Hello all,

Newbie here. A couple weeks ago our ‘09 King Ranch 5.4 with about 120k miles (owned by us since new) developed a misfire at idle, only with the AC on.

Checked for codes, no CEL, no codes showing.

Checked battery voltage at the battery and with my code scanner-13.9-14.1 volts.

Checked for combustion gas in coolant-none present, but it appears to be losing a small amount of coolant from somewhere. Top hose collapses on cool down and the level drops slightly in the radiator.

Checked for running misfire by pulling coil connectors at idle with AC on, 3 cylinders were misfiring.

Replaced all 8 plugs and coils yesterday with Denso (“OE” according to Rockauto) coils and NGK iridium unobtanium plugs, started it up, and the problem persisted-idle got rough as soon as AC was turned on, smoothed out when turned off.

Drive it two miles to work this morning and it’s not doing it, probably 30 degrees cooler…wonder if it might be a fuel thing. Could it be fuel/dirty injectors…??

I will happily entertain any suggestions at this point…thanks for looking!

 

wha'ts your idle rpm ac on and off? Perhaps the IAC or something isn't cooperating and kicking the idle up enough for the extra load. If that were the case I would think your idle RPM with ac on would be lower than with it off

 

Checked idle speed after getting home from work this afternoon, it was steady in drive at 500 rpm whether the AC was on or off. Put it in park, it jumped to about 600 and same thing, no rpm change when turning AC on or off, it just started idling rough.

I did notice this morning when the ambient temp was in the high 50’s that it ran fine whether AC was on or off, so it only appears to be doing it when the ambient temp is above a certain point and the engine is at operating temp…