Another Random Misfire Post

My 96 4.0 w/ 5 speed finally threw the CEL code for random misfire.

All of the secondary ignition is new including a cap with brass contacts. I removed my injectors and had them tested and they all came back fine. I put my noid light on each of the injector connectors and they flashed fine without pause. The distributor is new as the original one had slop in the shaft. My compression is even at 120 with the fuel pump relay removed. Oh yeah, I cleaned and greased all of the fuses and relays too.

It starts fine. I have an obvious mis/cross fire at idle, a slight surging under hard acceleration and when I coast in gear it feels and sounds like I've got an engine brake. There is no smooth transition from decelerating to accelerating. Abrupt and jerky. The throttle response seems delayed, not immediate. I also replaced the TPS.

I was all ready to replace the headgasket thinking it was leaking between two cylinders but the even compression has me holding off.

The fuel pressure was fine at spec psi which I have now forgotten. Can the pressure be ok but the volume not adequate? I replaced the inline filter but know it has the sock in the tank too.

Threads with similar issues are all inconclusive. Were the other vehicles all junked or parted out or what? This is a non-project vehicle that has quickly become one. And not a project with any immediately satisfying results such as customization or improvement. I like this XJ and would like to enjoy driving it, but my blood pressure rises every time I have to drive it. It now mostly sits in my way.

Any good next steps?

Brand of plugs and the gap?

Factory spec. gap Champions. Not to sound caustic but I am beyond that.

Jasper911 said:

Factory spec. gap Champions. Not to sound caustic but I am beyond that.

Click to expand...

Thats Ok, I still gotta ask (been a rash of E3-ittus lately).

Weird on the deceleration. If you had the AW4 I would suspect that the torque converter isn't unlocking, but you have the AX15. Under hard deceleration the PCM should go into open-loop mode and the fuel would be cut off, maybe something is causing that.

PM me your email and I will send you some info on the modes of operation, not for your year but it might give you some ideas.

Could be the crank position sensor is starting to go south. It obviously has a huge effect on overall engine smoothness, and it is also the sensor that generates the data used for misfire computation.

edit: If you can get your hands on a kV meter that'd be a good check of the secondary ignition circuits as well.

For sh_ts and giggles I replaced the CPS.

Jasper911 said:

For sh_ts and giggles I replaced the CPS.

Click to expand...

Too early for results?

Here's a good article on "random misfires" (P0300)

http://www.random-misfire.com/

i have the same exact xj and same exact problem.....I'm going out in a few to listen for vaccum leaks and change the spark plugs cause that's all that's left to swap out.


doug