86 xj6 idles very smoothly. I read that the instantaneous mileage on the trip computer should register 99 if you suddenly release the accelerator. Mine hasn’t done that in a long while. I checked for vacuum leaks using propane and also squeezed vacuum tubes around the intake with no suspects. I only know of the vacuum tube under the manifold that goes to the distributor “delay” thing and to the distributor. People have mentioned a drain under there but I can’t find it. That’s the only thing I can’t seem to find. Is the only thing that can prevent the 99 mileage a vacuum leak? And where is the drain.? Left or right side as you face the engine from the Passenger side?

 

in my opinion, the trip computers are by now, too old to work correctly or they stop working completely.
In my '84, it works sometimes, most of the time it's just a clock.
I had never heard of the "99" trick, must be something that was incorporated for 1986 and 87, the last two years of the XJ-6 Series 3.

 

It is probable that your TPS is out of adjustment. You are correct about the 99, the trip registers 99 (ie in effect zero) when decelerating on a closed throttle because the ECU shuts off the injectors until the revs drop to about 1400. If the TPS is out of adjustment, the ECU does not get the closed throttle message.

 

I agree with Greg that the fuel cut-off function is something to look at.

The throttle switch (not a sensor, actually) is on the underside of the throttle body. I'll try to find some info on how/what wires to check

Cheers
DD

PS-the drain is on the underside of the intake manifold. It's just a little nipple with a rubber cap

 

Possibly barking up the wrong tree here.

The wiring diagram shows that the switch is used for "full load" signal only on USA cars.

Plus, this text from the service manual, abridged:

"......closed throttle contact on the throttle switch. This contact is fitted, but not used, on cars to the USA and certain other countries."

I'll try to find more info. I recall confusion about the throttle switch coming up before. Decades ago.

Cheers
DD