Before every hater tells me to go search for it, it's been answered a thousand times. I have for like a month. The Wrangler is a 1998 with 4.0 engine, The issue is that it only acts up from just after start up until the engine gets to it's normal operating temp, then it runs like it's brand new. I've changed the pcm, re-ran the evap lines from the tank to the canister (including dropping the tank to make sure there are no bad hoses), new gas cap, new air intake temp sensor, new plugs and wires, cleaned the MAF sensor. It rarely throws a code, when it does it's P0455, which I know is large leak in EVAP system. It's driving me crazy, anyone out there got any ideas?? Thanks
an engine that runs bad upon first start up could be a symptom of a vacuum leak and this is especially true for MAP engines.. It may sound backwards but once the engine goes from open loop to closed loop, the PCM will notice the AFR is off and correct and therefore the engine runs better but won't necessarily throw a code for that. In fact, the PCV valve is basically a metered vacuum leak based on where the hose goes, to the intake manifold. Just a funny fyi. An evap system leak IS a vacuum leak btw which is what that code means. That code could also be as simple as a gas cap leaking but having your gas cap off won't change the way your Jeep idles so look elsewhere.
In a MAF engine, the engine may run fine with a vacuum leak during warm up but once it's warmed up it'll run horrible.
Just a thought to check for vacuum leaks. I'm a vacuum leak kinda guy because those pesky problems can account for all sorts of weird symptoms.
"What? my jeep is faster then your ricer? whoa surprise"
The PCM can take as little as 20 seconds to switch to closed loop from what I've seen on my scanguage. That could be considered just after startup. Since it runs normally once it's then warmed up it could well be a bad heater in the upstream O2 sensor.
The easiest way to tell if this is caused by the upstream O2 sensor is to unplug it and see how the Jeep runs. If the problem is still there the O2 sensor is not the cause. If the symptoms go away, replace the sensor. NTK recommended of course.