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Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go hereXJ (84-01)
All OEM related XJ specific tech. Examples, no start, general maintenance or anything that's stock.
Disconnect your O2 sensors...it will go into pre programmed fuel flow mode based on other parameters than your O2 sensors feedback. That programmed fuel mode runs very well, just not great for emissions. If backfire and glowing manifold still present...its not an O2 sensor issue.
I only have one O2 sensor. I dont know where or if the other one is supposed to be, but the one I do have would be pre-cat. I have Disconected it, but it doesn't stop the backfiring. It just causes it to go into open loop as it should.
Inability to rev/red hot manifold = Consider clogged exhaust.
Consider worn timing chain.
Would love to run a CPS/CKS waveform to see if that is unsynced.
This is true, but it is Catless, and the exhaust has been replaced from the crossover pipe back. Its all new piping. And the amount of airflow coming out of the tailpipe is a lot. So i do not suspect any cloggs.
I did think about the timing chain, but I guess other than pulling the front cover to see if it jumped, theres not really any other way to verify that right?
Yeah, Im still mildly suspicious of the CPS Because its not a mopar part. But I temporarily put a known good CPS in it for testing purposes that was running in a 98' xj with no problem. So I mentally dismissed it as an issue when the "good" part didn't change anything. But I could be wrong.
And I did not get the compression test done. I got off track testing other stuff and never did it. If I get a chance today, I can take some time and do it just to rule that out.
Yeah, Im still mildly suspicious of the CPS Because its not a mopar part. But I temporarily put a known good CPS in it for testing purposes that was running in a 98' xj with no problem. So I mentally dismissed it as an issue when the "good" part didn't change anything. But I could be wrong.
OK but not only do you need signals from the CPS and CKS but I **think** they need to be synced:
I say **think** because this is the CPS and CKS signals from a 2000 4.0, and the coil rail systems absolutely need to be synced, but IDK if that's a issue with distributor systems (like what the CPU does/tries to do with timing chain slop, distributor installed wrong, jumped chain (very rare, but have seen one), timing chain installed wrong (like if somebody slapped in a new cam)).
BTW how are the lifters?
Anyway, at some point you might want to consider an entry level o-scope to look at injector, spark, CPS, CKS, O2 sensor etc. signals directly.
OK but not only do you need signals from the CPS and CKS but I **think** they need to be synced:
I say **think** because this is the CPS and CKS signals from a 2000 4.0, and the coil rail systems absolutely need to be synced, but IDK if that's a issue with distributor systems (like what the CPU does/tries to do with timing chain slop, distributor installed wrong, jumped chain (very rare, but have seen one), timing chain installed wrong (like if somebody slapped in a new cam)).
BTW how are the lifters?
Anyway, at some point you might want to consider an entry level o-scope to look at injector, spark, CPS, CKS, O2 sensor etc. signals directly.
Ok, by CPS and CKS do you mean camshaft position sensor and crankshaft position sensor respectively? Ive never heard of them needing to be synced with distributor ignition, but Im not an expert by any means. I can check and make sure that my cam sensor is properly aligned in the distributor.
I dont know what condition the lifters are in, but they don't make any noise.
Ok, getting an oscilloscope would definitely allow me to do more precise diagnosis
Last edited by '99YETIXJ; Apr 11, 2026 at 10:24 PM.
If it still backfires in open loop, and is the manifold still glowing?..that means you do not have a computer controlled leanout . I would confirm that the manifold isn't hung up on an alignment pin causing a vacuum leak. All still leading to a prior mechanical mistime issue. I would make sure that the timing mark on the HB is at 0 when you confirm the #1 piston is at TDC. If so, then confirm dist rotor is pointing at #1 post inside cap.
Check your manifold bud. Hang up on dowels is common....
Should I be able to visually see if the intake manifold is hung up? I would assume it would be on the bottom portion where it would get hung up on the studs.
If it still backfires in open loop, and is the manifold still glowing?..that means you do not have a computer controlled leanout . I would confirm that the manifold isn't hung up on an alignment pin causing a vacuum leak. All still leading to a prior mechanical mistime issue. I would make sure that the timing mark on the HB is at 0 when you confirm the #1 piston is at TDC. If so, then confirm dist rotor is pointing at #1 post inside cap.
okay, I went back to double check that mechanical timing was exactly right. I verified cyl#1 on TDC on compression, and put the timing mark at zero. Then I pulled the cap and checked the rotor position. It was very close, but I was not 100% satisfied with where it was pointing. It seemed to be just past the #1 terminal on the cap. So I played with it for a minute and tried to set it back one tooth, but that put it a little bit behind the #1 terminal. I decided to go ahead and knock the roll pin out and spin the gear 180°. Reinstalled it, and it's dead on perfect. Don't know how I missed this when I installed it originally, but I must not have had the timing mark at 0°. So I cranked it up and it changed nothing. Same symptoms.
now it was time to check for vacuum leaks around the manifold. I started spraying around and couldn't find anything. So I decided to double check that it was actually working, and I pulled a vacuum line going from the rear of the valve cover to the intake manifold just to spray some carb cleaner in there to verify it was actually working. So when I pulled the vacuum line off, of course it idled up because it created a vacuum leak. It also smelled REALLY rich and the exhaust smoked a little. But when I sprayed the cleaner in the open port on the manifold, it did not change the idle at all. So I put the vacuum line back on, and the engine immediately died. So I cranked it back up again, and something seemed like it was running different. So I gave it some throttle and no more backfire! I thought this was maybe because it was compensating for the vacuum leak I had just created a moment before and running rich which was countering the lean condition that it seems to have. But I let it run for a while, and turned it off and back on several times and it seems to be completely fine. However, it does still stumble with throttle at low rpm, doesn't want to rev past 3500rmp, and has barely enough power to move the Jeep under load.
So I never did verify if there was a vacuum leak at the intake manifold, because the carburetor cleaner was not changing the idle even when I sprayed it in a known vacuum leak. I'm confused as to what exactly happened. What now?
As a side note, I know somebody asked earlier in this thread how my valves look. I did pop the valve cover off today just to look at them, and they all look like they're in great condition. Pretty clean, and no obvious issues.
Please verify with a number, the fuel pressure reading at the fuel rail ..good pressure is not enough. Then give us a part number of one of your injectors. Please give us all 6 cyl compression readings. All of these are vital in diagnosing your issues. Also on one of your earlier videos you sent timing advance compared to rpm. That was helpful. Can you put one together that has rpm/ timing and manifold pressure. Also need to know if its manifold pressure or vacuum since they're opposites thanks...and keep the O2 sensors disabled. The open loop fuel program is excellent for normal performance, just not emissions so it will give a good baseline without skewing the results due to emissions BS. Have you run this with the O2 sensors disabled...its important that you do so
Last edited by bluejeep2001; Apr 16, 2026 at 09:34 PM.
Please verify with a number, the fuel pressure reading at the fuel rail ..good pressure is not enough. Then give us a part number of one of your injectors. Please give us all 6 cyl compression readings. All of these are vital in diagnosing your issues. Also on one of your earlier videos you sent timing advance compared to rpm. That was helpful. Can you put one together that has rpm/ timing and manifold pressure. Also need to know if its manifold pressure or vacuum since they're opposites thanks...and keep the O2 sensors disabled. The open loop fuel program is excellent for normal performance, just not emissions so it will give a good baseline without skewing the results due to emissions BS. Have you run this with the O2 sensors disabled...its important that you do so
Fuel pressure:
Key on:17 psi
Running:49 psi
Key off after running:46 psi
After 5 min: 43 psi
After 20 min: 39 psi
Fuel injector part number as it appears on the injector in two seperate lines:
04854181
X103Y05216
Ok, I can make another video with those things. It may be a few days before I can get to it though. Also, when I was running the jeep today, it randomly lost oil pressure completely while running a couple of times. But I could see it still flowing through the top of the head with the oil cap off. So maybe just a bad oil pressure sending unit? Also, when I was looking at that, I soaked up a bit of oil from inside the oil cap with a paper towel, and it literally smelled like straight gas. Like REALLY bad.
Well you seem to have proper fuel pressure while running, so its not leaning out the whole system, and it probably means that one or more are not excessively leaking. Your injectors are listed as a standard mopar injector for the 99-01 4.0 so its not over or undersized. Your compressions are pretty wide but nothing that would make the engine run as badly as you say. The backfiring and prior dieseling seems to indicate a super lean condition, but your fuel numbers dont indicate that. The fuel smell in the gas sounds like super rich. Going to need that video with rpm and manifold pressures when you get a chance...no smoking gun in the stuff you just posted. Pull the dipdtick and put a lighter to it. oil will not burn, water will spatter and fuel laden oil will increase the flame.
Well you seem to have proper fuel pressure while running, so its not leaning out the whole system, and it probably means that one or more are not excessively leaking. Your injectors are listed as a standard mopar injector for the 99-01 4.0 so its not over or undersized. Your compressions are pretty wide but nothing that would make the engine run as badly as you say. The backfiring and prior dieseling seems to indicate a super lean condition, but your fuel numbers dont indicate that. The fuel smell in the gas sounds like super rich. Going to need that video with rpm and manifold pressures when you get a chance...no smoking gun in the stuff you just posted. Pull the dipdtick and put a lighter to it. oil will not burn, water will spatter and fuel laden oil will increase the flame.
Ok, I got the video. It was taken with my 02 sensor unplugged so it was running in open loop. The manifold absolute pressure DROPS as RPMs go up. Isnt that wrong? Shouldn't it rise? Because as the throttle is opened, it would decrease the vacuum bringing the pressure up closer to atmospheric pressure (I think about 29 inHG).
Also, I put a lighter to the dip stick and it didn't do anything. No spattering, and the flame did not increase. But it still smells distinctly like gas. As a side note, I do have a cracked exhaust manifold that I noticed. Not sure if that makes a difference. And the clicking noise that I mentioned earlier in this thread after you shut the engine off. I figured it out. I took the valve cover off, and I turned the engine over by hand. As it would start to turn, it would begin clicking again. I ended up finding it to be the lifters ever so slightly sinking down. And they would click every time they would move just a fraction of an inch down. I don't know if that's normal or ok with hydraulic lifters. They were only moving a very small amount, and they would stop after 30 seconds or so. They wouldn't keep going.
