I have a 2008 Malibu with about 5000 miles. In May or June I noticed a problem with engine speed control.

Consider driving along a flat highway at a constant 55-60 MPH with cruise set. The engine speed, on occasion, will jump up a few hundred RPM as seen on the Tach, stay there for a few seconds, then return tonormal. This happens with or without the AC on. Because the car is so quiet, it is hard to hear the engine speed change.

This problem actually happens with or without cruise control AND it happens at all vehicle speeds. The faster I'm driving the more the engine speed jumps.

I don't really feel the car surge or droop or anything.

 

I am having problems posting

 

unable to post entire issue in one message and it is not that long

 

They have tested the cluster and found it working correctly. They have confirmed that the ECM is sending the same data as displayed on the tach. They have replaced an ECM and a crank speed sensor.

They say they've been on the phone with GM and GM says that it is a corrupted engine speed signal. So yesterday I was suggesting how to troubleshoot that.

 

I told them I think they should use the transmission input speed sensor to confirm that the crank speed sensor signal is valid.

 

If they matchthen the problem is not a corrupted speed signal. I asked the question about the possibility of the transmission slipping. He said that it should set a code if that were the case.

 

I have experience in engine control from a previous job and have debated taking a day off work and telling them to let me use their Tech2 service tool while someone else drives.

 

Based on my previous work experience I could write up other test procedures to debug the issue, but I should not have to.

 

I would like to think that GM knows how to diagnose their products, but now I am not so sure.

 

Anyone else have any thoughts or ideas? I have 2 suspicions...