Radiator water level on cold motor.

[Bill M.]

I have a 1966 Coupe with a stock 427-L72 425hp in it. Non A/C. No alterations on the engine.

Runs great. No problems at all on any engine component.

Every time I take it out I check the flooring under the car and there is not one drip/drop of anything.

However, when I checked my radiator level (cold motor) I was able to see about 5-7 inches down into it.

5-7 inches of bare fins.. My thinking was it was low so I filled it up to a higher level to cover the exposed fins.

Fast forward a while later and again I see 5-7 inches of radiator fins not submerged.

I have checked the manifold, the radiator and when I park the car or get gas I do not see one drip of fluid coming out.

I spoke with an owner of the same matching car as mine and he said the same thing. His hygrometer was unable to reach into the water as his level was down the same as mine.

I have checked this Forum for answers but none posted.

Is it possible that this is where the level should be do to expansion and cooling.

If someone has an answer or a stock 66 427 L72/425hp can you let me know where your water level is.

[Mike T.]

Re: Radiator water level on cold motor.

Bill - A couple years ago I bought my NOM 66 L72 Roadster and early on, I had a bit of coolant finding it's way out of the overflow hose. Turns out the PO had slightly overfilled it. He owned it for about 30 years and in that time, he replaced the radiator and that new radiator did not have the stock/original side tanks on it. I found out the original tanks have what I'll call a 'fill line' a few inches (2.5 to 3") below the filler neck. I lowered the coolant to that approximate level and it's been good ever since. In your case, and maybe your friends L72, I'm wondering if you are losing some coolant out of the water pump, via the lower weep hole. It's typically hidden because of the water pump pulley but if you can use a mirror (not easy to access of course), you might see the telltale weep hole stains...'if' that is where you are losing coolant. Besides the coolant level issue with my L72, the water pump on there was a bit crusty and I found it too had been leaking from that weep hole. With a hot engine, a slight leak might evaporate more as it runs down the front of a hot engine and you might not see drops on the garage floor.IMG_8632.jpg

[Mark E.]

Re: Radiator water level on cold motor.

Two possibilities-

1) The system is leak free and holds pressure. In this case, what you're seeing is coolant passing through the overflow hose during each heat cycle until there's enough space in the system to allow normal fluid expansion. New owners of vintage cars sometimes mistake this normal "puking" as a problem. You say there's "no drip/drop of anything" which seemingly rules this out, but you may want to double check.

2) The system is leaking and not holding pressure. As mentioned, a leak may be small enough to not be easily spotted, or it may be an internal leak (e.g head gasket, intake to head passage, a crack).

Ways to diagnose-

- Does the coolant level continue to go down or does it stabilize? The latter suggests #1.

- Perform a pressure test. Holding pressure suggests #1. If it doesn't hold pressure, the leak is often easier to see while pressure is applied because coolant is being pushed out. A radiator leak tester is an inexpensive diagnostic tool for doing this and should be in every car guy's toolbox (it's helped me avoid going down rabbit holes numerous times). A roadside check method is to see how long the radiator hose remains firm. If hot, it should feel under pressure for at least 20 minutes.

- If it doesn't hold pressure yet there are no visible leaks while performing the pressure test, check the crankcase level. If it mysteriously rises over time, there may be an internal leak.

Let us know what you find.

[Mike T.]

Re: Radiator water level on cold motor.

Bill - As Mark suggested, there might be an internal leak. Some times when combustion gases escape into the coolant passages, you'll see a spike in engine temperature but there could be a slight leak of coolant internally that might not show up as a temp spike. I use Block Tester to verify the presence of combustion gases. Get the engine warmed up and you typically have to lower the coolant level but in your case, you're already there. The reason for that is they don't want coolant splash making contact to the tester, only vapors. With the engine running and coolant level lowered, you insert the rubber cone of the fluid tester and then use the squeeze bulb up top to draw up the vapors through the blue test liquid.
If after a minute or so, you see the blue fluid turning Yellow-ish, that's indicates the presence of combustion gases, meaning there might be a compromise in a head gasket or something similar.Combustion Gas Tester.jpg

[Mark M.]

Re: Radiator water level on cold motor.

Bill , The 66-68 427 harrison radiators have a fill line on the right side rear. the shroud covers it a bit. I learned on a 67 L68 I restored that if topped off this car would always spit out the overflow until it got around that fill line, which is around 5" down. If the car was parked with the nose down, it sometimes drizzled a little more out. They seem to seek just enough for expansion room since the top of the radiator does this. If it goes lower than the fill line I would be concerned.