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intermittent problem fun...

CommandoXJ96

NAXJA Forum User
Location
Tucson
1996 XJ, 4.0, AW4, NP242
well, when I say intermittent, i mean the problem was intermittent for a couple weeks and is now not going away. The jeep would start like normal, idle fine, and accelerate just fine. Everyone now and then I would notice it would not climb in RPMs and just kinda sit at 2.5-3k, but if i let off of it slightly and then got back on it, sometimes this problem would just go away. Also, just every now and then when coming to a stop the idle would drop to 200rpm or so. Sometimes this would fix itself right away, othertimes just shifting into neutral real quick would bring the idle back up. In the last week though, these problems started to happen everytime, and once the jeep got warmed up if I parked it and came back say 10 minutes later, it would barely start or not start at all. And sometimes when it would barely start, it would only get maybe 500 feet before it would stall. Once when it did this I unplugged the coolant sensor and it started up. So I went ahead and replaced the coolant sensor. The starting problem seemed to go away for a day, the sometimes low idle and sometimes inability to accelerate were still there on and off though. Very rarely, say from the driveway at idle to putting my foot on the peddle, it would just feel like it was gonna stall the more I put my foot in it and the RPMs would drop instead of rise. At that point I did all of this:

new cap/rotor
new plug wires
new spark plugs (champion coppers-not that platinum bs)
new ignition coil

no change...
now I think it should be mentioned that all the while I did have one (and only one) check engine light - regarding my Downstream O2 sensor as it got taken out by a rock. im pretty sure on the 96 that the O2 sensor only gives the computer information on the efficiency of the Cat, but what the hell... so I headed back to the parts store.

new downstream oxygen sensor
new throttle position sensor (thinking that might have been it)
new fuel filter (just in case the old one was clogged)
new air filter (while im at it)
still no change... just no more check engine light/O2 sensor code.

so I had no check engine light, and sometimes problems, sometimes it ran great... Then I thought, what if crap is moving around inside the Cat. Converter and causing excessive back pressure, but only enough sometimes to cause these problems. I removed the upstream O2 sensor from the exhaust but left it plugged in... and got on it. which burned rubber. So I thought that maybe that could be the cause of the problem and removed the exhaust from the cat back...

well, now I have an even louder vehicle with the same problems, no check engine light, and the problem seems to have even got worse as it feels like its going to stall trying to even move in the driveway. It also sounds like it is backfiring through the intake possibly.

went ahead and cleaned all the connectors on the MAP,IAC,TPS,CPS, and anything else I could find under the hood, including the connections to the computer.

also, the upstream oxygen sensor was replaced this May, the IAC (which wouldnt cause these problems i know but possibly contribute to the sometimes crappy idle) was replaced last november, and the CPS which im starting to suspect is a possibility was replaced last May. Also, all the grounds appear to be ok from the battery to fender and block to firewall... but I have no clue whats causing this now. Anyone have any idea as to what I could be missing or where to look next?
 
my dad is bringing over a fuel pump and cps borrowed off my brothers XJ tonight and we shall see what happens I guess. guess i can run some SeaFoam through it while the cats off of it and just pull the upstream sensor outa the pipe so i don't kill it. after that I guess ill just have to pull the entire wiring harness, do a compression check... or just burn the whole thing to the ground :D
 
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Commando, I'm unsure if you removed the exhaust from before or after the cat....? The reason I ask is that it sounds like you have a severly clogged cat. If you removed the exhaust pipe before the cat, then of course that theory is wrong......
 
ah... I think I have discovered the problem. the fuel filter is brand new as of a day er two ago as mentioned above, and I just put a fuel pressure gauge on the rail... Its only getting 20 PSI. which is probably just enough for it to idle and not do much more. so ill put the new pump in later and see what happens :D
 
Thanks for posting that fix. That's a new one on me; having a frayed wire give insufficent fuel pressure.
Good to know.
 
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