Hello everyone,

I have looked through the forums, and cannot find a solution to my overheating problem.

I have 1978 F-250 with a 400 and manual transmission. About a year and a half ago, it overheated. I was never able to find the cause.

The cylinder heads warped. I got them machined, and went through everything up from them (valves and rockers, etc.). Still overheated.
Replaced the water pump (the old water pump was 4 years old and had less than 3,000 miles on it) with a high flow Edelbrock.
I added a fan shroud just for good measure. It had worked for years without one, but just to make sure.
The thermostat has also been replaced with both 180 and 190 degree thermostats.
Coolant is flowing through the system properly.

It is still overheating, and I am out of ideas. Any help will be appreciated.
Thank you in advance.

P.S.-I have replaced the carburetor and replaced the distributor with one that has HEI.

 

Has this engine ever been rebuilt?

If it was overbored, they might have gone overboard.

Thin cylinder walls lead to an easily overheated engine.

Imperfect deck might allow combustion gasses to pass to the water, but that doesn't last long before a blown head gasket.

Any indication of fuel in the coolant? Rainbow sheen on it perhaps? Tiny bubbles? Gray?

 

how is the radiator (both internally and externally)?
is the cylinder block stock, or has it been bored?
what is your timing set at?
is it running lean?
do you have a fan clutch or solid fan? does it overheat at highway speeds, or just stopped in traffic?

 

Well there are only a few things and one is the radiator. It is possible to have too high of a flow rate too if you went to a non stock-ish water pump like you say you did. The fluid does need a certain amount of time in the tubes to transfer heat. Probably unlikely in this case or most cases because you have other restrictions but it can affect the flow characteristics

when I had my 360 rebuilt into a 390 the stock radiator wasn’t adequate. I couldn’t even get through the break in with a shop fan blowing on it in the summer and had to shut it down. I put a summit 4 row aluminum in with the stock water pump and a 180 F thermostat and it will idle at 180 F all day no matter how hot it is now. I think the stock radiators are barely adequate and if it’s original it’s probably all scaled over inside

 

Retarded timing will cause over heating. I've seen several times where old engines can push blowby into the distributor and cause the mechanical advance to lock up. HEI distributors can also have advance issues. Pull the cap and rotor and make sure the weights and springs are free to move.

 

OK, on the assessment that "coolant is moving properly through system" With that being fact-- I take it to be exactly that since that is what was said can only mean 2 possibilities, the engine is making too much heat for the radiator to digest, or the radiator is not capable of flowing enough air to cool coolant. That is the only possibility. What does the lower radiator hose temperature compare to upper hose, upper radiator tank temp compare to lower? Does it have coolant? does it have a radiator cap? Is the radiator cap installed? Is the radiator cap holding any pressure? I mean, The incorrect thermostat will cause exactly this however that would indicate that the coolant was bypassing internally through the block on a 400, and effectively contradicts that coolant is properly moving through system. Is the exhaust restricted, pinched closed, crushed? lots more recommendations however they mostly go into that the coolant is not flowing properly, like water pump, backing plate, hose sucking closed, conventional thermostat instead of correct one with internal bypass, water pump belt slipping....

Is the fan installed correctly or is it backwards and pushing air instead of pulling?

 

When It's hot take your hand and feel around the radiator's cores. If there are any cold spots then get the radiator rodded out.

 

Here’s a story then another suggestion. I had an 8N tractor that continued to run hot then boil over. Didn’t matter, glycol or straight water. Different radiatior, pump, thermostat, all same result. Pulled the soft plugs and was full of mud and grit from years of straight water. Rodded out the coolant passages until water ran clear, reinstalled the plugs and no more issue.

Have you flushed the system?

 

I will answer as much as I can. Thank you for all the response.

@85e150 Yes, it has been rebuilt, twice. I was not told it was over bored, but I think it is reasonable to assume. I did not see any signs of fuel in the coolant, and I have dumped it several times in this endeavor.

@dlburch As far as I can tell, the radiator is good. It is stock. I have been warned against going to a new aluminum one unless I have to. Speaking of the radiator, I installed an A/C system that has the heat exchanger in front of the radiator, but that is where it is supposed to be mount. As designed, should not cause a problem. Probably has been bored. Getting the timing right has been an issue the entire time I have owned the truck. After putting the HEI distributor in, I believe it is finally good. Solid direct drive fan. Over heats in traffic and at highway speeds. Once the temperature starts coming up it just keeps going past 180, 190, 210 to 230+.

@beardedcap I took it to a professional garage that believes the radiator is ok. Water does flow through the system. I could be convinced that the flow rate would be too high with the new water pump, but this was happening with the old water pump too. I flushed all the crude out of the system (it was bad before I took on a trip a few years ago).

@davelj As previously stated, timing has been an issue, but I have had several people time it better than me. It did not seem to make a difference. The HEI is new, but might be worth checking.

@F-250 WARHORSE I like the way you are thinking. Checking the upper and lower radiator hoses is a good idea to see if it is coming out of the radiator too hot. There is coolant and a cap, installed and holding pressure. The fan was installed backwards (after rebuilding the heads), but it was still overheating before being installed backwards and when corrected.

@mark a. I am thinking the only way I can check is replacing the radiator. I do not want to change things just to change something, but that might be the only thing I can do.

@broktruk I have flushed the system until it was clear. Took almost 30 minutes, but that was a couple of years ago. It has hardly moved since then. I do not know what the soft plugs are, but I will look that up.

Thank you all again.

 

Highway speeds should be one of the easiest situations to stay cool in, are you sure you aren't pinging? Do you have any idea of what your timing curve is like? Are you able to achieve decent idle quality or does it idle high/rough like a vacuum leak? Do you have a thermometer where you can measure the temperature differential between the upper and lower rad hose? That would tell you a lot about how well your cooling system is working.

Also I'm not an engine builder but I've read that the 400 was already prone to detonation, and it's been overbored, and the heads have been surfaced so you've increased your compression ratio in a few ways. Just a thought related to when I asked about pinging.

 

A backward fan sure will cause issues. The first place to check is the lower hose. If it is hot or warm or cool, then when it is overheating and sitting there idling, spray or dump cold or ice water over the top front of the radiator, forcing the coolant inside the radiator to cool down, and see if the engine temperature responds to that. If the radiator is cooled all the way down and the engine is not cooling off then the coolant is NOT PROPERLY CIRCULATING. might be pump issue, radiator issue, hose issue, belt issue pulley issue, restriction issue, thermostat issue??? All of the above.

 

@beardedcap I do not have a thermometer that can check the temperature except the gauge (which is new) and do not have a timing curve. When the timing is right, it does idle and there is no pinging. There was some pinging when the temperature was high or the timing was off, but that seems to be resolve. It does start to backfire when it gets hot before it shuts off as well.

@F-250 WARHORSE This is a good idea as well. I might see if there is a different radiator that I might be able to test with.

 

Since it's a 335 series pull the t-stat and look in the block for the donut looking thingy can't remember what there called it must be in there and properly there or you will be chasing your tail forever.

 

Exactly who's thermostats have you used, what part numbers? 351Ms, 351Cs, and 400s all use a thermostat many are unfamiliar with.

• When the engine is running and is cold or cool, the water pump is pumping, and as the thermostat is shut, a bypass is open allowing coolant to bypass the radiator ... to keep coolant moving and prevent cavitation in the water pump.
• When the engine reaches hot, the thermostat opens, and as it opens it has a feature to severely reduce flow through the bypass. Coolant keeps moving, but as long as the thermostat is open, the coolant is force through the radiator.
• The correct thermostats have a "hat like" featur that has a raised brim on the small end of the thermostat, it inserts into a hole in the block on the 351M and 400, or into a brass restrictor in the 351C block. It is smaller than the hole, but not a lot smaller, it restricts that path.
• If a thermostat is used without this feature, hot coolant from the heads will just go into the water pump through the wide open bypass just to get recycled back through the block, never having seen the radiator. The engine will just keep getting hotter and hotter.

The correct thermostats have the hat and incorrect thermostats do not have the hat.





Note that they all have "the hat" on the bottom. These above are 180* ... but Motorad also has the 223192 standard flow and the 2023192 high flow versions for 192*. Currently I'm using the 2023180 in mine, and while temps can drop to 180 on long cool down hills, typically I see 190-195 on level or up grades. No doubt, most all thermostats start to open at the designated temperature, but only reach full openness when maybe 10-15 degrees hotter.


Only the 351C has the bypass orifice insert under the thermostat, 351Ms and 400s have the bypass hole in the block casting under the thermostat.

 

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