1999 xj loss of power?

[Gman9596]

Hi, I am new to the xj world (just got my jeep last week) and I am proud to say it has lasted a whole 10 days without having a problem.

Btw, the rig is a 1999 Cherokee Sport, 4x4, 4.0, automatic.

Here's the short story: ive noticed that every few days, it takes a 5-10 cranks to fire up rather than the usual 2-3. I'm not sure if that is relevant here, but here is my main issue: Once it's started, it will run fine for 3-5 minutes. Once everything is up to temperature, it starts sputtering like it doesn't have fuel, then it will refuse to respond to throttle input unless I go WOT. It will sputter and surge and It won't die, and if I pull over and stop, it idles fine with no sign of a misfire. If I turn it off and let it cool down a bit, it will give me another 3-5 minutes of normal driving. Again, at any point if I go WOT, it perks up and pretends like nothing is wrong until I back off the throttle and it starts acting up again. Also, while sputtering, there are occasional pops that sound like backfires and feel like they're right under my feet.

Here's what I replaced so far:
-distributor cap and rotor
-spark plugs
-spark plug wires
-throttle position sensor

My next move is to replace the O2 sensors. I've researched enough to know to check the fuel pressure, but I don't know how, and will probably have a friend who works at a Chevy dealer do it if I can't figure it out.

What do you guys think? Any input would be helpful.

 

[Ou2mame]

I would check the fuel pressure and see if the power loss has any correlation with the pressure. You can buy a cheap gauge in amazon for like 25 bucks. It's totally worth having one around.

How did the plugs look when you took them out?

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[Gman9596]

The plugs looked pretty ok. Two of them had pretty rusty bases but all of the gaps were .035 and the tips were all fairly clean and consistent in color. They weren't white on the tips, which is what is leaning (no pun intended) me away from a fuel delivery issue that would cause a lean burn.

 

[Gman9596]

UPDATE
I had my guys at the Chevy dealer where I work look at it for a couple hours and their diagnosis is a failing ECU. When they said that, I immediately thought "oh sure, two Chevy mechanics with 40yrs of experience each would obviously go to worst case scenario when working on a Chrysler product". But here's the kicker, the jeep has a check engine light on, but no codes are present in the module. So here's my question: do jeeps have some strange tendency to throw lights without the codes to back them up or am I missing something here?

If not the ECU, what should I check next? O2 sensors? I'm lost on this one.

 

[Ou2mame]

Mine has had a cel with no apparent code. Disconnecting the battery got rid of it.

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[Ecomike]

Depends on the scanner, good ones will read pending codes, that do not show up as codes with a check engine light, I have seen codes come and go on their own, maybe an old code came and left but left the light on, I have no idea other than that.

5 main causes I know of,

1) cat converter guts busted and wandering around, blocking exhaust one minute and not the next. Tends to be random, not temp sensitive, thus back to #2

2) Low fuel pressure, should 49 psig, that leads to vapor lock and miss fires and lack of power during vapor lock. Cool down problem goes away is a good hint this might be it.

3) Problem with the Map sensor vacuum line or sensor or wiring. Low probability, but possible. Common on the 87-90 Renix vacuum hose, less common on the 99 hose, and not likely be temp sensitive, thus back to #2.

4) HV coil or ICM overheating and poor spark when hot, try the hair dryer test.

5) Maybe the CPS, try a heat gun blowing on the CPS to see it causes the problem. Test the CPS while heating it the same way versus factory specs (ohms?)

 

[Gman9596]

Ok thanks for you guys' input. I'll test some things and report back.