unknown problem

[teammonkeys]

I was wonder if anyone is having the same problem as me...Sometimes when i press on the gas in 4th gear, the car makes a knock noise. Its sounds like the car briefly downshifted and return.. really brief. This does not happen too often. It occurs more frequently when I drive long, going uphill, and hot weather (95 degrees +).My initial thinking is that it VVTI oil pump or VVTI mechanizism.I found that if I keep the car at constant speed, this doesn't happen. If I use cruise control, this doesn't happen.

[joatmon]

Do you see any movement in the tach that would indicate a downshift? Does it do it if the AC is off?

[Fialchar]

Check your oil, that sounds almost identical to what was happening to my Vibe before the engine blew due to the oil. It was when I would accelerate, decellerate, etc, but when I was going at a constant speed you couldn't hear it. It got worse over a few days, and ended up blowing on my way to work one night.

[Mavrik]

someone on here mentioned a concern like that where it downshifted under loads and it was the converter that had gone... there are threads in the warranty section that might help you out.

[teammonkeys]

I don't even notice the tach moving greatly... It happens either with AC on or off... For the past few days, I have been trying to duplicate the problem but I can't seem to duplicate it.

[ToolGuy]

This tool may help, getting one to use would be hard though. It is called a Vehicle Data recorder for GM and co-pilot for Chrysler. It is a tool technicians use or sometimes give customers to use and when the symption happens, the tech or customer pushes a button and it takes a snap shot of the engine functions at the time of the event. The tech or cust goes back to the dlr and analyzes it.The hard part is paying a tech to drive around with it or a dlr letting you take it home to use. Good dlrs will do it but possibly you should take the car to them and complain first. If you go away and it keeps happening, take it back yet again. After a few times of coming back unfortunately, you may get the tool to use. As all problems and dlrs, if the car is not doing it at the time a tech looks at it, it is hard to diagnose. Just of of curiosity, try a bottle of fuel system cleaner. Besides the GM cleaner, GM recommends ChevronTechron and it is what I use. Maybe it is the same stuff, cheap fix if it makes it go away.