1999 XK8 was driving just fine, engine stopped and now will not turn over. The valve covers were pulled and it appears that the timing chains and tensioners are OK. Tried to turn over with breaker bar on the crank bolt but it would not move.. The car was not overheated. Anyone seen this problem?

 

That doesn't sound good. When I swapped the tensioners I was able to turn the motor with a 3/8 in stubby. Since you have the cam covers off try pulling the plugs so you aren't fighting compression. I hope it's not what I am thinking.

 

remove all spark plugs,engine might turn over then.
Could be vapor locked.
Check your oil as well. Let us know what you find

 

The car has been sitting for a month now, would a vapor lock last that long? What do you mean by a vapor lock?

 

I think he means hydrolock - water or oil in the cylinder. That will bind it. This could mean a bigger problem somewhere.

If I read your post right it was running and just siezed in the middle of running? That is most likely an accessory, a bearing of some sort or a hung valve (now crunched). If it has been parked just and won't turn over now (fine when turned off a month ago) then hydrolock is the most ikely cause.

In any case here is what I would do:

1. remove spark plugs - try to turn over

If it turns over it is probably hydrolock which most likely either means a leaking valve seal or a leaking head gasket.

2. remove the belt - try to turn over

If it turns over with the belt off one of your accessories is siezed or the AC clutch is failed.

3. If it still doesn't turn over with the plugs out and belt off remove the cam shafts

Look for a hung or bent valve. If you don't see any try to turn it over with the cams out (you will have to retime the car). If it turns over with the cams out then one of the timing chains likely skipped some teeth. You probably have piston and valve damage I would consider pulling the heads.

4. If it still doesn't turn over remove the heads and inspect for damage. IF it won't turn over with the heads off I think you need to pull the engine because you have an internal problem with the lower end.

 

If you towed it to somewhere after the breakdown I guess it has nothing to do with the gearbox else it could be a clutch thing. Just a thought.