Hi all:

I'd welcome your advice. I have a 99 XK8 with 88K miles and here are the symptoms.

Yesterday and today, the instrument panel displayed Reduced Performance for about 10 minutes. During both episodes, the engine seemed to run rough as if there was a misfire. Yesterday, while parked at a light, the display blinked off and the engine ran smoothly for the rest of the day. Today, after 10 minutes, I stopped, put the car in park, revved up the engine to 5000 for less than 5 seconds. While doing this the display message turned off and again the engine ran smoothly for the rest of the day.

Going in my guess was a failing ignition coil, but now I'm stumped. I scanned the engine and there weren't any DTCs recorded. The local indy tells me that when California switches fuel mixtures in the spring and fall, this type of thing can happen.

What do you think?

Thanks

Thom

 

Thom,

You probably have the AJ26 V8 which is a self learning GDI engine. Any service history on your car?

I would always try the easy fixes first. Try adding some fuel cleaner/injector cleaner additive to the petrol when you next fill up the tank.

I'm very surprised at the lack of DTC codes esp. if the car check engine light came on. Did your local indy garage scan the car. Maybe head to a professional garage to get it scanned with correct software.

 

Hi David:

I purchased the car just over year ago and I have the service history for the last 15 years.

I scanned the engine using the same scanner and software version the indie garage uses. Are you thinking that a lean mixture might show itself as a rough running engine? The car did have a bad coil about 9 months ago, but I seem to recall that it threw a code then. The indie who commented that it might be a fuel blend issue, has 30 years fixing Jaguars and the shop has a good reputation. Would the ECU need to relearn things with the new blend. If I recall correctly, CA just switched to its winter formulation. I

In any event, 'll try some cleaner and see what happens.

Thanks

Thom

 

Trying some fuel injector cleaner won't hurt any. I've heard Techron is good. I assume you always use good 93 octane gasoline. It does sound strange that you have no CEL or OBDII codes with a restricted performance message. A fuel pump problem can appear without codes, an ignition coil will throw codes. You could also try doing a hard reset which will require the car to start over and relearn things that might have changed with any changes in fuel mix.

 

Thanks Jim...best I can tell from some threads is that I turn the car off and pull fuse F32 for say 15 minutes.

 

thom4782- I have a 2000 Coupe with about 72000 miles and lately having the same problem. At times I can drive an hour or so and no problems then a twenty minute drive and “reduced performance” message comes on but will clear itself after a little while and sometimes if I shut the engine off and restart it will clear. When the reduced message light goes off the car runs fine and as you have found I have no stored fault codes.

Hope someone can chime in on a possible cause..

 

"Disconnect the negative battery terminal and touch it to the positive for 15 seconds or so." A quote from the archives Actually, it's the cable you disconnect from the terminal. This is a hard reset. It discharges all the capacitors, etc. No, I will not fry anything. The negative post of the battery is not connected to anything.

 

The lack of codes is a bit mystifying to me at this point in this thread, especially with the engine MIL from the earlier poster, but I have been having a problem somewhat like this. I eventually diagnosed it as the ECM either shutting down or shunting to the secondary, emergency CPU in the ECM. CAN bus messages from the ECM disappear for about 3 seconds. Took quite a while to capture this by logging all of the CAN traffic. I can, to some extent, predict now the circumstances under which this happens. This is generally when it's hot (> 35C (95F)) and I have run the car a while, turn the car off for 15-30 minutes, and it can happen within 10 minutes of restarting and driving. I now have a thermocouple on the ECM too.

Running in reduced performance mode for about 30 seconds, then restarting, cures this, most often until the next rare and random occurrence. I generally make my way to a convenient position on the road where I could pull over if necessary, shift to neutral while rolling, ignition off and on.

A bit different than y'all, because I inevitably and most commonly get a P1797 on the TCM (ECM/TCM CAN bus error) and a U2500 on the ABS (CAN bus error) in addition to other codes occasionally. It's possible your codes have cleared, but not that quickly...

My fix will be a change of ECM, but I haven't gotten around to this yet, it's colder here now and has not happened since the last 36C day. I have also replaced the ECM/TCM box vent fan in the engine compartment. Those are probably not the first line of defense here.

First, perhaps, what code reader are you using? Jag specific?

On reflection, I suppose it's vaguely possible that INST is just displaying a message that is not actually happening, explaining the lack of code(s). So, is the reduced performance mode actually happening?

 

Hi crbass:

Thanks for sharing. The reader I use is a Creader Professional, model CRP126. The reader allows one to download diagnostic software for various cars. I've use the diagnostics for Jaguar XK8, 1999, automatic transmission. This type is one of two readers the independent shop uses. I don't recall the other type, but the CRP126 gets the most use because it is easily portable.

As I said, there were no stored DTCs. The reader also allows one to monitor values for something like 90 subsystems. I'm still trying to figure out which, if any, of these subsystems would be worth checking.

Last piece of information that suggests the ECU needs to relearn. I vaguely recall this happening a few times several months ago. The timing could be when California changed its fuel blend to the summer mix.

Thom

 

So, your reader seems fine...

Other than running roughly, did the reduced performance restrict your throttle/gear? I assume it did, but just checking.

As you know, if it's just relearning, that could easily affect idle and other engine performance as you note and as noted above. But it's just odd that it would throw a MIL without throwing and keeping a code. OBD was invented by CARB so that they can know whether your car is potentially smoggy without having to wade through 50 manufacturer specific diagnostics. It sort of defeats the purpose if it doesn't keep the code for at least some amount of time.

As always, this code mystery may end being still a mystery if the relearn is successful. Good luck with it.

Edit: Well, in looking through the DTCs to see if there are any non ODBII codes (Jag codes) that will throw a MIL and will only show up in WDS/IDS/etc, it appears that P0300 may happen transiently without being flagged if I'm interpreting it correctly. So that may be the solution to the 'mystery' of the nonexistent code. Engine management pdf attached.

 

Well, mystery solved in part.

The Reduced Performance light came back on this morning and a check engine light popped up as well. When I read the code, it was P0308 - a cylinder 8 misfire. My guess is its the coil pack. Now if I can just find an alternative the the Jaguar dealers $150+ per coil price.

Thanks all for the help.

 

I would recommend to clear the DTC´s and interchange the coils of cylinder 1 and 8. If the failure goes to cylinder 1 buy a new coil, otherwise look for the actual cause.

We had a Cylinder 8 misfire recently, turned out it was due to a broken catalysator. The high exhaust back pressure caused the backfire DTC. Can be checked with both hands behind the exhaust tips at about 2500 rpms.

regards , Fritz

 

Cleaning my mass air flow and the air intake tube and all tps connections solved my problem on my 99 xk8.
i had reduced performance and actual limp mode come into play. The vent tube had passed a lot of oil residue into the intake stream .