xj motor doesnt fire up some times

[gotta30]

I have a 2000 xj petrol and 50 % of the time it turns over but does not fire but if i turn the key back and start again it starts perfectly .Any clues?

[muddy]

Originally Posted by gotta30

I have a 2000 xj petrol and 50 % of the time it turns over but does not fire but if i turn the key back and start again it starts perfectly .Any clues?

Have you checked the Crank Positioning Sensor? They've been known to fail resulting in symptoms such as yours.

[ApacheResistance]

When you say "turn the key back" do you mean you switch it on (to right before it cranks) then switch it back off and then switch it back on to right before it cranks, then crank it?

A good (and simple) first step in helping you troubleshoot this problem is called “the poor mans prime”

1. Turn key to ON position (do not crank the engine!)
2. The fuel pump will energize and run for a couple of seconds
3. Turn key to OFF position
4. Repeat above steps 1-3 two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have a check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the fuel pressure leaking down and the subsequent stumble upon startup is the engine clearing the excess fuel that has leaked into the cylinder because of the faulty injector.

[Roler]

Originally Posted by gotta30

I have a 2000 xj petrol and 50 % of the time it turns over but does not fire but if i turn the key back and start again it starts perfectly .Any clues?

Does it do this on first start up of the day, or at any time, cold or warm engine?
The poor man's prime is a fine procedure, but I'd think that would only be relevant when the jeep has sat for a while (long enough to lose fuel pressure). I've been doing the PMP for nearly a year now, and will continue until I get tired of it and decide to finally tackle to issue.

Do I understand that it cranks, but will not run?

In that case, a few things can be causing that, and diagnostics (before any parts replacement) will guide you in finding it. The intermittent character usually point towards and electrical rather than purely mechanical issue.

[junglejuice]

The above points are all valid, more detailed info is required if you want a semi accurate dianosis!!!