Cooling system issues

[libarra82]

After recently rebuilding the top half of the engine, I started experiencing issues with the cooling system. I’ve had difficulty properly bleeding the system, and after pressure testing, I found that the radiator begins to leak at around 10 psi.

There were no cooling issues prior to the rebuild, although I did notice what appeared to be stop‑leak residue in the system during disassembly. Now that the system has been thoroughly cleaned, I suspect that a previously sealed leak has opened up. At this point, I’m trying to decide whether to have the radiator repaired or replace it entirely. If replacement is the better option, would OEM or aftermarket be recommended?

Additionally, I now have no heat from the HVAC system. Before the rebuild, the system only produced heat. The heater control valve was replaced with a 928 Motorsports steel heater control valve during the rebuild. This valve only opens partially compared to the OEM valve, which opens fully. Could this new valve be responsible for the lack of heat, or is it more likely related to the cooling system pressure issue?

[Tom]

After recently rebuilding the top half of the engine, I started experiencing issues with the cooling system. I’ve had difficulty properly bleeding the system, and after pressure testing, I found that the radiator begins to leak at around 10 psi.

There were no cooling issues prior to the rebuild, although I did notice what appeared to be stop‑leak residue in the system during disassembly. Now that the system has been thoroughly cleaned, I suspect that a previously sealed leak has opened up. At this point, I’m trying to decide whether to have the radiator repaired or replace it entirely. If replacement is the better option, would OEM or aftermarket be recommended?

Additionally, I now have no heat from the HVAC system. Before the rebuild, the system only produced heat. The heater control valve was replaced with a 928 Motorsports steel heater control valve during the rebuild. This valve only opens partially compared to the OEM valve, which opens fully. Could this new valve be responsible for the lack of heat, or is it more likely related to the cooling system pressure issue?

I replaced my 951 radiator with a factory new one when it was maybe 25 years old, and I was pretty surprised just how much better it cooled. Good radiator repair shops are getting harder to find these day (much harder) but if it's a simple leak you can probably get away with having it tanked and patched. Kind of depends on your goals for the car, but it's hard to do better than a factory new radiator. There are some aftermarket 'upgraded' aluminum radiators out there (Wizard, etc.) but between fitment issues, QC issues, and unknown engineering realities, I'd vote for OEM. For reference, I used to run my 3 liter turbo at 500hp, and the stock (new) radiator was more than enough.

As for the no heat, I'd start by pulling the vacuum hose off and confirming the valve opens all the way. With the hose removed, see if you have heat when you put the defroster on, which bypasses some of the other climate control box logic and is your best shot at heat. If you have a lot of trapped air in your system (which is likely with a leaking radiator) then often hot coolant just isn't circulating in the heater circuit -- in that case, sometime rev'ing the engine can get it moving. I'd probably vent the system over and over and then try the heat before much coolant can leak (or just wait till you sort the radiator). If it's not the valve and not due to trapped air, I'd run through the HVAC diagnostics in this link, as shown in these videos...

[libarra82]

Hi Tom,

The leak is coming from the metal to plastic seam on the driver’s side. Shortly after the thermostat opens, pressure builds and coolant begins spilling from the overflow line.

The engine is not overheating, I’ve been monitoring it with a temperature gun, and the coolant that spills out is not hot.

I’ll start shopping for a new radiator. At nearly 40 years old, it’s probably time.

[libarra82]

On the no heat. I’ll try unplugging the vacuum line, and double check the routing. No urgency on this with summer around the corner. Thanks

[libarra82]

Could it be that the radiator is also clogged, building back pressure? Idling, takes 20 -25 minutes to get to operating temperature, I really don’t think it’s overheating. Turning on the A\C speeds up the process. Something happens that causes pressure when the thermostat opens.

[jeyjey]

Are you sure the system is fully bled? Burping out the overflow line suggests air pockets, though it's not a definitive indicator.

[libarra82]

I may be bleeding it incorrectly. I’m using a pressure pump and might be over bleeding the system. I think I may have left the bleed screw open too long while the car was running last time and introduced air back into the system. I’ll give it another try later this week.

[danmartinic]

I am running the original radiator. Years ago, it started leaking where the side tanks attach.

I removed it & squeezed all the tabs that hold those tanks a little tighter.

It's been dry every since.

How many original radiators were tossed because those tabs got a bit loose....

[libarra82]

I thought about doing that, but I wasn’t sure how well it would work. I’ll try it this weekend.

[944m3]

I’ve bled my cooling system several times, both on my NA and Turbo. A few times I bled it by the book a few times I winged it because I was rushing. I’ve had success and failures both methods. It seems hit or miss. All that to say that maybe it still has air in it.

That being said, I installed a Wizard radiator and built in oil cooler and I love it. No fitment issues. Temperature stays consistent even in 90+ degree summer days in traffic.