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My 350 SBC is having some interesting cooling issues, looking for some guidance if any of this rings a bell with anyone.
A couple weeks ago I noticed a puddle under the car after it was parked all night. Popping the hood, I could see the upper radiator hose was leaking due to residual high pressure in the system (I manipulated the hose and could tell it was bubbling out from pressure). Hoping it was a radiator cap issue, I replaced the cap, refilled the coolant, and let it run. Right off the bat I could see significant bubbling in the overflow tank while the engine was still cold. Fearing combustion leakage into the coolant, I shut it down and went to get a combustion leakage tester.
I couldn't perform the test at the radiator cap port, as the flow in the top of the radiator is too high and it kept sucking up coolant. I tested the system at the overflow tank, and the results were negative for combustion gases.
I've got a few questions:
1. Is it legitimate to do a combustion leakage test at the overflow tank instead of the radiator cap port? I'm inclined to think that CO2 and other combustion gases would be present in overflow tank as well if I had leakage.
2. The excessive flow in my upper radiator occurs on cold starts as well. Thinking I've got a stuck thermostat? Seems odd, it's fairly new. Any known issues installing therms?
3. As soon as the cooling fan comes on, the level of coolant in my radiator drops at least 4 inches, why? The thermostat is a 160 degree therm, and the fan is set to come on at 185, so I don't see them engaging at exactly the same time.
4. How much flow should be seen at the top of the radiator with the cap off? I'm worried my radiator may be blocked and not flowing well.
It sounds like your car might have a bunch of air trapped in the cooling system after you changed the T-stat?
my takes:
1. Is it legitimate to do a combustion leakage test at the overflow tank instead of the radiator cap port? I'm inclined to think that CO2 and other combustion gases would be present in overflow tank as well if I had leakage.
From my experiences..yes...if the gasses go trough the coolant it'll end up in the tan as well and show up with the tester.
2. The excessive flow in my upper radiator occurs on cold starts as well. Thinking I've got a stuck thermostat? Seems odd, it's fairly new. Any known issues installing therms?
Flow? As in bubbles? Some is normal if your coolant level is super low..its air comig out....."excessive" could be your observation though.... Fill it, fill the expansion tank, burp it, clsoe it up, drive it a a few heat / cooling cycyles and the air will make its way out. I like to use a coolant funnel...that works EXCELLENT...you just attach it and fill and the air burps out. if you got a bvlown head gasket youll see the coolant sloshing around pretty violently in there depending how bad it is.
3. As soon as the cooling fan comes on, the level of coolant in my radiator drops at least 4 inches, why? The thermostat is a 160 degree therm, and the fan is set to come on at 185, so I don't see them engaging at exactly the same time.
How do you kmow the level drops? At this point Id get too hot to have the radiator cap off?
4. How much flow should be seen at the top of the radiator with the cap off? I'm worried my radiator may be blocked and not flowing well.
It shouldn't be excessive. I know s0me have the heater hose connected near the cap, but even then the flow isn;t like a raging river.
Thank you for your responses. I was able to run it without the cap on at operating temp, and when the fan comes on that's when I observed the falling coolant level. I'm talking about flow as in a steady stream of coolant in the upper part of the radiator coming past the cap area. At any time while running, cold or warm, I see quite a bit of fluid coming from the top of the radiator down to the bottom. I'm thinking the radiator is blocked and the coolant isn't able to cycle through the cores so it comes over top of them then back down. My upper radiator hose is on the opposite side as the cap, so it's odd to get so much flow running by the cap. I'll post a picture in a few days when I'm back home.
Thank you for your responses. I was able to run it without the cap on at operating temp, and when the fan comes on that's when I observed the falling coolant level. I'm talking about flow as in a steady stream of coolant in the upper part of the radiator coming past the cap area. At any time while running, cold or warm, I see quite a bit of fluid coming from the top of the radiator down to the bottom. I'm thinking the radiator is blocked and the coolant isn't able to cycle through the cores so it comes over top of them then back down. My upper radiator hose is on the opposite side as the cap, so it's odd to get so much flow running by the cap. I'll post a picture in a few days when I'm back home.
Pics would help!
Honestly though, the side tanks go straight down, so they will definitely fill up first vs say the core as air is worked out.
And if you got that heater pipe under the cap on the side tank that makes sense to see all that running liquid.cause it goes from the motor, to the heater and empties back into the radiator. My guess is you are ok.
How old is the radiator? Theres a point..IMHO...that with a certain age, these kinds of things just become maintnenance items and need replacement.
At the very least, no harm and removing the radiator and reverse flushing it if its really old though....if you do that might as wel lflush out the entre bloclk too. / heater core too.
You cannot run the car with proper cooling with the radiator cap off. It needs to be pressurized.
You will not get "excessive pressure" with a normal functioning radiator cap. All "excessive pressure" is vented to the recovery tank. The cap is set at a certain number of pounds (I forget how much) 16-18 psi I think. Then the spring in the radiator cap will allow the pressure to push the stopper up and vent the pressure into the recovery tank which is at atmospheric (bleeds it off).
You may have had a stuck thermostat which caused overheating and the boiling. You should never run a 160 in a daily driver. It does nothing but cause more maintenance and repairs.
Make sure your coolant is the green stuff and its mixed 50-50 with distilled water.
The puddle under your car may be from the weep hole in the water pump when the pump seal fails.
Just some thoughts.
Last edited by Tootie Pang; 12-13-2021 at 05:37 PM.
I appreciate both of your feedback. Unfortunately I've been working pretty much every day and I don't get home until dark, so I don't have any pictures showing the flow I'm seeing by the radiator cap. I know I'm not supposed to run with the cap off, but how else do you check to make sure the thermostat is closed on cold start? Good point about the 160 therm, I'm going to get a 185 and that will solve my suspicion that I have a stuck thermostat either way. I was not overheating, but the high pressure seems to have gone away with a new radiator cap. I'm pretty close to 50-50 on my mixture. The puddle was definitely from the upper radiator hose, I would manipulate it and watch the coolant leak out due to the pressure.
I've only been running the car for about 1500 miles since I finished the build. The radiator is original, so it's entirely possible I need a new one. I extensively flushed the block, radiator, and hoses when I filled the system up originally. With no overheating problems, I'm going to say my radiator is probably OK. I think I'll change to a 185 thermostat was suggested, and chalk it up to an open thermostat and bad radiator cap causing a high pressure leak. Thank you both for taking the time to help me out, I was worried I had a bad head gasket but it seems I'm OK.
As someone who has done a stupid amount of Vortec-350 Builds (Late-880 Cylinder-Blocks)...
I would recommend always installing a Thermostat Element that has some small Bypass-Holes.
Most Vortec Cylinder-Blocks do not use a Bypass-Passage (in the Block) like SBCs traditionally do.
Early in my Engine Building Career, I started Tapping and Plugging both of the Bypass-Passage Holes in the Cylinder-Blocks.
It helps to provide much more consistent Coolant Temps among all the Cylinders.
I also usually run external Coolant-Lines from the Thermostat-Passage in the Front of the Intake-Manifold to the rear of the Manifold.
Then of course using a Thermostat Element with Bypass-Holes (prevents any trapped air/ air-pockets).
One of the benefits of having Coolant Temps among all Cylinders being more consistent...
is it make Tuning much easier!
Here are some Images of what I am referring to:
-Water-Pump mating surface (plug the Cylinder-Block, and the Water-Pump):
-Cylinder-Block Deck surface is also Tapped, Plugged, and Re-Surfaced.
(Hole in Deck shown below):
-SBC Thermostat Element with Bypass-Holes:
-Intake-Manifold with external Coolant-Lines (Front to Rear Passages):
Last edited by vorteciroc; 12-13-2021 at 11:47 PM.
Something that I do with all SBC Engines Gen-I, Gen-II, Gen-III, Gen-IV)...
is an external passage in the rear of the Cylinder-Heads.
Even when I do not use the external passages for the Intake-Manifold...
I do still make an external Coolant-Line from Head to Head.
Gen-II Engines had one of these Stock (a Metal Pipe) and it also connected to the Steam-Passage.
Here are some examples below, that use Hoses:
I personally make separate Steam-Lines, that do not connect to this external Line from Head to Head (on Gen-I, Gen-III, and Gen-IV Engines).
Gen-II Engine however do connect into the Steam-Line.
Edelbrock recommends a bypass when using vortec heads and their manifold and has pictures in their instructions. usually a heater core can simulate it but I added one to my engine really helped make the temps steady. I was having issues on the expressway where the T stat would not open much
Wow, you guys have clearly done a million jobs like this. I plan to upgrade my water pump soon, I just have a cast iron Napa one off the shelf. I'll look into plugging those bypass holes when I do.
Changed the thermostat today, my old 160 was definitely stuck open, changed it with a 180 and I've got way more stable temps and no flow thorough the radiator when cold. Seems like that did the trick! Thank you all for the help and advice, I was real worried I had a bad head gasket but it ended up just being a bad thermostat and radiator cap manifesting at the same time. Phew!
12-04-2021, 12:03 PM
