hey fellas,
I am having a missfire issue on my 98 xjr supercharged. I have a thread going in that forum but i got a suggestion from Graham to check with you guys over here to see if you may be able to shed some light on my problem.. ok so here goes,,,, under a moderate to heavy load i have a missfire on at least 2 maybe even 3 cylinders..im judging that by the power loss i am getting. under normal to light load the car is spot on, no missfire at crusing speeds either. it seems to be worse the longer i drive it. when its cold its def better. when its cold the missfire is much more intermittent...it will miss then a microburst of good power for a split second then miss then good power over and over up throught the rpm range as it gets warmer the missfire is more constant with no bursts of good power, mind you, this is under a moderate load up to wot only,,runs great at normal loads..I would say that at 30% throttle and under its perfect and from 30% and up is when i have my issue...I have taken all the plugs out and cleaned up the oil in the plug wells..only a few had a small amount of oil in them. I changed the fuel filter..also. there are no dtc codes present!! i did a compression test and the lowest cylinder was at 130 and the highest was at 145 not horrible thats for sure..so i guess what im thinking now is the coil packs..i have read that you can get a missfire from a not so good coil pack and it wont throw a code..is that true...is there anyway of testing a coil pack all the plugs look great..a nice chocolate brown color and dry...
If you have any suggestions as to what to try next i am sooooo open to advice

thanks in advance!!!

 

One other possibility may be the fuel pumps. They have been known to cause a few problems with the XK8/R

I believe (on the XKR) one is used for initial fuel pressure prime and boost under acceleration, and the other is used for normal running. So it may be the boost/start one that is playing up.

Do you have to crank more than you used to when starting from cold?

 

do you know what the preasure should be at the fuel rail? i don't have a fuel preasure tester but ill get one

 

From JTIS:


Disconnect and plug the vacuum hose from the pressure regulator.
Start the engine and record the fuel pressure reading.
As a guide, the fuel pressure should be approximately 3.0 bar (44.1 psi) with the regulator disconnected.
With the engine still running, reconnect the vacuum hose to the pressure regulator and record the reading.
The pressure should drop to approximately 2.6 bar (38 psi) when the vacuum hose is reconnected,

Hope that helps.

Though if it is a fuel problem under load/acceleration it will likely read OK at idle.

 

thanks stumpy, not sure if there is 2 on the xjr ill try to find out.. honestly i cant even find the fuel preasure reg on this thing...cant really even see the fuel rail as its under the the intake tubes.....im starting to put some things together in my head right now and im starting to think its not a fuel issue..i changed the fuel filter today. when i got the old one off i cleanded it up a bit and tried to blow throught it...it was very difficult to get air to go through it, i had to blow extreamly hard... i could blow through the new one with alomost no resistance at all....so i was thinking, yes this must be my problem...turns out it didn't change a thing. so if i had weak fuel preasure you would think that i would at least be a little better with the new filter because of the less resistance now right?? so that leads me to believe that its not a fuel preasure issue..

 

When blowing into a dry filter you will meet with little or no resistance. As for the car stumbling and no codes I would be looking at the possibility of your plugs breaking down or the wrong plugs I would also be looking at poor fuel presentation to the cylinder (your fuel injectors). It is possible that one of your fuel pumps might have failed but I would think the symptoms would be more serious. Had you changed your plugs recently? I would add a can of BG44K to the tank and see what takes place.

 

Rocket,

These guys here are great, and I'm sure that they will get you what you need.

With that said, I'm not an expert on my Jag (XKR) as of yet, but I had a similar issue with my Grand National many years ago. After a bunch of troubleshooting (and parts) I found out that a coil pack was bad. in fact, had to replace them 3 times over the 11 years I had her.

just food for thought.

Best wishes,

Ron

 

Gus,

I have not changed the plugs yet. im gonna do that next along with a fuel preasure test. the car ran fine last friday when i test drove it. it seems to be getting worse as time goes on. just went out for a ride and the missfire is seemingly getting more severe. it is becoming more constant with less surges of good power durring the missfire situation. im starting to think this car is going to be more trouble than i want to deal with...... i think the blower is on its way out as well. its very loud with a rattling sound at idle, it sounds fine just off idle but at idle it sounds pretty bad. the blower is only a yr old with around 1000 miles on it. it was a reman blower that was put on and the guy said it sounded like that when it was put on. i think its still under warranty though

Ron.
yep it could def be a coil pack, im going to eliminate a couple other things first then ill prob start replacing the coils



thanks for your words guys

 

Hey Gus,
YAAAAAAY!!!!!!

I put a new set of bosch platinums in it yesterday and cleand out the plug wells... runs so good now!!!!

I just gotte a small yet somewhat annoying exhaust leak coming from the drivers side up by the manifold/cat area..its not leaking between the manifold and head and not after the cat...sounds like its somewhere under the shields...cant really see in there very well. is there a tube that comes off the manifold on the drivers side...the passenger side has one that goes to the egr valve but i cant see in there on the drivers side,..

 

Kudos to you!