So in the last month or so I have noticed a tendency for the car to run rough or downright stall when putting it in gear. It only happens after a reasonably long run (today it was 10 miles at 70 MPH, then stop and re-start 10 minutes later). The car will run very rough/lumpy for about 30 seconds during which time if you try to drive it will most likely stall. There is no smoke during this episode. If you leave it to idle for a while it is fine. The car recently had a 50K mile service including fluids change. No codes appear to be produced and no lights on the dash. FYI the car is an 04 XJ8 with 52K miles.

Any ideas of what to look for?

Thanks

- Tim

 

There have been reports of throttle position sensors going bad in the throttle bodies at this mileage, this could cause the problem.

 

Oldmots, you probably nailed the problem,

Not many things that cause this symptom with out
a CEL. The TPS will get worse and worse til
you just cant start the engine anymore.

One other thing may do this and that is the
EGR valve. No lights or codes and the rough running
changes with engine temperture.

 

Oh boy that sounds expensive. The car is at the dealers today and I'll get it back tomorrow. They think it is dirty/gummed-up injectors from the Ethanol fuels. Car is under warranty until May next year so if it is the TPS then I guess it will eventually show itself.

Being a pilot, I see exactly the same operation with fuel injected piston engines. Most continentals and lycomings are an absolute pig to start when hot due to fuel vaporization. It leads to difficult hot starts and lumpy running until the fuel gets into the lines. Of course the only ECU is the human brain. Cranking with mixture at idle cut off and the throttle wide open and swapping the two lerers around as the engine starts to fire is the normal solution. Not easy to do on a twin engined Aztec when you have your left hand on the starter switch and the right hand juggling the mixture and throttle.

- Tim