singularity

My 1999 XJ started idling rough like a year and a half ago. Last summer I had to change out the exhaust manifold and while I was at it, I changed out the injectors from single holes to four. At first everything was great, but after a few hundred miles, the idle was really getting rough. It was showing periodic misfire codes, but the codes stopped even though I can feel it misfiring. The issue became worse with periodic sluggish take off (doesn't matter if hot or cold) and a backfire through the intake manifold (which I thoroughly cleaned when I changed out the exhaust manifold). I changed out the ignition coil. It ran great for about 150 miles, then the same symptoms returned. Thinking I made a mistake on cheap injectors, I changed them out again with Bosch 4 holds. Again it ran great for 150 miles then back to the same symptoms. I changed out the cap, rotor and spark plug wires. Yet again, it ran great for 150 miles and now it's back to it's misfiring ways. Arrgh! I am wondering maybe the problem is my ECM/PCM. Every time I changed something out, I disconnected the battery then turn the lights on to be sure the ECM would reset and then adjust to my changes once done. I am thinking that everything is good after I reset the ECM, then as I drive near the 150 mile mark, the ECM has finished it's reconfiguration but somehow completely goes into fail mode?! Running great for 150 miles, then misfires after that point over and over has just one thing in common, my ECM resets. Any thoughts? Anyone know where to get a decent ECM not outrageously priced, as in over $300? .

lawsoncl

It's very rare that the ECM is bad. It's far more likely you have damaged wiring where the wiring harness crosses from the top of the engine over to the firewall. Rotten motor mounts causing excessive flexing of the wiring harness can cause frayed or broken wires. Also clean all of the ground points. After 20+ years, refreshing all of the ground connections is a good idea.

singularity

I changed my motor mounts out three years ago. I am thinking that if it were wiring, why, after every change I make, does it run great for around 150 miles, then start misfiring? Any electrical issue should be more consistent or have some sort of easy pattern (i.e. cold/hot engine, road bump). It seems the proof that it's the ECM will be target it. Tomorrow, I will reset the ECM (disconnect battery, light switch on), then reconnect. No other changes. I am betting it will, again, run great for 150 miles, then start misfiring. It seems it takes around 150 for my ECM to start malfunctioning after a reset. I will report back on what happens. The curious things are the misfires and intake backfire and no codes. I believe the ECM is supposed to send the code when a misfire is detected.