So, first, the facts. I've cleaned the IAC and throttle body, replaced the spark plugs, wires, cap, rotor, fuel filter, air filter, upstream O2 sensor, some vacuum hoses and battery on my 1990 Cherokee (4.0, auto). I also flushed, filled and bled the coolant, as well as changed the oil, not that that should be a real factor. The other relevant info is that the previous owner cut out the muffler and welded on a straight pipe, which I'll be getting replaced tomorrow at a muffler shop. Beyond that, the cat is utterly shot and it actually spit out pieces of blackened honeycomb before I tuned it up. That's also getting replaced tomorrow.

Anyway, here's my problem. She actually starts fairly decently and idles pretty well right now. The problem is, yesterday, I was waiting for it to get fully warmed up to 210 degrees so I could bleed any air from the cooling system - I had installed a bleeder valve on a heater hose earlier. For what seemed to be about 4-5 minutes, it idled fine and then it seemed to start to labor and the RPMs dropped from about 7-800 down to 250-400. Though it never stalled out, I did intervene and tap the gas pedal. The idle wasn't quite as smooth after that, but not in danger of stalling out.

Long story short, I took it to the gas station to fill it up and it ran great until I got a couple blocks from home. Then it started running at low RPMs again like before, and though I gave it plenty of opportunity to stall out on me, it never did. So it's really more of a nuisance than a driveability or reliablity problem. I'm at a loss as to what else I can do since the XJ doesn't have an AMM to take the blame. I also don't think it's the fuel pump relay since it does this consistently, whether it's warmed up or not. Anyone have any suggestions?

 

I would check the fuel pressure first when it is running poorly and make sure it is up to spec. After that if it checks out, then I would check or replace the O2 sensor. If it runs fine when cold but bad when warm, it is likely the sensor as the computer doesn't look to it for info until the engine has been running for a few minutes.

 

Thanks for the reply. I had been suspecting the FPR or pump/check valve. Of course, I'd like to avoid throwing money at a phantom problem. But just to clarify, I did replace my upstream O2 sensor last night with a brand new Denso...the old one looked coked up a bit, so I figured a new one ought to help it run better.

 

Even though I cleaned the IAC, I'm going to try running it tonight and then unplug it to see if there's any difference. I'm also going to try the same approach with the MAP sensor. Beyond that, I can only imagine it might be a failing fuel pump or FPR. I'd really prefer it's not the CPS, though I think if it were, my idle would be irregular/crappy regardless of whether she's warmed up or not. Ideally, I'd like to avoid having to replace as many as four different components, but hopefully I can get it narrowed down.

 

Mine does the same basic thing. At idle in the morning the rpm's will drop to around 700-800, then jump up to around 1500 then back down for a minute or so. Then once its warmed up itll be fine. I just figured it was a sensor or something.

 

another thing to check is exhaust/intakemanifold bolts when engine get hot things expand possible vacuum leaks there, common problem with older xjs

 

Well, shoot. I spose that can't hurt anyway. Is it really as simple as applying some penetrating oil to the bolts and giving them a quarter turn or two to the right? Usually there's a catch.