My 04 f150 has developed an idle surge when sitting stopped w/ my foot on the brake. Only happens w/ a/c or heat. Rpm will drop for couple seconds, then shoot up suddenly, and settle back down. Truck will try to creep fwd if not pushing on break pedal hard enough. Does this in 30-40 sec intervals. Any insight would be appreciated, thanks.

 

May be low on freon. This changes the compressor load on the engine and the pcm over compensates the idle. try un plugging the low pressure switch on the accumulator and see if the problem goes away.

 

I had the same problem the other day with my 2007 5.4 4x4 KR with 29000 miles. The weather was clear and about 50 outside and I had 3/4 full tank. In addition my AC was off.