Cooling system pressure test - do gaskets breath?
 

Howdy boys,

I have a 1989 Mercruiser 320 EFI sbc, built by Mercury Racing. Last summer I had a local machine shop rebuild it to the point of being a long block. They put the heads on. They're a reputable shop and do lots of boat motors for local marine dealers.

The engine sat on the stand for a year, now I'm finishing it. I did a pressure test on the cooling system. First to 20 psi then 15 psi. It slowly leaks. By slowly, I'd say it drops about a 1 psi every 3 to 5 minutes. After about 20 minutes it's down about 5 or 10 pounds (starting at 15).

So i sprayed all around the engine with a very very soapy water. What I see is foam forming around the head gaskets. But it's weird, it's like the air is coming through the gasket, not on the top or the bottom. And it evenly leaks most of the way around, on both heads. And, I see the exact same thing on the thermostat gasket housing. It's like the gaskets are "breathing" some air.

Is this normal? Or should this thing hold pressure for hours? I find it hard to believe this shop did a crappy job sealing the heads on. Would sitting for a year matter? Or do the gaskets behave differently with water than the do air? I suppose I could just pull it all apart again, but Id rather not chase something that's not actually a problem.

thanks!
Stan

It should NOT leak pressure down that QUICKLY and definitly not leak air and foam out around the head gaskets and thermostat gasket. First try re-torquing the heads to correct specs. for the engine from Merc. Racing and reseal the thermostat gasket, then retest at pressure again and see what happens. If it still leaks down after a few minutes or in an hour or so, you are going to have to replace the gaskets (use good stainless MLS type Cometic) after checking block deck and heads for straigtness and good finish. Sitting long periods is never good for engines but that is not totally your issue here in my opinion. Are the head gaskets composite type material or are the MLS type?

Best Regards,
Ray @ Raylar

Hi Ray,

Thanks for the quick reply. I re-torqued the heads, but it didn't help. So I just pulled them. The gasket on the thermostat has a rivet, so it didn't have any RTV on it. Maybe I'll put a little and avoid the rivet.

The heads looked fine, They have been resurfaced of course and everything looks in order. I don't know why those gaskets are seeping air. I don't plan on running this engine hard because I'm hooking it up to an alpha drive (just for this summer). So do you think the stock Mercruiser gasket would be okay to go with? I'd have to order the Cometic, which is fine, just wondering if I can go down to the dealer and get stock.

http://i93.photobucket.com/albums/l5...ler/photo3.jpg

http://i93.photobucket.com/albums/l5...ler/photo2.jpg

I cant remember, but the t-stat rivet I believe is for continuity for your temp gauge.

Yeah that's what it's for, that's why I didn't put RTV. But I can put a little around it I think and be okay.

In your application and use the standard GM head gaskets or good Fel-pro or Victor head gaskets should work fine. Make sure thread sealer is used on the head bolts and torque to standard GM engine specs and then retest. I think you'll be fine with that.
The rivet is for the ground continuity on the sending unit, ground continuity needs to be maintained for the sensor to work.

Bestr Regards,
Ray @ Raylar

Cool.. thanks Ray!