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If the electric fan kicks on, the shroud no longer pays a part. The fan is forcing cool air through the radiator. The shroud helps the clutch (or non-clutch) fan to draw air through the radiator. If the truck slowly overheats with that electric fan, you have a different problem - thermostat or clogged radiator. You can check the radiator for cold spots indicating clogged tubes (which run up and down). May give you an idea if the radiator is the issue or the thernostat is. Normally a thermostat sticks either closed or open, but in your case it may be stuck halfway open not allowing enough water to flow through the system causing a slow overheat.
 

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Discussion Starter · #22 ·
I think the factory spec thermostat is 195°. Tried that, then a 180°, then without any. The latter obviously lead to a longer warm up period while driving, but the same overheating condition at standstill. Once everything is said and done I am going to go with the 180°. A little below factory spec won`t hurt, right?
 

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yes a stuck thermostat will cause stop light over heating [ idling ]
 

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Discussion Starter · #24 ·
Yes, and I was confident that by replacing or removing the thermostat the problem would go away. Unfortunately, it didn`t. (The thermostat is fineby the way, checked it in a pot on the stove.)
I removed the radiator yesterday and I will have it tested.
 

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I once had a water pump on a 318 cause intermittent standstill overheating. You would think a bad pump would overheat all of the time, but this one didn't. Only reason I changed it is because I had changed everything else in the system and figured I had nothing left to lose.

New pump and it went away.
 

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yes the fan blades [ impeller ] on a W/P do corrode and dont pump as much coolant except at higher rpm's , which would cause stop light over heating

l had one where the impeller blades [ fan] fell off
 

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Some aftermarket pumps have way too much space between the impeller and the casting, and just spin the water around inside the waterpump at idle.
 

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'79 Macho 360 Magnum, Comp 480 cam, Hughes springs, 650 Thunder AVS, Pertronix Flamethrower ignition
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I think the factory spec thermostat is 195°. Tried that, then a 180°, then without any. The latter obviously lead to a longer warm up period while driving, but the same overheating condition at standstill. Once everything is said and done I am going to go with the 180°. A little below factory spec won`t hurt, right?
No it won't hurt it but you will get better fuel emissions from a hotter engine and maybe even a little bit better fuel economy also.
 

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Hi guys,

Thinking I´d do good to my vehicle`s OEM style appearance I got a thermal fan clutch for the now bolted on fan. Just to find out it won`t fit. The fan is stamped 3462190 and the little cylindric spacer it is mounted on has a ´78 production date stamping. So this actually looks like factory MOPAR. Now here`s my question: Was the thermal fan clutch an option on those vehicles? (1979 Trail Duster, 360CID, 4WD, A/C). This is clearly not a matter of life or death, but I am curious...
The vicous fan works better. I have 2 '79's both with them and no spacer. Grade 8 bolts used. On mine the fan and clutch share the same 4 bolts mounting to the water pump.
 

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Discussion Starter · #30 ·
I did replace the water pump. With great regret, because it turns out it still had the factory one on it, and it looks just fine to me. Anyway, now I have the best (or at least most expensive) pump rockauto offered. Airtex high volume. Didn`t change a thing about the overheating though...
The good news is that I found an OEM style radiator on ebay. I took the chance and ordered it right away, even if I didn`t hear back from the shop yet whether my old radiator is clogged or not. I sure hope it is clogged, because otherwise I might be looking at blown head gaskets as the cause for overheating (aside from the missing shroud). Fingers crossed!
What points to the clogged rad theory is the fact that the upper coolant hose is hot and under high pressure, while the lower hose is lukewarm at best...
 

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a hot upper hose and a cold lower hose is the way it should be , you do have a 195* thermostat in there right

to me it sounds like the coolant is flowing thru the motor to fast or your looking at head gaskets
 

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But why would your rad be clogged? Does your engine have lots of scale rust inside (brown/red water?) I could see the original radiator Being clogged from decades of scale but that aluminum radiator can't be all that old. I'd check to see how much flow your engine has. No shroud with still cause a hot engine though.
 

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Discussion Starter · #33 ·
The radiator is five or six years old. You are right, it shouldn`t be clogged. The coolant didn`t look too bad either. But I have to eliminate options here. The thermostat is fine, so is the pump. The missing shroud surely has its role in the problem, but there is an auxiliary electric fan right in front of the radiator. That should make up for a missing shroud and therefore ineffective stock fan. So the next guess is the radiator. It sure is worth the go before dismantling the heads...

Of course I do want a hot upper hose and a cold lower hose, but when the upper one is almost boiling and the lower one is hardly warm to the touch the radiator either is working perfectly - or not at all, blocking the flow. I have to ask though: How can coolant be circulating through the engine too fast?
 

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If it heats up at idle, but cools down again when moving, it is an airflow through the radiator issue. They also say that too much advance in the timing can cause heating at idle, I do not understand how.

Coolant going too fast, is a theory from back in the day, claiming that the coolant is going too fast to pick up or drop off heat. I have heard many different theories as to how true it is. One ounce per minute grabbing and dropping off two degeres of heat, vs two ounces per minute each grabbing and dropping off one degree of heat. Simplified model, but I hope you get the point. I have looked, but never really seen any great tests that definitively say one way or the other.

Way too much flow, where you get cavitation inside the block, I can see, but that would take a lot. I think.
 

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Also, if the cap is not letting pressure build up, you could get boiling inside the engine block, which can also impede heat transfer.

If you still have issues after you get the rad back, get a digital laser infared thermometer, and test many locations to see what the actual temps are in the hoses, water pump, ect.
 

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If it heats up at idle, but cools down again when moving, it is an airflow through the radiator issue. They also say that too much advance in the timing can cause heating at idle, I do not understand how.

Coolant going too fast, is a theory from back in the day, claiming that the coolant is going too fast to pick up or drop off heat. I have heard many different theories as to how true it is. One ounce per minute grabbing and dropping off two degeres of heat, vs two ounces per minute each grabbing and dropping off one degree of heat. Simplified model, but I hope you get the point. I have looked, but never really seen any great tests that definitively say one way or the other.

Way too much flow, where you get cavitation inside the block, I can see, but that would take a lot. I think.
No, advancing the timing cools the engine, it's too much retardation that causes hot EGT's. Having the timing retarded too much causes the engine to run hot because of the late timing and late firing which burns the fuel in the middle of the cylinder (as opposed to the top of the cylinder where it should be) and therefore heating more of the cylinder walls which heats the water more. Cavitation usually happens at the water pump and not in the block so much. When they say that the coolant is moving too fast they mean that it removes too much heat by constantly flowing. In a regulated system, the water sits stagnant in the engine for a certain period of time and then circulates to the radiator to cool off but if the water is in a constant state of motion then it will never fully heat up.
 

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The radiator is five or six years old. You are right, it shouldn`t be clogged. The coolant didn`t look too bad either. But I have to eliminate options here. The thermostat is fine, so is the pump. The missing shroud surely has its role in the problem, but there is an auxiliary electric fan right in front of the radiator. That should make up for a missing shroud and therefore ineffective stock fan. So the next guess is the radiator. It sure is worth the go before dismantling the heads...

Of course I do want a hot upper hose and a cold lower hose, but when the upper one is almost boiling and the lower one is hardly warm to the touch the radiator either is working perfectly - or not at all, blocking the flow. I have to ask though: How can coolant be circulating through the engine too fast?
How many blades does your pump have and have you checked too see how close it sits to the timing cover housing? Also, is that auxiliary fan spinning in the right direction? What if it's blowing the air away from the rad and not towards it? Is the thermostat in the right way? Have you performed a coolant pressure test to see if you have any leaks?
 

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No, advancing the timing cools the engine, it's too much retardation that causes hot EGT's. Having the timing retarded too much causes the engine to run hot because of the late timing and late firing which burns the fuel in the middle of the cylinder (as opposed to the top of the cylinder where it should be) and therefore heating more of the cylinder walls which heats the water more. Cavitation usually happens at the water pump and not in the block so much. When they say that the coolant is moving too fast they mean that it removes too much heat by constantly flowing. In a regulated system, the water sits stagnant in the engine for a certain period of time and then circulates to the radiator to cool off but if the water is in a constant state of motion then it will never fully heat up.
I knew it was something with timing, had a 50/50 chance. Thanks.

Cavitation can happen anywhere there is sharp turns, moving objects, and heat. With big trucks it is common on the outside of the cylinder liners, `especially if the cooling system is run a lot with no pressure. I also have a water pump (I'll try to get pics), that had severe cavitation and wore a ton of pits on the inside of the water pump housing, where the pump spins. Once the pitting starts, it continues rapidly. I see it a lot here with the irrigation pumps I rebuild, Too many sprinkler "Pros", sell too big of a pump, if the water flow is not what the pump wants, then system backs up, and creates a lot of cavitation inside the housing.

I can show you a ton of tests that show too fast is more of a myth. And a ton that say it is real. But none that use the same setup, and test all the possibilities. I think system pressure is more important, Higher pressure will help eliminate hot spots, and formation of steam pockets. Slower coolant speeds will increase the possibility of both.

Also, if slower was better, then why do so many folks have more overheating issues at idle, the when running at 2500 RPM? ;)
 

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If the system has no thermostat then the water pump just pushes the water as fast as the water pump can spin. It's not that it's moving too fast per say, it's more that it's not regulated and not having enough time to absorb the heat. The cooling system is designed to flow and then not flow depending on the temperature of the thermostat by holding the water in the engine for a given length of time and allowing it to absorb the right amount of heat before it's off to the radiator to cool off to go back into the engine to remove more heat. If the water/coolant moves too slowly, it then absorbs too much heat and doesn't let the engine cool properly much like having a hot water bottle next to you in the summer or even in the winterwith too my blankets on you.
 

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No body said anything about not having a thermostat. That is a whole different can of worms.
 
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