I have a Xj6 x300 4.0 litre sport 1995 model. I changed the spark plugs as it was running rough when cold and hesitated when accelerating intermitently. After changing the plugs the car runs rough and the exhaust fumes smell strongly of petrol. I even put the old plugs back in the car and it was the same. The breakdown guy initially told me that two of the solonoids are not working. So he changed the front two solonoids to the back of the engine and the problem was the same. He then told me that there is a spark on all the plugs but it appears there is no petrol in cylinders 5 and 6 nearest the windscreen. He also said the fuel injectors may be blocked. I tried using injector cleaner with no luck. The car runs rough when idling and cannot be driven at the moment. I got the car taken to the garage they said they isolated it to a ecu fault. They said it was not pulsing on cylinders 5 and 6. So i located a second hand one with all the same codes except the date code was different. After fitting the new ecu the car ticks ten times when the ignition is turned on. The car starts but runs rough the same as before. The garage told me that the new ecu may have the same fault as the old one. Which I doubted very much they suggested that I send off the original ECU and get it repaired. Im not sure where to start with this problem I do not believe its a ECU fault as its very unlikely that the problem would be the same on both ecu's. I did think maybe the timing has gone out but then its not overheating. Previously the car has been run with low fuel and also the fuel filter has not be changed. This is because when I ordered the fuel filter its to big compared to the one on the car and this is the part that is recommended by searching by reg number and make/model. I have tried a few different shops also the part is always the same. Any suggestions would be gladly appreciated as I do not want to spend money on anything that is not causing and related to the problem.I have already spent £120 and got no where at all. My number is 0756-5399313 and my name is Mat.

 

Try numbering the coils and moving cylinders 1 and 2 to cylinders 5 and 6 and see if the problem follows the coils. It could be the coils or possibly a ground wiring fault to the coils.

 

Very few things happen by coincidence. From your post I think you are saying that the problem started when you changed the plugs.

My bet is that you have the coil connectors for 5 and 6 the wrong way around. It's something I've seen before as the cables for 5 are shorter than the cables for 6 so an easy mistake to make.

Coil 6 is likely to have a piece of tape attached to it to identify it, please check.

See this image for what I mean
http://www.go-lpg.co.uk/X300_Coil_6.JPG

 

Beware of the mechanics that talk about a faulty ECU. They tend to do that when they can't identify a simpler problem. Swapping ECUs usually does not solve problems. Each ECU is set up for it's specific car and usually will not work well unless reprogrammed with Jaguar Dealer equipment from that era. Don't spend any more money on your ECU until you have exhausted all ideas.

"Plus One" to Brendan (b1mcp) for the suggestion of swapped coil feeds to 5 and 6! That was my first thought. It has happened before. That would cause the symptoms you describe.

A dirty or old fuel filter most likely would not result in such poor running, although it is wise to replace it when you can.

Good luck, be careful when dealing with repair shops regarding this car. Post up an intro in the New Members section when you have time, for a proper welcome. There is plenty of advice and assistance here for you.

 

+1 on the swapped coil connectors, quite common.

Also +1 on Richard's suggestion of being wary of mechanics suggesting an ECU replacement. Generally it means they are just guessing!! which is a quick job for them, an expensive one for you and probably won't solve your problem to boot.

 

Thanks everyone especially Brendan for your help it was the leads round the wrong way on cylinders 5 and 6!. The new ecu works fine also so will keep it in the car as it is most likely better than mine. I was going to give up and scrap the car.

Many Thanks Mat