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Cooling system pressure help needed

Wolfshark

Wolfshark

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Hey guys, we got a cooling system problem over here that has us stumped. Hoping someone can help us out.
1987 K5, 350 TBI.

Having constant leak issues due to pressure, but it's not the head gasket. We've changed out & fixed just about everything, and it just keeps leaking(due to pressure).
What we've done so far:
-new radiator (nice copper 4 core)
-all new cooling hoses
-new thermostat
-new rad cap(x2)
-heater core bypassed atm(due to being blown)

Temps never gets high, it just keeps springing leaks. We fix one, it leaks somewhere else. Over and over. It's blown the heater core, a hole in the new radiator, and at damn near every hose. Just keeps finding the weakest leak and blows. Had the head gasket test and it passed fine(plus no head gasket symptoms), so it's not that.
Coolant circulation is fine so it's not the water pump.

Any ideas what could keep causing the pressure build up in the cooling system?
Possibly vacuum/vacuum hose related?

I don't think so, but maybe there's some blockage (maybe in the block somewhere) and should have the entire system flushed again (did this back when first installing radiator).

In short, what could keep causing excessive pressure other than head gasket?
We're running out of ideas, any help would be GREATLY appreciated.
 
For bypassing the heater core? Yessir.
Was very careful when doing that.

Problem definitely originated before that. Heater core went after doing the radiator (and T-stat) swap.
 
I must be missing something. The radiator cap pretty much controls the pressure in the cooling system unless there is a blockage that does not let the pressure get to the radiator. And since its blown the radiator, the pressure is getting there.
If the cap is working, then it should start venting at whatever pressure is marked on it. Does it vent at all?
I would try leaving the cap off for a little while to see what happens.
The only way the pressure in the system could exceed the cap setting, is if the volume of the pressure buildup is greater and faster than the cap can vent. Since it vents out the little overflow tube, you should see something coming out there. It is open, isn't it?
If it cannot get rid of the pressure fast enough, then leaving the cap off should be fairly spectacular. I would not stand too close while it warms up.
If you get a geyser, then I suspect a cracked block or head letting compression get into the cooling system.
 
Fordum said:
I must be missing something. The radiator cap pretty much controls the pressure in the cooling system unless there is a blockage that does not let the pressure get to the radiator. And since its blown the radiator, the pressure is getting there.
If the cap is working, then it should start venting at whatever pressure is marked on it. Does it vent at all?
I would try leaving the cap off for a little while to see what happens.
The only way the pressure in the system could exceed the cap setting, is if the volume of the pressure buildup is greater and faster than the cap can vent. Since it vents out the little overflow tube, you should see something coming out there. It is open, isn't it?
If it cannot get rid of the pressure fast enough, then leaving the cap off should be fairly spectacular. I would not stand too close while it warms up.
If you get a geyser, then I suspect a cracked block or head letting compression get into the cooling system.
Click to expand...
Exactly.
I would try to change the cap though instead of leaving off. The system is supposed to have a little pressure to keep the temp stabilized.
Excess pressure should come out of the cap.
 
I agree, normally I would say change the cap, but in the original post, he said it had been changed twice!
Of course, he did not say what pressure cap they put on it.
 
Fordum said:
I agree, normally I would say change the cap, but in the original post, he said it had been changed twice!
Of course, he did not say what pressure cap they put on it.
Click to expand...
I guess it's time for a pressure check of the system.
If it holds the required pressure something is wrong and building up pressure, and if it doesn't hold them the parts used are not good quality.
I have learned in the couple of decades that I have been dealing with new car parts is that putting new parts is never a guarantee that the parts are good
 
I can't imagine the vent line would be plugged either, I would think those clamps/hose would burst before the other items mentioned, but if pressure if building in the system, it HAS to come out that location unless the pressure isn't being allowed to vent for some reason.

IIRC you can turn the cap to "half notch" and it will not leak, but will still allow coolant to overflow before pressure really starts building.
 
dyeager535 said:
I can't imagine the vent line would be plugged either, I would think those clamps/hose would burst before the other items mentioned, but if pressure if building in the system, it HAS to come out that location unless the pressure isn't being allowed to vent for some reason.

IIRC you can turn the cap to "half notch" and it will not leak, but will still allow coolant to overflow before pressure really starts building.
Click to expand...
Didn't think of that.
 
You stated "plus no head gasket symptoms", building pressure IS a symptom. How did you test the head gasket?

Autozone and the like will rent a radiator pressure testing kit for $0, have you tried that? If so - Can you build pressure and it hold for a period of time? Will without pumping up the system, will the gauge build pressure on it's own while running?
 
nvrenuf said:
You stated "plus no head gasket symptoms", building pressure IS a symptom. How did you test the head gasket?

Autozone and the like will rent a radiator pressure testing kit for $0, have you tried that? If so - Can you build pressure and it hold for a period of time? Will without pumping up the system, will the gauge build pressure on it's own while running?
Click to expand...
That is what I suggesting but there is a kit to test for combustion gas in the cooling system I have it and it was easy to use.
 
Fordum said:
I agree, normally I would say change the cap, but in the original post, he said it had been changed twice!
Of course, he did not say what pressure cap they put on it.
Click to expand...
Yessir, cap was one of the first things I tended to.
Put on a 16lb cap, later tried a 13lb cap, then recently put back on a (new) 16lb cap(at recommendation of mechanic friend).
 
imiceman44 said:
That is what I suggesting but there is a kit to test for combustion gas in the cooling system I have it and it was easy to use.
Click to expand...
I had a shop test it with the little syringe/liquid device they put in radiator. They also pointed out it wasn't showing the usual other signs of a blown head gasket(like exhaust smelling like coolant, etc).
 
imiceman44 said:
Didn't think of that.
Click to expand...
dyeager535 said:
I can't imagine the vent line would be plugged either, I would think those clamps/hose would burst before the other items mentioned, but if pressure if building in the system, it HAS to come out that location unless the pressure isn't being allowed to vent for some reason.

IIRC you can turn the cap to "half notch" and it will not leak, but will still allow coolant to overflow before pressure really starts building.
Click to expand...
This is one I haven't fully looked into but I definitely will. Seems to be doing what it's supposed to, but I'll absolutely take it all apart and clean hose & reservoir out.
At this point I'll try everything.
 
Wolfshark said:
This is one I haven't fully looked into but I definitely will. Seems to be doing what it's supposed to, but I'll absolutely take it all apart and clean hose & reservoir out.
At this point I'll try everything.
Click to expand...
Did you put a high volume water pump?
 
imiceman44 said:
I guess it's time for a pressure check of the system.
If it holds the required pressure something is wrong and building up pressure, and if it doesn't hold them the parts used are not good quality.
I have learned in the couple of decades that I have been dealing with new car parts is that putting new parts is never a guarantee that the parts are good
Click to expand...
Yessir, I think pressure check of system is definitely needed. Probably should've done it much earlier. But I had no idea it would be this difficult to troubleshoot, and that the above fixes/usual culprits should've fixed it.
 
imiceman44 said:
Did you put a high volume water pump?
Click to expand...
I bought a new pump(plus new fan & fan clutch) but haven't installed them yet, as the old pump is moving water fine & not leaking. Wanted to fix problem first before putting new parts on there(as to prevent further variables).

imiceman44 said:
Also did you check that the new thermostat is fully opening?
Click to expand...
I haven't fooled with it since installing, It's been holding perfect temps per gauge and heat gun. I figured I'd have more temperature issues if it was acting up.
 
There is no reason a 13 or even 16PSI cap should have you blowing radiators or heater cores up. That's essentially standard pressure nowadays, the cap is what vents the pressure above its rating, period.

Where did the new radiator leak? If this is an abused system, its entirely possible all the hoses, the heater core, and the radiator needed to be replaced. With new radiators, if it is plastic tanked, those will leak out of the tank to core joint fairly commonly.
 
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