Hi. I have a '98 xjr and it initially felt fairly quick. It already had a larger bore cat back stainless system and sports silencers. Since then I had it decatted, removed the middle resonator and effectively have 2 completely separate banks. it sounded great and was definitely a bit sharper. After a while though it didnt feel as quick especially below 3000rpm but I assumed it was me getting used to it. It did not sound as good any more mostly under load and when accelerating its less v8 rumble and more harley davidson. I had bottom pulley changed for 16% one and had it dynoed. It is significantly down on power and one of the coils was showing a different ohms reading. This has now been changed, it sounded better but the power is still not right yet. The dyno operator said the fuelling is inconsistent for each bank and this could be various things including coils. Here are the printouts. Any thoughts and help would be greatly appreciated as I dont want to replace loads of good parts. Thanks

 

Welcome to the forum Paul,

I've moved your question from General Tech Help to X308 forum. Members here with the same model will be able to help.

Please follow this link New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum to the New Member Area - Intro a MUST forum and post some info about yourself and your vehicle for all members to see. In return you'll get a proper welcome and some useful advice about posting to the forum.

Graham

 

You stated you had the pulley changed to provide more power. Was the ecm calibrated for this condition?

 

The charts you really need are the Air Gas ratio under load. You might also check the slack in your throttle cable. At released pedal, it should be just barely slack.

 

I didn't have it recalibrated as I have been told repeatedly that the denso ECU fitted to the 98 xjr is murder to remap and various experts have said that to a fair degree it is self regulating so when the air increases, in this case because of the pulley then the cars fuelling will automatically adjust. The car wasn't right even before the pulley I reckon. The main problem noted was that the fuelling seemed to be markedly different from left bank to right banks and at various points in the rev range it would be under fuelling then over then under.

 

I will contact the dyno guy as he did have all that info and try get it and as I recall the jag guy I used made some adjustment to the pedal I believe for similar reason as you have said.

 

Here is the fuel air data. the two top lines are the left bank which had the odgy coil pack, but I was unable to do another run with another coil fitted. Before the coil was replaced you could clearly hear the left pipe different from the right. The best way to describe it was you could hear every individual fire, while the right pipe was more of a deep rumble. This sound improved as soon as coil was changed but although better, still wasnt right. the sound from both sides still isnt as it used to be.

 

Maybe the rest of your coils are failing ? Or just getting weak.
Generally at the dealers we would recommend changing all of them, but that was outside of warranty. Funny though that jaguar would only pay for the ones that had failed.

 

My initial thoughts were coils, given that they would cause unburnt fuel which would throw the levels out and cause the car to underfuel and so on. Its also my understanding that they can often be failing or weak and show no obvious signs and are hard to detect other than changing them. I just wanted to rule other stuff out before I bought a set as they aint cheap. Have you ever seen any similar sympotoms with coils being the culprits then?

 

Coils could be an issue, so for top performance better make sure they are ok.

Here is a tool I use:
https://www.jaguarforums.com/forum/j...tester-143357/

That the only way I know to detect coils that are slowly on their way out.

 

Thanks, I'm probably horibly misinformed but I thought that coils that were not completely goosed were extremely difficult to detect and apparently main dealers and even manufacturing plants threw away tons of good coils each year and machines to detect them are only just coming out but I will have a look at this. I have noticed that the sound of the car, not immediately after the exhaust mods but gradually got to a point where at idle it sounds good and stationary revving and gentle acceleration its all good but under harder acceleration its horrible but again this problem was worse until one coil was changed so I will definitely start here. If I wasnt from scotland and "careful" with money I would have a twin screw kit like yours :-)

 

Have you plugged into a reader and checked the fuel trims, could be a O2 sensor on it's way out.

 

Yeh the dyno guy put it on one as I recall and nothing showed up. I have no idea about fuelling so have no idea if the right bank graph is normal and ok but the left one looks all over the place which again would point to an issue mostly on the left bank so any non shared component potentially could be dodgy.

 

We used to test them by pulling them slightly away from the plug with engine running. The good ones would arc and you could hearv them jumping the gap. The weak ones wouldn't fire.

Bit hit and miss but it's a start.

Considering you're from Scotland ��������

 

hear them as in an electric "cracking" like pulling a plug out and eathing it on the block? iv never worked on anything modern so forgive my lack of knowledge but I am guessing you would do this with plastic pliers or something or would you not get a wallop?

 

Yes like that. No pliers needed. I've never had a shock from one. It shorts in the plug hole or not at all. Make sure you have ur ears clean to listen. See how far you can pull each one until it stops firing. Compare each cylinder.

You could do a compression check while you are there.

 

Like avos, the only way I know to test (COP) coils is a COP tester. Well, apart from the car which is quite good but not as good. The old ways will pick up a bad coil after the car does.

 

So, a known bad coil was replaced and it sounds better but yet wasn't running as good as it was prior to the previous coil failed.. Is this statement correct?

Did you gut the cats before or after the failed coil and because you thought you might have a clogged cat(s) and being down on power?

How many miles are on the car? Besides the performance exhaust and overdrive crank pulley, can you list your mods?

Sorry for all the questions, but I need all the information I can get in order to help you out. There's nothing worse then getting a customer that comes in and they tossed parts at it (usually inferior parts at that) and nothing made a difference, their wallet is almost empty and just about fed up despite their love affair with the car. LOL

A few thoughts about what I've 'red' so far... I would do as suggested from another member and perform a compression test. You have to be able to dismiss the possibility of a blown head gasket. Sometimes you have to take a step back and ask yourself.. "Is the engine mechanically healthy enough prior to accepting any modifications and asking it to perform more than what it was designed for?"

I believe you need to have an equalizer tube somewhere in your exhaust system. The original exhaust did. I've seen people isolate their exhaust system such as you have done and the car ran worse. I also think you need to have functioning cats back on the car. Unless your putting out serious horsepower, you need to have some form of back pressure.

 

No, the very first thing I done was hollow out the cats and isolate both banks by making it straight through with the middle resonator removed. sounded great and as far as I could tell was a little faster. It's hard to tell how long after this before I became disappointed but it was gradual and at the point I was booked in for the bottom pulley it became apparent it sounded bad under load, and the guy who fitted the pulley felt it should have been faster and he's a jag specialist so he fiddled with the pedal as I recall and the throttle body set up. It was given a full service minus plugs at 94k and has FJSH. It was after all of this it became more obvious that one exhaust sounded significantly different from the other and this is when I out it on dyno, and subsequently changed the bad coil. After this it did sound better and I would say slightly sharper but still not right. Funnily enough the jag guy felt it was the lack of equaliser in exhaust but as I say, immediately after this mod it was all good, at least as far as I could tell. Oh and the system is 2 1/4 inch pretty much straight to twin sports silencers. thanks

 

Obviously since I'm not there to listen to the car, something in the back of my mind is sending red flags about the individualized exhaust and burned exhaust valve(s). I hope I'm wrong but, many many years ago I seem to remember a friend or coworker of mine having very similar issues that you're having. Went to individualized exhaust and found out that it had burned a valve or two as a result of. I also can't explain why that would happen. However, the compression test and cylinder leak down test would rule that out.