Only overheating on the freeway.

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Teds63dart

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I have a 1963 Dodge Dart convertible. It had a 225 Slant six and a 3 speed push button transmission. The temperature gauge goes to three quarters after about 10 miles on the freeway. Always stays about half way when riding around town. Not losing any coolant. I'm wondering what the water temperature should be in the radiator. Last Saturday the temperature gauge went almost to the edge. I pulled it over and run some water across the radiator it cooled right down. Any ideas would be appreciated. Thanks maybe I'm a worry wart.
 
Needs more water flow. Check the bottom hose. If it does not have a support spring, it is probably sucking itself closed and restricting the system at highway speed.
 
If the lower radiator hose is collapsing, it's not because the lower hose has no spring, it's because the cooling system is badly neglected: won't hold pressure and the radiator's severely clogged.

Read this.
 
You also might want to verify that the gauge is accurate; it's probably easiest to use an infrared thermometer.
 
Use a analog temp gauge for temp accuracy and tighten all the hose clamps. Do a pressure test and record the pressure every 10 minutes.
The small amount of air being sucked into the system through a small leak can cause cause a hot run situation. Auto Zone has a pressure tester you can rent for free.
 
Timing may be to advanced for highway use. Check your thermostat. Some thermostats will stay open when the coolant temp gets to hot with the assumption that nonstop water flow will cool the engine which does just the opposite. Also check what thermostat temp you have. Is it 195,190,185,180....180 is a nice compromise especially where you live. And lastly....do you have a fan shroud? If not you should try to source one and install it.
For leaks in the system let the engine idle at night time for 10 minutes and turn on a black light. The black light will pick up any leaks in the system. Leaks don't have to be in the form of liquid to hit the ground. It may evaporate before it hits the ground. And I know Sacramento is running 100 to 105 degrees for the last few days. Is it happening all the time or just when it gets hot like now. Make sure what /6 Dan said. Is everything new on your cooling system? There isn't much to them but a lot of things that can affect them. A crusty radiator can cause all kinds of problems. You might try replacing the sending unit as it may be giving a false reading. Or installing an aftermarket gauge. Good thing is that cooling systems are relatively cheaper to fix than anything else on the car. Check one thing at a time and keep the engine oil full at all times just in case.
 
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Thanks everybody for all the information. I will check the bottom hose and I'll check the water temperature when I get off work this afternoon.Thanks once again
 
With the car hot, feel around the radiator core. Temp should be even. A cold spot or area indicates clogged tubes.
If its all even, pull the thermostat.
 
fan shroud, even a crude one helps. Are you losing water?
 
whitepunkonnitro said:
Fan shroud..as well as the fan itself, are out of play on the highway.
Click to expand...

Yep, strictly a flow volume or incapable system all together.
We usually go on the assumption that since most cooling systems are fairly normal stuff for the job that there is another problem.
Hot only on the freeway almost always means a coolant flow volume issue, and the thermostat, radiator and possible block full of crud is the problem.
 
Yup. Old rule of thumb. Over heat in town or at idle, not enough air. Over heat on the road, not enough water.
 
Are you running a thermostat? When was the last time the system was flushed and cleaned. Is this something that just started to occur? More info would be helpful.

Thanks
James
 
One other cause could be a poorly adjusted fan belt. It might be slipping at highway speeds (higher rpms). Back in my shop days, quite a few do it yourself customers did not have the right amount of tension/adjustment on the new belt(s) when they were replaced.
 
Yeah, let's not just shout out names of parts, folks. That's not gonna help him.

The OP almost certainly has a used-up radiator, a faulty thermostat, and/or other clogs and faults in the wet part of the cooling system. The symptoms do not point to a faulty sending unit, a missing fan shroud, or inadequate system pressure (which can cause boilover but no, is not going to cause a hot-run situation). Ignition timing? Sure, that's on the list of possible causes, but the thing to look for is retarded timing, not overly-advanced.

A fan shroud is not going to help this problem. Fan shrouds are for getting more air across the radiator and A/C condenser when the vehicle is standing still or moving slowly. At road speeds above about 25-30 mph, the forward motion of the car pushes more than enough air across the rad and condenser and a fan shroud is irrelevant.

Low speed/stopped/stuck in traffic overheating = inadequate airflow. High speed/highway overheating = inadequate water flow.
 
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Proper diagnosis also requires a history. Did this situation just begin to happen out of the blue? OR has it only begun since summer temps hit?

If it happened quick, it's likely something broke. Thermostat, vacuum hose, etc. If it happened gradually, or doesn't happen in the early morning or evening (when temps are cooler) then it may just be that the high temps are the 'tipping point' that makes the 60 year old cooling system show it's warts.

As dan pointed out: used up radiator, coolant, water pump, thermostat (wet parts) are the likely culprit unless things just changed overnight (fine on the freeway one day and not the next, with similar outside air temps).
 
Crusty radiator, my 66 Belvedere got this way even with a 3 core.
Took the top off myself and rodded out the crud, and soldered the top back on and happy.
See if you can find the old oxalic acid cleaner that is two step.
The second step neutralizes the acid.
If not, buy a gallon of CLR, add to system with distilled water only.
Run it 2 days, be prepared to change water pump afterward.
Dissolving the crud is easier than rodding out a radiator, even if you have to change the water pump.
 
I bought 5 lb citric acid on ebay and use 1 lb per engine flush. That is what used to be in the better Prestone flushes until they went wimpy. Drive around with it and pure water for a few days, then keep changing water until it comes out clear. You can tell a clogged radiator by how well the water pours out the lower outlet (w/ upper hose off), at least old radiator guys used to judge them that way. In my vehicles, after a good flush I have been drying (Sacramento summers are great for that), then refilling with pure Evans Waterless Coolant so no more corrosion worries.
 
I had an old Valiant with the same problem. Turns out (as stated above) it was the negleted radiator even though I thought it had sufficient flow. I got the radiator cleaned up at a radiator shop and the needle never moved from normal temp again.
 
I wouldn't rule out worn water pump impellers or an impeller loose on the shaft. Either one could cause inadequate flow at road speed but might still be OK for idling. I've seen that cause this issue before.
 
Ditto. All slant water pumps I have seen have a sheet-metal blade and sheet-metal divider plate. Those could be rusted away, especially if the system has been sitting. Even with the proper 50/50 coolant, if you don't run the engine a few times a year the rust inhibitors can settle out so you have corrosive pure water in areas.
 
if you end up with a new rad i would suggest a Gano filter. By far the best investment i've made in my cooling system to date.
 
Me, I considered the Gano but wound up going with the Tefba instead (easier to service). It worked very well for the ~10k miles between when I bought it and when I sold my Dart; I kept it but I don't have any more cars it'll fit, so if anyone wants a deal on it, send me a PM.
 
True the tefba is easier to service I guess I'm partial to the gano because I got it at a yard sale new for $10 the guy didn't know what he had I guess. But either way buying either is a very good investment.
 
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