Hey. I've got a cold idle issue in a 98 I'm trying to pin down. I've been using a fuel pressure guage to try to diagnose it.Heres the deal: ​​​​​​When she's warmed up, the idle is steady and the needle on the guage holds at 49. However, on start up (cold), the idle is erratic and at times it wants to stall. When it does that, the fuel pressure will "bounce" exactly as the idle does. It "pulses" and matches the ups and downs of the idle. My thought is, that means the cold idle problem is a fuel pressure issue coming from the pump end. Whaddya think?

( I've already cleaned the TB, installed a new IAC and both O2 sensors)

Well, whadarya askin us fer! You done figgerued it out already!


Now, why would your fuel pump flake like that when cold? I have not heard of that before. Could there be a sensor that is malfunctioning when cold? Hmmm. Odd. Most malfunction when hot.....

[QUOTE=BlueRidgeMark;3610096]Well, whadarya askin us fer! You done figgerued it out already!

Ha! I guess I was hoping I'd finally figured out one of these problems on my own for once.

First thought was a failing engine coolant temp sensor, or ECT. With Ford, when that sensor fails, the engine may have trouble starting when cold as the PCM thinks the engine is a different temp than it actually is. It might cut fuel, or add fuel, when it should be doing the opposite. Cold engine = more fuel. Warm engine = less fuel. Ive seen the same with the IAT on atleast one. You say the fuel pressure fluctuates, but im not sure how/if the ECT, or even an IAT, plays a part in that. I never had a pump act up when cold so i have no idea whats going on there. Maybe the wiring is compromised and the changes in temp are screwing with the resistance? Id think any issues would occur when hot tho, not cold.

Another thought was the TPS, but i never had trouble on cold starts because of it. Or fluctuating fuel pressure. That, in my experience, was always the pump itself. Or failing to seat the connector. But that was with Ford. The XJ is still a mystery to me in some ways.

[QUOTE=fb97xj1;3610106]First thought was a failing engine coolant temp sensor, or ECT. With Ford, when that sensor fails, the engine may have trouble starting when cold as the PCM thinks the engine is a different temp than it actually is.

Hey so I put a new ECT sensor in and there's no change. However...I read somewhere that a way to see if the IAC is working is to turn the A/C on and it would adjust the idle higher to account for the load. I've already put a new IAC in but I thought wth. So turning the A/C on made the stumble/stall symptoms go away! When my son turned the A/C off, everything was fine. Any thoughts about this??

OMG! This happened a couple of times with my XJ, but it was after restarting when it was very hot. I shut it off and restarted it and everything was normal. I never figured out why it did that.

FIXED! So all this time, the new IAC I put in was bad. Go figure. I got frustrated with it yesterday, pulled the TB off, pulled the sensors, cleaned it again and noticed the tip didn't look right. Plugged it in, turned the key and nothing. Grabbed the old one, plugged it in, turned the key and it moved. Cleaned er up, reinstalled it and da,da... car cold idle problem is gone. Not for nothing, it's already a challenge diagnosing these things without having to deal with new parts that don't work! Thanks to all for your input!

Another claim for genuine Mopar parts, I guess. Doesn't it just bust your ***** when the old part cures the problem? What's really odd is my WJ the other night was sitting idling while I reloaded my front seat with newspapers and all of a sudden it started idling at 1200 rpm all by itself! I blipped the throttle a couple of times and the idle didn't drop down, yet as soon as I put it in gear and started driving, it was fine, until I slipped it into neutral again. After driving it for another 30 minutes, I decided to shut it off. On restart, it idled at 750 like normal and has stayed that way for a couple of days. The IAC is a Standard Motor Products part that my mechanic put in it about 5K miles ago. Maybe it needs cleaning or a Mopar replacement. It got an SMP part because one of his guys broke the original one while cleaning it. That guy no longer works for him. He said that was the last straw. Must be he did some other stupid stuff.

Just a word about that IAC. I had not cleaned it for the first 200K miles and it worked perfectly, then I cleaned it just because. It still worked fine until the engine change at 252K. It got broke when putting the new motor back into the car and setting everything up. When I cleaned the throttle body at 200K, the IAC housing looked like the inside of a chimney, but it still worked!

Yep. Lesson learned. It'll kill my wallet, but I'll never go with another non Mopar part again. Thought Standard was good enough... Thought wrong. Also reinforces the old saying, "if it ain't broke....". And of course, Dave that's what u get for helping me out... Now you've got the idling problems.

Next up is a new fuel pump. Hmm...Bosch or Carter... Hmm Carter is cheaper...

Carter made carburators for Chrysler for over 90 years so you can trust them, hopefully. The Edlebrock is a remanufactured Carter.