Hi all, I drove my 77 f150 4x4 351M auto trans to work this morning. It was a bit cool, -7 this morning & my temp gauge was pegged, my heater was blowing... warmish air, not hot by any means.

I've been chasing this problem for years & It's really getting old.

Seems the colder it gets outside the hotter that engine runs.

Things i've done so far ar system flush, new coolant (50/50) and I've tried several different thermostats thinking i've maybe been getting bad ones or something.

What the heck is going on?

Dave

 

Might need a new heater core as it could possibly be plugged up. I did mine myself so that means you probably can too! Remember to get new hoses if you do it!

Is your fan working at all speeds and/or correctly. What temp t-stat are you using?

That's all I can offer, someone else might have to chime in.


Someone correct me if I'm wrong on anything I posted!

Hope you figure it out- I drove around freezing my butt off last month until I put my heater core in. It wasn't fun.

 

I'm running a 195 I believe.

yeah the fan works, It's not a blowing issue, just seems my coolant isn't circulating since the engine runs hot & the heater core seems to be getting very low if any flow.

That's why I kept changing stats... thinking it just wasn't opening.

 

Back flush your heater core. I take the hoses off the core and cut the end off an old garden hose, then clamp it to your heater core outlet and flush it out with clean water.

 

but could the... I was going to ask if the heater core could be interupting the flow for cooling the engine but i know it cant.

See, the dang engine goes straight to hot & stays there, and it seems the colder it is outside the worse it gets. And no the coolant isn't freezing in the radiator, the mixture is good I tested it.

I'm going to run out to the parking lot pretty quick here & give a squeeze on the bottom radiator hose, make sure there is a spring in there... could be pulling the bottom hose flat (I could never be so lucky to find such an easy & inexpensive fix tho)

 

Well, I just ran out & gave a squeeze on the lower hose.

No spring in there.

So, I'll be stopping by the auto parts store on my way home to pick one up.

I'll be crossing my fingers & hoping that my problem is this simple.

 

Also,be very certain of the style of regulator(thermostat)you are installing in your engine.The 351m/400 is a "335" series engine.It requires a unique style regulator.When the engine coolant is cold,the regulator is closed and allows the coolant to travel in the block circuit.When this particular thermostat opens,it also has a copper pellet that moves into position to close the bypass port so that coolant flow is now directed to the radiator or open full flow circuit for cooling.Motorcraft P/N is RT139.Do not confuse this with Motorcraft P/N RT1139,as this is a Windsor style and a different story.Let us know what you discover.

 

I had a 94 ranger and the heater core on it was almost totally plugged, I bought this truck from new and am very **** about my maintenance. So if the heater core is partially plugged the rad. might have blocked passages as well. I run one of these on my 429 CJ Torino, autocoolantfilter.com, I put it on right after the motor was rebuilt and the rad. was flushed. I still get pieces of rust, gasket material, silicone, and just junk in the thing after 12,000 and 11yrs.

 

I knew the 351M/400 took a special stat, this last one I ordered from rock auto & in the detail specs the parts addressed the 351M/400 bipass design.

So I'm pretty certain I have the right stat but your motorcraft part number is very much appreciated in case the lower radiator hose spring doesn't cure the issue for me.

Thank you for that.

 

well, I spoke too quick when I stated earlier that the coolant tested out ok. I must have been thinking of my plow truck but I was sure I tested this one, must have missed it.


on the way home from work she went straight to hot and boiled on me.

boiled bad.

I added 2 straight gallons of antifreeze.

The truck seemed to come out of it & settled into what is "normal" for the truck.

It's a 77 so the factory dash gauge says "TEMP"

the needle sits at about the M, about touching the P.

I dunno, I guess I need to get an actial gauge so I can know for sure what's going on.

 

Make sure you don't have a leak somewhere. Can you pressure test the system? could be running low. After it warms up, with heater on, feel both heater hoses, if same temp, core is passing fluid. I actually had a water pump years ago that the fins on the pump had rotted off, not giving enough flow at road speeds but was fine at idle. Also had the vacumn water shut off valve in my 78 go bad and at road speeds the pressure would close off the valve. At idle the heater would heat fine. Like to froze on that trip till Ii figured it out, spliced it with a deep socket. Stayed warm the rest of the way home.

 

moose4x4

I'm thinking on the same lines as you with that water pump thought.

I have a spare 351 on an engine stand in my shop out of a 79 bronco. I had tore it all down once to see what it's troubles were & found it had 2 bent push rods but that was all. I reassembled it with bolts just started finger tight so last night & grabbed the water pump off of it & took a look at it. A healthy pump definately has some water tossing fins on it that's for sure. There has got to be something wrong with my pump on the truck like rotted fins or something like you said because it just doesn't seem to be moving any water.

I'll get a gasket set for swapping the pumps & I'll dig into that as soon as I get a chance.

Funny you mention the vacuum actuated heater hose valve. I had the same thing happen on my old 78 lol!!

Mine had some exposed linkage after the vac diaphram on the valve so I was able to pull & tie mine open with a piece of wire.

The one on the 77 I drive now seems to be functioning fine, it's gotta be the damn pump.

 

I had mine tied open also, but it went bad inside the valve. Me and my dad were on a road trip to pick up a truck, would not have any heat going down the road. We would stop for gas and the cab would heat up fine, I tried changing thermostats that night and was pulling my hair out, nothing was working. After a gas stop we started down the road, in about 5 miles it went back to cold air. Told dad to pull over and I checked the temp of the hoses, it was hot up to the valve and cold after the valve. BINGO! Put a splice in the hose with a socket and Eureka. Good luck Bruce

 

Forgot to add, when the engine is hot & while it's running I can look in the radiator & there is no visable movement of the coolant.

It's gotta be the stupid pump.

 

Sounds like you are on the right track. As long as the hoses are not sucking shut like one poster said. Bruce