I’ve seem to have lost some power. I recently had an oil change and tire rotation done in town and on the way home it started taking more and more pedal to maintain speed, then it would downshift till I reach highway speed and the cycle would start all over again. It dropped to 2nd on an overpass. I haven’t gotten any OBD codes. Engine runs smooth. I changed the spark plugs (iridium) about 10K ago along with the timing belt. The vehicle has 116K miles on it.
Since this started I’ve cleaned the K&N air filter, removed the secondary air filter, cleaned the entire cold air intake, cleaned the throttle body and MAF sensor, checked all vacuum hoses and disconnected the battery to reset the computer. The engine runs better with less down shifting but it still down shifts for overpasses which it never did before.
My next plan of attack would be the fuel system. There is no fuel filter on the 4th gen V8 limited and I really don’t want to drop the fuel tank if I don’t have to… I have run a can of injector cleaner and used mid-grade gasoline for a tank full.
Is there something in the fuel injection system that might cause this problem? Or could it be something I’m overlooking or ignorant of? I did switch to a semi-synthetic 5-30w oil when it was changed and saw the guy in the pit check the diffs and assumed he checked the auto trans also (sealed unit?). I also checked torque on all the tire lugs to make sure I didn’t have a wheel dragging, and I don’t think I have a caliper locked up since it stops straight and even. I’m not losing any anti-freeze.
Suggestions???

Make sure they did not drain the transmission fluid instead of the engine oil. Check your oil level. This has been done several times before on this forum.

1. Check for dragging brake or brakes
2. Have a compression test run
3. Check diffs/driveshaft

You are struggling somewhere and before you pop an engine you should figure out why.