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I have a 1965 Plymouth Belvedere II, 273 V8 engine, automatic transmission, power steering, standard brakes. Engine was rebuilt 5 years ago, electronic ignition, new be cool aluminum radiator, everything else stock. Car is well maintained. Coolant changed out ounce every two years, 50/50 antifreeze and water. No problems for 5 years. New water pump when engine was rebuilt 5 years ago.

Recently the engine over heats after you drive the car about 4 miles. On a cold engine i pulled the radiator cap off, started the engine, let it warm up, and checked for bubbles in the coolant. None. Also there was no loss of coolant. Turned the heater on and it worked fine. Checked the fins in the radiator, belt, pulleys, and hoses and all looked good. Drained the radiator and caught the fluid in a jug so i could examine for contamination. Did not see anything. Poured the coolant thru a cloth and did not pick up anything at all. Engine starts right up and runs smooth. When i had the cap off the radiator, i speeded the motor up and got an increase of water flow thru the flutes in the radiator. This is a cross flow radiator so this was easy to check. I ran the car on a test run with no thermostat and the car still over heated.

I am running out of ideas. Anyone know what I should do next? Thanks
 

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True Hotrodder
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There appears to be two versions of this engine in 1965. One has a compression ratio of 8.8 and the other is 10.5. Just a guess but our fuel is getting worse. And even though it's been rebuilt 5 years ago, there will be some build up of carbon on the pistons and compression chamber of the head which will increase the compression ratio slightly. I wonder if trying a tank of higher octane fuel might help the overheating situation and/or a slight reduction on total timing?
 

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And there you have the most likely culprits......
Worn-out thermostat (replace with a good quality unit, don't go cheap)
Return hose at bottom of radiator old and weak and sucking flat (replace with a hose that has a coiled spring in it to prevent collapse)
And the long shot, a water pump that has the impeller spinning on the driveshaft when the water gets hot as a result of eliminating the press fit. You might remove the pump and run it under hot water to see if the impeller loosens up on the shaft.
My best advice in any case is not to purchase any cheezy Chinese parts.
 

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Old(s) Fart
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Failed water pump or failed headgasket. If it's not losing water, likely a failed pump. Replace the thermostat while youre in there.
I would really like it if someone could explain to me what failure mode of the water pump will cause overheating. Yeah, there's the one-in-a-million chance that the impeller came off the shaft. You're more likely to hit the lottery. The normal water pump failure mode is a failed front bearing, which immediately takes out the seal, which results in a large puddle under the car. If the pump isn't leaking, it isn't causing overheating.

Despite this, everyone's first impulse is to replace the pump if the car overheats...
 

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True Hotrodder
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Not my first impulse. The OP already said that with the cap off and raising the rpm he sees an increase in water circulation so a busted pump it isn't. He also said he tested the car without a thermostat, being that the thermostat doesn't do much more than help the engine warm up I think that eliminates that as an issue too. And if it's a headgasket then more than likely we would be pressurizing the coolant system and seeing something in the form of white smoke exiting the exhaust pipes. There is the possibility of the bottom hose causing a decrease in water movement - the OP will simply have to take a look at that when it overheats for verification.

The radiator was said to be new - but how new and are we electric fan or stock engine driven?
 

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Discussion Starter · #10 ·
Thanks for all the replies. Yes i have a catch can. I checked the bottom radiator hose and it has the wire coil inside and did not collapse. Car still runs hot with the thermostat removed. I have the 8.8 compression ratio so I will try the higher octane and timing adjustment. The cooling system does not leak a drop but I will take a compression test.

Is there any way to check the impeller of the water pump without removal? Since i got a surge of water in the radiator when the motor was speeded up and the heater worked i felt the water pump was probably ok.

I will keep working on this and let everyone know if i fix it. I had an engine years ago that did this and it turned out to be a blown head gasket between two cylinders. The odd part about it was that a compression test did not reveal the problem. I had to take the heads off to discover it. I always try the simple things first before doing the more complicated procedures so i will put that off as long as i can.
 

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Race it, Don't rice it!
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You want to do a leak down not a compression test. Unless it really really really bad a compression test won’t tell you much beyond how fast the starter turns.
 

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If you suspect a head gasket / cracked head , obtain some coolant test strips , testing will tell you if combustion gases are present in the coolant . Stop speculating/ guessing/ waiting for divine intervention . Test it , correct the problem ..:)
 

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It’s either a coolant flow problem or a tuning problem.

Obviously if you took the thermostat out it isn’t that as a cause. That still leaves pump, plumbing and radiator. Watching flow all your seeing is the top tubes that doesn’t mean all are up to snuff. Corrosion is a constant problem with aluminum radiators and iron blocks. Coolant should be 50/50 with either distilled or deionized water. The grubbier the water used the faster the inhibitors in the coolant get used up.

Tune from the cam and/or ignition drifting late to fueling falling lean all can result in heating issues. One sign of a timing set and/or distributor drive gear failing is a frequent need to ‘tune’ the base timing. A failed vacuum advance can will do similar things by not advancing timing when needed and adds a vacuum leak. Vacuum leaks from any source will do the same thing. A failing fuel pump or carburetor can drag the mixture lean which adds heat. Also if this still has the exhaust bypass valve in the exhaust system if this fails closed it will force one banks exhaust through the intake crossover which will put a lot of heat into the mixture.

There just is no simple answer you have to test these things.

Bogie
 
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