New 1994 XJ12 - Idles but misses with throttle


(Tim Wasson) #1

Hi All,
New 1994 V12 owner here. I had an '84 Vanden Plas with the XK engine, so not a stranger to Jag foibles, but bought my first V12 at an auction on New Year’s day.

The car starts and idles fast (~1100 RPM), relatively smooth, sometimes will surge up and down from about 1800 down to 750 and back up but it starts stumbling and missing, losing RPM as I apply throttle off idle. Further throttle travel only exacerbates the situation. Occasionally, it seems to be OK, and happily revs and pulls great up through the gears for a minute or two, then it starts stumbling, missing, and backfiring. Sometimes shutting the engine off and restarting it will clear the condition for 30 seconds or a minute.

The alternator doesn’t appear to be charging, as the idiot light is on and the voltmeter is sitting at about 10.5 V.

My initial plan is to get the battery charged and tested, then pull the alternator and get it tested. Once the charging system is working (giving the electronics an chance to do their job with proper voltage), I’ll move on to the high idle, which I’m guessing could be vacuum leaks, and see where that leads.

Does this approach make sense?


(Philip Lochner) #2

Welcome to the forum!!

Yes, sort out the alternator first, low voltage (specially THAT low) to all the electrical systems needed to run the engine could cause all kinds of trouble.

Next thing to do is check the throttle position sensor output for correct output voltage ) should be between 0.32 and 0.36V IIRC, and for a smooth and continuous increase in voltage as the throttle is opened.


(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #3

Remember that one common cause for the alternator not charging properly on these cars is a sheared crank damper.


(Tim Wasson) #4

Thanks kirbert, I’ve got the charger on it now, will start fiddling tomorrow. I appreciate the heads up on the crank damper!