78 F150 351mod 4sp top-loaded 4x4 no AC
I purchased this truck several years ago from a fire department down south, and brought it north. The heater never worked very well. Had to replace the radiator (twice), heater core (original not plugged), water pump, several thermostats and radiator cap. Still no heat and it looses coolant out the overflow. There isn't an overflow tank. It just drains out the tube under the radiator cap after shutting off. There isn't any controls in the coolant flow, just the doors in the heater unit. I'm beginning to think I might have a leaking head gasket. What test do I need to perform to check the head gasket theory? Will a compression test tell me anything? What else do I need to check?

Thanks in advance for you help.

 

A leaking head gasket will generally vent while its running, not after shutdown. The tests I know of are 1) air up one cylinder at a time with a sparkplug air hose adapt or. Listen for leaks and watch the top tank 2) There are test strips you can buy to check for combustion products in the coolant. Never used them but understand they work.

If it is not a head gasket leak then what else might it be? A couple of questions. What is the temp like while running... OK or hot? If you feel the heater hose are one or both hot to the touch?

One member fought cooling problems for most of a year and found out the backing plate for the water pump was omitted by the PO. The 351M, like my 400 has and needs the backing plate. Another possibility is the block is plugged up... although I would think the fire department had decent coolant maintenance but maybe not.

Good luck.

 

Thanks for taking the time to help and give me more to think about!

Making a "spark plug air hose adapter" sounds like something for a weekend. I need to pay more attention to when the coolant comes out of the overflow tube. I need to know if it is coming out while running or just after it shuts off.

The temp runs normal on the gauge. When stressed, like towing up hill, I've seen the temp go up, but it comes down when normal driving resumes. Both heater hoses warm up before the upper radiator hose warms up. I need to make sure there is flow across the top of the radiator after the thermostat opens.

I think I know the plate you are talking about. Doesn't it mount between the water pump and the block?

I think the truck had pretty good maintenance. They were required to change oil every 6 months no matter how few miles so I would think they probably had other maintenance practices, too. (I hope!)

You did make me think of something else. They changed the intake manifold so they could put a Holley 4bbl on it. If it was the wrong manifold could the coolant flow across the intake be blocked? If the water pump was trying to push coolant through but couldn't that would create pressure that is released when the engine was shut off and pushes the coolant out the overflow tube.... Just thinking... That goes back to your comment about the block being plugged.

Wow.... Too much thinking and more work to do!

Thanks again!

 

Without an expansion tank and radiator filled to top of neck, heat soak will push coolant out after shutdown. It should have one. Also there should be a heater control valve somewhere, though I'm not familiar exactly where for your truck. If this is stuck closed, no heat. Not saying you don't have other issues, just mentioning simple possibilities.

 

and an expansion tank is a cheap and easy add

 

The radiator fill instructions for my truck (no expansion tank) indicate that you should only fill to 1.5" below the neck - if it's not dropping below that level, you may not actually be losing coolant that should be in there, just the amount you overfilled.

Have you tried flushing your heater core? Could just be plugged up...

 

Never mind on the plugged bit - just saw you'd replaced the heater core.

 

The 351M/400 is a dry intake manifold....meaning no coolant goes through the manifold.

Are you sure you have the correct thermostat for the M block? They're different than the ones for the Windsor, but look almost the same.