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77 F -150 Overheating

Ok so here is the latest problem with my 77 F-150. This truck is an automatic 4x4 351 modified.

I had noticed a while back that this thing was getting hot, so I replaced the radiator. I knew the radiator needed to be replaced anyways because it had the nasty green and every other color in the world coming out of. Hints probably about a million leaks. I replaced that with one of the best radiators I could get from Advance. I wanted to put in a good one since I plan to keep the truck.

I thought it seemed to be work ok but it would always be a little on the warm side, thinking this is just how this truck is. Well finally the thing had it one day and overheated big time, blew my radiator cap, and the radiator flooded over, and even blew the temp sensor. I replaced the temp sensor, radiator cap, and thermostat. This did not help it from overheating.

I then moved to the water pump thinking maybe there is something wrong with that even though it seemed to be fine. I replaced that one night and it still overheats.

Now frustrated with it I let it sit for two weeks before I take a hammer to it. I’m starting to think it is a headgasket. I say to myself let me try one more thing before I decide it is that. My last attempt was removing the thermostat so I can tell myself the thermostat is not stuck and I take the heater hose off and have it go back in the engine to see if the heater core may have something to do with it. Still the thing overheats.

Now when I replaced the radiator I flushed then engine so I know it has good clean stuff in there. I am now down to two thoughts… 1) The radiator that I put in is not good, or 2) I got a head gasket issue. I also did a headgasket test with the blue chemical stuff. When I did that it was suppose to change from blue to yellow. When I did it change from blue to a light blue/clear color. I would have to say more on the clear side of the two but not really white. While I was doing the test, it got so hot that the test tube I was using melted in the middle. Luckly I took it off before it was destroyed.

I have seen these things to put in the engine for a blown head gasket and they are guarenteed to work or your money back. So should I try that, or look more at possibly a bad radiator? One concern I have is, if it is not a head gasket issue, will I mess up my engine in any way by putting this stuff in? The antifree in the radiator looks normal, with nothing to think there is oil in it. The oil in the engine looks fine also, no reason to think there is antifreeze getting in the engine.

Thanks in advance for any ideas.

When the engine is cold, remove the radiator cap and start the engine. Watch the coolant in the radiator and if bubbles appear in the coolant as the engine is running there’s a blown head gasket.


Would the engine need to get warm for me to see the bubbles or would I start to see them almost immediate? My thermostat is out, so it wouldn’t need to get warm for that to open but just wondering for other reasons it would need to be warm to see the bubbles?

If the a head gasket is breached the combustion gasses will produce the bubbles immediately. You don’t have to wait for the engine to warm up.


how is the fan cluch ???

You stated that the engine has been modified. If you have installed a serpentine accessory belt drive system, you have to make sure you installed a water pump for a 351 with a serpentine belt or the water pump will run backwards and the engine will overheat. Of course, if the engine has been seriously overheated you may by now also need new head gaskets and hopefully nothing more.

You have to check the engine timing with a timing light. Too far advanced and the engine will overheat. The timing chain is easy to check once you get the cover off. It’s a tough job due to the time and effort involved but it should be done if you’ve never been in there.

The engine has not been ‘modified’.
It’s a 351-M , as opposed to the smaller block 351-W, and that’s what they call it.

My 80 Bronco, also 351-M, had over heating problems too and I threw everything at it like t-stat and all but it didn’t stay cool until I gave it a massive cleaning.
Cooling system chemical flush ( not just a drain out & refill ) running a garden hose attatchment for hours, engine block degreaser, a/c condenser and radiator fin cleaning and combing.

Since yours has a new radiator check for coolant flow rate ( water pump or internal obstruction ) …
Clean clean clean. Greasy build up on the block and other places acts as an insulating blanket…
Air flow. Clean bugs and mud from the fins of a/c condenser and trans cooler and see that the fins aren’t all bent over or pushed in.
Fan clutch operation.
Air / fuel mix ratio. My 80 Bronco would over heat at lower altitudes ( 6500 ft at home ) due to the resulting lean mix.

Sorry I suggested the engine had been modified, as in, by the owner, when it’s really apparently just a factory tall deck 351M. I primarily grew up around Chevy guys and have never heard of the “M” in “351M” lengthened to “modified”, although a little research on the meaning of the term brought sense to this designation. “Mutt” would also make sense since it’s a mutt of FE and Cleveland parts. Learn something new every day.

Ok, so I think I’m going to try to work on it this weekend. I’m planning on getting the cleaning chemicals and doing the super clean. Is there a way that I can test the coolant flow rate myself? Also, does anyone know if you do use that stuff to fix a blown headgasket and the headgasket is actually ok if it will hurt the engine or not?

Thanks for the ideas everyone!

Don’t over look the water pump running backwards comment from another poster. This is possible and everything else can be perfect and you’ll still overheat. You can get LH (left hand) and RH rotating pumps that look the same externally.

You need to make sure you’ve got good water pressure and flow from the pump. You also need to check that there is nothing restricting water flow through the new radiator.