2006 Tahoe Z71, well maintained with 170k miles. Runs and drives like new, and just passed both safety and emissions inspections. Engine light has NOT been on. Today on the highway I set the cruise around 72 MPH and everything seemed normal, until I got to a moderate incline.....it downshifted one gear, then decided to downshift again and the RPM jumped up pretty high, but it didn't seem to really make any more power AND the engine light started flashing. I took it off cruise, it calmed down, and the engine light never came on again and it drove fine the rest of the way home.

I know when the engine light FLASHES it means a misfire, but what's up with only doing it at higher RPM?

This is a pretty hilly highway, but I don't think it should need to downshift more than one gear due to the decent torque it has - in fact my 91 Mustang GT never downshifts at all when on cruise on the same highway. OTOH my 98 Prizm and 03 MPV will both go nuts on cruise and do crazy high revving downshifts. They don't do it when NOT on cruise, never could figure that out.

Anyway what could I be looking at with the Tahoe??

 

My 2005 8.1 did exactly this. I replaced the spark plugs with new AC Delco platinums and it resolved the problem.

Hopefully it's that simple for you as well.

 

sometimes there has to be a high load on the engine for a misfire to show itself, start with the basics and if you have a scanner check the data to see which cylinder is acting up

 

Good thoughts, thanks. With a little luck good ole Chevy reliability will hold and it just needs plugs n wires - certainly couldn't hurt either way. I will have AutoZone check for codes first.

 

THIS!


when you isolate the misfiring cylinder; go to that cylinder and remove and inspect the spark plug. if it looks ok, swap the spark plug from a non misfiring cylinder; see if the misfire follows the plug.




if not swap the coil...
if not swap the plug wire...
if not compression test.
fuel demand is greater under load, so check your fuel trim under load or fuel pressure with a pressure gauge. it may be a lean misfire.


this way your not replacing good parts...shot gun diagnosis can be expensive if your aim is bad.