odd miss fire

[TGFH]

My 1998 XJ - automatic, 4.0 with A/C has a miss fire at about mid warm up. No other time. Starts fine - runs smooth, then abot 140 to 160 degree mark it starts running rough. Under power it back fires. Then a 200 - 210 it stops missing and runs great. Help?

History - caught a deer the hard way. Replaced the pass. fender, the hood and all the nose parts. The deer went as far as bending the PS pump pulley. Also replaced the coolant temp. sensor - that connection was broke loose. I replaced the vacuum canister and new vac line.

Am I missing something?

 

[birchlakeXJ]

Any check engine light? If so, pull codes.

My strategy for misfires is always the same. #1 is to verify tuneup hardware. When was the last time the plugs, plug wires, distributor cap, and rotor were changed?

A backfire in the intake is often a lean condition. A backfire through the exhaust is often a rich condition. Getting a fuel pressure gauge on the schrader valve on the fuel rail would be one of the first things I would do. You're looking for 49.2 psi at idle plus or minus 5 psi.

If you get stuck on this one, getting a scan tool hooked up and watching how everything is performing real time as it becomes symptomatic would be really valuable.

Good luck and keep us posted.

 

[TGFH]

no check engine light - and it runs great other wise.

I know I need a good code scanner. My 95 xj was easy to check codes on

 

[TGFH]

oops - forgot to add the last response. When it misses and at idle - the idle goes up and then back to normal.

 

[McQue]

I have read other posts with these exact same symptons...problem turned out to be a O2 sensor , or fuse for O2 sensor heater circuit, or bad O2 sensor wires.

 

[TGFH]

thanks - I will check that today

 

[birchlakeXJ]

I know I need a good code scanner. My 95 xj was easy to check codes on

Code readers are fine for reading codes but I was referring to a scan tool, which (along with the ability to read/clear codes) monitors different sensors and engine management parameters real time while the engine is running. Can't do that with a code reader.

I always recommend digging in yourself, but if you get stuck figuring this one out, a scan tool and a test run (from cold) while monitoring the scan tool closely for clues might really be helpful.