I recently acquired a 1987 XJS that has been sitting in a garage for the past 15 years with 69K miles. My local garage who has a very competent mechanic on staff has done quite a bit of work to get this beast to run. The most recent repair is replacement of the distributor & pickup coil, new plugs and set timing and new AAV. After this, below is the newest problem. If anyone has some ideas on where to look for troubleshooting I would appreciate it.

Thanks,
Bob

 

A couple quick thoughts....


I hope they're running fresh fuel !

After 15 years of non-use there are probbaly a whole slew of things that need attention.

As for the issue at hand, if the car was in my garage I'd be tempted begin by to making sure the throttle bodies were clean, the throttle blades set at the proper .002" gap, and the linkages synchronized....and check that the throttle bushings haven't crumbed away. Sometimes the easy stuff actually fixes problems

An oscillating idle can be caused by extra air being drawn in. This increases the rpm *until* the ECU cuts off fuel. Then the engine speed drops way down....at which point the ECU turns the fuel "on" again, and the engine speed increases. Then the ECU cuts off the fuel once again....repeat, repeat.

This often points to an AAV problem...but you say it was already replaced. So, extra air might be coming in from elsewhere. One thought: if any of the air-cleaner-to-throttle-body-bolts are removed the engine speed will increase quite a lot!

Cheer
DD

 

According to the mechanic's description, its not an oscillation issue, it is when the throttle is applied that this problem arises. We cured the oscillation when we replaced the AAV. This started after they replaced the distributor and set the timing.

 

Ahhh. Ok. I misunderstood what this meant:


"....then respond to the throttle spiking the rpms, the rpms will hang high and then settle back to spec, and then repeat. "




I'd go back to what I mentioned about throttle gaps, etc. Gotta start somewhere....and this is one of the easiest places



One of the frustrations on these V12 Jags is that it is SO easy to inadvertantly cause a problem with Widget-B while working on Widget-A. Close working quarters combined with a plethora of brittle wires and hoses is a recipe for trouble. It's not a matter of being careless or ham-handed. It just happens.

Cheers
DD

 

At what engine speed did they set the timing?
Unlike most engines, which are timed at idle, the V12 must be timed only at 3,000 rpm.

 

The work is being done by a very experienced mechanic, so I will assume he did the timing correctly. How would this contribute the to the stated problem?

Thanks,
Bob

 

Not to cast aspersions on your mechanic or to elevate XJS repair to the level of out-n-out black magic....but even very experienced mechanics don't know the tricks of *every* car made.

If you tell us that your guy has umpteen years of "Jag V12" work under his belt we can problably dispense with a great deal of speculation. :-)




Well, if he set the timing at idle speed rather than the required 3000 rpm he'd probably end up with way to much advance....which could account for the pinging. Plus, increased initial advance can increase the engine speed/idle speed.

In any case, when trying to ferret out mysteries there's no point in adding new variables....such as ignition timing being incorrectly set.... to the mix

Cheers
DD

 

Ok, I'll pass this forward to him and let you know if progress. Thanks to both of you for the experienced advice!

Bob