So we bought a project '82 xj-s after conquering (temporarily of course) our other xj-s. After repairing the totally non-operational engine we thought we were set. The engine runs great! Of course as is the fashion of these cars, a new MAJOR problem arose with the car. The transmission started to fail in upshifting. It would go to second and not go up into third. Then you'd let the throttle down, rev it back up and no more problems. It would go away for a few days and there were no more problems. HOWEVER, It progressively got worse and worse until it one day it would not leave 1st at all. There was no way to coax it out of first. So... We let it sit for 2 or 3 weeks. I decided to take a fresh look at her at that point. over the 2 weeks it had somehow leaked most of the fluid onto the floor! I filled it up, drove it and It worked fine! errr.... for the first five minutes that is. Five minutes later it was back to its old ways, so I parked it. This time I decided to monitor the speed of the leak and it leaks about a quart a week sitting. AS it sits it is stubbornly is stuck in 1st. Am I resigned to a rebuild or could it be external---- vacuum, vacuum modulator? Could the leak cause ALL the trouble alone? What could it mean that after it sits for long periods, it heals itself briefly? Any suggestions before a rebuild are greatly, greatly appreciated!

 

First find the leak ! (it could be something very simple)

As I recall this is a GM400 gearbox, normally bulletproof, so it is unlikely to need a full rebuild, but maybe some seals. However to replace the seals will require you to drop out the transmission.

 

The "typical" failure mode for a vacuum modulator is no upshift until an abnormally high rpm is reached and then only if the throttle is released. This sounds pretty much like what you've described. I'd replace it, top off the flud, and see what happens. If it fixes the problem then I'd go ahead and fix the leaks.

As far as the leaks go you'll probably need to replace the pan gasket, dispstick tube seal, and selector shaft seal. When the car sits unused the fluid drains out of the torque converter and into the sump, rasing the fluid level so that it back up behind old seals and gaskets....which can't hold it in.

The 400 is a good old trans but I respectfully disagree with Fraser's description of it being bulletproof....especially after 28 years. I'd prepare for the worse and then be happy if it is an easier fix :-)

Cheers
DD

 

Thanks Doug and Fraser! Doug, how difficult is the removal and refitting of the vac modulator? I always get nervous about jacking that 2 1/2 ton car in the air! But if it can save me 2k + I'll gladly take any advice!

 

The modulator is held into the side of the trans with a simple single-bolt hold down clamps, so that part is easy.

You'll likely have to lower the right side exhaust pipe to gain access, though. That's where the work is :-). There's an exhaust joint just ahead of the rear suspension. Undo it and you might be able to twist-n-turn the exhaust pipe free of the catalytic converter. If not, you'll probbaly have to drop the pipe at the exhaust manifold with the cat converter attached.

Before you buy a modulator check the vacuum hose. It runs from the rear of the RH intake manifold down to the modulator. Maybe it has split open or something.

With good jackstands or big wood blocks there's not much to fear about working under the car....although a little bit of fear does help keep us safe :-).

Nitpick: The curb weight on an XJS V12 coupe is almost exactly 4000 pounds, by the way....not 2½ tons.

Cheers
DD

 

Update: After testing the car again today, there was a "hiss" coming from under the car that I noticed. Something is definitely going on under there. I plan to take a look tomorrow and update with whatever I may find. I will check for split vacuum hoses as well. And my bad Doug! I've never been very good with numbers