I have an XK8 Coupe, 4.2 litre, MY 2003, Engine No. 0207021307, 74000 km.

During the warm up from a cold start, and particularly in the last few minutes of the warm up, the engine RPM oscillates, say about 50 to 100 rpm at light throttle and as much 100 to 200 rpm at heavier throttle settings. As soon as the engine has reached the normal run temperature the problem disappears and does not return during that day even if the car stands idle for a few hours.
The problem will be there again with a cold start the next day. Ambient temperature and humidity have no influence. No fault codes come up.

Has anyone experienced this problem? Can you suggest a cause and cure?

I would appreciate any advice.

 

First thought is cleaning the throttle body valve. Check the part load breather tube orifice.

 

Same thing happened on mine, various vacuum leaks present...passenger side, between PCV to the throttle body lower elbow was the worst, driver side TB to breather line also slight leak. I removed each line, carefully cut off the norma connectors, replaced the rigid plastic lines with rubber fuel lines (5/8" passenger, 3/8" driver) and worm-clamped em' back onto the norma connectors...replaced the dry brittle o-rings on the normas and re-attached. Huge improvement.

Clean your MAFS while your at it so readings are true to the ECU.

 

Thanks for your advice. I cleaned the MAFS and the throttle body during an annual service last month. I will tackle the part load breather and the vacuum leaks and let you know how things work out. May take a few days.

Thanks again!

 

Have checked the vacuum lines and part load breather as you suggested and cleaned the MAFS again for good measure. Problem remains. Tried another cold start today with the transmission in "Sport" mode and if anything that exacerbated the problem.

This prompted me to look up the "transmission surge" threads on the forum and found 3 Jaguar Technical Bulletins covering surge during warm up, harsh transmission shifts and transmission noise during gear shifts. All suggest software updates should fix these problems. I will get the car to a ZF transmission specialist (which may mean a trip to Sydney 220 km away) and see whether that leads to a solution. I wll keep you posted.

Thanks again for your help to date. Regards. Colin T. Bathurst NSW Australia

 

The reprogramming etc. that you found in those bulletins are always the first step. If the surge/hunt condition is still present after that, a failed torque converter is probably the culprit. I've attached a diagnostic screen that illustrates the issue. I have a setup in a scantool I use to verify what's going on. In the lower graph you can see the temp has been rising through warmup, and in the upper graph is the road speed and engine speed. In this case, all the reprogramming and adaption resetting has been done first, and the surge still remained. You can see that at level road speed and constant throttle position, the RPM is jumping up and down about 200 revs. This is happening when the engine and the trans are supposed to be locked together by the torque converter clutch. It's obviously slipping. Took a torque converter to fix the surge in this one.

It's much easier to do this with a graphing scantool of course, but you can prove it with live data, too.

Good luck! Hopefully the repair per the bulletin with do it for you.

Cheers,

 

I've replaced a ton of torque converters for this very issue. I have not been very successful with any SW updates.