Confusing misfire

[Maxxyjeep]

Hi all,

I have 1999 4.0 sport - has run perfectly for years, however recently it has developed a misfire when first driving the car. No issue starting the car but once I get off the drive and take it up the road it pops and bangs and judders until I floor it and it all seems to clear and then drives perfectly for the rest of the journey.

If it's popping and spluttering and I pull over the revs seem to be very erratic, up and down. Like I say if I rev it up though it revs nice and smoothly.

Has anyone come across this before?

I have replaced plugs, leads, coil, rotar and distributor cap so far.

I'll add, no codes or engine lights are logged at all which makes diagnosis very difficult.

Thanks in advance

 

[RobertJ]

There could be pending codes, and the cheap code readers won't alert you to those. It's a good investment to get one that can read pending codes as well as live data.

If I had to guess though, I'd say it could be the Crankshaft Position Sensor, but I'd be hesitant to just throw parts at it. I will say that I keep an OEM one on hand, because I'm afraid they'll get harder and harder to find.

 

[RMETeeJay]

If you've never cleaned your IAC, give that a shot.

How to clean your IAC (Idle Air Controller) valve

 

[Maxxyjeep]

If you've never cleaned your IAC, give that a shot.

How to clean your IAC (Idle Air Controller) valve

Thank to both for the advise!

I have replaced the IAC. Forgot to mention that. Never thought of the Crank Position Senser, can't imagine they're overly expensive so will try that out.

Are we agreed OEM is the way to go with these cars?

 

[RMETeeJay]

Perhaps a bad O2 upstream sensor? See the following thread; seems similar to your issue:

Misfire when warming up