Hi all,
My 2007 honda fit (manual trans) with 121,000 miles shutters a little when the heat or a/c turns on. It's like everything gets bogged down because the rpms will briefly drop and you can visibly see the engine shake for a second or two before things even out. Vibration is felt inside the car. There is no CEL and no other issues with the vehicle. It idles smooth in gear and neutral except for when the heat or a/c kicks in. Note: when the radiator fan turns on with the heat off and a/c off there is no shaking. I recently replaced all 4 NGK spark plugs and coils with OEM parts. When the alternator died I replaced it with a remanufactured unit and also replaced the serpentine belt and battery. Thank you for helping me diagnose this shake.
How old age but do check the alternator and battery have a low test if they're weak they can do that if not make sure the valves are adjusted Hondas need manual valve adjusting unlike most other cars. If not that I'd say have the map sensor checked perhaps cleaned and look for vacuum leaks
