XJ40/X300 wont start?

I used to own a jade green 4.0 litre Sovereign (My first Jag) 1990 year. After some years of ownership I found that occasionally it wouldnt start. There was no particular time, morning or evening that it happened. It would just turn over and turn over with no chance of the engine catching. I would leave it for an hour and quite often it would start.

I did a massive amount of head scratching on this one. Even pro help was no good- no error messages were stored and when left overnight at the auto electricians, the !Removed! thing started perfectly the next morning.

In the end I determined to check every single connection that I could find on the car under the bonnet. After some long time checking I found the connector for the crank angle sensor. The sensor is an electronic device that looks at the crank damper on the front of the engine and tells the computer what position the engine is in when being turned over by the starter. This allows the electronic wizardry to determine the correct fuel/ ignition setting.

The connector- one of those big jobs guaranteed never to fail that Jaguar used on the forty. Well it was melted onto the front of the engine, out of sight below the over hang of the cylinder head. The plug was shorting out onto the engine, presumably when humidity (or dampness?) in the air was just right. Thus the computer never got the important message about the crank angle and wouldnt start the car.

I replaced the plug with a tough waterproof item from an electronics web site and the problem went away.

Electronics-its always in the connections, either bad ones or dry solder joints.