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Rough idle questions??

H8D-XJ

NAXJA Forum User
Location
San
Hey guys, i just bought a 96 XJ sport, automatic trans, 4 door, 2WD with a 4.0 HO. Actually i helped a friend buy it and now im def looking for a 4wd for myself i love these things.

Anyways, the car blew smoke and had a CEL. It was the upstream 02 sensor, so i replaced it and no more smoke and no CEL. I also replaced the air filter and spark plugs and ran some seafoam through the TB. It runs perfect and idles perfect when started up cold. But when it warms up it idles rough, runs and drives just fine when warm just idles rough. It sounds like its cammed and sounds like its sputtering/popping through the exhaust. I cant seem to find out what the problem is, im thinking maybe it needs the downstream 02? What are your guys' thoughts?
 
my ranger acted like that i while back ended up being my egr valve got hit on the fire wall off roading befor i replaced my motor mounts. but they can just simply go bad so look into that
 
No EGR post-1990 XJ 4.0s.

Something making it remain in open loop and running rich would be more like it.
 
I also replaced the air filter and spark plugs and ran some seafoam through the TB?

I'd recommend finishing the tuneup with new plug wires, distributor cap, rotor, fuel filter.

As mentioned, no EGR on this one. The downstream oxygen sensor is mostly there to monitor the efficiency of the catalytic converter. The upstream one is the one that figures into fuel/air ratio.

In the absence of any more check engine light/codes, you could start testing of fuel pressure (with a GAUGE, compare your numbers to factory spec) and of individual sensors. Map sensor, coolant temp sensor come to mind for running rich.

A SCAN TOOL can come in real handy here to see how your engine management system is operating.
 
I'd recommend finishing the tuneup with new plug wires, distributor cap, rotor, fuel filter.

As mentioned, no EGR on this one. The downstream oxygen sensor is mostly there to monitor the efficiency of the catalytic converter. The upstream one is the one that figures into fuel/air ratio.

In the absence of any more check engine light/codes, you could start testing of fuel pressure (with a GAUGE, compare your numbers to factory spec) and of individual sensors. Map sensor, coolant temp sensor come to mind for running rich.

A SCAN TOOL can come in real handy here to see how your engine management system is operating.

Alright so the downstream 02 wouldnt matter even if it was bad or if the cat was bad?

Im going to try cap, rotor and wires tomorrow. Where is the fuel filter located on the XJ?

I did just get a check engine light today, it was P0171 (Fuel trim Bank 1: System too lean) I find it hard to beleive its lean though because the car sounds like its rich. sputtering/misfiring at idle and lightly blowing black smoke. And it seems to be doing anytime at idle now cold or warm.
 
I did just get a check engine light today, it was P0171 (Fuel trim Bank 1: System too lean) I find it hard to beleive its lean though because the car sounds like its rich. sputtering/misfiring at idle and lightly blowing black smoke. And it seems to be doing anytime at idle now cold or warm.

The OBD system is pretty darn good at leading you down the right path.

First thing I would do is to test fuel pressure WITH A GAUGE at the schrader valve on the fuel rail. Compare your numbers to factory spec. Could also be an oxygen sensor or a map sensor at play here or even an exhaust leak.

Google P0171 + Jeep and you are bound to get a few hits with ideas of possible causes.
 
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Started it up this morning and the xj ran perfect, figured it would go bad as soon as it warmed up.... It didnt, drove it all the way to work and it never idled crappy like it did last night? weird. I did notice though it doesnt seem to warm up. the gauge is 100 then a mid line then 210. The temp never gets above that mid line which im assuming is around 140-150 degrees. So maybe its a thermostat or bad coolant temp sensor which is keeping the car in closed loop?
 
Job #1 is to get your engine temp in the 200-210 range. That has to come first and then see where you are at with any symptoms.

You could verify your actual temperature by getting a point and shoot infrared thermometer and pointing it directly at the thermostat housing for a reasonably accurate verification of temp.

The thermostat is the biggest player here. I like thermostats purchased from Jeep and the Stant Superstat. 195 degree only. Stay away from the cheap stats. I'd really consider replacing this; it is inexpensive and doesn't take very long.
 
Yeah, tonight im doing cap, rotor, wires, coil, thermostat and a oil change. Well see how she runs after that.
 
Yeah, tonight im doing cap, rotor, wires, coil, thermostat and a oil change. Well see how she runs after that.

Replaced the cap,rotor,coil,wires and thermostat today. She starts and runs good and actually warms up to operating temp now. But it eventually started to idle rough again and misfire a little :cry: What i think it is now is that the cat is clogged a little and at idle the downstream O2 is reading lean so its dumping more fuel into the motor. But when you give it gas its fine so i dont know. The exhaust seems a little hotter than normal so thats why im swaying towards a clogged cat.

Also just found out my OFA is leaking! Man when it rains it pours haha
 
Downstream o2's do not effect fuel trim. They are only there to monitor the condition of the cat. The cat is probably melted from the failed upstream o2.

To further confuse matters, if you get into an ignition misfire this can cause a lean code even though you're really running rich..
 
it was mentioned to change a fuel filter....not sure on 96, but my 98 and 97 are both in the tank...attached to the fuel pump, which also has a sending unit attached to it....they go for 190 bucks. sooooo if you do a fuel test and all is well id leave it alone.
 
you have a vacumm leak. wait for it to do it then spray brake clean around prone areas such as vacumm lines intake gaskets injector o-rings. when you spray over a given area and it changes the idle you have found your leak. its also a good idea too vissually inspect all vacumm lines and hoses for for falling apart, cracking, rotting if this doesnt help you it might have a burnt valve you can put a vacumm gauge on it and see if it fluctuates rapidly
 
it was mentioned to change a fuel filter....not sure on 96, but my 98 and 97 are both in the tank...attached to the fuel pump, which also has a sending unit attached to it....they go for 190 bucks. sooooo if you do a fuel test and all is well id leave it alone.

I bought a fuel filter but pressure at the rail is fine so i left it for next time.
 
you have a vacumm leak. wait for it to do it then spray brake clean around prone areas such as vacumm lines intake gaskets injector o-rings. when you spray over a given area and it changes the idle you have found your leak. its also a good idea too vissually inspect all vacumm lines and hoses for for falling apart, cracking, rotting if this doesnt help you it might have a burnt valve you can put a vacumm gauge on it and see if it fluctuates rapidly

I keep looking for a vacuum leak but its all hooked up correctly ill spray brake clean on my intake gasket and injector o-rings and check some more. I hope i dont have a burnt valve that would suck. Its weird though cuz randomly it will idle fine, other times it wont.
 
Did you clear out codes? If so, does check engine light return and what code numbers now exist?

Testing is highly recommended from this point forward. Vacuum testing with a gauge, compression test all cylinders.... exhaust gas analysis and vacuum testing can help you isolate an exhaust restriction or disconnect cat temporarily to see if that makes any difference.

Troubleshooting is not a straight line, it is more a "process of elimination". Good luck and let us know what you find!
 
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