Idunno

1998 XJ 2WD 4.0 - I recently changed the IAC and cleaned the seat (housing) to fix a poor hot idle problem. It went from a normal cold idle to a poor (rich) idle when it reached the half-way point to normal operating temp. Changing the IAC corrected that but now the opposite occurs; it idles poorly at start up (runs rich) until it reaches the half-way point to a normal operating temp and then settles down to a normal idle.

I also changed the TPS so both the IAC and TPS sensors are new. I also replaced the fuel pump/filter unit.

I've read that another symptom of a poor idle could be the mass airflow sensor.

After reaching the half-way point to normal operating temp the idle is fine. The engine is very responsive to driving power demands; no hesitation or under-powered performance with zero upper or lower engine noises.

I just got this thing on the road tonight after a 2 year non-running period and it seems to pull just as strong as my 96, after warm up. But, I've had it running various times over the last couple weeks (parked), on a new tank of gas with some Seafoam additive, trying to ferret out the poor hot idle problem until the changing of the IAC (mentioned above) fixed that. Thinking I nixed the idle problem completely with the IAC change, tonight was my first true cold start after that only to discover this new fly in the ointment.

I'm just a little baffled as to how poor cold/hot idle conditions can reverse themselves. Obviously there's a lack of air in the cold idle mixture, or a case where too much fuel is being introduced. Is there an air/fuel mixture sensor on this buggy?

Ideas?

Thanks.