99XJ Odd Misfire

99XJ 4.0 auto

Has a misfire that only shows itself on startup when i put it in gear. Once im moving it isn't noticeable and at stoplights it only happens once in a while. Doesnt matter if its a cold start or hot start.

Has brand new plugs, rotor, wires and cap. Old ones were pretty junky. Regular plugs no fancy junk.

Doesnt seem to be a fuel issue because its fine while driving and WOT.

Any ideas? Doesnt throw a CEL either.

iratefla ive been dealing with a similar prob. on my 98 2.5L. it progessively got worse, stays missing. traced it to bad injector on #1 cyl. thanx to threads on .

Injector would be my guess too. A bad injector isn't likely to show up at WOT, but it will show as a subtle miss at other times. If you're' not getting a code, it's likely to be clogged or mechanically bad. If it's only partly malfunctioning, it can be pretty hard to track down without an injector tester. But if it's actually not injecting, you can test for it pretty easily. At least this worked for me on a 93... Have the engine idling, and pull one injector plug at a time. If the injector is good, the engine will briefly bog, then speed back up to normal by itself. Put that one back, and do the next one. If nothing happens, you've pulled the bad one.

There's enough slack in the injector harness to allow you to swap neighboring pairs. If you find a bad injector, the next step is to turn off the engine and swap its plug with its nearest neighbor. (Injector timing is not that critical for idle speed, so it should still run the same.) Now repeat the above test. If the problem stays at the injector, it's the injector. If it follows the plug, it's the harness or the ECU.

on the other hand double check the new plugs ive gotten bad plugs in the past. (twice , honest.)

MUDDPUPPY78 said:

on the other hand double check the new plugs ive gotten bad plugs in the past. (twice , honest.)

Click to expand...

Good point. So have I.