I have a 96 Grand Cherokee that ran great when I bought it a few weeks ago but now it will barely run at all. After it runs for a few minutes it starts missing and backfiring violently. It will not drive more than 25-30 mph at the best, and it cuts off at stoplights. The PO replaced ECM, plugs, wires, cap, rotor, air and fuel filter and o2 sensors. It set a code for the cat not functioing properly, but I wondered if there is a way to test that before spending the $$ to replace it.
1996 Grand Cherokee 4.0- wont go over 2000 rpm
- Thread starter gradymotorco
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does it run hot??
My '96 had a plugged cat. Dealer replaced it and updated PCM under a recall. The PCM programming would not detect catalyst inefficiency, and would not set a P0420 code when cats started to clog.
Call the dealer for potential recall repairs.
Call the dealer for potential recall repairs.
I had a '97 Dodge Ram that did the same thing. It's the Cat!
It doesn't run hot, but it does everything else consistent with a bad cat.
The engine doesn't necessarily need to run hot with a plugged cat, but, after driving for a few minutes I bet the cat is glowing.
i should have been more clear i meant is the converter cherry red
Originally Posted By: crazyoildude
i should have been more clear i meant is the converter cherry red
The converter will turn red for two reasons. The engine is running pig rich or it's plugged.
If the engine is running pig rich, it won't affect maximum RPM - it'll be able to run at 5500 RPM with no problems ... but burn a bunch of gas and foul the plugs pretty quickly.
A plugged up cat will affect max speed of the engine.
Sounds like that's the problem
i should have been more clear i meant is the converter cherry red
The converter will turn red for two reasons. The engine is running pig rich or it's plugged.
If the engine is running pig rich, it won't affect maximum RPM - it'll be able to run at 5500 RPM with no problems ... but burn a bunch of gas and foul the plugs pretty quickly.
A plugged up cat will affect max speed of the engine.
Sounds like that's the problem
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Originally Posted By: Warstud
Remove the oxygen sensor before the Cat and see if it runs any better.
Try this, it's worth testing before you make a conclusion.
Also, check your fuel pressure if you can.
It's likely the catalyst but the temperature sensitivity makes me wonder about the chance of a completely different issue.
The catalyst code doesn't really mean much as far as it being clogged. That code pops up all the time on older cars without the cat being very bad. The limited RPM is stronger evidence though.
Originally Posted By: salv
My '96 had a plugged cat. Dealer replaced it and updated PCM under a recall. The PCM programming would not detect catalyst inefficiency, and would not set a P0420 code when cats started to clog.
Call the dealer for potential recall repairs.
That sounds like a bonus feature. That code pops up easily on older cars when they still have a clean tailpipe output. But having the code forbids it from being registered. That code is oversensitive and many cars have to be hauled to a junkyard because of it. Having it not work would be a blessing IMO.
Remove the oxygen sensor before the Cat and see if it runs any better.
Try this, it's worth testing before you make a conclusion.
Also, check your fuel pressure if you can.
It's likely the catalyst but the temperature sensitivity makes me wonder about the chance of a completely different issue.
The catalyst code doesn't really mean much as far as it being clogged. That code pops up all the time on older cars without the cat being very bad. The limited RPM is stronger evidence though.
Originally Posted By: salv
My '96 had a plugged cat. Dealer replaced it and updated PCM under a recall. The PCM programming would not detect catalyst inefficiency, and would not set a P0420 code when cats started to clog.
Call the dealer for potential recall repairs.
That sounds like a bonus feature. That code pops up easily on older cars when they still have a clean tailpipe output. But having the code forbids it from being registered. That code is oversensitive and many cars have to be hauled to a junkyard because of it. Having it not work would be a blessing IMO.
Well today I drove the Jeep nearly 30 miles before it started acting up at all. Even then it wasn't as bad as it had been before, the missing/backfiring was most obvious from 20-40 mph. After you got it past 40 and in overdrive, it calmed down. If it is the cat, why would it occur intermittently?
I removed the front O2 sensor as suggested and it didn't make any change.
Occasionally the broken cat core will move around....sometimes causes a complete blockage and other times free. It happened to our sons Tahoe....when it was blocked it would barely run above an idle, backfired terribly and zero power.
nobody has mentioned burnt valves. It is possible that the valves are burnt or cracked, not sealing properly to the valve seat. without touching the valve seat, the valve will not be able to conduct all of the heat off to the seat, therefore becoming very hot, and warping.
two things happen--- the valves stop sealing, and they retain enough heat to start causing preignition and/or detonation. The engine will not run for too much longer if this is the cause, and it's not addressed.
As a rough test, gas it up next time with high octane. If it runs better, I'd start looking at getting the head done. This could also be related to waay excessive carbon buildup inside the cylinders--- probably too much for a seafoam or steam treatment to help, but could be worth several tanks of techron treatment.
two things happen--- the valves stop sealing, and they retain enough heat to start causing preignition and/or detonation. The engine will not run for too much longer if this is the cause, and it's not addressed.
As a rough test, gas it up next time with high octane. If it runs better, I'd start looking at getting the head done. This could also be related to waay excessive carbon buildup inside the cylinders--- probably too much for a seafoam or steam treatment to help, but could be worth several tanks of techron treatment.
Originally Posted By: meep
nobody has mentioned burnt valves. It is possible that the valves are burnt or cracked, not sealing properly to the valve seat. without touching the valve seat, the valve will not be able to conduct all of the heat off to the seat, therefore becoming very hot, and warping.
two things happen--- the valves stop sealing, and they retain enough heat to start causing preignition and/or detonation. The engine will not run for too much longer if this is the cause, and it's not addressed.
As a rough test, gas it up next time with high octane. If it runs better, I'd start looking at getting the head done. This could also be related to waay excessive carbon buildup inside the cylinders--- probably too much for a seafoam or steam treatment to help, but could be worth several tanks of techron treatment.
I doubt this is the issue. Usually a valve sealing problem would be worse at low rpm and it would run better at higher revs (less time per cycle so less leakage occurs).
nobody has mentioned burnt valves. It is possible that the valves are burnt or cracked, not sealing properly to the valve seat. without touching the valve seat, the valve will not be able to conduct all of the heat off to the seat, therefore becoming very hot, and warping.
two things happen--- the valves stop sealing, and they retain enough heat to start causing preignition and/or detonation. The engine will not run for too much longer if this is the cause, and it's not addressed.
As a rough test, gas it up next time with high octane. If it runs better, I'd start looking at getting the head done. This could also be related to waay excessive carbon buildup inside the cylinders--- probably too much for a seafoam or steam treatment to help, but could be worth several tanks of techron treatment.
I doubt this is the issue. Usually a valve sealing problem would be worse at low rpm and it would run better at higher revs (less time per cycle so less leakage occurs).
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