Running Hot on the Highway

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340sFastback

340sFastback

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My car runs hot on the highways, around 200-210 or so degrees driving a steady 65 mph. Before someone says "that is not hot" I don't like it and don't want it running that hot. When I get off the highway it slowly creeps back down to 180 - 185 or so. It was 85 degrees outside that day. It can idle all day long and never get hot no matter how warm it is outside. Never runs hot when driving around on the back roads. Here is the set up:

Flowkooler water pump (new).
Original crank and water pump pulleys for non-AC car.
Mr Gasket 160 high flow thermostat (new)
Mopar Performance viscous fan package (new)
3:91 gears
414 small block stroker w/Edelbrock heads, 500 miles
TTI headers and TTI 2 1/2" exhaust
Original 340 w/AC radiator with NEW 3 row core
Reproduction fan shroud
Spring in lower hose (spring is 4" too short)
727 trans with 2500 stall.
50/50 mix anti freeze and distilled water.
8 pound radiator cap (recommended by radiator shop).

Wondering if that spring being 4" too short on the lower hose is allowing it to get sucked shut at high RPM? I can pinch the hose shut with my fingers no problem but I can't find a longer spring. Is the radiator not up to the task? I would think a new three row core would do and it only runs hot on the highway cruising at high speeds.

Any suggestions greatly appreciated. Like I said above, I don't want it running that hot on the highway and want to find a solution.
 
Why did they recommend such a low pressure cap? I would also put a 180 degree thermostat in it, that will help it stay closer to the temp you want without such a large fluctuation. I put a new radiator in my Dart a couple of years ago, and had a problem similar to yours, even though it had the same number of cores as my oem leaker, the cores were MUCH smaller. Is it running lean at all at 65mph?
 
metallidart said:
Is it running lean at all at 65mph?
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That is a good question. I don't think it is because throttle response is great on the highway. Just touch the pedal and it takes off no hesitation. The car also ran very strong on the highway in the cold weather and I would think if it was lean it would hesitate when I gave it more gas.

Timing is locked at 32 degrees.
 
Sounds like your coolant is not satying in the radiator long enough to cool it down. Sounds the most likely culprit is the water pump.
 
When I first put the engine in the car I had a stock replacement 6 blade water pump on there with the stock AC crank and water pump pulleys. The temps climbed right up on the highway and that was when it was 40 degrees out. The non/AC pulleys, which spin the pump slower, and the high flow pump helped some but did not resolve the problem.
 
Change it to a milodon/mr gasket 180* t-stat. 160 is probably being held open.
 
They're are only 3 reasons that come to mind that would make it run warmer at hwy speeds than at idle or slower traffic speeds. First is a restriction on the flow thru the radiator, this usually happens on a older partially block radiator. You say yours is new so? second is the water is not staying in the radiator long enough to transfer the heat to the air, Need to run a 180 degree thermostats, yours maybe flowing to much water. Third is your motor is running lean. Don't rely on the throttle response for it not running lean. A engine runs it's best at a leaner than rich condition, especially throttle response. My 2 Penny's
 
I have not put a wide band on it so I can't say for sure if it is lean or not.
 
Looks like you've got a lotta cubes there in that small block, and then the 3.91's too. Sounds like you're doing everything right, though. If it were mine, I'd probably throw an engine oil cooler on it as well and maybe a Dodge van (oversize) radiator. Unless you're going after the stone "sleeper" look. Keep in mind that the one thing you mentioned....slowing down the water pump helped a bit......I hope that water pump isn't cavitating with the super-high RPMs on the freeway. 3.91s are insane on the freeway without overdrive.
 
The Flowkooler water pump has a disc on it to help prevent cavitation.

WaterPumps.jpg
 
Just a couple suggestions..
180 thermostat,
Do yu have a fan shroud?
Plugs?Which ones are you running?
I put a clutch fan on mine,and helped cooling.

Good Luck!
 
My fan was installed backwards, car ran hot on highway. Just a thought that came to mind.
 
The cap may be keeping the pressure too low, causing a vacuum effect and if the spring isn't in the lower hose, or its too short, it may be sucking the hose shut. I'm curious as to why the shop recommended this pressure rating on the cap, I always use a 16 lb cap. Just something to try, or ask the rad shop that re-cored the radiator. Also, its kind of a crazy question, but...is the fan belt tight enough?
 
You didn't mention if you're running an auxillary tranny oil cooler and if so how is it plumbed, eg. oil goes to the aux. cooler first then the rad lower tank. With the slippage of the high stall converter (non lockup) you're putting a lot of thermo load into the rad. Go with the 180 deg. high flow thermostat and check it in a pot of boiling water with a thermometer first, 3 or 4 times open and close. I had one fail to open right out of the undamaged package. Sounds like you've done most everything right except pull one sparkplug and tell us the color of the electrode, white, tan, brown or black. You can also check to see if the lower hose is collapsing by simply have a friend bring the engine to the same highway RPM and see if it does, I'd also check it from underneath just don't go getting yourself run over by your friend.:D

Terry
 
My first thoughts leaned toward "pump turning wrong way" "serpentine vs. v-belt" sorts of things. I'm not sure if the spring would tolerate being stretched a few inches, but if it's fairly new this may be worth a try. The low-pressure cap and lean main circuit resonate the most.
 
Like the others have said, I would start with a 180 or 185 thermostat and a 16 lb. cap. You`ll experience less engine wear with a warmer engine. Check for a tight belt that isn`t glazed and if your fan clutch is working like it should.
 
Thanks for all the suggestions. Belt is new and it is tight. Fan is on the right way (pulls air through). I am going to try and stretch the spring in the lower hose some and center it in the hose to try and keep it from collapsing. I'll try running the 16 pound cap too.
 
340sFastback said:
Thanks for all the suggestions. Belt is new and it is tight. Fan is on the right way (pulls air through). I am going to try and stretch the spring in the lower hose some and center it in the hose to try and keep it from collapsing. I'll try running the 16 pound cap too.
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What about the thermostat?I,d change to a 180 first.My 2 cents.
 
When the car is warm and you shut the engine off, does the fan keep spinning for a awhile? If so, bad clutch. If it stops within 1 turn of the engine, its good.
 
pettybludart said:
What about the thermostat?I,d change to a 180 first.My 2 cents.
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I agree about the 180. A 160 buys you nothing in the hot weather and then in the winter no need to run cooler then 180. But what ever thermostat is in there during the summer it will be staying open anyway so I don't believe it is causing it to run too hot on the highway.
 
redfastback said:
When the car is warm and you shut the engine off, does the fan keep spinning for a awhile? If so, bad clutch. If it stops within 1 turn of the engine, its good.
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Fan stops fairly quickly when engine is off. When engine is idling you can put your hand by the front of the valve cover and feel a strong breeze blowing by. Can also feel the air going through the radiator from the front too.
 
I took the spring out of the lower hose and stretched it out. Then I pulled it through the hose with a string. The spring is now the full length of the hose. Now there is no way for it to suck shut. I have the 16 pound cap on there. Drained out two gallons of coolant and put distilled water back in to dilute it. If the roads dry up I'll take it out on the highway and see what it does.
 
Just took it out and cruised it 65 mph on the highway and it got hot again so I pulled off into a gas station that was right off the road. Radiator and hoses were not hot at all but you could burn your fingers by touching the intake manifold under the thermostat and on the intake coolant passages coming from the heads right where the sending unit is located. But when I go onto the back roads it runs nice and cool. I think the thermostat lets coolant through but not at a fast enough rate. So I'll pick up a 180 degree and put that in there next. I am going to test it in boiling water before I put it in.
 
The 160 stat is letting the water run thru the rad so fast it can't dissipate much heat.go at least with a 180!
Sorta like holding you hand over a lit match compared to waving it acroos the flame.
 
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