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Idle Issues

Turncoat_Reno

NAXJA Forum User
Location
Reno, NV
Hey guys, problem here that hopefully somebody can point me in the right direction on. I've searched around and found some ideas, but still not quite sure.

First, the basics: 89 xj with a 4.0 sitting at 205k miles, pretty much bone stock top to bottom. I've owned it for about a year, haven't had any major trouble. Previous owner had it for 15 years, took really good care of it, I have his (very good) records.

For a couple weeks now, I've been having some hard start problems, but only once in a while. Whether the engine is hot or cold doesn't seem to affect the issue. Probably 75% of the time it'll start within 4-5 cranks, rev to about 1600-1700rpm, then taper off to idle about 800 or so, sounds like it always does, drives around just fine. The other 25% it'll take 10-12 cranks and will only start if I keep the key turned. When it does this it'll just sputter up to about 800-1000 and run rough, wheel shaking and such. The rpms still stay relatively stable, but the engine clearly isn't running the same. This seems to be worse when it's cold, but you can still feel it running rough when it's warmed up. I've also noticed a more potent smell in the exhaust while this is happening.

I've been dealing with this as it comes up, it's never left me stranded anywhere so I've sorta been ignoring it (Good plan, right?). However, tonight I was heading home and stopped by the mailbox while my girlfriend grabbed the mail. I was idling there for a few minutes, maybe 5, and I noticed the temperature gauge going up slightly. Not much, but a little. It never goes above 210* usually, not even in the 5 hours of gridlock heading out of the bay area on a road trip last month. I got into our parking space and let it sit for a bit longer, probably 10 minutes, and over that time the temperature got most of the way to the 3/4 mark (245*?) For reference, it was about 95* outside. I checked the coolant level, it was fine (replaced all radiator/heater hoses about a month ago).

This made me think that the engine was possibly running lean, so it'd be running hotter. If this was related to the rough idle issue, could it possibly be the fuel pump dying slowly, failing to deliver enough fuel to the cylinders? I don't think the overheating is related to any sudden deficit in cooling ability (radiator was replaced with a three-core about 150k, pressure bottle has been replaced several times, doesn't appear to leak at the cap, both fans work, water pump was replaced about 150k also)

I've looked back at the PO's records for what else he might have done related to the idle. What I know has been replaced is as follows: new EGR valve at 132k, O2 sensor and entire emissions wiring harness at 140k, replaced IAC stepper motor and adjusted TPS at 183k, fuel filter and air filter are pretty recent.

My first suspect would be the fuel pump or fuel pressure regulator because it doesn't appear either have been replaced from the records I have. Other posts I've looked up about idle issues have pointed out the IAC and TPS, but those have been serviced fairly recently from what I've read, although I haven't actually dug in there to see how clean they actually are. I've read O2 sensors can cause some rough idle issues, but this one only has 60k on it and in my experience I would expect them to last longer than that.

Anybody have any other ideas as to what would explain an intermittent rough idle with slow overheating? Am I on the right track here? Anybody experience something similar?

To anybody who read all of that, thank you. Any questions/suggestions welcomed and appreciated.
 
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