Intermittent Overheating at Highway Speeds

Ian Curtis

Ian Curtis

Senior Member
On long drives, I have recently experienced an intermittent overheating problem that I can't find a good physical explanation for.

My pickup normally runs comfortably cool as indicated on the MotoMeter, maybe 1/2 inch below the black line. However, after about an hour of highway driving (even on level ground), the temperature begins to climb and after about 5 minutes the temperature climbs to about the top of the MotoMeter. If I pull off for 5 minutes, when I restart the car and get back on the road the temperature on the MotoMeter quickly falls back to normal, and stays there for about another hour. If I pull off onto a slower road (25-35 MPH), the temperature quickly falls back to normal. This weekend, I even pulled off into stop and go traffic in town and the temperature fell back to the normal range! I didn't touch the spark lever as traffic slowed either, so it can't be that.

Someone suggested maybe there is some crud in the cooling system that is occasionally lodging in a bad place, and getting dislodged with the speed change? Can anyone else come up with a good explanation of what is going on? :confused:
 
Test the actual temp when it's reading high with a thermometer... The motor meter is an inaccurate gauge mostly for looks or a half *** guide..
 
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Do you have a thermostat installed? Could be your motometer. When the engine is shut down the temp should go up not down. With the water no longer being circulated the heat collects in the high point of the radiator and should send the temp up.
Next time it does this feel the radiator, is it really hot or does it feel normal?
Do you have any out flow at the tube? If it was getting that hot you should have boil over.
Is it foaming? I have found with my Model T, some water pump grease has caused a bit of foaming and if that foam touches the motormeter causes the temp to go up on it. Motometers read the air temp above the water not the actual water temp.
 
Replies in Bold.

Do you have a thermostat installed? No.

Could be your motometer. When the engine is shut down the temp should go up not down. With the water no longer being circulated the heat collects in the high point of the radiator and should send the temp up. Yes, the temperature on the MotoMeter does climb when you stop. It quickly falls back into the normal range when you're back on the road after the stop.

Next time it does this feel the radiator, is it really hot or does it feel normal? I just got back from a 350 mile trip, I'll try this out when I'm ready to cram myself back in the cab for an hour. :D

Do you have any out flow at the tube? If it was getting that hot you should have boil over. Yes.

 Is it foaming? I have found with my Model T, some water pump grease has caused a bit of foaming and if that foam touches the motormeter causes the temp to go up on it. That is plausible. I have noticed that sometimes on inclines the temperature will spike & then quickly decline, and I was wondering if it was because the water was temporarily touching the MotoMeter itself.

Motometers read the air temp above the water not the actual water temp. Yep. Out of curiosity, does anyone know what the air temperature itself is in the normal range when the water is normal operating temp?
 
first steps to take when there is an overheat issue:
1. use a temp gun or actual temp gauge to assess what you really have, as Mitch states, motometers are notoriously innaccurate
2. be absolutely certain timing is correct and that you are running the spark rod in the correct place

"does anyone know what the air temperature itself is in the normal range when the water is normal operating temp?"
I doubt this info is known by anyone. get an IR temp gun; it is worth its weight in gold

BTW your temp differential from top to bottom of a good radiator s/b 30°

You said this is a recent problem. What is the back story? what was most recently done to the car before this began
 
After my vinegar treatment a few years ago I removed the radiator, turned it upside down and back flushed it with a sump pump in a large tub of water. I then used the same sump pump to back flush the block. I got a lot of rust flakes out, and installed a screen in the top hose to make sure I trapped any more junk before it could block off radiator tubes.

Unfortunately many of these old engines have lots of rust from people thinking plain water is a good idea. :mad:
 
I too had the same problems when I got my coupe in 1985. Club took a tour to Daytona Beach. Up US 75,. about a 450 mile round trip. Cured it by getting rid of the motometer. Never had a problem since. My blood pressure went up with the motometer went up and went down when the meter went down . Never added any water during the whole ordeal.Tony
 
Art Newland said:
Haven't I read somewhere about some waterpumps pumping to much, to fast?
Click to expand...

No water pump can pump too much too fast so long as:
  • The water doesn't enter the top tank so fast it can overflow it
  • The pump doesn't cavitate
The notion that if the water circulates too fast, it doesn't spend enough time in the radiator to cool down is rubbish. If it circulates that fast, it doesn't spend enough time in the block to get hot. So long as the water does circulate, things will be OK. On our recent National Model A rally, one fellow travelled 3,500 Km (2,200 miles) with a broken water pump shaft which meant the pump did nothing. His engine survived on nothing more than thermo siphon for water circulation. Anything from that situation to pumping water so hard that it NEARLY surges out of the top tank is fine. IMHO
 
"Can anyone else come up with a good explanation of what is going on? :confused:"

Don't over think the problem. With the engine running, what reduces coolant temperature? Radiator. As Tom mentioned, I think you will find a thorough flushing will cure your overheating. But first verify you don't have late timing as Tbird suggested.
 
As I mentioned earlier before jumping in the pond confirm that its running hotter than it should be using an accurate manual check...
 
With a really good Boyce Motor meter, Minervas' temp would go UP & DOWN on regular cycles. She was LOW on water! A ROOFING nail in the overflow fixed it. (TIP# 11)
Bill W.
 
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I had a problem with my four door I had a original rad that leaked and had repairs on it so i replaced it and water pump and i was still having issues till i found out that the overflow tube go plugged up by some fuze from and old gasket that had gone bad and ended up in the system blew out over flow tube and thing runs cool as a cucumber now
 
Ian,

Go out and buy a cheap dial meat thermometer. (or borrow one from the kitchen) Pull the radiator cap and see what the temperature actually is. Your shooting in the dark until then.
This is one tool I think all A and AA owners should have in their tool kit.
 
meat thermoter. This is the tool crosscut is referring to. Bought it at Walmart. takes about half minute to reach running temp in dial. ken
 

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I installed a 160 deg thermostat and temperature gauge yesterday (from Snyder's). Fired her up and no leaks after idling for about 15 minutes so went for a test ride. It's 94 deg today. The APCO gauge was at 180 when I started down the street. I went about 15 miles round trip with a few stretches to 55 mph(Mitchell OD). Mostly 45 to 50 though. The gauge climbed to 190 and stayed there. I didn't lose any water. Question is, does this seem reasonable? I have searched but not found what normal should be. Does 190 under load sounds reasonable?
Hugh
 
Gud mawnin VFR. I don't know what norml should be. Outside temperature will have some bearing on that. "lecoupe, the one picturesd in my avatar above will cruise at 45 to 55 (4th gear overdrive) and temp will be consistently at 175 to 180. That is in 94 - 98 degree Texas heat. On an 80 to 90 degree day she will crusie at about 165 degrees.

Cruising at 190 is risky. That only gives you 20 degrees of margin until boil. Maybe 25 degrees if you have 50/50 antifreeze.

Compare the APCO temp gauge to the meat thermometer pictures above. Sometimes I wonder how accurate some of th Temp gauges are.
 
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