Overheating/Boiling over on at highway speeds

[markedbigk10]

Hey Guys, im new to the forum and im looking forward to helping and showing my progress along the way! but at the moment im having a major issue that I cant seem to nip in the butt. A little back story, I bought my '84 K10 swb with a 350 ci and a th350 from a cousin who had previously wrecked it and let it sit afterward. Approximately 4 years. So, I bought it and so far this is what Ive done. Water pump leaked through weep hole so I replaced it. new lower hose. new thermostat x2. new radiator because the old one was busted and leaking and replaced with a used aluminum with plastic tanks that was the correct part number off oreilly website. also ive replaced the fan clutch, and fan shroud is intact. now, the problem is that I can drive around town all day long and it runs fine with no overheating or boiling over issues. but as soon as I get out on the highway above 55mph and around ten miles, it starts to boil out the antifreeze through the overflow and into the overflow tank and out the tank into the engine compartment. To attempt to fix the issue ive tried the thermostat with two different ones, ive replaced the distributor and timing chain which were both wore out and took it to a mechanic to have the timing correctly set thinking that maybe the timing wasn't advancing at higher speeds and causing the cylinders to lean out and overheat. no change. ive also replaced head gaskets and top end gaskets. on that note I didn't take the heads to a machine shop to have checked due to funding issues at the time. head bolts were replaced and retightened in the correct order. still no change. also, I figured out that the water pump I installed the first time was the incorrect impeller rotation so I replaced it with the correct one as well. any thoughts would sure be appreciated as im at a loss now. thanks in advance!!

[blazer74]

Lower hose have a spring in it?    Could be collapsing at higher rpm's.

[FlatBlack77]

Lower hose have a spring in it?    Could be collapsing at higher rpm's.

my first thought.

last year i had a heck of a time finding the correct replacement lower radiator hose for my C10. i tried 4 different onces from NAPA all very similar just not close enough. finally found the right one but my point is that out of five hoses, none of them came with a spring. and the guy at the store had no idea what i was talking about. so i reused my old spring. it was quite a battle to get it in correctly i had to use some silicone spray and plyers to twist the spring in

[markedbigk10]

That's not a bad idea at all. The new hose doesn't have a spring in it and I can't remember right off if the old one did. I'll check on that. That'd be nice if that's all it was. I was leaning more towards having a bad head. But I'm trying to make sure I cover all my bases before I spend that kinda cash. I'll let y'all know if that does anything for it!

[rich weyand]

It's definitely not the fan.  At highway speeds, ram air through the radiator is more than the fan would pull anyway.

[Irish_Alley]

you say its boiling over but whats the temp gauge reading at?
have you replaced the rad cap?
the reason i ask about the temp is cause with a 15 psi cap it should increase the boiling point by 3°/per psi so a 50/50 mix should boil at about 267° (227° + the 40° from the cap)

[SkinnyG]

With the engine running and up to temperature, look through the rad filler neck.  If the coolant moving through has a zillion tiny bubbles, like champagne, or maybe even frothy, you have combustion gasses getting into the water jacket - head gaskets.

If combustion heat is getting into the coolant, ain't no cooling system in the world going to cool that - and it shows up on the highway, not in town driving or idling.

[markedbigk10]

I say "boiling over" but its really just pushing the antifreeze out through the overflow. not necessarily boiling. plus, I did replace the radiator cap, twice. once with the 16lb it had before and then with a 20lb. Even with the twenty it still pushes coolant out. the temp gauge never shows it running hot until its finished pushing all the coolant out. as far as the bubbling goes, I only notice bubbling if I have the cap off and rev it up some while looking down into the fill neck. I wasn't sure if that was normal due to the speed up of coolant flowing through the system or combustion gases in the system. but it was one of the main reasons why ive been leaning towards having a cracked head. I wasn't sure mainly because I figured if it had a cracked head, youd see pressure build up even at idle with time at full operating temp. and ive let it sit running for hours trying to mimic the symptoms with no luck. but according to SkinnyG that wouldnt be the case anyway. so maybe I oughta start looking for a set of rebuilt heads

[blazer74]

Rent a pressure tester at a chain auto-parts store and check your cooling system for leak down.

Any white smoke on startup after sitting.?

[gunrac]

I say "boiling over" but its really just pushing the antifreeze out through the overflow. not necessarily boiling. plus, I did replace the radiator cap, twice. once with the 16lb it had before and then with a 20lb. Even with the twenty it still pushes coolant out. the temp gauge never shows it running hot until its finished pushing all the coolant out. as far as the bubbling goes, I only notice bubbling if I have the cap off and rev it up some while looking down into the fill neck. I wasn't sure if that was normal due to the speed up of coolant flowing through the system or combustion gases in the system. but it was one of the main reasons why ive been leaning towards having a cracked head. I wasn't sure mainly because I figured if it had a cracked head, youd see pressure build up even at idle with time at full operating temp. and ive let it sit running for hours trying to mimic the symptoms with no luck. but according to SkinnyG that wouldnt be the case anyway. so maybe I oughta start looking for a set of rebuilt heads

[gunrac]

Like I was told not long ago, do your best to figure out what caused the problem, at least figure out where the problem lays. Yes, could be a cracked head, could just be the head gasket. You mentioned water pump issues, are you sure it is really correct? I would bet you do have an issue with at least one  cylinder. Figure out which one. I wouldn't just get another set of heads till you check out what you have. I would do leak down test. Go from there.

[markedbigk10]

I had a friend tell me that I can take off the thermostat housing and thermostat out and pull off the water pump belt and turn the truck on and if the heads have a crack you'll have bubbles in the coolant there at the thermostat hole. So I'm gonna do that tonight or this weekend and if it does then have the heads checked and repaired if possible.

[Irish_Alley]

you shouldnt see bubbles when the water pump on, you will see some but not much. your plugs should tell a story you can also test the antifreeze for exhaust gasses

[markedbigk10]

alright. well ill see what i can figure out now that were coming onto the weekend. i really appreciate everyone's input.

[frotosride]

+1 on the lower rad hose. the head gasket seems plausible as well but you would likely experience these symptoms at lower speeds especially at extended driving times. If you think of the constant higher RPM due to highway speeds, a collapsing hose is a highly likely scenario.