After having my xj6 87 not start for 2 days, I finally had it towed into my mechanic. Then something told me to try to start it, and guess what...it started. Is the starter going bad? Solenoid?

 

For a 23 year old car, I would think your starter motor will have lost a lot of its efficiency over the years. The commutator and the brushes wear so less current passes, hence the torque is reduced. I had this trouble with my 1980 XJ6 in around 1998. I replace the battery to no avail, and it was only when I took it to a local Jaguar specialist that the fault was revealed.

 

Replaced the starter and it won't start.....ugh!

 

The ignition switch may be the cause. My car needed one when I bought it, the owner couldn't start it. I figured out that it tried to start when the key was turned all the way to the START position, but it died as soon as it was released to the RUN position. Replaced the ignition switch (mounts on the back of the key cylinder) and it fired right up.

I have also heard of people taking apart their ignition switch and cleaning the contact surfaces to restore operation. Basically, poor contact in the switch prevents current from passing through, preventing the engine from running.

You didn't really detail what happens when you try to start it. Does the starter engage and rotate the engine like it should, but the engine fails to start? Or is there no response from the starter at all?

 

Tap gently on the starter relay and see if that doesn't solve the problems.

 

This might help
http://www.jag-lovers.org/xjlovers/xjfaq/nocrank.htm

Cheers
DD