1998 Mercruiser 5.0 Intake manifold gasket mismatch issues

[finsterx8]

I have been chasing a rough running problem since I bought this boat, I have confirmed that I have an intake manifold gasket internal leak. I pulled the manifold today and have found that the gaskets do not line up with the casting for the ports causing a leak from the crankcase. I took the intake to the marina and the gaskets referenced to the VIN are the same, it appears to be the correct manifold (same as the other engine in the boat) and there has been no material removed.

This engine was rebuilt before I bought the boat and I have been told by others at the marina that it has never run right. Seeing the gap at the edge of the gasket makes it pretty obvious that it could not have. I was also at a local Napa and the owner had several mercruiser 5.0 engine apart and none of the gaskets would match the runners on the intake. Perfect fit on the cylinder head, tried OE mercruiser gaskets, used Felpro, OE GM, and one other gasket and all would not match up.

I can try to post pictures in the morning, it is a carburated motor. The gaskets are the vortec style with black plastic and the thin silicone seal. The manifold has 8 bolts, 2 in each corner.

Just wondering if anyone has been down this road and has any suggestions.
Thanks for any help.

 

[tank1949]

I have been chasing a rough running problem since I bought this boat, I have confirmed that I have an intake manifold gasket internal leak. I pulled the manifold today and have found that the gaskets do not line up with the casting for the ports causing a leak from the crankcase. I took the intake to the marina and the gaskets referenced to the VIN are the same, it appears to be the correct manifold (same as the other engine in the boat) and there has been no material removed.

This engine was rebuilt before I bought the boat and I have been told by others at the marina that it has never run right. Seeing the gap at the edge of the gasket makes it pretty obvious that it could not have. I was also at a local Napa and the owner had several mercruiser 5.0 engine apart and none of the gaskets would match the runners on the intake. Perfect fit on the cylinder head, tried OE mercruiser gaskets, used Felpro, OE GM, and one other gasket and all would not match up.

I can try to post pictures in the morning, it is a carburated motor. The gaskets are the vortec style with black plastic and the thin silicone seal. The manifold has 8 bolts, 2 in each corner.

Just wondering if anyone has been down this road and has any suggestions.
Thanks for any help.

The heads may have been swapped out too and don't match block. Any chance you have gaskets flipped? If my old memory serves me correctly, manifold gaskets are "keyed" to only fit one way. If my memory is correct, GM changed 350 head designs around 1995. Are heads vortex, pre-1995 or 1995 and newer??

 

[jimmbo]

Pictures showing the Mismatch would help.

 

[finsterx8]

The gaskets have pins to locate on the head. We have tried them both sides and with pins up and the won't line up to the intake.

 

[jimmbo]

No pics show up

 

[finsterx8]

No pics show up

When I look at the feed there are 3 pictures. Do you just see 3 blank images or nothing at all?

 

[jimmbo]

They have appeared since my earlier post. Earlier, it said "View Attachment" at each spot

The first pic appears that they fit/line up well, the second ??. the 3rd ?? However, How do they match the ports on the Head? Every intake gasket I have used for the later SB Chevy, had the silicone bead on the divider between the ports.

 

[finsterx8]

 

[finsterx8]

They have appeared since my earlier post. Earlier, it said "View Attachment" at each spot

The first pic they fit/line up well, the second ??. the 3rd ?? However, How do they match the ports on the Head? Every intake gasket I have used for the later SB Chevy, had the silicone bead on the divider between the ports.

The first picture is in position where locating pins would have it, second picture I moved gasket on the intake to show where the casting is behind it, 3rd shows the gap when the gasket is in place. The pictures are of the gasket on the intake manifold which is upside down. They have full contact on the cylinder heads.
I have had 4 sets placed on there and none had dividers between cylinders, nothing I can find online that has dividers with silicone. I would agree they should have with a dual plane manifold.
These are the Sierra gaskets online and Mercruiser OE are the same with orange silicone. The Mercruiser's had the thinnest black plastic framing that I have ever seen, considering that's what usually fails and let's the silicone seal move and leak I would expect OE to be superior.
I have changed 100's of this style on GM V6 cars and vans and that's always what fails at the coolant passages.

 

[jimmbo]

When you said gaps, I was lead to believe there was a port being exposed to Atmosphere or Crankcase.
Different Intakes and Heads can have different sized Ports, so the Gaskets are generally made for the Biggest Ports used in Production Intakes, so with intakes/heads with smaller ports, there will be Metal inside the Gasketed Area. Some people will get out the grinders to widen the Ports, to match the Gasket, but in some cases that actually resulted in poorer Performance, as some Intakes, the port on the intake was deliberately offset for Reverberation reasons
It looks like Intake Gaskets for the Vortec SB omit the Silicone between the Ports.
To me "later" SB Chevys were the 2nd Gen Lt1s, which were pre-Vortec

 

[finsterx8]

When you said gaps, I was lead to believe there was a port being exposed to Atmosphere or Crankcase.
Different Intakes and Heads can have different sized Ports, so the Gaskets are generally made for the Biggest Ports used in Production Intakes, so with intakes/heads with smaller ports, there will be Metal inside the Gasketed Area. Some people will get out the grinders to widen the Ports, to match the Gasket, but in some cases that actually resulted in poorer Performance, as some Intakes, the port on the intake was deliberately offset for Reverberation reasons
It looks like Intake Gaskets for the Vortec SB omit the Silicone between the Ports.
To me "later" SB Chevys were the 2nd Gen Lt1s, which were pre-Vortec

Yes the gasket has no casting to seal against on the intake and the port is therefore opened to the crankcase. There has been no porting done, the second picture shows the actual casting shape and the gaskets sits too far over to the left of the edge of the intake manifold port. The blue sealing edge is no where near to sealing edge/casting of the intake port.

 

[jimmbo]

Okay I see that once you explained it.
I wonder if someone removed some metal off of the Intake at some point. You said the engine had been rebuilt before you bought it. If the rebuilder had removed a lot of Metal from the Heads, it would change how the Ports on the heads lined up with the ports on the Intake. Intakes sometimes had to be machined to fit properly again. However without another intake to compare it to, that is difficult to prove

Does it fit like that on both sides of the intake?

 

[finsterx8]

Yes both sides are the same and no signs of any metal being removed. It is as if the carbureted intake ports are smaller on the intake and the gasket needs to be a little narrower, I saw another intake at a machine shop that had much more casting in that area but it was a throttle body intake.
The marina I took the intake to told me to put a blob of silicone in those corners and stick it together?? That must be why they charge the big bucks.
(might be my only option though)

 

[tank1949]

you might want to take manifold and gasket to a hot rod shop and pray some old schooler can identify what you have.

 

[itsathepete]

I don't think the silicone molded into the gasket is needed for the intake port dividers but there does appear to be an issue with the manifold casting on the edge of the runner. It might be a casting defect. Is it OEM or aftermarket? Probably need to replace the intake. I have used inexpensive automotive aluminum manifolds for freshwater without a problem but if you're in salt definitely get marine aluminum or cast iron.

 

[tank1949]

There have been too many replies to the Op original post for my old mind to filter. However, back in the day, I had a 95 proline with twin MC 350 block motors. It was salt water cooled. It had the newer heads and block w/o a fuel pump cavity. Block was designed for Throttle-Body FI. Obviously, the heads were the newer style. I discovered that the intake manifold was an aluminum after-market or special $$$$ marine manifold to allow a carburetor onto the block. I am suspicious that former owner tried to make an older carbureted manifold fit, rather than paying the big bucks for the proper manifold. You might be able to find correct manifold at a speed shop.

 

[finsterx8]

The manifold is original to the boat and the same casting numbers as the starboard side motor. If I look up the serial number on mercruiser it is the same number as what is on the manifold.
If you look at the application list for the intake manifold it only shows several carburetor applications, for the intake gaskets listed by serial number there are a whole bunch of fuel injected applications but I do not see carburetor listed anywhere.
The catalogue comes up with 860100A1 for the manifold and
the number on mine is 860100-C, which is the same as the second motor.

 

[tank1949]

The manifold is original to the boat and the same casting numbers as the starboard side motor. If I look up the serial number on mercruiser it is the same number as what is on the manifold.
If you look at the application list for the intake manifold it only shows several carburetor applications, for the intake gaskets listed by serial number there are a whole bunch of fuel injected applications but I do not see carburetor listed anywhere.
The catalogue comes up with 860100A1 for the manifold and
the number on mine is 860100-C, which is the same as the second motor.

so then why doesn't it fit?

 

[tank1949]

The manifold is original to the boat and the same casting numbers as the starboard side motor. If I look up the serial number on mercruiser it is the same number as what is on the manifold.
If you look at the application list for the intake manifold it only shows several carburetor applications, for the intake gaskets listed by serial number there are a whole bunch of fuel injected applications but I do not see carburetor listed anywhere.
The catalogue comes up with 860100A1 for the manifold and
the number on mine is 860100-C, which is the same as the second motor.

I have never seen a serial # on intake manifold. Block and heads only...

 

[finsterx8]

The number on the intake manifolds is a casting number. Same number as the part number on Mercruiser site except it has a 'C' instead of A1.