Ok a new thread for a problem that has come up on here before.

I have an XJR that started to run a little rough on idle about a week ago. I thought I was very low on fuel so I put some in and went for a run and the car ran great. It started and ran perfectly two or three times since. Today it started perfectly and drove out of the garage. When I restarted it about half an hour later it started to run rough again, as if not firing on all cylinders. Increasing the revs seemed to make no difference. Thinking it might be lack of fuel I put some in. After that the car fired a couple of times but wouldn't start. Now there is no sense that it is even firing.

The car cranks over nice and fast, the battery is fully charged. So I checked that I have fuel and there is a high pressure on the rail when cranking. Removing one coil pack there is also a spark although it looks a little weak.

Thought I better do a bit of research before posting. Checking the archives here and elsewhere suggests that it may be the crank sensor and one test for that is whether or not the tacho reads (200 or so) while cranking so I will check that first.

I think an hour or so of reading previous posts as well as attached links and web searches has given me something to go on, but I thought I would just see if that (crank sensor failure mode) is consistent with anyone else or if there is something else I should check.

Will report back.

 

Crank sensor is a good bet, then consider water in the gas. Your gas cap drain isn't plugged up, is it?

 

OK I decided to do things logically! First I rechecked to confirm there were no fault codes stored. That suggested to me that it was not a sensor failure? Then I checked the tacho and the revs were registering on cranking. That also suggested that the crank sensor might not be the cause. I reconfirmed good pressure at the fuel rail (loosened the nut and fuel started to spray out when cranking). I also reconfirmed I was getting a spark. When I did that I also heard one cylinder fire. Thinking that might be the sign that it would start I excitedly tried again - no joy.

So I decide to do one thing at a time so that if it was fixedc I would know exactly what the problem was. First thing I removed the plugs and cleaned them. I cleaned them by washing in petrol and blowing dry with the airline. I refitted them and tried to start the car again. Instant success, fired straight away and has been running as sweet as a nut ever since!

So the problem was plug fouling, but the cause is still a mystery. It has to do with starting and stopping the engine after less than a minute or running the car with little or no fuel or a combination of both with cold temperature thrown in for good measure!

Thank you for a great forum!

 

The records that came with my car include a starting problem that the dealer diagnosed as fouled plugs caused by repeated shutting off before the engine could warm up.

 

Yes, I am pretty sure that is all it was too. Also it seems that there is a point of no return when the more you try and start the worse the fouling becomes.

I have an X308 as my everyday car and that engine has a serious problem with cylinder wash if started and immediately shut off. When I was moving both cars I left that one running and shut off the XJR. Now I know never to start either without letting run for a bit!

 

This has come up before.

My morning routine often requires a couple "cold stops" then re-starts and I've never had a problem....but others do.

Why some cars are affected and other not is a mystery to me. Maybe "repeated" means several times in short order and not just a couple?

Variance in cold fueling enrichment maps?

The V8 cars in particular had a problem with this. Less so, I think, on the six cylinder models.

Anyhow, good that you have 'er running :-)

Cheers
DD

 

Yes Doug, to cause this problem I think there needs to be a few cold stops in succession (not necessarily within a short time, but not driving in between) and we are talking running the car for a few seconds each time. This seems to be very much compounded by only short journey use as well. I suspect that cold weather and very low fuel do not help either!

In my experience the big problem for the V8 engine is if the car has been standing for a few days and is run for just a second or two.