I have a 2008 Equinox LT FWD with the 3400 motor in it with 89,000 miles. A couple of weeks ago I noticed it would miss while at highways speeds(50-60mpg) under load(going up slight hills) and it would do this while the vehicle was in 5th gear at around 1500-1800 rpm. If the vehicle downshifted to 4th it would stop missing and run normally. Then today it started to miss almost constantly. The funny thing is that the check engine light does not come at all. Which I think it is wierd. I had a belt replaced about 1 month ago just for preventative maintenance reasons and asked them to go ahead and replace the spark plugs while I was there. They said the vehicle had 88,000 miles on it and the spark plugs are good till 100,000 miles and there really was no need to replace them. I was pissed off because after asking them to replace them and now possibly have problems with a bad spark plug, I called the dealer today and asked them if they think it is a plug or plug wires and they asked if the check engine light was on and I said no. They said they would have to run a diagnostics check on it because they said it may not be a misfire it may be the transmission lunging. They said when they hook it up to the machine it will be able to tell if it's misfired and how many times and what cylinder. But my thing is that if it is saving these misfirings in the computer then that means the computer does detect a misfire and that means there should be a check engine light on giving a engine misfire code. Does anyone know if it will save a misfire in in the computer but not trigger the check engine light? I just wanted to know if anyone else has had this problem and what it might be? It does seem to point to the transmission. Only does it in 1 gear(5th gear) at low rpm, it doens't do it at all driving in town(up hills or anything)and ofcourse driving in town you are only using lower gears. And ofcourse that would explain why there isn't a misfire check engine light either. But then again 9 times out of 10 if you got a bad wire or plug it will only misfire under load and low rpm. Maybe while driving in the city i never really am under enough load at lower gears to put a big enough strain to cause it to misfire. Any help would be nice. I do have a powertrain warranty on it so luckily if it is the transmission I am covered. But i'd rather not have the headache either