common cold start misfire plague :(

Hello ,

I've been plagued with this cold start issue and I've read countless threads on various forums regarding this issue to no avail.

Symptoms: After jeep sits overnight or an extended period of time, it will exhibit a really rough idle for about 5-10 seconds and smooth out on its own and drive perfect. It got worse today (steering wheel was shaking it was idling so rough) so I'm looking to finally fix it.

Knowns:
- Replaced injectors & heat wrapped all 6 of them to prevent common heat soak issue
- Fuel pump check valve is bad, been poor man's priming for the time being. This shouldn't cause a rough idle, just slow start.

Possible fixes:
- Front o2? Seems like I've read a few cases where the o2 sensor fixed it... but they were experiencing misfires on warm starts / cold starts, all the time.
- Clean IAC / TPS, doubtful it will fix it but I wont rule it out.
- Fuel filter?

Any other ideas / things I should check?

Thanks!

What year is it? and when was the last time you did a tuneup on the ignition system?

its a 2001, sorry for not specifying.

I did brand new champion plugs a few months ago along with the injector replacement.

This might help: http://www.wjjeeps.com/tsb/tsb_wj_1801001.pdf

I would also check the fuel pressure when this happens.

Another possibility is the check valve in the fuel pump allowing all the fuel in the rail to drain out. That will give hard starts and rough idle until the pressure builds back up.

ehall said:

Another possibility is the check valve in the fuel pump allowing all the fuel in the rail to drain out. That will give hard starts and rough idle until the pressure builds back up.

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If you don't have enough fuel pressure the fuel will boil in the rail and create air pockets which can cause what you're talking about.

Vacuum leak at intake? Warms up at intake expands and reveals. Seen this on a few 4.0s. How many miles on it?

trippled said:

Vacuum leak at intake? Warms up at intake expands and reveals. Seen this on a few 4.0s. How many miles on it?

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about to hit 150k.

I have a CEL but its for a minor evap leak.