Moondocker

So I upgraded the intake and carb on my Century inboard to a Edelbrock aluminum and 4 bbl. I ran the boat and discovered that the engine oil was milky and the level was rising. Assuming that I did not seal the intake correctly, I removed it and re-sealed it. It did not solve the problem. I started looking further and started pulling spark plugs and found that I had 1 that was ultra clean. A compression check revealed 145 lbs on the clean plug cylinder and 140 on the one right next to it. I am thinking head gasket or exhaust manifold??? Thoughts???

billinstuart

The compression values are virtually the same. There are different intake gasket patterns for 351's, I believe. The problem started with the manifold swap, that is the issue.

Moondocker

That was my original thought but I resealed and the problem arose again. What could be the problem with the intake?

Flybull

Are there even any oil valleys at all affected by an air intake? I had a 351W on my 69 Cougar back in the day, but don't remember what was all involved on the intake.

Could have been making oil before the swap but just not discovered until after?

One cylinder plug was "super clean"... is it even firing? If not, lots of unburned fuel will get past the rings and make oil... of course 7 of 8 cylinders firing would be pretty obvious...

jhackl

As billinstuart said, there are different intake gaskets for the 351W. Check to make sure you don't have the wrong one. You must use the correct gaskets on the early heads or you will get four major water leaks! The early 351W intakes had 4 extra bolts that went through the 4 water jackets. You must use the correct gasket on early 351W heads that have the "L" shaped water ports.

Edit: Also, if you put a later model 12 bolt intake on earlier model 16 bolt heads you need to make sure that you have a gasket that blocks water passage to the 4 empty bolt holes in the heads.

Wrong gasket:


Correct gasket:

dsharp

billinstuart

Thanks. It's been a long time since I did this.

jhackl

Me too! I didn't think of it till your post reminded me.

1breakinit

this!!.

also if the intake was a used part someone may have machined it to fit and utilized OS thickness gaskets.

Moondocker

The replacement (aluminum) intake is identical (12 bolt) to the stock one and correct gaskets were used both times. I spoke with the Edelbrock tech and explained the situation including the fact of the "washed" spark plug. He said that was the "tell tale" sign. He said it is highly unlikely that water would find its way from the galley down to an intake port and enter a cylinder. I honestly believe it is not the intake but either a cracked head or just the head gasket. As much as I don't want to, it looks like I have to pull the head off. I should also mention that the motor ran fine and the oil level went up about 1/2 quart in 2 hours running at moderate (2500) rpms.

1breakinit

start measuring !.

very unlikely that a head gasket would blow by changing an intake manifold..

as has been mentioned on another thread pertaining to "water intrusion 351 Windsor" there is a metal plate where the water pump faces the engine block that is known to be a cause of water in oil , even so "changing a manifold will not start this leaking.

Moondocker

I just wanted to get back here and let everyone know what the cause of the water in the crank case was. I pulled the exhaust manifolds and heads. Pressure test showed all good but the heads were warped and machinist took about .008 off each. He speculated that the motor was overheated at some point. Put everything back together with new top end gaskets and 2 oil changes. Approximately 6 hours on it and no evidence what so ever of water. I am confident that this problem is solved.
Thanks to all for your input.
Mark

billinstuart

So the problem did NOT start with the manifold swap?

Moondocker

While the manifold swap was the obvious place to look, apparently it was not the issue. Coincidental? Yes. I still don't get it but it's an old engine that when I acquired it, it was right on the edge of this failure. That's all I can come up with.