A friend of mine has an LT4 with about 50k miles on it. We hit the dragstrip a couple weeks back and noticed his car had some coolant sprayed under the hood. It didn't seem to leak in normal driving, so we did some tests.

With the car stationary, we brought the engine revs slowly up. As the car got closer to 4k RPM, you could see coolant start to push out the radiator end tank on the passenger side. So we figured he'd need a new radiator if nothing else (this was a rebuilt one from the previous owner).

We've installed the new radiator, new hoses, and a new thermostat. But we wanted to make sure the old radiator didn't fail due to excessive pressure vs just being a poor rebuild.

So we hooked up a gauge to the system as such. We pulled off the air bleed line on the top passenger side of the radiator, plumbed in a "T" fitting, and then sent that off to a pressure gauged taped to the windshield. I've done this before on my LT5 to ensure the bypass valve was working properly:





We then took the car out on the road to warm it up (this is with all new rad, therm). The first thing we noticed is the static pressure never built up past about 8psi. So I suspected a faulty pressure cap. We still did the test, revving the car up to about 6,000 RPM, at which point the pressure rises to about 26psi. This is way too high for the stock setup.

We decided to replace the radiator cap, because clearly it was bad. I didn't know if it could bleed off the dynamic pressure change much or not, but what the hell, it wasn't holding proper pressure anyway.

So with a new cap, the system now holds ~15psi static pressure when warmed up. The problem is now the dynamic pressure increase from raising the revs climbs even higher. Here is a video of the car bringing the revs up:



Notice it climbs to about 30psi by only 4,500 RPM!! This is way too high for the radiator to endure! The bypass valve on the thermostat is supposed to handle this kind of thing, isn't it? Has anyone seen this kind of behavior before? And any thoughts on what might be the problem, or on whether or not this type of troubleshooting is correct?

One thing I thought is, wouldn't the air bleed side of the radiator be the low-pressure side when the pump is really whipping up? Since it's the radiator itself that presents the restriction and creates the pressure on one side. The passenger side is the inlet side to the water pump, it should be lower in pressure I'd think.

Anyway, to be clear, when I've done this test on my LT5, the static pressure would be around 17psi (It's a 17psi cap on that car), and would raise to about 21psi at which point the needle would rise and fall, rise and fall, as the bypass operated properly, keeping the pressure right around 21psi. This is what I expect for correct operation, and what I expected to see on the LT4.

 

sounds like a blown head gasket, my toyota was acting the same way i found the culprit both times to be a blown head gasket.

 

That would be the only reason I would think of.

 

Please keep us posted on what you find as the root cause. This is a good one to increase or confirm knowledge. Good luck!

 

You are probably looking at "pump" pressure and not Necessarily system pressure. The header tank would be the best place to test system pressure, but how would you tap into it??

On a second note, I just replaced a blown head gasket on my LT1, and the symptom was an recovery tank would actually overflow do to excess gas pressure build up in the Engine cooling system.

 

Do you mean the black plastic pressurized surge tank by the passenger side windshield? If so, this is where the line I tapped runs to. I would think on the passenger side of the radiator, it would not be pump pressure, since it pushes fluid in the driver's side. But I'm not 100% sure on what I should be seeing here.

Just to know, what made you aware you had a head gasket problem? Was it just the filled up recovery tank? So far we've not seen that happen, but the car has also not been driven very much since this problem with the rad leaking developed.

 

I'll also just point out, I don't know that this pressure is a problem. Nothing leaks. But it seems pretty high. And the owner doesn't want to blow up the new radiator. I suppose he could just drive it and see, but that's a $100 chance there.

I'd love to find another LT1/4 in the area and see what kind of pressure readings that has.

 

the line you tied into is the Cylinder head(s), (steam pipe) return, this line has constant flow/pressure.

The pressurized header tank, on the Passenger side, just forward of the windshield is what I had in mind. If you had a way to measure the fluid pressure at the top of the tank (internally) this would be a more accurate measurement of internal pressure.

 

Interesting. I will look again next time I'm at the car. I was fairly sure I traced the line back to the surge tank, but maybe not. If not, this could explain the pressure. Perhaps it is not totally unusual...

Thanks!

 

I read this thread this morning twice, left work and while I was on the freeway I was thinking about it. I wonder why is it so high. I assume the Vette you tested has a mechanical pump, I wonder what an electrical pump would read.

 

Yes, mechanical pump. Electric should have fairly constant pressure, since it doesn't speed up with the engine.

We did a CO test and it was negative. No coolant pushes into the overflow either. So it really doesn't seem to be a headgasket problem. It may not be any kind of problem. So he's just gonna drive it.

 

I can't believe that 30+ PSI anywhere in the cooling system is good.....

Do you get this only with a cold engine when the the thermostat is closed or when the engine is warm and the thermostat is open or both hot & cold??

 

Hot engine, thermostat open. The pressure doesn't reach 15psi until it is warmed up, and testing a cold engine didn't seem to offer much benefit as the initial problem of the radiator leaking occurred when he drove the car hard (engine warm).

I'd love to see someone else duplicate the setup and see what their pressures are.

So far the car is driving just fine, nothing irregular and no coolant loss.