Restricted Performance-it comes and it goes and comes and goes?

This is one problem that never seems to go away. 2001 XK8 and now the check engine light is on, along with restricted performance warning. I did find the 81,000 mile air filter so clogged that you could not see light through it, and bought a new filter. No help there. I was wondering how common a problem this is, and what causes it 9 out of ten times. It is a beautiful car, but can’t be enjoyed running like this. Any and all help appreciated.

Are you getting any codes? Could be several things. I know on an old episode of Wheeler dealers it was the throttle body, but could be a few things

Thanks for the response. I will have to get the codes and go from there. I have heard of the throttle body being the culprit and don’t know if it can be cleaned or need replaciing.

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You might also check old posts from the Jaguar Forum site…

XJ XJ8 / XJR ( X308 ) - Jaguar Forums - Jaguar Enthusiasts Forum

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Sounds like the throttle body needs to be rebuilt.

I have twice has these restricted performance hit me, with no engine light on the dash. Both times, the oil level was near the low mark ( but not below). The engine uses oil pressure to activate the Variable Valve Timing, and I surmise that the low oil level causes this to not activate properly. Both times, adding less than a quart of oil brought the performance back .

Just something simple to check.

That occurred to me too, and sure enough, the level was about a quart low, and I topped it up, but no change happened, but sometimes it takes a while for the computer to ‘catch up’ with what took place. My codes showed ‘misfire’, and is likely the twenty-year old coil packs needing replacing. My last go-around involved replacing two coils, and that temporarily cured things up. Now I am back to the restricted performance woes, and I read somewhere on here that ALL coils need replacing when things start going south. I have purchased 8 new coils and will try to discover which new ones were installed in the last expensive ‘service’. Just one note on oil levels: all Jaguars leak, and some leak more than others.

If you are going to replace the coils, might as well inspect and/or replace the plugs as well. Depends on how old they are. If you had a misfire, it could be the plugs might need it too, and it’s easy at this point.

Good idea on the plugs, as they may be as old as that air filter at 81,000 miles. The top will be off, so why not go for new plugs? It is just another expense, and that is normal for all cars besides Jaguars.

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Looked up the codes, and a misfire on cylinder 5 looks like the culprit. I bought all 8 new coil packs and will first replace #5 and see what happens. A while back, I had the same issue, and they replaced 2 of the coils and that fixed it (temporarily) until I let it sit again, and the troubles returned. I was told to drive the car much more often; that they have to be driven. With more cars than sense, what is one to do?

You could always try swapping coil 5 to a different position and see if the fault follows the coil. If it doesn’t then it would be something else. My restricted performance due to misfires was actually my ecu. I sent it out, had it gone over and repaired, then no more issues. The capacitors leak and corrode the internals. I couldnt see mine, but they said it was bad and did the repair, and since then no issues

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Well, the kitty is at the vets for a checkup and a shot or two. My best mechanic friend has taken it for a general go-over. He also said he had contacts with folks that buy and sell these cars, and I told him to fix if first and let me decide. They are nice cars when they run reliably, but when you can’t trust it to get you there and back, then that is a whole different reality. I do have the advantage as being a senior, and just standing along side it on the road will almost always get someone to stop and take you wherever you were heading. If you get real lucky, someone will take it and you just file a claim with the insurance company.

I had a severe restricted performance and engine stutter if I would try to raise the RPMs over 3,000 under load on my XK8 I rescued from craigslist. Turned out nobody had ever changed the fuel filter or clean the mass airflow sensor with CRC mass air flow sensor cleaner.

The fuel filter I could barely blow air through it was so old degraded and blocked. These are the first things I tell owners to check as it seems not many ever clean or replaced the mentioned areas.

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I did change out the air filter, as it looked to be the original with the car with 81,000 miles. If you held it up to the light, you could not see through it. Amazing that people do no take care of essentials on cars, and wonder why they have problems. My mechanic is going to install the 8 coil packs and says he changed all 8 spark plugs the last time the car was serviced. I mentioned the throttle body and if he cleaned it. Also, the mass air flow sensor needs looking at too. The ABS light being on is due to bad solder joints in the module, and hopefully he will fix that too.

Well, well! I just bought it back from the mechanic, and he spent hours and hours on the car to get everything right again. It will be delivered tomorrow, and I can get it washed and take it out for a spin. He told me to drive the car often and not let it sit for too long. He told me that last time, and of course, I ignored the advice. THIS TIME, it will get ‘exercised’. As they say; ‘If you can’t afford a Jaguar, you don’t need a Jaguar.’

$600 rebuilt throttle body fixed mine…

As they say, “If you can afford a Jaguar, buy two …one for parts.”

A bit late reaction: I had restricted-performance issues on my '99 XK8 cuased by a faulty coil according to the code reader. I swapped the coil with another to find out wether the fault code would swap too. It didn’t no faults any longer, no restricted performance messages. I suspect the connectors need a bit of a good clean/wiggle. My advice: don’t buy new parts too quickly. Check the connectors first.


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Biggest problem with XK8’s is keeping the battery up to charge. These things eat batteries. I am currently looking for an in-line cutoff switch like the one in my Aston. It was mounted below the rea lower seat cushion and you were always wise to turn the battery off. It was probably intended to be a theft preventive device.

I have a solar panel connected to my car XJR ‘99 when I am not going to use it for a while, saves the hassle of resetting everything when you turn on the battery if you use a cut off switch.
When I swap the cars out to rest the X type I will transfer the panel over.

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