Overheating on the Freeway - What Are Your Possible Causes?

My '65 Skylark keeps its cool (around 200 degrees) when either sitting at idle or moving along the boulevards at 45mph. However, within a couple of miles after getting on the freeway (moving at 65mph), it starts to overheat. The coolant mixture is where it should be and the system (including radiator cap) holds full pressure. Also, after each attempted run on the freeway, I lose barely enough coolant to notice a drop in the level (it still looks almost full) and there's no sign of it in my oil.

I'm getting ready to either flush or replace the radiator and/or core depending on how much debris might have choked them up. (How much do the higher rpms translate to a higher coolant flow rate?)
I've checked the distributor advance's vacuum port and found a tiny leak (is that normal?) which I'm going to investigate futher.

Back when I started driving this car 27 years ago, I need to drive it faster to help cool it down, not heat it up. (That was my excuse and I stuck to it. :Brow: )

Who's had the same experiences with overheating at higher speeds and what have you found to be the cause(s)?

 

I had that long ago and a new rad was the only fix, my heater core was the same, only a change got good heat but was free and clear, almost as if something has coated the inside that you can't see, but inhibited proper heat transfer yet water flowed free.

maybe a pro harsh chem cleaning, some kind of acid that will strip a micro thin layer inside.

[ I even tried that CLR cleaning stuff with no luck ]

 

you didn't mention if you have air conditioning. makes difference. could be fan clutch. do you have the factory fan shroud in place? when was the last time you changed coolant? if its a non ac car your more than likely going to need a radiator.
get a lower hose also. as it could be weak. and sucking closed with any rpm.

 

I only had a problem sitting still or cruising in traffic. So i opted for a new thermostat, new 4 core radiator from the old 3 core, new 7 blade fan with new clutch from a fixed 4 blade and new radiator cap. I also removed the A/C compressor, lines and condensor which helped a lot too. This yr i am installing a 16 electric fan to help with stand still traffic. It still gets up to 230 when sitting long periods in traffic and if i raise the rpm it will slowly drop the temp. so i know it is lack of air movement. I also lack a radiator shroud, so i am thinking of fabricating one to help channel more air through the radiator.

 

If over time nothing significant has changed other than the overheating complaint, it sounds like radiator inefficiency to me. While it's a good idea to check the fan clutch (if you're running one), the fan isn't a factor at highway speeds.

Devon

 

I chased a problem like yours, and it turned out to be weak mechanical advance springs in the distributor.

Weak springs will allow too much mechanical advance at highway speeds.

 

What is your definition of "overheat"?
You said it maintains 200 deg. when driving around, but starts to overheat when on the highway, how hot is the temp gauge indicating, 220, 240, 260 deg.?
What temp. thermostat?
Radiator? New, old?

 

High speed overheating? >confirm your vacuum advance diaphragm works. Larry is the expert on timing among so much else. In California to be smog control compliant we are required to disconnect vacuum advance; this decreases NOx emissions. The retrofit kit comes with a sticker for the windshield pointed at the driver. "Warning: car is prone to overheat at freeway speed."

At high speed your mechanical fan is completely redundant (i.e., not even necessary).

 

Our '65 Skylark (300 2bbl) had the same issue. I had the radiator rebuilt. Took care of the problem.

 

A lot of good points and ideas here. Just to help narrow things down, here are my answers to some of your questions:

My Skylark's been stock for the last 50 years. No changes to the engine short of maintenance stuff and replacing what broke with an equivalent part.
It's never had air-conditioning.
No fan clutch.
Factory fan shroud is still there.
Coolant was changed around January/February along with all hoses, thermostat, water-pump, and radiator cap. The radiator and core appeared clean enough when I last drained it, but were NOT treated to a thorough flush or cleaning.
What I'm calling "overheat" on this car is 225 degrees. As the needle approaches that number, I can feel the engine start to struggle a tiny bit and the radiator cap vents a little (which is easy to see since I'm driving without a hood and getting the mist on the windshield.) This one brings up a chicken-and-egg situation... Is the engine starting to struggle because of the heat, or is the heat coming about because of whatever is causing the engine to struggle?

Oh yeah... AND the reason I changed the coolant (and related parts) in Jan/Feb was because I was bringing this car back to life after it was out of commission for 15 years. :shock: (Let the flogging begin.) Despite the gallons of water I sent through the cooling system during the flush, the odds of rust/corrision/oxidation/crystals still being trapped in the radiator and core are still high.

Any other experiences to be shared?

 

I'd recommend checking your vacuum advance to see if it works, first. This will give similar symptoms. Otherwise, all things being equal (you haven't really made any modifications, and springs and such don't just go bad, as a rule), have the radiator rodded out. I'd suggest a real flush of the cooling system, as well, with an acidic or caustic cooling system cleaner before taking the radiator for rodding. Even if some of the tubes are blocked, it is better to get the rest of the crud out before putting the cleaned/new radiator in and immediately clogging the tubes.

 

Well the rad cap sounds like its pooped out, and the rad IS probably gooped up.
When the engine is warm, and thermostat is open, feel the front of the radiator, slide your hand over the entire surface and feel for spots that are cooler than others.

 

My experience after a fan clutch and a water pump was the radiator. Shop confirmed a clog internally through several of the rows and it was flowing, but didn't pass the flow test.
New radiator, 3 row but man are they BIG.
no more problems.

 

Use a laser thermometer to check the radiator. You should have a temp difference between the inlet/outlet but you can easily find cold spots quickly with it and safely.

 

Thanks everyone.

I've hit the simpler solutions so far, but I'm definitely going to pull the radiator and core soon for a thorough cleaning/replacement.

Even after everything is cleaned out of there, there's going to eventually be more debris blowing around in the system, so it'd be nice if I could install something to filter this out before it builds up in my radiator/core again. Is there a coolant-filter kit I can install someday? I checked around, but did not find any products intended for a '65 Skylark. Was anything ever made for that or a comparable car?

 

Chicken vs. egg.....

Both.
Leaking or insufficient vacuum advance absolutely makes the engine run warmer.
Whether or not it's taxing the cooling system over it's capacity is is the other part of the "both".

 

With the car sitting for 15 years, good chance the water jacket in the block has loose scale rust built up. You need to get out as much as possible.

In the car this is a PITA.

I would suggest remove all the hoses you can, therm and housing, water pump and any simple to get to core plugs (freeze plugs) get a garden hose or pressure washer wand in there to help break up and flush out all the crud.

Back flush the heater core too.

You will see mud water for a bit of time.