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I went to start my 71 today. It starts effortlessly every time (as long as the distributor is hooked up).
It ran for about 5 minutes and started leaking at the thermostat bypass. It currently doesn't have a thermostat installed. I quickly topped off the radiator after starting the car and installed a new radiator cap. The upper radiator hose is rock hard.
I've done 2 compression tests since I put the heads on the engine (both with the same gauge). It has good compression. The water pump is a known "good" pump off of my other 455. The radiator is a new parts store cheapie. Both of the radiator hoses are new.
I suppose the hose clamps for the thermostat bypass could have been loose and caused the leak. I could always go out and start the engine and let it run unit it blows a radiator hose or cap.
A cracked block would be IDed with a cylinder leak down test, right? I would get bubbles/leakage in my radiator or crankcase if my block is cracked? Can you get poor leak down test results with good compression?
Sort of at my wits end. Might be time to swap in my spare 455 since Oldsguy doesn't seem to want it anyways.
Is there a way to pressurize the system without running the engine and getting it hot? maybe with compressed air? Then you could listen as well as look for a leak. Just speculating.
As I mentioned to Oldsguy on the phone, it's been 3 or 4 hours since I ran the 71 and the upper radiator hose is still hard. It's not even warm anymore. I think whatever problem the 455 in my 71 had that caused the head gasket to blow and the parts store cheapie radiator to exploded is still present.
I cleaned the deck and chased the head bolt holes when I had the heads machined. There were no cracks present to the naked eye. The machinist magnafluxed the heads and cut new valve seats and faces.
It takes a while for the system to lose pressure. What you're seeing are good things. Hoses tend to leak when the temps get cooler out and a simple hose clamp tightening usually fixes it.
Last edited by oldcutlass; Sep 21, 2023 at 09:19 AM.
Jesse - Before you get too far into it...buy one of these kits and have at it. Fast, inexpensive & they work. Suggestion - after you've ran the car for 12-15 minutes, use the flashlight in a very dark garage as the dye shows up much better. Me, I'd perform this test first. $16.00
Get a small bottle of Radiator UV dye ($8.99). Radiator UV dye is miscible in coolant i.e. not same as oil UV dye. Take the guess work out, find the leak.
If there isn't coolant in the oil and there isn't white smoke out of the tailpipe the block and heads are likely not cracked. A leak down test may not show a crack if the block is cool.
A test for exhaust gases in the coolant would provide the answer.
This is perplexing to me. If you have combustion pressures going into your cooling system, then you would think the pressure would bleed off soon after the engine is shut off... Right?? If there is a bad head gasket/ cracked head/ cracked block wouldn't the crack/leak allow the compression to go into coolant when running and then go right back thru the crack/leak and pressure go away when shut off as it dissipates from where it came??? But you say the pressure is still there after engine has been shut off and cooled down after 3 or 4 hours??? I don't get it...?? You say the heads were magnafluxed?? So they don't have any cracks ( if you can trust the magnafluxer) . So it has to be gaskets or block. Can you get the pressure tester and put on radiator, start engine and watch gauge. If it goes up immediately after starting, then yes combustion is going into coolant. So then shut off engine and see if pressure starts going down. It should. Good luck .
A test for exhaust gases in the coolant would provide the answer.
I agree. The local auto parts store will rent a combustion gas test kit, all you buy is the test fluid for under $10. I did this on my Jeep and it was very simple.
WTF, I took the radiator cap off this morning (15+ hours later) and there was still pressure in the radiator. Enough to spit coolant all over my hand and onto the ground. Should have taken video of it. 🤯
I have a combustion gas tester I bought when the head gaskets "BLEW" on the 5.4l Triton V8 I owned. MAW give it a try.
WTF, I took the radiator cap off this morning (15+ hours later) and there was still pressure in the radiator. Enough to spit coolant all over my hand and onto the ground. Should have taken video of it. 🤯
I have a combustion gas tester I bought when the head gaskets "BLEW" on the 5.4l Triton V8 I owned. MAW give it a try.
Wow, Jesse that is really strange, like a one way passage that lets compression in cooling system then holds it- Not impossible I guess but very strange. What else could it be? I think you should test for the combustion gases also.
WTF, I took the radiator cap off this morning (15+ hours later) and there was still pressure in the radiator. Enough to spit coolant all over my hand and onto the ground. Should have taken video of it. 🤯
Makes me think head gasket. When the air/fuel charge ignites, it generates a lot of pressure (150-200 psi) that could push past the head gasket, then the gasket seals well enough so not all of the pressure flows back into the cylinder. Kinda like a one way check valve.
Were the heads tested for cracks? My 5.9 in my Dakota blew head gaskets. I replaced them, about 100 miles later, rock hard upper hose and violently blowing coolant out of the rad cap. Replaced the heads, Dodge Magnum heads have known issues. No more violent coolant escape!
I agree that the symptoms seem like a blown head gasket again. I have a combustion gas chemical tester I'm going to try. I suppose if that indicates combustion gasses in the coolant I'll conduct a cylinder leak down test or buy a new compression gauge. The one I have now I bought when I owned my 64 98 before Momma and I got married.
You can have good compression checks cuz your compression is around 100 psi or so. But then you add gasoline mixture and spark it and it explodes- I'm sure way more than 150 psi. I think Ken is prolly right. Keep at it Jesse, you'll get it.
WTF, I took the radiator cap off this morning (15+ hours later) and there was still pressure in the radiator. Enough to spit coolant all over my hand and onto the ground. Should have taken video of it. 🤯...
This is confusing, how/what can keep pressure in the coolant jacket/hoses/radiator after that long of a period. It seems it would bleed out someplace.
Is there a way to pressurize the system without running the engine and getting it hot? maybe with compressed air? Then you could listen as well as look for a leak. Just speculating.
1. you can rent a pressure tester from Autozone... you buy it 1st, then return I think within a few days and you get your money back. I
The engine idled for 20 minutes today and the upper radiator hose only ever reached 140* F. I set the electric fan to HI. Apparently, the LO relay for my electric fan is faulty ( cheap imported electronics).
This weekend I plan on starting & tuning the engine and taking the 71 for a test drive.
