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Idle sputter gets worse with A/C on
I have a headache this big (imagine a M6 sedan here) and it has a 5 pound sledgehammer written all over it!
2004 M6 V6 manual trans fusion swapped:
Has a slight but noticeable roughness in idle from cold startup. It sounds more like a "sputter" than a miss, although an occasional miss can be heard.
Turn on the air conditioner and the sputter gets noticeably worse.
The engine continues to run and does not die.
Increasing the throttle at idle smooths out the sputter
Further depressing the accelerator and the engine spools but a miss is heard.
When driven poor acceleration, "instant" and momentary surging and bucking present after about 25% throttle
WHAT HAS BEEN DONE:
New spark plugs installed
Ignition boots inspected
Before initial startup I removed and cleaned the throttle body, re-installed with new gasket
Air intake boot is OK
No vacuum leaks found
Exhaust is not restricted (thanks to 2 new cats)
Unfastened and lifted the top of the air cleaner box and manually restricted the air coming in to the air intake tube and the idle smooths out. If the air is restricted too much there is a "snap" noise at the throttle body and the idle picks up....appears to be the IAC functioning.
After startup the air mass sensor was cleaned, no help and exchanged with a known good air mass sensor, no help and runs identical to original air mass sensor.
When air mass meter is unplugged the engine idles butter-smooth, with or without air conditioner engaged but there is a constant idle surge from about 700 to 1100 RPM. Have not tried accelerating engine with air mass sensor unplugged as I don't think the engine will accelerate without it sending a signal.
This appears to be an idle air situation, perhaps a throttle position sensor amiss, but what I don't understand is why the idle smooths out when the air mass meter is unplugged.
Anyone have any ideas or have maybe run in to a similar problem?