xjr6 problem please help
Discussion
Hi my jag recently developed a bad misfire and very slow starting (engine needs cranking a few times when cold or hot). I serviced the car and found one of the plugs completely burnt out. The misfire didnt cure after the service so i took it to a specialist where they replaced 4 coils.
The car now misfires much less than before but still a bit and the slow starting hasnt been cured at all. I am also getting rev hunting when the car is idling (in N and D), when i m stopped in Dthe revs drop to 500 and pick up to normal when in N but still hunt.
Any thoughts please?
The car now misfires much less than before but still a bit and the slow starting hasnt been cured at all. I am also getting rev hunting when the car is idling (in N and D), when i m stopped in Dthe revs drop to 500 and pick up to normal when in N but still hunt.
Any thoughts please?
Which plug had burnt out? That's usually caused by a too lean mixture and coupled with the other symptoms you describe suggest a vacuum leak somewhere.
Firstly were the other plugs OK? They should all be an even grey / light brown colour. If the other 5 were all right then I'd suspect a leaking inlet manifold gasket. If the car drives and pulls well under load but has a rough idle then that's enough confirmation you have a leak to spend some time looking for it.
Check all the air and vacuum hoses under the bonnet are all OK. Listen carefully for air leaks with the engine idling and see if you can cause or cure the problem by twisting and mauling all the pipes around. See if there's any nuts missing off the manifold. Put some gaffer tape on any suspect hoses as a tempoary fix. Clamp off the vacuum pipe from the inlet manifold to the servo and see if that improves the idle. If it does you have a leak within the servo.
Other than that unless there's anything obviously wrong with the pipework I think you'll have to get the manifold off and fit a new gasket. One trick you can try before you get the spanners out 'tho is to dribble some oil from a can along the joint between the manifold and head. The oil will make a tempoary seal so if doing so makes an improvement then you've found the leak.
Eve if you do all of this to no avail at least you've ruled out several possible causes and can then get some diagnostics done. Don't be tempted to leave it though - burnt plugs are easy to change, burnt out valves, valve seats and melted piston crowns are not.
Firstly were the other plugs OK? They should all be an even grey / light brown colour. If the other 5 were all right then I'd suspect a leaking inlet manifold gasket. If the car drives and pulls well under load but has a rough idle then that's enough confirmation you have a leak to spend some time looking for it.
Check all the air and vacuum hoses under the bonnet are all OK. Listen carefully for air leaks with the engine idling and see if you can cause or cure the problem by twisting and mauling all the pipes around. See if there's any nuts missing off the manifold. Put some gaffer tape on any suspect hoses as a tempoary fix. Clamp off the vacuum pipe from the inlet manifold to the servo and see if that improves the idle. If it does you have a leak within the servo.
Other than that unless there's anything obviously wrong with the pipework I think you'll have to get the manifold off and fit a new gasket. One trick you can try before you get the spanners out 'tho is to dribble some oil from a can along the joint between the manifold and head. The oil will make a tempoary seal so if doing so makes an improvement then you've found the leak.
Eve if you do all of this to no avail at least you've ruled out several possible causes and can then get some diagnostics done. Don't be tempted to leave it though - burnt plugs are easy to change, burnt out valves, valve seats and melted piston crowns are not.
Edited by Jaguar steve on Friday 12th November 16:06
Thanks Steve, it was the 4th plug counting from the windscreen outwards, the other plugs were as you describe but that one was quite bad. i will give your suggestions a go, and see how i get on, but i know my manifolds are also cracks which wont help whne tracing air leaks. is it hard to change the manifold gasket?
thanks
thanks
Edited by QuiteQuietCerb on Saturday 13th November 00:10
That's number 3. If all the other plugs were the correct colour then that points to a problem somewhere in just that one cylinder - as the plug had burnt out I'm guessing from your description that the electrode had burnt away - that the most likley cause is either a failing injector or an air leak causing the mixture to go too lean under light load and high vacuum conditions on just that cylinder and not the whole engine. A lean mixture burns both too quickly and way too hot, hence the shagged plug.
You can get the same effect with either the ignition timing too far advanced, which is very unlikley or a fault in the injection system or a air leak somewhere else on the induction system causing the fuel mixture to go lean but I don't think that's your problem. If it was all 6 plugs would be affected and you'd get detonation (pinking) under load and chances are the car would be dog rough under light loads.
It's a difficult one to be sure about but as the problem appears confined to one cylinder and you have symptoms of an air leak my prime suspect is the inlet manifold gasket. Oil from a can usually does a good job in diagnosing leaks - try that first. It's always best to spend more time on diagnosis to be absolutely sure you've found the problem 'tho before you tear everything to bits.
IIRC the inlet manifold gasket is not hard to change. I've had one off before but it's a long time since I worked on an AJ16 engine and I can't remember the job in detail. Just take your time and FFS don't drop anything down the intake ports.
If you are not sure about the job then a workshop manual - the genuine JDHT one is worth the extra cost over the cheaper copies you'll come across on ebay- is a good buy.
You can get the same effect with either the ignition timing too far advanced, which is very unlikley or a fault in the injection system or a air leak somewhere else on the induction system causing the fuel mixture to go lean but I don't think that's your problem. If it was all 6 plugs would be affected and you'd get detonation (pinking) under load and chances are the car would be dog rough under light loads.
It's a difficult one to be sure about but as the problem appears confined to one cylinder and you have symptoms of an air leak my prime suspect is the inlet manifold gasket. Oil from a can usually does a good job in diagnosing leaks - try that first. It's always best to spend more time on diagnosis to be absolutely sure you've found the problem 'tho before you tear everything to bits.
IIRC the inlet manifold gasket is not hard to change. I've had one off before but it's a long time since I worked on an AJ16 engine and I can't remember the job in detail. Just take your time and FFS don't drop anything down the intake ports.
If you are not sure about the job then a workshop manual - the genuine JDHT one is worth the extra cost over the cheaper copies you'll come across on ebay- is a good buy.
Steve thanks very much for your advice, looked at the old plugs again today, and the difference is not actually that bad even though the 3rd plug is a bit worse, if i do have an air leak somewhere, will this also cause the slow starting of the engine? to start the car i need to crank it for around 5-10 seconds
Ah... A bit worse is not quite the same as compltely burnt out, which was what I based my posts suggesting you may have a inlet manifold leak on.
What do you actually mean by worse? - That's not me being a pedant BTW (for a change) but checking plug colour and condition is a useful tool that often helps with diagnosing problems.
Is the plug concerned more or less worn or a different colour to the rest? If so is it lighter or darker? Is there a build up of soot or gum on that plug and not the rest? Are they all the same number and thererfore heat range? Are you sure they are still in the same order you took them out? What colour are the plugs generally?
It's still a possibility you may have an vacuum leak somewhere so I'd have a good look round the engine compartment trying all the things I suggested to see if you can find any leaks. It's unlikley to be an electrical contact problem but may still be worth unplugging and re connecting all the commectors (to clean the contacts) on the injection system.
Clamping off the servo vacuum pipe is worth a try too - that's often overlooked as a source of vacuum leaks.
Even if you don't have a leak, you're not at the stage yet where you know that as a fact. Once you have proved for sure there's either no leak or found one and fixed it then you can move on to running the car to see if the problem is fixed or getting some diagnoistics done. You may have a component such as the air flow meter or throttle position sensor failing or out of spec - which could be the source of the problem.
What do you actually mean by worse? - That's not me being a pedant BTW (for a change) but checking plug colour and condition is a useful tool that often helps with diagnosing problems.
Is the plug concerned more or less worn or a different colour to the rest? If so is it lighter or darker? Is there a build up of soot or gum on that plug and not the rest? Are they all the same number and thererfore heat range? Are you sure they are still in the same order you took them out? What colour are the plugs generally?
It's still a possibility you may have an vacuum leak somewhere so I'd have a good look round the engine compartment trying all the things I suggested to see if you can find any leaks. It's unlikley to be an electrical contact problem but may still be worth unplugging and re connecting all the commectors (to clean the contacts) on the injection system.
Clamping off the servo vacuum pipe is worth a try too - that's often overlooked as a source of vacuum leaks.
Even if you don't have a leak, you're not at the stage yet where you know that as a fact. Once you have proved for sure there's either no leak or found one and fixed it then you can move on to running the car to see if the problem is fixed or getting some diagnoistics done. You may have a component such as the air flow meter or throttle position sensor failing or out of spec - which could be the source of the problem.
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