Long time lurker. First time poster. I have searched the forum and have seen where it has been mentioned about a plugged heater core can cause overheating. I tried about everything mentioned in my searches with flushes, checking thermostat, replace radiator cap and cleaning and fans. My conclusion was a plugged heater core caused the entire problem. After bypassing, my overheat problem is gone. Could someone please explain to me how this causes overheating? I thought the water through the pump ran in parallel to the heater core, so theoretically, reduced flow in the heater core would increase flow in the primary passage. Right??? Boy, was I wrong...

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1967 Dodge A-100 90 inch wheelbase panel van with a 225 /6 three on the tree Holley 1920 carb

Welcome here!
Many things are mentioned…some of them are true and correct. A clogged heater core cannot cause overheating.
No, it was something else.
Correlation does not mean causation. You bypassed the heater core and the engine is no longer overheating, but the one did not cause the other. Be mindful not to assume you are definitely seeing what you think you are seeing. For example: How certain are you that the engine was actually overheating? What indication did you have?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Radiator boiling water. Sad to say, I had several instances of this happening. With sevearal new 13 # radiator caps, due to one overheating can cause cap to fail. Was pretty sure heater was plugged but could not fathom this causing an over heat. Pulled hoses from heater core and tried a water hose through the core. I finally got through and also out the front of the core. Decided to bypass core and voila, she has been running at the E of temp. No boiling, and tested head temps at temp sending with temp gun. Running about 180 where before with heater core in the line it would get up to 180 and slowly seem to heat soak unit overheated up past the line next to the P in temp 230. I am having a hard time understanding this. Maybe coinscidence. I had flushed radiator several times and before heater bypass was getting clear water. After bypassing heater, the water got rusty again, leading me to believe increased flow through the engine. Maybe faulty reasoning on my part. Have not been able to drive it more than a few miles without it getting hot, but this weekend drove it on a 24 mile round trip setting at the E of temp and checking with temp gun.

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1967 Dodge A-100 90 inch wheelbase panel van with a 225 /6 three on the tree Holley 1920 carb

Should have said 230 degrees plus. Water boiling in a 13 # pressurized system would have to be 251 plus if cap functioned correctly. Failed to mention this van had sat for a few years. Thanks for reply and maybe one big strange coincidents. Just glad it is not overheating, for now.

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1967 Dodge A-100 90 inch wheelbase panel van with a 225 /6 three on the tree Holley 1920 carb

so theoretically, reduced flow in the heater core would increase flow in the primary passage.

Maybe the flow did increase when you bypassed the core and the rusty/dirty water you saw upon starting was an obstruction clearing because of the increased pressure and the previous flushings?

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ski

Good point! So I guess my question now is would an obstruction in the flow to the heater core cause reduced flow in the primary flow through the engine? If so, then what is the mechanism to cause reduced primary flow? I agree that in a system that is optimal this reduced flow may not cause overheating. But in a system that is compromised, could this happen? My guess is an increased localized pressure or turbulence in the water pump to decrease its efficiency??? Or am I blowing smoke???

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1967 Dodge A-100 90 inch wheelbase panel van with a 225 /6 three on the tree Holley 1920 carb

I would bet that you have a failing thermostat that is not opening completly.

When you "by-pass" the the heater core by re-routing the hoses, you are, in essence, bypassing the thermostat also... basically, hot water from the head is routed directly into the suction side of the water pump, for another 'cycle' thru the engine.

Doing this type of bypass will usually result in continued overheating because the bypassed coolent never goes thru the radiator.
DD

No.
No.
No.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Hmmm... I guess it shall remain a mystery as to what caused the van to suddenly stop overheating. Just glad it isn't for now. The more I know, the more I know that I don't know. Thanks for all the replies. Certainly has me scratching my head.

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1967 Dodge A-100 90 inch wheelbase panel van with a 225 /6 three on the tree Holley 1920 carb

Are you just using water for coolant?

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The car I am restoring is a 1971 Valiant 4 door with a slant 6 - 225 engine.

For now I am running straight water. I had planned to until I figured out what was causing the overheat. I plan on one more good flushing then replacing with antifreeze this weekend. I am at a lose. All indications point to some kind of increased flow, maybe coincidental.

I had flushed system just before bypassing the heater core, took it for a drive and motor got hotter than it should but did not boil, opened radiator cap to inspect and see clear water. Unhooked heater core and bypassed. Topped off radiator to replace the water that was lost due to unhooking and bypassing and took it for a drive. Engine ran noticeably cooler but still got warm enough to make me think this did not totally fix the issue. Got it home and inspected water in radiator and found rusty water in the system. New flow through heater core bypass passage may have created this.

Flushed one more time and now it sits at around 175-180 when driving. I entered it in a parade last weekend in 75 degree weather and it ran for a solid hour just creeping along and warmed up to 190. As soon as I was able to get into second gear, the temperature dropped back to 175-180 and stayed there on the 13 mile trip home.

These temperatures were all checked with dash gauge and verified with a temperature gun at the sight of the temperature sending unit with vehicle not in motion. Seems like the E of temp on my dash gauge is anywhere from 150-180 degrees. M is 190-200 P is 200-210 with the H line at 230+.

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1967 Dodge A-100 90 inch wheelbase panel van with a 225 /6 three on the tree Holley 1920 carb

About 15 years ago I had a customers 90 ish plymouth FWD 2.5 engine, while driving at steady speed temp gauge would peg to hot then fall down to halve continuously. First through has head gasket problem,checked and found no combustion gas in radiatior so changed thermstat and flushed cooling system even thoght I didn't think that was the problem. Road tested with no differance, pulled thermostat and ran no differance, debated pulling water pump to check impeller when I noticed heater hose going into the housing behind water pump and also noticed poor heat from heater. By passed heater and it fixed problem, I installed new heater core and it fixed problem and customer now had good heat. Don't no about a slant six but on this 2.5 a plugged core definitely disturbed the cooling system to make the temp gauge peg. I beleive I had a cavalier with a 2.2 haveing a similar problem a few years ago also.