Strange Motor or Transmission Reving on Highway

[flattire]

I have a SL with CVT. I drive to work and back about 40 miles every day, mostly on the highway. The problem is that after a cold start I drive about 5 miles to the highway and then after I drive about 15 minutes on the highway at about 65 miles per hour (120 km) the motor starts to strangely rev up and down, up and down, up and down – 6 or 7 times. It feels like the transmission is hunting for a gear. At this time I loose some of the power and the car starts to slow down. After about a minute of this, everything goes back to normal. It happens about 2 or 3 times per trip and it is getting worst. It is unnerving because it is just asking for an accident, especially because I have a hard time getting out of the way of other cars. Has anybody else had this problem – or anything similar??

Flattire

[BenDupre]

I've noticed the car revs a little higher when it's cold. That is it rev's about 400 RPM higher than it normally does. After it warms up, it goes back to normal. During that time, it also seems a little more sensitive to changes in terrain (grade).

If your transmission is hunting on flat ground or oscillating at all, that's not normal. It should downratio when climbing a hill, and then upratio when you crest the hill. It shouldn't ever go up and down and up and down unless the grade is changing that rapidly.

Ben

[flattire]

Ben,

Higher revving when cold or going up a grade is understandable. I’m talking about flat ground and a noticeable jump in revs from say 3000 to 3500 rpm. So much so that you really feel it. Now, of course, this is an automatic with a CVT transmission, but the problem feels as if you were cruising on the highway and had a standard transmission and you quickly pressed the clutch in and then let go again. If you did that, the revs would jump and you would loose power momentarily until the clutch engaged again. That is what if feels like.

I am mystified. I have no idea if it is the transmission, injection, fuel pump, the high tech electronics or something else. Personally, I suspect the transmission as per the above analogy of a standard transmission – but I am no mechanic. Strange, but around town it’s OK. The problem seems to be only on the highway after about 10-15 minutes.

I took it to the dealer and they have no idea. They said if they cannot reproduce the problem, then it is next to impossible to diagnose. Great.

Flattire
Modified by flattire at 8:36 PM 12/18/2006

[Jrscherer]

You will learn very quickly, as did I, that most mechanics are not used to this car. Took them 2 hours to figure out how to get at the fuel ratio sensor when I needed it replaced. The books told them about 30 minutes. Lets you know it's a brand new engine that most haven't dealt with.

[flattire]

Jrscherer,

Why did you need to replace the fuel ratio sensor? How was it behaving or did it just die?

Flattire