Join Date: Aug 2004
Location: South Bay Los Angeles, CA
you could try insulating plug #3 with some heat wrap; your problem could be a worn out TPS or oxy sensor. once warmed up and in closed loop operation, the TPS and oxy sensor signals are used by the ecu to control ignition and injector pulse width along with the idle air motor.
here's a quick check: disconnect the TPS and drive until fully warmed up. yes, the CEL will turn on, but it should run fine, if it smooths out your idle then your TPS is an issue. as for oxy sensors, usually the first one in the exhaust manifold wears out first, and if you have 120k miles with original oxy sensor then it's beyond it's service life.
your rough idle is probably a heat soak issue with either a plug or injector, imo. since the 3924 plugs did not do anything for you, i'm a little shocked since that's my planned fix (have them but not installed them) but then again i've had a rough idle all along with my XJ.
you could use a volt ohm meter to check out the tps for wear, but probably cheaper to spend 40$ on a new one , again, if you have a lot of miles with original parts it's an issue.
here's a forgotten item: if you can get to it, a fuel filter change can do wonders for idle smoothness and acceleration power. on my 96, it was a $12 part and 10 minutes labor. felt like i dropped a new motor in!
anyways, try one thing at a time and don't go nuts. if it runs well so far, then be happy. if you try to fix it all at once, you'll never know what the main problem is.
i will try to heat wrap the #3 injector in a few days, and after that, i've got an oil change with the 3924 plugs scheduled.
for you, try the fuel filter, then heat wrap, then TPS, then oxy sensor if all else is running well with the motor.
how did the old plugs look? clean, gray, or brown, with ash? if it's the heat wrap solution, i would suspect that all the plugs look like they're running a bit hot.
Newly Painted! Emerald Green '96 XJ 4x4 4.0 HO 5-spd
3" Lift + 31" Tires / D35 TracLok / NP231
Drilled out Stock Airbox / Gauge Cluster Upgrade
Magnaflow Muffler / High-flo Cat / MSD Coil