Overheats on the freeway but not when cruising around town?

[Demonic]

It's likely flow, if you're overheating at speed. Higher rpm= more heat output. All the aftermarket thermostats, like the ones pictured, are smaller opening=less flow than what Mopar intended in the 60's and 70's. If you find on original thermostat like 2128994 or similar later versions, the opening is nearly 33% bigger.

The other rule of thumb for most radiators is - they're dirtier inside than what you believe.

 

[cosgig]

Shroud in place? Hood to radiator support seal in place? Lower splash pan to radiator support seal in place? Without those seals, air can just go around the radiator opening, it’s like sucking through a straw with a hole in it. The idea is to trap air in front of the radiator, and use a conical shaped shroud and clutch fan that’s at least 1/3 of the way in the shroud to create a vacuum and draw air through the radiator.

 

[dvw]

I'm assuming all parts are back in the way they were before the rebuild. So block was clean. Same type of head gaskets. Same shroud, sealing strips at the core support, hose size. Air flow is greater at speed. Overheating at speed is generally due to reduced coolant flow. The fan does very little if anything at speed. Seldom will a t-stat stick closed. Most failures are stuck open. In fact I've never seen one stick closed. That leaves a few options. The lower hose sucking closed. May need a internal spring. Radiator tubes clogged and restricted. Water pump impeller blades coroded and are to small causing cavitation. Water pump impeller slipping on the shaft. The last Mopar big block I saw with this condition was the impeller slipping on the shaft.
Doug

 

[747mopar]

I'd pull the thermostat completely out and drive it just to see what it does, quick easy way to tell if the system is up to the task. If everything is good it should run cooler than you'd want. Timing is the other thing I'd look into. Like you said, fans, shrouds etc don't mean anything if it's doing it at cruising speeds, it's somewhere else.

 

[eldubb440]

prob not your situation, but I had an AMC engine that always ran hot....... finally pulled it apart and found the water jackets in the block were partially packed with a hard, rusty, carbon type material......... we dug it out with ice pics and coat hangers and sent it off to the machine shop

 

[747mopar]

I'm assuming all parts are back in the way they were before the rebuild. So block was clean. Same type of head gaskets. Same shroud, sealing strips at the core support, hose size. Air flow is greater at speed. Overheating at speed is generally due to reduced coolant flow. The fan does very little if anything at speed. Seldom will a t-stat stick closed. Most failures are stuck open. In fact I've never seen one stick closed. That leaves a few options. The lower hose sucking closed. May need a internal spring. Radiator tubes clogged and restricted. Water pump impeller blades coroded and are to small causing cavitation. Water pump impeller slipping on the shaft. The last Mopar big block I saw with this condition was the impeller slipping on the shaft.
Doug

When I first got my Charger running I had a thermostat sticking closed, the temp would climb until I pulled over and gave the thermostat housing a whack.. I'd climb back in an watch the temp dive back down to normal. After I replaced it it never did it again but now after 19,000 miles it's sticking open. Time for another one.

At least you've heard of it now.

 

[Mackman]

I agree with taking the thermostat out and driving the car. Isolates the thermostat as the problem.

 

[Don Frelier]

When I first got my Charger running I had a thermostat sticking closed, the temp would climb until I pulled over and gave the thermostat housing a whack.. I'd climb back in an watch the temp dive back down to normal. After I replaced it it never did it again but now after 19,000 miles it's sticking open. Time for another one.

At least you've heard of it now.

I had one that would occassionally stick closed.
One day after work (going to the parts store for another car) I drove about 4-5 minutes stopped at a light and looked down.
****! 220 degrees and rising fast. I knew what the problem was but I didn't want to shut it down at that light (it's kind of a busy intersection).
The light changed and I think I was around 230 or 240 deg. F.
Went about a 1/4 mile to a side street turned and just as I reached to turn off the key the lower hose blew to bits and dumped everything out in front of someones house.
Pretty big mess under there.

I called my son and he came by and quickly pulled off the hose and went and picked up a universal.
We filled it with water and I started it (still with the same stat).
I whacked it a few times and it opened so I drove it home and changed it that evening.

When stuff is broken this kid is the best guy to have around:

Yes they do stick shut but mine never did once it opened initially.

 

[eldubb440]

I also had an issue where a higher temp thermostat held the coolant in the radiator longer and cooled better than a lower temp thermostat or no thermostat at all

 

[zyzzyx]

Start the car and get it warm. Open the hood and watch the lower radiator hose as you bring the engine
to 3 or 4 thousand RPM and see if the hose collapses.

 

[PurpleBeeper]

I did still wonder about the hole in the thermostat. I've seen some with a small spring loaded sliding valve/pin.
View attachment 1456432



For now, I'm going to swap some parts in the hopes that I can drive it back home on Sunday. I could just default to trailering it back home. That is a smack to the face for a guy that wrenches on his own cars though!

You can drill a 1/8" hole, or have a little "wiggle piece of brass" in an existing hole or similar. It's just something to allow any potential trapped air to not get stuck behind the thermostat....hot air bubbles won't let the thermostat open....so you get the bubbles out.

In general, it sounds like you don't have enough air flow through the radiator on the highway. That A/C condenser doesn't help. Your radiator is a good one. A shroud is more important in traffic. You don't happen to have an electric pusher fan in front of the A/C condenser to you? (that made mine run hot on the highway)

 

[dvw]

The only way I could see a t-stat that could stick closed is if the pin was covered with corosion or it was physically broken. The bimetallic springs get weak with age and don't hold as much tension. Then they open at t oo low a temp. Not saying it couldn't happen. But I've been in the repair business since 74. Never personally seen one stuck closed.
Doug

 

[Kern Dog]

You can drill a 1/8" hole, or have a little "wiggle piece of brass" in an existing hole or similar. It's just something to allow any potential trapped air to not get stuck behind the thermostat....hot air bubbles won't let the thermostat open....so you get the bubbles out.

In general, it sounds like you don't have enough air flow through the radiator on the highway. That A/C condenser doesn't help. Your radiator is a good one. A shroud is more important in traffic. You don't happen to have an electric pusher fan in front of the A/C condenser to you? (that made mine run hot on the highway)

Jigsaw does not have Air Conditioning.

The red car does.
I'm heading over to the venue soon. I'll have more to report in a little while.

 

[eldubb440]

Jigsaw does not have Air Conditioning.

it appears Jigsaw is also lacking a fan shroud

 

[Mackman]

Thermostats fail to open because the wax motor, which is what makes the thermostat operate, leak or corrode. The spring closes the thermostat.

 

[Kern Dog]

it appears Jigsaw is also lacking a fan shroud

I ran without one in the red car for years without overheating. It wasn't even a hot day yesterday, maybe 85.
I know it isn't ideal but this problem came on suddenly. An hour after shutdown, the hood and top of the bumper were still really hot.

 

[Gus chiggins]

This car:

View attachment 1456361

.....is a hodge podge of used parts, hence my nickname for the car, Jigsaw.
I put it together with a lot of parts that I had stashed away. I used a lot of used parts but bought new where it was needed. The 383 has been rebuilt, runs 9.3 compression with a 280-474 cam, Edelbrock RPM intake, Holley 750 Vacuum secondaries, 1 7/8" headers, 2 1/2" exhaust, the radiator is a Griffin aluminum 2 row that I used to have in THIS car:

View attachment 1456363

Prior to today, Jigsaw only overheated once. That was due to being low on water. I simply didn't top off the radiator all the way after first fire. The cam and lifters were used and reinstalled in order so there was no cam break in needed.
I have driven the car maybe 150 miles since the engine install and all the driving has been local.
Today, on the way to a car show, my friend had it on the freeway and the temperature gauge was pegged. We pulled off to a gas station and let it cool down and added water. Once the needle on the gauge was to the left of center, we moseyed on to the venue. While idling in the line to park cars, it spiked again.
My experience with overheating is limited. I always place a high priority on using good radiators and stock belt driven fans.
This radiator worked fine in the red car and was only swapped because the A/C install I did required a radiator with a stock (1970) hose arrangement.

Common sense tells me that if the car runs hot while the radiator is getting enough air flow, it means that there is a coolant flow problem.
When I built this engine, I used a used thermostat, used water pump, pulleys, fan clutch and even the belt. I have only used straight water in the system though.
I'm going back to the venue tomorrow. I suspect that the thermostat is to blame. Maybe it is stuck closed. I'll know more once I dig in.
Anything else to look for? I'll replace the thermostat regardless. Some suggest to drill a hole in the thermostat to improve flow. What size hole?

1/8 inch hole, it's been awhile, so I'm not sure.

 

[Photon440]

Do you have an infrared digital thermometer? Maybe check for cold spots on the rad while it's running hot to make sure that there aren't any blocked areas.

 

[dvw]

Thermostats fail to open because the wax motor, which is what makes the thermostat operate, leak or corrode. The spring closes the thermostat.

I stand corrected. Been doing this stuff for years and either forgot about the wax chamber or never new it. I read up on it and educated myself today.
Doug

 

[Kern Dog]

Ooof. What a day.
I got back to the car hoping to find an obvious problem.

The thermostat looked fine but I didn’t know if it was stuck so I installed new one with an 1/8” hole drilled into it.

Next up, the water pump. Turns out the one I had also looked good. Aluminum GMB with a plate on the back of the impellers.

Working in this dimly lit building as temperatures are climbing… hard to see with low light.
The lower radiator hose had a groove in it through the outer layer. I figured that with enough pressure, it could blow. I don’t know how it held with as hot as it got on the way there. I went to a store to look for random hoses with a similar shape. This is one big disadvantage with non stock parts. One change requires other changes. I bought two hoses at $35 and $41!

Neither were close enough. I cut one so no chance for a refund there.
I decided at this point to just patch it back together and default to trailering it home on Sunday.