XJ_Atheist

My mom has a 98 ZJ and when you start it up, it starts hard but idles fine. as you drive down the street it bucks and knocks and wont drive until its warmed up...then it drives fine. We couldnt figure it out so we brought this to a couple shops and neither of them knew what was wrong, at first they thought it was the CAT but it did the same thing with the cat off...Long after these symptoms began, It spit a code for the EGR but i dont know if that would cause this...

Any help would be appreciated,

Thanks. -Roger

Bustedback

If the EGR is stuck open it will cause a vacuum leak and make it run as you described.

OverlandZJ

I was under the impression the 4.0 ZJ's had no EGR and the later model V8 Grands eliminated the EGR?

My 98 4.0 is doing this as well. I plan to replace the TPS and clean the IAC and see if that helps.

Bustedback

I'm assuming he's talking about a V8 and that there is an EGR valve because of the code it spit out, but it could be a bogus code.

XJ_Atheist

Crap im sorry, I meant to say it spit a code for the Evaporative System, the motor is a 4.0l

Bustedback

OK, sounds like a vacuum leak someplace in the evap system.

XJ_Atheist

The vacuum leak would actually cause that? im having trouble understanding why it would only do it cold though? looks like its time for a smoke test...

Bustedback

A cold engine requires mor fuel than a warm engine, a vacuum leak leans the air fuel mixture out too much for a cold engine to run.

Willys55

don't rule out a faulty coolant temp sensor that would send false signal to the computer resulting in an incorrect pulse width on the injectors.

XJ_Atheist

Cleaned the IAC, it helped alot for a few days, but then returned to normal. So i replaced it, but it didnt help...i guess i need to fix that evap leak