97 TJ hesitation/bucking/stutter/rough idle/backfire on initial start up
I've got a 97 TJ with the 2.5L and 5-spd.
It has a funky issue that only occurs right after the engine has been started (warm or cold, it doesn't matter). 150k on the odometer. No Codes are stored.
After starting the engine, the rpm's drop and almost kill the engine. This can be remedied by pressing the gas pedal enough to keep it from stalling out.
If you try to drive the Jeep normally, as soon as you take off the engine starts to hesitate and sputter while backfiring through the intake, which makes a loud pop noise, which continues until you let off the gas.
The only way this can be avoided is to rev the engine past 3k rpms in neutral and then put it into gear and then gradually let out the clutch.
After this has happened once, the Jeep runs pretty well and does not have the issue anymore until the next time it is restarted. Although it does not seem to be misfiring at this point, The only symptom that remains is a very small bounce on the tachometer needle at idle.
This problem is really annoying me, I cannot let anyone drive my Jeep because they don't know how to handle this situation when it occurs.
What is causing this problem?
I've already replaced the plugs, wires, distributor cap+rotor, and air filter...I replaced the front O2 sensor too, I have looked vacuum leaks, and swapped CPS/TPS sensors as well-
None have remedied this issue...Please Help!
__________________ Red 1992 Jeep Wrangler - 2.5L/5spd, lifted 4" sitting on 33s
I had the same problem, took me a year to figure out it was the upstream O2 sensor. replaced almost every sensor. Replaced O2 and no more problem. Unfortunately, any of your sensors could be acting up. Do the free stuff first... Gas cap closed and not broke, no vacuum leaks, no cracked header, rotor/cap/plug wires OK, throttle body clean, Idle speed motor clean etc. GOOOOOD Luck!
Jeep took another piece of my finger today...
Hmmm. You said you replaced the O2 sensors? I had very similar issues with my TJ, same year at roughly the same mileage. I replaced both oxygen sensors and the coolant temperature sensor (it regulates the PCM based on coolant temp - it's cheap and I replaced it based on advice I'd heard here), replaced the cap, plugs, and wires, and it solved my rough start and idle problems. The engine runs awesome now. It seems like misinformation from bad sensors was causing the PCM to go haywire especially before the 2.5L warmed up.
Coincidentally, my sensors never threw a MIL (check engine light) code. I confirmed it by looking at the codes on the odometer using the three key turn trick, which simply showed "55" indicating no stored problem codes. Some here have said sensors can run crappy in a TJ a very long time before throwing a code, and based on my experience, that's definitely true. (As it turns out, I discovered around this time that my check engine light didn't even have a bulb in it, so I wouldn't have discovered a code unless the jeep failed inspection. I bought the jeep brand new so I don't know why the bulb was missing.)
Most cars do not use the O2 sensors for the first minute or so after start up. This is to allow them to heat up and work properly. Do you have a CAI on it? My guess is that the MAF is reading wrong. (are these jeeps maf based?) You can clean it with electrical or maf cleaner. After a minute or so it will switch to closed loop and base fueling off the O2 sensors instead of the maf. Going WOT will switch back to open loop. I would also double check the timing considering the backfires. It would have to be one hell of a vacuum leak to cause these issues so you would hear it for sure and it wouldn't go away after driving for a bit. The IAC valve is another possible cause of issues (sometimes they heat up and work better), but i would concentrate on the MAF and the timing. (sorry if I'm wrong, I'm a mustang ECU tuner and just got into jeeps so i'm not familiar with their strategies)
I'm having the exact same problem. Looks like I have a better starting place now! So far I have changed all plugs and wires, installed new manifold and gaskets, Cleaned fuel systems, no luck with these except for the exhaust leak being fixed. Thanks for your help!
TJs don't use the O2 sensors immediately so if the issue is right off the bat it isn't O2 related.
How old is the battery? Bad batteries can cause idling problems just after starting. First remove the clamps from the battery, then clean the battery clamps and the battery terminals. Bad connections can create similar problems. If the problem still persists charge the battery up and get it properly load tested at an auto store. If it's bad replace it.
I had the same problem. It got to where mine didn't even want to start. I took the throttle body off and cleaned it up real good with throttle body/carb cleaner. There is a good detailed write-up somewhere. I had what looked like chunks of carbon build-up coming out of mine. I replaced the TPS as well. Now it runs like a champ.
Having the exact same issue on the exact same car/engine/transmission/year as zer0321.
Did anybody resolve this issue? My plugs, wires, and O2 sensors are all new.
On a possibly related note (but probably unrelated), there is an antifreeze leak from the radiator which is causing some overheating.