2015 f150 lariat supercrew fx4 3.5 ecoboost
truck has 350k miles on it but engine was rebuilt last year. and the problem persisted before and after
new water pump and no thermostat

active radiator damper new
Turbo shutter new
both damper and shutter have been wedged wide open to eliminate suspect

boils over fairly quickly when not at highway speeds with a/c off.

will overheat on highway as well, but seems slower

i did have some misfires that i think have been corrected by new plugs. I noticed gradual overheating with heavy loads with moderate turbo boost even with a/c but I have not tested that since new plugs

I'm guessing that the a/c compressor triggers high cooling fan speeds.

what can I check to start eliminating possibilities or narrow it down?

 

I dont recall, does it have 2 engine cooling fan motors?

turn ac on & pope the hood, if both fan motors aren't spinning, your high speed fan has failed.

350k, external or internal radiator can be plugged.

external can be cleared with strategically flushing the radiator & ac condenser with high volume garden hose.
Before doing that I like using high pressure air nozzle using a compressor.

may need to reposition plastics to get access in-between the radiator & ac condenser thats where debris like to get trapped.
especially if you see dirt roads, cats in the. garage, live out in the sticks, etc....

head gasket isnt out of the question, since you mentioned misfire.

it was rebuilt, by a shop?

or a shop installed a reputable rebuild?

 

2 fans both running
Condenser radiator intercooler new

 

Rebuilt by me. Problem existed before rebuild
Compression is good and leak down good

Loosing no coolant unless it blows out radiator cap

 

Why did you rebuild it?
Did the heads get re-surfaced, checked for warpage, cracks, etc?

Did you lower the coolant level get its smoking hot by taking it around the block a bunch of times & chemical test the cooling system to be certain head gaskets are sealing correctly.
See if the blue fluid turns yellow after a nice drive & quickly taking the cap off & putting the tester on there after HC accumulate.

Certain the Tstat isnt in backwards?

New cap & bottle??
Maybe its warped & not sealing?

Assuming it has a bottle, im a visual guy.
I cant recall every config at this point.

new water pump too im assuming, they havent been problematic inregards to impeller issues.

Sounds like you replaced all probable causes, thats why I question engine.

Respectfully, no idea of your ability & thoroughness.

Ford doesnt even want us to replace rod bearings on these things or machine surfaces....

 

If you rebuilt it, why are we replacing spark plugs again?

 

Rebuild triggered by bad vacuum pump. The connector shaft broke off from the camshaft causing what sounded like no oil pressure .. High mileage , no good reason not to do it. I have a nice shop.

The heads were all lapped by hand and vacuum tested on the bench. Resurfacing wasn't needed , surfaces flat with no defects. No concern with engine internally from an assembly perspective.

Again. This issue exited begore and after the rebuild.

I'm not saying the issue isn't in the engine.

I never intentionally turn the air (compressor) off even with heat on.

I'm not opposed to putting in a new water pump. Easy enough, but I'd rather pinpoint the issue before replacing anything else

Typically the only check engine lights I see is cat converters efficiency below threshold... I've never seen a DTC relating to cooling

Maybe some familiar can explain how to test the fan that's not tied to a/c ... What it should do vs what it actually does

 

if its truly overheating to the point its boiling over, both fans should come on at some point in attempt to cool the engine.
What makes you say it's actually overheating? do you have an actual temp degrees #?

So see if water pump impeller isnt wiped out from electrolysis & high mileage.
take a heater hose off somewhere & start the truck, water should profusely pump immediately,

 

is the system building pressure & sealing, maybe the cap to the coolant Degas bottle aren't sealing any longer & distorted.
All stuff im suggesting is all stuff ive personally seen over the years, Ive seen it all more than once!

 

So it overheats with ac off.
focus on finding out of the high speed fan comes on when its prompted to, when it gets hot!

the low speed fan kicks on & off as required & when the engine is still hot, the high speed fan kicks on, so both motors are running at the same time when the high speed fan is running.
Did you also install new fan assy also, need to ask.
Hopefully if you did it was a complete ford factory unit.

thats not something ide cut corners on.


thats the simple way to check fan operation.
if its truly overheating / boiling over,
both fans must be running.

if theyre not, that where the problem is, but why??

that takes some diag