So driving my new (to me) XK8 over the weekend, this was the first time I drove it with the top down. Trying to take advantage of some beautiful fall weather here in Cleveland before Mother Nature sends us into the depths of Winter Hell. The car idles great and accelerates normal. When I get to cruise speed (let's say 40 mph), as I try to maintain that speed, I notice that the car appears to be "searching" for the right RPM. I hear it (via way of exhaust sound) and also notice the tach bouncing ~100 RPM's up and down. It's nothing violent and frankly if it wasn't for having the top down, I probably wouldn't have even noticed (the tach moves, but barely). I hear it faintly via way of the exhaust. It's minor enough that my wife doesn't notice, but I do. In any event, wondering if anyone else has ever experienced this, and if so, what do you suspect to be the culprit? Thanks in advance for any recommendations.

--Andrew

 

Hey Andrew, what year is she?

John

 

Sorry - that would have helped, wouldn't it? She's a model year 2004 w/ a 4.2 naturally aspirated (XK8)

 

My '03 does the same thing when first started and it stops once the the temps get to normal. I always thought that it was trying to decide which gear it wanted for that speed 5th or 6th. I once tried putting the shifter lever in 5th and it worked fine.

 

It wouldn't hurt to give the MAF a good cleaning. They sell spray stuff for that very purpose at AutoZone or NAPA shops, and it's relatively cheap and easy. There is a bulb inside the MAF that detects the incoming air temperature.

 

Start with checking the tire pressure, especially on the rear wheels. Make sure it is the same pressure on both sides. The rear driver side is a major input into the transmission logic.

Barring that, put a full transmission maintenance cycle on your schedule. Phase I is to do pan and filter, fluid (Check Mercon SP), connector sleeve, bridge seal and tube seals (if you have the stomach to pull the valve body off). Be sure to do the refill the proper way (it is temperature based). Follow that with possibly a transmission control module reflash if it has not been done (get the newest code), but also a reset of the adaptations. These are 2 diagnostic port procedures with the "factory" software. A dealer might do that for a standard diagnostic fee of maybe an hour or two of labor. Phase II is to do a drain and refill of the fluid a few weeks later because the drain part is never complete (the torque converter typically stays full of old fluid). This is where the cheaper Mercon SP comes through as the ZF fluid is 3 times more expensive. Mercon SP is considered OK because the same transmission model (Ford 6R) is fitted on some american cars, and that is what they run.

You can start looking for parts, this is not terribly pricey. The transmission code is 6HP26 by manufacturer ZF. There are genuine ZF parts of course, and also other sources much cheaper.

 

+1 on rear wheel air pressure. I saw some pretty scary symptoms when driving with a bad rear wheel bearing. It was a lot worse than your description, but similar for sure.

John