Engine Runs Hot At Highway Speeds..!

RonD said:

You need to do a Glove Test to take cylinder to cooling system leak off the table, i.e. blown head gasket.

It is an easy, simple and FREE test
Cold engine
Remove rad cap
If coolant is at the very top drain some out, so it is about an inch down, lower is fine.

Remove Overflow hose, and plug that outlet on rad, use gum, putty, vacuum cap, ect.........

Find the Coil Pack(spark plug wires) unplug it's 4 wire connector, you want a no start

Get a latex glove and rubber band, or a balloon or even a condom
Put latex glove on rad cap opening, seal it using the rubber band.

Cooling system is now sealed, and a cold cooling system has no pressure and can generate no pressure, water pump just spins, it isn't a "pump"

Now crank the engine and watch the glove
If it starts to bounce then you have a leak from a cylinder into the cooling system.
A cylinder generates 150+ psi of pressure when starter motor is turning the engine, it's compression.

If glove just lays there no bouncing then all is well, move on to Radiator or water pump circulation problem.

If glove does bounce, remove 1 spark plug at a time and crank engine, when glove stops bouncing the last spark plug removed was from the leaking cylinder.

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RonD said:

Your symptom points more towards a circulation issue, i.e. old water pump or clogged radiator, even a failing thermostat(not opening all the way)

But better to test for a cylinder leak first, because that is a black or white test, so no wasting of money, replacing cooling system parts when that isn't the problem.

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wildbill23c said:

Ron, I tried your glove test on my 88 B2 works every time. No special expensive tools needed. Ruled out a blown head gasket within minutes. I was kind of afraid it may have one, but your test proved otherwise.

Another cause for overheating could be air in the system. An air bubble in the system could cause all sorts of overheating trouble...usually it happens even when parked, but is far quicker to happen while trying to drive the vehicle.

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RonD said:

Yes, a leak can suck air into the system as an engine cools down.

The overflow system on vehicles is pretty simple.
The radiator cap has TWO valves in it, a larger one that has a pressure rating and allows coolant to flow out of radiator into overflow tank and a smaller one that lets coolant flow back into the radiator as engine cools down.

When you heat a liquid it expands in volume, this is how a Pressure Cooker worked.
So as engine heats up so does the coolant, as it expands the pressure in the radiator will get up to 14psi(rad cap rating), at 14.1psi the radiator cap will be pushed open and hot coolant will flow to the overflow tank, then it will close again until pressure goes up above 14psi again.

After engine is shut off the coolant cools down, and shrinks back to its cold volume.
When pressure in Rad goes below 0psi, the smaller valve in the rad cap opens and coolant is sucked back in from the overflow tank.

So very simple operation.

If there is a leak in the engine/rad system then two things happen, you lose coolant of course, but when engine is cooling down the leak point can now also become a suction point to pull air into the system, instead of coolant from the overflow tank.
The air is much easier to suck in than a liquid(coolant).

So you get air in.

Easy way to check for that and if overflow system is working as it should is to remove rad cap before starting engine after it has completely cooled down, i.e. overnight, coolant should be at the very very top of rad, if system is working as it should.

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