I have a strange engine issue with my XJR, and would welcome any advice. Whenever I run the car – and only from being totally cold (overnight or longer) – about 3 or 4 miles into any journey, the revs constantly fluctuate up and down by about 50-100rpm, searching… It only happens at low engine stress when ‘coasting’ – around 1,500rpm give or take @ whatever speed (i.e. in different gears, whether doing around 40-50mph, or 60-70mph). If you gently put your foot down it sometimes feels like it can’t decide what gear to be in. If you really floor the accelerator, it pulls through it, but comes back after you ease off.

After about 3 or 4 miles of this when the engine fully warms up, it just stops doing it, and doesn’t return unless/until the car is left to fully cool down again. I leave it for up to 6 hours when at the office, but it doesn’t happen when I drive home again. I recently had the gearbox serviced in the hope it would cure it, but it didn’t. Perplexed

 

I had something similar, and it was solved with a transmission oil change.

Automatic transmissions are very sensitive regarding the oil level and age of the oil, so a transmission oil change (including the oil pan, as it has an integrateg oil filter) is advisable as a first step.

Check whther the harness connector is leaking, it can only be replaced after draining the transmission, so you could do the job together with the oil change.

In your case, it might be that the transmission oil level is too low. When warming up, the transmission oil expands, and that's why it might be that you only see the problem when starting after an overnight stop.

Best regards,

Thomas

 

Thanks again Thomas. I had the gearbox serviced last week and the oil changed (he did say the level was low and poor quality oil). I'm unsure about the pan/filter though, but will ask Chris who did it. Is it possible that the problem could take time to disappear, or should it have rectified itself immediately after the gearbox service if low oil was the issue? If it isn't going to be cured by this, where do I go next? Thanks.

 

If it was only a level problem, the issue should have disappeared immediately.

As your mechanic said that the oil was of poor quality, it might be that the transmission needs either a flush, or a few more oil changes to get rif of the old oil. The problem is that with an oil change you cannot remove all of the old oil, a certain quantity remains in the torque converter and othe innards of the transmission.

With a correct fluid level after the service, I would have said that you should have noticed an improvement, even if only a slight one. Did you notice any improvement?

And: did your mechanic try to see whether there are any fault codes?

Best regards,

Thomas

 

As soon as I saw the year of your car, I though, "yep, that's the same as I had on my 2003". This 'hunting' of the transmission was a hot topic on the Jaguar forums when I bought my XJ. I was told by one of the partners at a local Jaguar specialist who maintained my car, that it was a Jaguar attempt to get the transmission fluid up-to-temperature, by cycling the torque converter lock-up clutch on and off whilst the engine was warming up. This was done to save about "10 nanogrammes" of petrol and thus reduce fuel consumption. Sounds crazy, doesn't it, and yes, it was crazy. Apparently if one changes the torque converter to a later type, the 'fault' disappears. I had this 'hunting' syndrome the whole time I had the car. (2010-2016). The second XJ6 I had, a 2007, owned 2016-2019, never had this 'hunting' as it had the later torque converter, and, I think, different software in the TCM. Of course if the car is used for lots of short journeys, it will be noticed more than use on long trips where one is on higher speed main roads, (or "highways" as you say in the US)

 

True, there was this problem, but, as far as I know, it was only related to very early 2003 XJs with VINs up to somewehre in the G1xxxx range.

My SV8, VIN G20252, built in September 2003, already does not have that basic problem anymore. As I stated, the problem was cured with a transmission oil change.

I seem to remember that also a TCM re-flash could help?

Best regards,

Thomas