2007 f150 5.4. Since winter started I’ve had 4 “reduced engine power” messages (on different days), the temp gauge pegs to hot, but the truck is NOT overheating. The coolant is fine. If I turn off the turn and crank it back up it goes away and the truck runs fine. It’s throwing code P1299 (cylinder head temp overheating). Today, I was driving and the check engine light started flashing and the truck was stuck in limp mode for about 3-4 miles. After that the light went off and the truck started running fine again. Throwing code P0308 (cylinder 8 misfire) it sat for about 3 hours and then I drove it for about 20 minutes. No problems, runs fine. The truck also sometimes has a really loud humming noise when taking off, but only after it’s warmed up. I just don’t understand why I’m having all of these problems simultaneously. Are they somehow related? Why am I only having spurts of them? Why don’t they continuously do these things? Sorry if these are dumb questions, I don’t know much about trucks.

 

Honestly, if you don't know much about the way trucks/cars etc. work then it would be in your best interest to get your truck to a reputable shop asap.

 

I think they are ALL related. And not knowing much about these trucks is no sin - it is a condition any mechanic at your 'reputable' shop might have also.

Several things you mentioned provide good clues. ("it's NOT overheating but temp gauge pegs out - and goes into 'limp' mode") ("Check engine light started flashing for 3 or 4 miles") ("after that started running fine") ("throwing code p0308") ("Sat 3 hours then ran fine").

A quick 101 on these trucks before going to your trusted mechanic. The computer on these things are ALMOST smarter than we are, but here are sime basics on engine temperature. The Cylinder Head Temperature sensor screws into the engine block under the intake manifold between cylinders 3 & 4. HEAVY HEAVY responsibility is placed on the CHT because it does more than just provide reading to "CALCULATE" engine coolant temperature (there is NO engine coolant temperature sensor on these trucks). Because of it's importance, the device's design is almost bullet-proof. Even so, each drive cycle, the PCM runs a 'complete full range' calibration test' on it and electrically monitors it for "Opens", "Shorts" and "Rationality Check". Your symptoms, being intermittent, suggest possible electrical connection or harness issues.

The device's location routes the CHT harness behind an underneath the intake manifold. It runs real close to the CMCV mounted on back of the IM. If the CMCV linkage rubs the cable, over time it can wear thru insulation and cause intermittent short circuit - or the movement can cause loose connection in electrical plugs.that function along with more combustion temperature monitoring. The PCM's reading from the CHT is used to calculate ECT that is sent to the instrument cluster for display on dash temp gauge.

_WHEN_ the smart PCM determines engine is overheating, (or if wiring problem causes bad high temp reading) it does two things. Limits power and shuts down injectors on four cylinders - alternating between the other 4 cylinders in an effort to cool the engine by PUMPING FRESH air through it. *IT IS LITERALLY RUNNING ON 4 Cylinders. That's why you felt it run horrible for 3 - 4 miles. That isn't registering misfires - but when injectors are re-enabled - I can well imagine you could experience a misfire on some cylinder before things clear out and start running perfectly again. So address the CHT circuit test /checks first - then worry about any misfires.

I would suggest looking carefully at wiring harness on the firewall just behind cylinder 4. Make sure it isn't (can't) rub CMCV linkage. If CMCV is stuck open, it can result in a weird carburetor 'moan' sound under light load when cams retard. ((I had to tie that harness up to my fuel rail with a stiff piece of 12GA solid insulated wire).

Good luck. Hope this helps