XJ random power loss and pop

1998 XJ, 4.0, auto, lifted


A few months back I parked my XJ in storage because it had a very plugged catalytic converter, tailpipe and muffler were on their way out as well, AND it did not always read fuel. So over the weekend I installed a new cat, new flowmaster, tail pipe, o2 sensor (downstream), dropped the tank, ground out any surface rust, painted above the tank and installed a new fuel pump (delphi).



The XJ runs the best it has in a long time, runs like new! I drove it around for the remainder of the weekend before making the 2 hour drive back home last night. On my way home last night I was accelerating up a hill and suddenly it started to loose power. I then heard a couple pops (sounded like in the intake or manifolds) and then all the power came back and I did not have another issue. It did this on occasion just before I parked it due to the cat, I assumed this was the issue. I also think my MPG went down a bit. Going through nearly half a tank in 2 hours seems a bit much. The problem did not repeat itself for the remainder of the drive. Highway speeds around 75 when not in bumper to bumper traffic and it ran as it was supposed to. Also, no codes thrown.



It has had a tune up less than 4k ago. Today I am going to pull the plugs and see what those tell me for starters. Anyone have any tips as to what else to check? I have a feeling this is going to turn out to be a sensor issue :looney:


Thank you in advance for any input.

Since it's a intermittent issue it will be hard to determine what's causing it. I suggest that you inspect all the wiring for chafing/abrasions, after that about the thing that would tell you anything is a data logging scanner.

Rubbed wire loom is suspect, tps will cause similar symptoms, but usually when it starts going it continues to act up until repaired.

So I took a look at the wiring, I didnt find any problems. I did however find problems when pulling the plugs. The tune up was done under 4k ago and mind you this cat has been plugged for a while. So I expected things to be a little ugly. All cyls were consistently okayish aside from 5. Cyl 5 was pretty bad. I have tried to upload a photo via tapatalk but it keeps telling me network error. So I will try later.



Is there any way my distributor could be a hair off? To my understanding you cant really mess it up. Set engine to tdc, set the position for the oil pump and then the distributor slides into place. I dont think it will bolt in if its not perfectly in place. I am just trying to rule some things out.

Cyl 5

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The rest all look like this. Not great, but not full of buildup.

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That #5 does not look good at all, I would start just by replacing them. Make sure you use Champion copper plugs!

RCP Phx said:

That #5 does not look good at all, I would start just by replacing them. Make sure you use Champion copper plugs!

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Yes I'm going to have to replace them as these are pretty gnarly. When this stumble pop issue started, it was after I determined the cat was bad. But this jeep has had a detonation issue going back a few years. After hitting it with a timing light, fuel pressure check, verified my upstream o2 was working, cleaning the throttle body, replacing tb gasket, verifying I didn't have a heat issue by replacing the fan clutch AND hitting it with a thermometer to verify no hot spots. I started running premium and it stopped pinging.

I am leaning towards this being a mechanical issue as opposed to a sensor or something like that because it has seemed to get worse over years, not weeks. It would be a big stretch to say that I've had a slowly worsening cat for years and now I'm just suffering from fouled plugs. I'm going to go back out and double check egr lines. But I think that would throw a code if it had an issue.

Overall I'm not sure if this is a new issue, or an existing one getting worse. I didn't really connect the dots on that possibility until just now. Either way, bad mpg and plugs that foul in well under 4k makes this daily driver a real problem so it now has my full attention.

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Two things that come to mind, first is it takes a while for the computer to relearn the changed parameters. The second is does this happen on a cold motor (first few minutes of running), if not I'd look at the front O2 sensor.
Could be other things, many would throw some kind of code. O2 sensors are notorious for not causing a code to show until they are completely gone.

8Mud said:

Two things that come to mind, first is it takes a while for the computer to relearn the changed parameters. The second is does this happen on a cold motor (first few minutes of running), if not I'd look at the front O2 sensor.

Could be other things, many would throw some kind of code. O2 sensors are notorious for not causing a code to show until they are completely gone.

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So the one and only time it has done that stumble pop since the new exhaust and fuel pump was installed happened while the engine was cold. This was the first time starting or driving the vehicle since I cleared the bad cat code.

I also have checked my egr valve connector (although if it was not working I am sure I would have gotten a code). Tomorrow when I go to the store for spark plugs I am also going to get some brake clean and test for vacuum leaks.

Sadly my code reader is 2 hours away but I will require it asap to revisit fuel trims. Although, when I first had ping years ago neither the long term or short term fuel trims ever exceeded 7.8. So I don't think anything would have changed now even in that department.

The connector for the upstream o2 is a little damaged but I have it wrapped up well, and according to live data has been functioning. Yet anything thing I will verify yet again once I have my code reader. The only thing I will have a difficult time verifying is the spray pattern for each injector however going by the plugs I think injector problems are very low on the list.

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8Mud said:

Two things that come to mind, first is it takes a while for the computer to relearn the changed parameters. The second is does this happen on a cold motor (first few minutes of running), if not I'd look at the front O2 sensor.

Could be other things, many would throw some kind of code. O2 sensors are notorious for not causing a code to show until they are completely gone.

Click to expand...

When it had a bad cat (which is what I initially assumed was the reason for the power loss and pop) it was not consistent to any temp. Had happened at both cold and operating temp.

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The computer re-learn takes a while if I'm remembering right, I know I mine runs like crap and gets horrible mileage while re learning, but mine is a 95. You seem to have a persistent rich condition, the coolant temp sensor could be sending wonky readings keeping it in warm up mode, im not sure if the intake temperature sensor will send it that far out of parameters if it's reading bad.

Maybe check the distributor cap for carbon tracking?
Look at the rotor and the center carbon tip in the distributor.
Also check the coil to distributor wire; it shouldn't have more
than 3K-5K ohms resistance.

These intermittent glitzes turn up when the engine has high
cylinder pressures when under load...

Unclewolverine said:

The computer re-learn takes a while if I'm remembering right, I know I mine runs like crap and gets horrible mileage while re learning, but mine is a 95. You seem to have a persistent rich condition, the coolant temp sensor could be sending wonky readings keeping it in warm up mode, im not sure if the intake temperature sensor will send it that far out of parameters if it's reading bad.

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I don't think it would be the coolant temp sensor. It reads accurately I'm confident in that. I can't say for intake though.

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Runnin'OnEmpty said:

Maybe check the distributor cap for carbon tracking?
Look at the rotor and the center carbon tip in the distributor.
Also check the coil to distributor wire; it shouldn't have more
than 3K-5K ohms resistance.

These intermittent glitzes turn up when the engine has high
cylinder pressures when under load...

Click to expand...

Threw a code this morning and smelled like fuel rich exhaust all the way to work. I will borrow an auto parts store code reader while I'm in for spark plugs later today and start there. I'm going to check for carbon tracking later as well

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Okay long story short I think I have had an injector that has slowly gotten worse over time and is firing, but not closing. I have great fuel pressure but as soon as I shut the key off it drops fast. Has anyone used these? Are they reliable?

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I never recommend anything but "matched" sets of injectors and not off Ebay!

RCP Phx said:

I never recommend anything but "matched" sets of injectors and not off Ebay!

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These are on rock auto. I was going to get a set of 6. Supposed to be oem

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BTW, from your description it would likely be the fuel pressure regulator.

By "matched" I mean they have been packaged as a "set" after being flow tested and flow the same rates.