My recently purchased 87' 305 carburated Trans Am only overheats on the highway.
I've changed the thermostat, flushed the radiator, timed the engine, changed the
coolant temp. sensor, and I'm sure that the
water pump is in perfect working order. The car has a compression leak, however, and I'm
wondering if the overheating could somehow be
caused by worn piston rings.

If you have a compression leak, carefully remove radiator cap and look for bubbles in coolant.

If there is bubbles, good reason for overheat, also, excessive blow-by not really a reason for overheat.

Others here may have some good suggestions also

Everett 68/350/PG/11.90/115mph

Just a couple of thoughts here. Check your fan clutch and make sure it is not worn out and slipping. Do you have a coolant leak over a little time, maybe a cracked head/gasket and compression is going that way, look for white tipped spark plugs to determine which head. Typical water pumps have a weep hole under the bearing shaft and when they start to fail you can see fluid coming from there, run your hand under your pump and feel/look for fluid.

Yea, I actually have noticed bubbles in my coolant, like a foam. What causes?

I have a small coolant leak, like tiny. I don't think it's enough to cause overheating though.

If your coolant leak is coming from your water pump, more specifally from the weep hole on the bottom, I recommend you replace it. If you check your spark plug tips and you have a white one, that means water is mixing with the compression and that could be your bubbles from a cracked head or head gasket.

Wouldn't most of my coolant leak out if the water pump went out?

No... The coolant will leak past the bearing housing and out the weep hole..this is due to bearing wear, which means the pump impellers aren't pumping the amount of water it was desinged to push, thus causing an over heat problem in the long hauls. This is (only) if the leak is from the waterpump weep hole. Your problems sounds as though it only happens on the highway and around town it is fine or short distance it is fine as from what I read. Don't forget to check your plug tips for the white color which means water if the cylinder via the head or gasket.

Thanks for the help hillbilly

If you have foamy bubbles in your coolant, you've blown a head gasket, and combustion gases are going into the coolant, causing the highway overheating (assuming your airflow isn't blocked, like the front chin spoiler missing - it directs air into the radiator at highway speed, and without it, you'll overheat).

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JohnZ
'69 Z28 Fathom Green

I may be totally wrong here but it's something to check on. I saw a car and I believe it was a T.A. that would only run hot going down the highway and ran good and cool at idle and at slow speeds. Turned out that there was an air dam that was underneath the front of the car that was missing that forced air through the radiator while rolling down the highway. Call the dealer (or a couple of them to get a straight answer) and ask them if your car was equipped with one.
This is just a suggestion as I can't remember what make and model car it was but I'm thinking it was around that year Trans Am.

Is it at all possible that my broken overflow tank cap could cause the system to lose pressure and overheat?
By the way I didn't notice any bubbles on my oil dipstick, so I think that the bubbles that I see in my radiator are caused by the flush that I used about three months ago.
It also does slightly overheat on the street too, it can get up to about 200-220 on the street. But, it can only boilover on the highway.

If the front chin spoiler or the air dam is the part of the fender that curves up under the car to the top of the radiator, then I still have it, thanks for the input though.

If your radiator is getting plugged, a flush probably will not get it all out. A blocked radiator may cool OK off highway, but when it needs to flow more and cant, it can overheat on the highway. I've had this happen on customers cars. Might try having it rodded out.

Well, as it turns out I was missing the air dam on my Trans Am, ordered the part from the dealer and now it's runnin' at a nice and cool 190 on the highway. Thanks for the help everybody!