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1979 Ford Bronco (351M engine) overheats, tried everything I can think of

Got my 1979 Ford Bronco (351M engine) from my father. He had it in Colorado, mostly in a garage. Had 82,000 miles when I got it and has 67,000 miles now. Runs fine when the outside temperature is less than 80F. Above 80F, the will get hotter and hotter and the temp. gauge will climb. When the temp. gauge is 2/3 of the wat between Cool and Hot it will start to buck and cut out. Eventually the engine will stop. Then I have to wait for it to cool down to repeat the cycle. I have replaced all hoses, radiator, thermostat (3 times), radiator cap, carb. I have installed SS louvers in the hood. \Heck, I have even “burbed” it (see Internet). . Any ideas?

How.s the water pump? Impellars erode over time, and some can wear completely off.
Do you turn the heat on when you burp the system?

Overheating won’t cause it to cut out unless it is siezing up. For the cutting out part, I wonder if you aren’t getting vapor lock from the fuels. Maybe need to check fuel pump, use an isolation gasket under the carb, insulate the gas lines over the engine, etc. to keep the heat away from the fuel supply.

On the cooling side, it just seems like you have inadequate cooling capacity. Fan clutch or electric fans working ok? Otherwise do you have a heavy duty radiator or larger capacity radiator? Way back in the 60’s a guy we knew had a new Chrysler that he’d pull a camper with and it would overheat. He had an extra tank extension (looked like about a gallon) brazed onto the upper radiator and never had a problem after that.

Water pump test: Remove the thermostat and re-install it’s cover. Remove upper radiator hose from the radiator. Arrange to have coolant directed into a bucket (with a spliced on large diameter hose). Start engine and gently rev it up a LITTLE…Water should be pumped out of the engine with CONSIDERABLE force… Another test. Does the heater produce really hot air when the vehicle approaches overheating? It should.

Last but not least, after 35 years, engine blocks can become plugged up with rust, the passages into the heads completely blocked up and proper cooling impossible…

Good point about the age of the engine and the possibility of blocked passages. Clearing that problem if it exists along with checking out the water pump should get to the heart of the matter.

Not just the block, but the radiator as well. These older cars that sit a while tend to have fluids overlooked as maintenance items. I was offered my grandmother’s LTD with 60k on it, it was 18 years old at the time. Noticed that the temp gauge didn’t rise too much, so I did a coolant change with new thermostat and rad cap. The coolant was so rusty brown, it took several cycles of clean water to get the block clear. Luckily, the radiator rinsed right out. Fresh coolant, and it was running like a charm. Should have done the water pump, 'cause 6 months later, it started to leak.

He said he replaced the radiator to no avail.

Assuming this problem is not related to the water pump, maybe there’s an ignition system problem that is heat related.
This vehicle should use the Duraspark system and there were a number of versions determined by the color code of the grommet on the module.

The DS system module allows a wide swing in the ignition timing so maybe the system is trying to retard the timing too much which progessively worsens and eventually leads to the temp gauge climbing and performance going utterly into the tank.

You might consider an ignition timing check after the engine is fully warmed up along with making sure the timing is advancing like it should.

The DS system was not necessarily a bad one but with age and heat those modules can act stupid at times.

In Denver eh ?
Where are you now ?

I had an over-heating 1980 Bronco 351m.

My 80 Bronco was highly succeptible to altitude and while adjusted to my 6500 ft.alt. it would certainly heat up just going to Albuquerque ( 4950 ) and would OVER heat on a cross country trip back to Ohio @ 1050 FT.
– Owning a Cessna 172, I was schooled in adjusting air/fuel mixture and knew it overheated if too lean and knew the Bronco needed that, so I asked my mechanic to show me the adjustment on the carb.

Late in its life it started vapor locking with only medium overheating.
A three port fuel filter solved that…running the return hose all the way back to the filler neck wher I could attach a hose fitting.

But as it continued its quest to drive me to the nut house, and replacing the t/stat did nothing, I put a coolant system flush in and ran it extra time with no stat blocking the flow.
– AND —
Cleaned ALL the greasy dirty build up off the block, pan, and intake. ( this greasy build up acts as an insulating blanket. )
– AND –
Although I did not replace the radiator or fan clutch ( both proven good ), I DID take a fin comb to the front fins of the A/C condenser, radiator, and trans cooler to guarantee air flow.

Something worked…or all of it…because it ran normal after that…
for years untill…

Using the cheapest low octane gas I could buy it would cackle/ping now and then under heavy acceleration and hills but being only occasional I thought nothing of it…
Then one dark and not-stormy night,
on the way back from albuquerque which requires quit an altitude climb over the continental divide,
I had it cackling way too much time …and this time it added to the total to be too much and blew a hole clean through the top of piston #3 and I puttered to a steaming cackling spewing stop on the side of I-40 just 10 miles from home.
Long story short, a $10 used piston head fixed it and it purred like a kitten till I sold it to the mechanic who put the used piston in it.

Moral of the story ?
You fuel octane could be contributing to the over heating too.

I echo mountainbike’s idea that it could be the water pump impeller. I’ve seen impellers break lose from the shaft, essentially allowing it to freewheel.

Also, where the heads on this engine ever shaved down during a valve job to make them flat again? If more material is taken off than what is allowed, you’re asking for a hotter running engine.

Carry an extra spark plug in the vehicle. The next time the engine stalls and won’t start, remove a spark plug wire from one of the spark plugs on the engine. Insert the extra spark plug into the plug wire and lay the spark plug on the engine to ground it. Now have someone crank the engine over and watch for the spark on the end of the spark plug. If there is no spark then the ignition module or the magnetic pick-up in the distributor could be effected by the engine heat causing the engine to shut down.

Afterall, we don’t know if the engine is actually overheating until runs long enough to do so.


@TSM, I caught that reading it closer. Sorry. Couldn’t log back in. I concur with the water pump theory. The radiator may also be too small. Unless you already have a 4-core.

All above great comments. A couple of other things to consider. Make sure all the routine maintenance suggested in the owners manual is up to date. Check the ignition timing. Retarded timing can cause overheating. A lean mixture can also cause overheating. A tail pipe emissions test could tell you for certain if the mixture is too lean. A leaking headgasket is a possibility too. Ask your mechanic to check for signs of hydrocarbons in the coolant. The fan might be spinning, but not as fast as it should. Compare yours to another Ford truck of similar vintage maybe, see if the fans are spinning about the same speed. Finally, there’s a possiblity that the old radiator was clogged, but the new radiator isn’t up to the task of cooling this particular engine. You might simply need a bigger radiator with better cooling capacity.

Check the ignition timing too. Your vacuum advance may be stuck too. If you can’t rotate the distributor to adjust timing wait until engine is cold; it may move easier. I agree with earlier post to change that big ignition module if you have one. If it gets very hot, change it. You probably have one with a blue grommet which may look black, so clean it or turn it over to see the cleaner side. It’s that big aluminum box near the left front fender and master cylinder.

Oh yeah; if the engine starts to buck and then quits, it probably is the module causing it. Been there, done that.


When it’s running hot, you should hear the fan ROARING, trying to cool the engine down.

If you don’t the fan clutch may be faulty. Not uncommon.

I have a 1978 f250 351M. I did everything that that everyone wrote. The final fix, that has worked for 17 years, is I put a big fan in and also an electric fan. When I drive up in altitude or idle in heat to long I flip the fan on and run them both. Never had that problem Again.
Now, I had a over heating problem from a 1 1/2 inch pin located in the distributor. After the truck had set for long period. This pin did not function correctly in the distributor.
Two entirely different problems.

Those 351M’s just need more cooling off. I put a new engine, hoses, giant radiator, extra side jug for the water, several different thermostats, caps and etc … the big fan was the only fix for that vapor locking to stop. Yes I know how it sounded when the engine was trying to cool off.