body? Most don't ever do it at all. It could be a PCV valve that is damaged and has dropped the valve portion down into the engine creating a big vacuum leak, a really dirty throttle body or a bad spot on the TPS. When you clean out the TB, use a very soft toothbrush and a soft cloth, along with TB-friendly cleaner, not carb cleaner. Work with the engine OFF, and take your time cleaning all the gunk off the sides and the butterfly valve.

While you're in there, pull your EGR valve and give that a good cleaning as well. Sometimes they get so gunked up that they stick open, creating a vaccum leak.

Also, on those TPS's, aftermarket units are junk. Buy only an OEM replacement. Same goes for the EGR valve, if you ever need to swap it out. I found this all out the hard way.

The 4.0 doesn't have an EGR, just a CCV. When it's tube plugs, oil blows into the air filter.

Mike

Where is the CCV located? I'm not finding it in my book.

~Casper '93 GC Ltd.

Casper did pass the time by typing:

You're right, Mike. No EGR at all on my rig, and it's probably the same on all 4.0's. No wonder it runs so well. EGR is the technological equivalance of throwing a wet blanket on a fire, and they're difficult to clean when they get gummed up. Smog check techs love 'em though; they're good for retest business.

I'm going to put my money on the TPS and a dirty throttle body. Some older computers will throw a code for a low TPS signal, but on a '90, it's doubtful. Another symptom of TPS failure is an automatic transmission OD that keeps dropping in and out when cruising. One could hook up a DVM and check it manually, but they're cheap enough that replacement makes more sense.

Let us know what you find, Harry.

-JD

The early 4.0's had EGR's but they dropped them at some point. I know the 87 - 89 XJ/MJ using the Renix computer had them - I've cleaned mine a couple of times.

My MJ had real idle problems and cur> You're right, Mike. No EGR at all on my rig, and it's probably the

Hi Guys: Thanks for the good info. It's been too cold and windy for me today to do too much, but I did tap into the O2 sensor and that seems pretty non-responsive. The voltage slowly drops from about .45 to .3 volts, motor still running and no engine lights. Shouldn't it be closer to .5V? Even with that, it seems to run fine after its ran a few minutes. Humm. Maybe that engine light is burnt out. Any quick ways to check for codes, with a laptop or something? Thanks! Harry

Hi JD: I don't think I have a PCV valve, but I do have an EGR which I changed. I cleaned the throttle body valve not too long ago. The TPS is a couple of years old. At that same time I changed the MAP sensor. The Jeep runs fine and smooth axcept when it's hot-started for 2 minutes or so. Something I've been suspecting: When it's warm, the alternator puts out lower voltage. I think the same time the fuel pump is pumping less pressure. Lowering RPM and voltage and even lower pressure. I've had a pressure gauge on the injection manifold. The pressure does drop, but the pressure regulator does seem to work also, I suppose with no feedback of the pressure, but just today I tested pulling the vacuum off this regulator, and the engine does speed up, even with the line plugged. I think the O2 sensor should quickly correct the rich mixture and lean it back out, but it didn't. While doing this I noticed little change in voltage coming from the O2 sensor. Harry

What are you going on about now... Our 88 has the CCV tube, I have already had to clean it out...

Mike

bllsht wrote:

You are right, the letters got messed up in my head...

Mike

bllsht wrote:

Oops, you are right, I was thinking PCV.....

Mike

Mike Roma>

The O2 sensors for the renix computer system that you should have in your 90 are under recall. If it hasn't been done you can get it replaced free still.

The renix system has no codes.

Mike

Maybe that's the problem. O2 voltage should always be changing between 0 and 5 volts.

Low fuel pressure would be more likely to cause problems under load, not at idle.

If the TPS and MAP sensors have been changed, and TB is clean, I agree: next thing to look at is the O2 sensors and their connections.

On that EGR, just for laughs, disconnect it temporarily and see if the problem goes away, esp. if it was an aftermarket unit. After failing a smog check on the beater last year, I discovered that the problem was a $90 Napa EGR that I had just replaced. The shop guys told me that all aftermarket EGR's were pure junk, and after installing an OEM unit, it passed famously. I had another shop re-diagnose and repair the thing to insure an accurate diagnosis. No obvious symptoms like you describe, but it's worth looking at.