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The GM Factory Service Manual specifically warns against this. When the coolant cools in the radiator after you shut off the engine, the coolant contracts and creates a vacuum. GM warns to only use the OEM cap, which is pictured in post #8.
I understand the correct cap lets pressure out of the radiator (like any other cap) but is also specifically fitted to the siphon, and also prevents any hazardous vacuum from forming.
I also understand the vacuum is a bigger problem on the F bodies because our radiators have the plastic sides. If a vacuum builds up to a low pressures, it can crack.
I don't think you have the right cap. Do you still have the old one? If so, can you put it on and see if it solves the issue? It should look like this:
The stock caps let pressure escape and also prevent a vacuum from building up when the coolant contracts and cools down. I'm wondering if your current cap is letting fluid expand in to the expansion tank but is not letting it siphon back when things cool down? Another symptom of this would be that your overflow tank would be overflowing or very full.
I don't know about that cap you purchased, but if it allows the siphon to work, you should be fine. Not all caps of that style do.
For anyone else that has this issue. I just fixed this problem and it was the radiator cap on mine. System held 20 lbs for 30 minutes on a pressure test cold. Drive the car and it doesn't overheat but, it keeps losing coolant. My old cap was also not hard to open or close, not a lot of pressure to release it. Just replace the cap with the factory cap and see if the problem goes away. It cost me $11 for a AC Delco cap shipped. Cheap test and fix.
11-14-2016, 06:58 AM
