454 Mag MPI Milkshake With Low Compression on 1 Cylinder
 

I wanted to get your thoughts on my latest issue this season…. I went to replace the riser gaskets on my 3 year old fresh water GLM exhaust system and noticed the water in the manifold jacket was low, I tore them off and realized both sides were rusted out at the bottom of the manifold (the middle two cylinders on both sides) which I believe was leaking some water into the cylinders when it would sit. You could see that one of the exhaust ports in the head was pretty rusted up when I tore the exhaust off. I bought and installed a brand new EMI Thunder exhaust with the standard short risers. I had been fighting some strange idle issues prior to noticing the exhaust leak and did a compression check while doing the exhaust and the one cylinder was only about 80 psi (I’m assuming this was from the exhaust leak which then rusted the exhaust valve and sealing surface). My plan was to try to salvage the last month or so of this already shot season and just lightly run the boat and see if the rust wears off and the compression comes up (probably wishful thinking) and then tear the heads off this winter and have them redone if needed. The first trip I took it out I noticed a small amount of moisture under the oil cap but nothing major. After the last trip I took I noticed a good amount of moisture forming and there’s the telltale milkshake forming inside the valve cover that I can see through the oil fill. The oil on the dipstick still looks ok but I’m not going to risk anything so I’m calling it quits. My question is could the low psi exhaust valve issue be causing the reversion? I’d be blown away if the brand new EMI system was leaking since it has the dry joint riser setup, also I doubt the head gasket is bad but I’ll find that out when I tear the heads off. I just want to make sure I don’t have anything else that I’m overlooking…… is it possible that since the exhaust valve isn’t sealing 100% that it’s allowing some exhaust moisture to be sucked back in on the intake “suck” stroke? My engine is a bone stock 454 Mag MPI so it doesn’t have a large cam. Thanks

Do a leak down test and you can see if the valves are seating properly. is it possible you are sucking water back with a bad exh valve? absolutely...but it would depend on how bad the leak is and how close the water is getting introduced into the exh stream.
A little milkshake under the breather is normal is some cases due to condensation.....if the oil on the dipstick is still clean you should be ok as the moisture is burning off before it gets down into the pan.
How are the compression reading of the other cyl's?

I have a high degree of confidence your motor is sucking back through the improperly sealing exhaust valve. The exhaust valve stem is likely corroded which isn't allowing the valve to fully seat. I've seen it happen in one case where it hung up enough to have PTV contact and bend the exhaust valve stem.

Intake and exhaust valves are two way valves...not just one way.

I appreciate the info....all of the other cylinders had good compression when I tested them, it was just 1 low and that one had the ugliest rusted exhaust port in the head. I could visually see the valve somewhat through the port when I had the manifold off and it had some rust on it. I did put some oil in the spark plug hole and then rechecked the compression and it didn't change so I don't think it's the rings. I religiously check the oil fill cap after each run as preventative since my buddy trashed his engine due to rusted manifolds and I never had any signs of milkshake before (maybe just the slightest bit of moisture under the cap) but nothing like this. Now when I took the cap off at least a teaspoon of water ran out of it, also it was all yellow and the same with inside the heads that I could see through the breather hole.