We have a 2006 F150 w/new engine, new everything but transmission and cat. converter. When driving in warm weather with the AC on and at low speeds or idle the engine will knock and want to cut out but you rev and turn off the AC and it stops. Only with AC on. We replaced fuel filter, fan throttle took to dealer and updated the software which it stopped for 3 days and started again today. Really frustrated! Could it have to do with AC compressor or low refrigerant? Please help my husband is at his wits end and I know way more than I ever cared for with truck repair! Thanks.

 

Is the truck equipped with skid plates?

I had a similar issue and it turned out the skid plate under my transfer case was bent upwards and rubbing on the exhaust making it seem like it was knocking and causing excessive vibration to the cab.

 

But would it do that only after the truck had been running for a while and only when the AC is on? I would think with the skid plate rubbing it would do the knocking more often.

 

that's what I thought as well but these motors have a fairly loopy idle.

what happened to me was it would when the idle was loopy it would intermittently make a knocking sound and when I turned on the AC it would vibrate plus it transfered the loopy idle shake to the cab.

check underneath the truck on the passenger side right under the T-Case there's a skid plate and the exhaust is very close to the skid plate, I had bent mine helping out a friend with farm work and bottoming out over a hill and ultimately I had to scrap the plate because I couldn't get it bent out right and even after going crazy with a high speed cutter it still rubbed.

 

It could be something really odd or out of the ordinary like Charz is saying.
Being a new engine and the dealer having checked it over, I wouldn't think it would be spark plugs or Coil Packs going bad.

I had a miss due to bad plugs and it would shudder and vibrate in low RPMs in OD. Changed the plugs and it is perfect.