1984 Mercruise 470, 3.7L Exhaust Manifold

I'm new to this site, so please bare with me. I have a 1984, 25' Wellcraft with the original twin Mercruise 470's. I am loosing coolant in the starboard motor, but only after I have been running for about an hour and a half or so and after the forums I have read over time, I see that I'm not the only one battling this problem. I decided to tear the motor down and start with a new head gasket, working my way back out. I had all the parts cleaned, checked, and pressure tested by a licensed Mercruise Mechanic. When it was all said and done, I was still loosing just as much coolant as before. We have narrowed it down to the exhaust manifold because it is cracked from the top of the baffle, down inside, but when it was checked everything was ok. We figured it didn't leak because it wasn't hot and under pressure. I am looking for 2 parts at this time. If I can find a good used CAST IRON exhaust manifold then I can use the original elbow (It's still in good condition). My mechanic has an ALUMINUM exhaust manifold, but the ALUMINUM elbow that fits it is on back order till the end of September. Can somebody let me know where I could find either of these parts? I have been looking around on the internet and I am finding the right parts but they are either the wrong metallurgy, they're asking an arm and a leg for them (can't imagine why), or of course they are on back order. I would HIGHLY APPRECIATE any help I can get.

Re: 1984 Mercruise 470, 3.7L Exhaust Manifold

rreed04 said:

My mechanic has an ALUMINUM exhaust manifold, but the ALUMINUM elbow that fits it is on back order till the end of September. Can somebody let me know where I could find either of these parts?

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The riser (elbow) used on the aluminum exhaust manifold is cast iron, just like the one used on the cast iron manifold. They arent interchangeable, but they are both cast iron. The difference is in the bolt pattern and length of bolts used to hold it to the manifold. Oh and the gasket. Use only the Mercruiser gasket for an aluminum manifold, dont use an aftermarket one. The Merc one is graphite and allows for the aluminum manifold to mate against the cast iron riser.

I see them on eBay from time to time, also my local Craigslist. Search around, you'll find one eventually. I wouldnt buy a new one, waaaay too much money.

Re: 1984 Mercruise 470, 3.7L Exhaust Manifold

New ones are around $1,000. I am not sure why, but that is just crazy. As guyaverage said, ebay is a good resource. I have seen them for around $300 there. How much coolant are you losing? Does the engine heat up as it happens?

Re: 1984 Mercruise 470, 3.7L Exhaust Manifold

Have you checked your front cam bearing weep hole to see if is leaking coolant? One of the 4 or 5 main 470 problems.

Larry

Re: 1984 Mercruise 470, 3.7L Exhaust Manifold

Hey thanks for the replies! I can run the boat for about an hour and a half to two hours before it needs to have coolant added. There's been a couple times I noticed the temp start to go up but that was before we realized we had a problem. I keep a close eye on it now. After that time span, you can normally see the bottom of the reservoir or close to it so about a 1/2 gallon to a gallon. We haven't found any coolant in the bilge, so I can't be loosing it externally. When we pulled the head the back 2 intake valves were a dark rusty color and the 2 front ones were clean so we know water was getting to the back ones. I even had a push rod that was bent and didn't know it. The boat runs GREAT at 3500 rpm's but as soon as you back it down to about 1200 to 1400 rpm's, the starboard motor starts running really rough and dies. We're thinking the exhaust is pushing the water out at 3500 but when we throttle back there's not enough pressure so the water runs back on top of the heads.