The 1990 XJ6 has been sitting for the last 2 years and now won't start. The engine turns over fine. The dash gages don't seem to operate. I filled the gas tank with 5 gallons of premium gas. HELP!!!

ALWAYS check the body grounds first. Ground to the dash gages could be faulty, dirty, or just not tight enough to make a good connection.

If the engine does not crank sometimes, you would need to test the clutch safety switch.

not sure if any off this will help you o The starter won't crank. If nothing happens when you turn the ignition key to the "Start" position, it means that the starter motor doesn't turn over the engine. Most commonly this could be caused by a dead battery; here is How to check the battery. ... The ignition switch could be bad - it's a common problem.13 Aug 2013

Why a car won't start - troubleshooting tips - Samarins.com

www.samarins.com/diagnose/ut...or try this link. Car wont start Google Search

Usually on idle (warm engine) this is OK.

First check the oil level by means of the dipstick. If it hasn't got any oil it cant leak either. Under no circumstances should it be started without oil up to the correct level.

have you had the BCM(body control module) checked for any service codes? it sounds like your BCM may be experiencing memory loss or failure.

I don;t know the exact answer, but $3000 to fix an air bag sounds preposterous. I don't know where live, but if you in California, as far as I know, the only test a car has to pass is the smog test.
Second, I have been able to turn off some red lights (the engine) by using a $50 computer diagnositic meeter, which fits under the dash and any body who can plug in a lamp can use. Some red light messages are errors. If the car has not been wrecked or crashed then it would seem to be that air bag is still there and intact.
Also, you can see the car as is for a little less, approximately $3,500. I just bought one in perfect running order with only 82,000 miles on for $4,000 in CA. I bought one last year for $5000 that 120,000 miles on it.
You can advertise on Craigslist and autotrader. From 1995 to 1997 XJ6 are supposed to be the best year of Jaguars.

forget the fuel filters you are barking up the wrong tree here ,you have either a perished engine breather pipe ,or a fault with a air leak on the inlet manifold ,providing that your 1990 year of manufacturer is correct

id check petrol , and clean plugs,allso while plugs are out check that they are sparking

Is the car immobilized? if so you may have to lock and unlock the car with the key and not the remote.

Is it turning over or just nothing, also starter motor may be jammed. put the car in gear with the hand brake off and push the car forward until you feel the engine turn over. You dont need the keys in the car!

This should free the starter off, sometimes they jam very tight and will not move at all, on these jags. they get rusty quickly if sat about. I had a XJ many years ago great motors.

Good luck.