Recently my 1996 XJS with the AJ16 engine goes nuts when cold. I start the car and it idles at 1,500rpm.

When I put it in gear and start driving it really wants to rev higher, so while moving, if I shift into neutral it will soar up past 3,000rpm, sometimes higher. When the car comes to a stop, again, still in neutral, it will drop down to 1,300-1,400rpm. While the car is in motion, it wants to do more.

As the coolant temp approaches 188F, things calm down, and once it hits the 188F mark, everything is fine.

I have hooked it up to an OBD2 reader and the I've noted the following peculiarities:
1. The TPS, when cold seems to be at 14.9% open when at idle. When the car warms up, it will read 12.9%-13.1%, which is normal.
2. When shifting the car into neutral while driving, the MAF numbers follow, but do not seem to lead the car into increasing rpm.
3, When shifting the car into neutral while driving, the timing advance numbers go up quickly. Not sure if it is a cause or effect though.

The peculiar thing in all of this is that the car behaves normal once hot. The warmer it gets, the lower the rpms go at stop or drive, when in neutral. I noted that at 170F, the car only idles, in drive at stop, at about 800-900rpm. Once hot, it's at 580rpm.

I haven't done much to the car, in terms of parts other than the following:
1. Replaced camcover gaskets, including spark plug gaskets. Plugs looked good, so I didn't replace them.
2. I used my newly acquired PDU to reset base idle settings and TPS.

That's it.

While the TPS is original to the car, I did swap out the camshaft (and crankshaft) position sensors chasing down a strange blip in idle (documented in these forums, and resolved thanks to a replaced intake manifold gasket), but that was back in 2014. Idle, while high when cold, is smooth.

Thanks for the help.

 

You must have extra air getting into the intakes somehow, when the car is cold, the engine cannot race without extra air getting into the intake manifold somehow.. Check out whatever mechanism the engine has for cold starting. Also check that the throttle butterfly is not somehow sticking open when the engine is cold.

 

Well, I replaced the TPS, and along the way found the rubber boot that connects the elbow to the throttle body has begun to split at the seam. It seemed to have stopped at the clamp, even if it got past the clamp, it couldnt have been by more than a fraction of an inch.

In any case, they still offer those new, so I have ordered one and will replace it.

After installing the TPS, I hooked up the PDU and reset it no problems. However, I decided to reorient the oxygen sensors...because I was there, and I have started a new thread on what I found...please take a look when you have a moment.

https://www.jaguarforums.com/forum/x...1/#post1945379

 

Greg is on the mark.

ANY air that enters the system AFTER the airflow meter, is un-metered, and will cause all sorts of grief, and silly closed throttle revs is only one of them.

TPS holding the throttle disc open is common, and easily seen. Loosen the 2 screws and allow the disc to close.

Both of mine, albeit X300 sedans, one 3.2l and the other an "R," start at 1200rpm, and quickly settle to 800ish, via the idle stepper motor. But that stepper can only control so much, and if there is an air bleed, it will not have the scope to compensate for that much air.

Also, many markets, especially those with EGR systems, complain of stepper motor clogging with carbon, as a regular issue. We dont have EGR, so not an issue.

TPS flaky "could" cause some issues, but usually a hunting idle as the engine warms up, and settling to smooth as temps get close to Normal.

O2 sensors are usually a trimmer, on the X300 they are, as we only have them Post Cat. Pre Cat are the prime, as in my S Types.