engine misfire / hesitation.

[whippetpaul]

hello, I am having a problem with my 1990 3.6. The engine began to misfire / judder ? Inevitably it would not do it at the garage until now.
They have replaced the crank position sensor, plug leads and plugs, fuel filter . and checked pressure at fuel delivery . Also compression test. all ok.
I have just collected the car , but it is behaving just the same.
The engine appears to falter more on any gradient? and almost stalls at stops / junctions. But does not quite .
The engine starts easily and idles fine, will show no symptoms .
Does anyone have a suggestion , getting a bit worn down.

Many thanks Paul.

[poprivet]

Hi Paul and welcome to the forum.

It's always a bit difficult to say exactly where a problem like this lies without knowing history and seeing it in detail, but here's a few things to check.

Undo and clean all electrical connections and reassemble with contact spray, the temp sensor on the thermostat housing, MAF sensor behind the air filter, ignition amplifier on the front R/H wing, CPS behind main fan and coil and ECU at end of the passenger footwell which can suffer from water ingress.

Same for earth connections, especially the one on the MAF, the two main posts on the radiator cross member at both ends and the main earth strap to and from the front subframe. Don't just assume they are ok, take them apart and clean them well.

Although the main fuel filter has been changed its vital to clean/replace the one in the fuel pick up in the swirl pot under the battery. The pick up was modified to stop it being sucked up and blocking the pipe, you may still have the original. In fact I would almost bet that this filter is your problem.

These later straight sixes are pretty reliable so I doubt it's anything serious, not sure where you are located but there are helpfull owners here that may be able to lend a hand.

Keep us informed how you get on.

[whippetpaul]

Hello Pop Rivet, thank you for your message.... that's a lot of possible faults. So the problem could still be either electrical or fuel?

I only bought the car in February, but has a service book stamped until 2 yrs ago , and many bills. appears to have been maintained well. I had it serviced and MOT recently. it passed with just a broken handbrake pad.

I have moved to North Wales about a year ago from East Sussex. So have no helpful friends / garages nearby anymore.

My knowledge is a bit limited, to cars with no electronic systems , 60s models or brand new that one cannot do anything at all with. This era of car is a bit difficult, early electronics with no diagnostics.......

Many thanks again for your suggestions.....Paul.

[whippetpaul]

Hello, I have checked what electrical connections I can locate. and have investigated the swirl pot filter . This was perfectly clean , but how can I tell if this is the modified filter ? There is no reference to this item in my workshop manual at all .

Many thanks Paul.

[poprivet]

The filter is a push fit on the pipe, there should be lugs to prevent it going too far else the end of the pipe can be obstructed. Give it a shove, if the pipe end stops well short of the end of the filter it will be ok. CBC5649 part number 18 below.



It is covered in Kirby Palm's book which is worth downloading (free)

http://www.jag-lovers.org/xj-s/book/Jaguar.html

[whippetpaul]

Hello Pop Rivet, thank you again for your reply. And the link to the Kirby book. An incredible document..... an awful lot beyond my technical understanding, but very amusingly written.

Paul.

[whippetpaul]

Hello Pop,
I have been using the car a little and don't seem to be getting as much trouble from the hesitation. But I am not yet convinced. I have ordered a new coil as this has been suggested as a possible cause. Although I would have thought the garage would have thought of this already?

One strange thing that has happened since checking the electrical connections is the temperature gauge now registers mid way up its scale. Where since I acquired the car it always showed quite low , something I thought odd but did not consider it a serious fault.

Could it be some electronic sensor has been reading the engine temperature incorrectly ?

Kind regards Paul.

[davidr]

Hi Paul - coolant temperature is one of the major inputs to the ecu controlling the fuel injection system. If it thinks the engine is cold it will increase fuel flow which may account for the car starting fine from cold but then not going well warm. However several things make me wonder whether that is actually your problem. First I think the sender for the instrument pack is a separate one to that for the fuel injection? Secondly I'm not sure that it would give you the symptoms you have and thirdly if the temperature sensor was seriously under reading it would make the car very hard to start when hot.
My best still guess is still either crud in the swirl pot being picked up whilst running and clogging the filter or the coil breaking down under heat.
The crud clogging the fuel filter in the swirl pot builds up as you run the car and gradually starves the fuel supply but will fall off again once you stop, turn off the fuel pump and remove the suction holding it to the filter. You may well take this filter out and think it doesn't look too bad but unless you've taken the swirl pot out and given it a good clean you won't have dealt with the issue.
Here's a picture of the crap that came out of my V12.

And the filter

Coils are also known to break down under heat so it will be interesting to see if that solves the problem.
Regards
David

[whippetpaul]

Hello, there has been another woeful incident I am afraid. The car broke down completely a few weeks ago . I noticed the oil pressure drop and pulled over to find clouds of vapour / smoke coming from around the bonnet . The car would not restart. It was taken away to a local garage , who diagnosed the cylinder head gasket had failed.

Despite having previously done a compression test. I have just collected the car and it drove home without any misfire. Could this have been the fault all along ?

Paul.

[peter5884]

Hello everyone, thought I would add something to this misfire / hesitation issue.

My car is an Xjs 1991 3.6 auto and is well maintained, however it's old and needs TLC to keep it purring and you have to expect the unexpected on occasion. But never give up on your love and joy.

Less than 3 years ago my car would just about start and was running very badly, tried all the usual things and made no difference, also I checked the 'Swirl' tank and replaced the fine mesh filter within, however it did not require it and engine still ran very badly.

My friend and a wealth of knowledge on most things Jaguar suggested the crankshaft position sensor be replaced, did that and wow it fixed the poor start and misfire issue which made me very happy.

Move forward to now with only 4,500 miles and less than 3 years since my last misfire problem and the gremlins have returned but slightly different this time round.

Checked all the usual things again and you guessed, the 'Swirl' tank filter was found to be spotless. This time round the engine started well, idled ok and ran smoothly unlike before with the duff crankshaft position sensor. Things were ok till you reached 3000 RPM then bang pop splutter and cut out till you backed off the throttle. Phoned my friend and he said replace the crankshaft position sensor as it's gone again, was a bit doubtful as the replacement previously fitted was a genuine lucas part but I did as he suggested. Am so happy now as my car is now running perfectly and have now got an improved idle thrown in.

Moral of this story is the crankshaft position sensor, when faulty, manifests itself in different but always nasy ways to keep you entertained.

Cheers from Peter