Chevrolet Repair/Pontiac montana misfire
Just bought a 2006 pontiac montana sv6 90k lmiles. It has a "misfire " while at steady speeds, doesn't matter if cruise control is on or not. Changed plugs and wires and used fuel,treatment - didn't help. I thought it was tranny , bit when the tranny shop scanned it all that came up was engine misfire, but when I took to a shop they couldn't get a code at all. This "misfire" only causes rpms to fluctuate about 200-300. Thank you
Crank engine over with fuel pump fuse out. indicated on cover of fuses.While cranking, do you hear a steady sound pattern of the starters work turning the engine over [a continuous pulse/beat ie.[..........]with no gaps,or does it have a missing pulse/beat ie.[... ..... ..] or similar.[A quickie, ballpark,compression test.] If you determine the latter to be evident, your problem is a dead cylinder[low compression]. Most likely then if this is the case, the offending part would be a burnt exhaust valve. It could be something else, odds are it isn't,that is,if indeed the compression is low. I am assuming it has this miss fire across the spectrum, idle too. If comp. is good, I would take a look at the fuel system, the injectors in particular. Run a good cleaner through system. SeaFoam is a good one.If you get no improvement, give it time,patience matters here . I once treated a Grand Am to the SeaFoam diet, and 22 days later, after continuous treatment, when I was at the cusp of other options, as to cause, B A M, car cured. So let her "soak". Best of luck!