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Merc Exhaust Riser Question

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Old 07-24-2004, 08:13 PM
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Default Merc Exhaust Riser Question

If I was to remove both hose's from the riser/exhaust manifold and run engines would that destroy anything? I'm going through process of elimination with water in oil. I filled the engine with water plugging the discharge & bypass hose's and hold's pressure steady. Only getting water in the oil when it's running. Also while pressure was holding, I did a leak down on each cylinder observing any changes in water psi...none. Any suggestions?
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Old 07-24-2004, 08:37 PM
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Default Re: Merc Exhaust Riser Question

what kind of risers? If cast iron there ****,rip em out.if there stock just take out the 4 bolts on the riser and look down center for water spots.thats the cheap quik easy way the gaskets are the first to go.by the time you jerk around testing you could be back up and running.
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Old 07-24-2004, 09:11 PM
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Default Re: Merc Exhaust Riser Question

I wouldn't run the motor for more than a minute or two without the hoses hooked up to the risers. Actually, I wouldn't run it at all. They will get real hot real fast, then they will crack for sure..
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Old 07-25-2004, 06:51 AM
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Default Re: Merc Exhaust Riser Question

The risers are brand new. Changed the gaskets anyway and still stumped. Pressure tested the oil cooler & holds steady. While the risers were off, I made up a plate to seal the top of the exhaust manifold. Pressure tested that too...holds steady. Just last night before I gave up, I left the pressure tester to the block again pumped to 15 psi. Just checked on it, held steady. I am stumped!
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Old 07-25-2004, 07:55 AM
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Default Re: Merc Exhaust Riser Question

Stock cam? How much water? Any rust on the plugs?
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Old 07-25-2004, 12:58 PM
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Default Re: Merc Exhaust Riser Question

Marc, Yes for 365 mag. Just replaced that and thought the same thing. GM PN# 14096209 220/220-500-115. Looked at this mess again today. I did find #2 very wet and while cranking water was evident from that hole and no other. Took the the tiny view cam through the spark plug hole and noticed the rust on the exhaust valve. I pressure tested the exhaust manifold..AGAIN and held steady. However, when I blocked the mating surface I noticed a slight warpage from the water jacket into the exhaust port itself. I was using the stock Merc riser gaskets with the "locktite" sealer as recomended by Merc. This time I'll try the aftermarket cardboard gaskets with hi temp silicon.

I just got done freshening the engine and did not have this problem prior to rebuild. Hell, I kept the risers on the exhaust manifolds to prevent something like this happening. (if it ain't broke, dont fix it) Got to get a manifold gasket on Monday and at a stand still. Pumped the block again up to 25 psi while looking through every hole with the view cam...nothing and gauge holding steady.

Drained the oil and ran the drain hose out the transom drain into a glass jar while block is pressurized. No evidence of pure water, just a little milk for the coffee.

Last edited by cig1988; 07-25-2004 at 01:01 PM.
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Old 07-25-2004, 01:12 PM
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Default Re: Merc Exhaust Riser Question

Try just sealing up the manifolds off the motor and pressurizing each one seperatly. I have seen a few brand new manifolds that leaked right out of the box. Just a thought.
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Old 07-25-2004, 01:28 PM
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Default Re: Merc Exhaust Riser Question

Raylar, Did that. Made up a steel plate and drilled 4 holes in it. Made a thick gasket & bolted it on the manifold. Filled it with soapy water and pressurized to 25 psi. Did notice a minimal leakage on the threads on the lower fitting but no significant drop in psi.
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Old 07-25-2004, 02:34 PM
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Default Re: Merc Exhaust Riser Question

Unfortunately there is really no way to check the manifold/riser interface for leaks. Sounds like you are doing all the right things. The newest merc gasket is graphite, and no sealer of any kind is used.

Just for grins, make sure you have not rounded a lobe on the cam, or some other valve train issue. If there are no leaks, then it could be reversion, but as that is a very mild cam I would only believe that if there was something wrong with the valve train.

Back in the 70's I did not know about reversion, kicked my ass learning the hard way.
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Old 07-25-2004, 05:27 PM
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Default Re: Merc Exhaust Riser Question

Marc, I've been using the graphite gasket. The local Merc dealer said that Merc recomended the loctite sealer with using that gasket. Perhaps that may be a problem?
The cam only has about 1 hour top's on it it, the 1st half hour was at high idle to break the cam in. Inspected the cam when I removed the intake manifold to change 'em gaskets last weekend. Seem's I'm under the hatch than more in the water during our short lived season ):
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