Intermittent overheat, 1990 350 magnum

500dollar744ti

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Jul 23, 2012
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At the start of the season I like to take my boat out for a test trip before I go on any journeys or invite people out. Yesterday I put in at the local ramp and went about 10 miles down river at moderate speed. I began to smell a 'popcorn' burning smell for lack of better words, I slowed to idle and shut down the engine.

I found the port side rubber exhaust couplers smoking but only that side. I let it cool until I could touch both risers and then restarted. The port side immediately heated up like a stove burner while the starboard side was just warm. I knew that I wasn't going to be able to limp back.

In preparation for the season, I had replaced my drive bellows, water pump impeller, shift cable, and one of the exhaust couplers on the port side. I idled the engine with water hooked up in my driveway and made sure I had adequate exhaust water flow and there was no overheating. I thought for sure it was something I did with the port exhaust tube so I wanted to take it back apart.

Thanks to what I've learned on here, I know that water could come into the boat if I remove the exhaust elbow so I took my oar and rowed my 23ft cuddy to the shoreline and beached it.

I took the exhaust elbow out and tried to clear any rust/debris from the riser and I removed the flapper in case it was getting stuck. I put it back together and pushed back into the water deep enough that I could run the engine. I ran the engine at idle for a while and both risers stayed at normal temps.

I went back out and headed back to the ramp. Everything was good at cruising speed for about 15 mins then the temp started to rise to 160 so I came back to idle and shut it down. I waited a couple minutes and started it back up, the temp came back down and I continued on. Five minutes passed and it started to rise again so I shut it down and did the same thing. I made it back to the ramp and loaded up the boat and brought it home.

My guess is that the manifolds and risers are clogged with rust that dislodged over the winter. I think the water is pushing rust into the ports until it clogs, then when I shut it down, the rust subsides and it can flow again for a while.

My repair will be to replace both manifolds and risers, does this sound like the appropriate course of action? Also, I've heard the exhaust flappers cause more harm than good, is that true? Should I leave the flappers out on both sides or move to replace them?

Thanks in advance for your help.
 

500dollar744ti

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Re: Intermittent overheat, 1990 350 magnum

Also, is it worth it to buy OEM quicksilver parts for the manifolds and risers or can I substitute the GLM or Sierra stuff?
 

dubs283

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Re: Intermittent overheat, 1990 350 magnum

did you replace the water pump base assy and gasket when you replaced the water pump??

glm, osco, sierra are all good replacement manifold/risers
 

Bondo

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Re: Intermittent overheat, 1990 350 magnum

My repair will be to replace both manifolds and risers, does this sound like the appropriate course of action?

Also, I've heard the exhaust flappers cause more harm than good, is that true? Should I leave the flappers out on both sides or move to replace them?

Also, is it worth it to buy OEM quicksilver parts for the manifolds and risers or can I substitute the GLM or Sierra stuff?

Ayuh,.... It sounds like yer headed the Right way,... Include the shutters, they're Necessary....

I've had Bad luck with off-shore manifolds,.. GLM's,... the gasket faces weren't flat....
 

500dollar744ti

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Re: Intermittent overheat, 1990 350 magnum

So would you say it's safe to go with Sierra parts or is it worth the extra money for OEM mercruiser? I will order new exhaust shutters, do they come with the little rubber bushings required to mount the shutter in place?
 

500dollar744ti

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Messages
691
Re: Intermittent overheat, 1990 350 magnum

did you replace the water pump base assy and gasket when you replaced the water pump??

glm, osco, sierra are all good replacement manifold/risers

I did replace the base assembly and all the gaskets using OEM quicksilver parts.

I purchased new OEM mercruiser manifolds and risers. I used to opt for the cheapest alternative but I've worked at a Volvo dealership for 10 years now and I have learned that the quality of OEM parts is worth the extra cost in most cases.

I was looking at around $500 for aftermarket manifold/risers, I was able to get OEM mercruiser for $650.
 

500dollar744ti

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Jul 23, 2012
Messages
691
Re: Intermittent overheat, 1990 350 magnum

So I got all my parts here and I took the starboard side riser and manifold off. The manifold bolts were rusty and I had to force a 1/2" socket over the two middle bolts but I got them out.

It was generously clogged with rust.
IMAG0230_zps0cc63645.jpg


Then I took the port side riser off and the two outer manifold bolts but the two inner manifold bolts were so rusted I had to drill them. I spent all of yesterday drilling with a right angle drill and a mirror trying to get the manifold bolts out with no luck. I came up with this idea that I would drill from INSIDE the exhaust chamber through the manifold and directly through the shank of the bolt thus splitting it in half. That actually worked and I got both bolts extracted.

Here's where I drilled to split the bolt. I wish I would have done that in the beginning as I would have saved a lot of time.
IMAG0232_zpsa65739c7.jpg


I'm waiting on some valve covers since mine are pretty rusty and I'd like to replace them while I've got it apart but last question..

I was unable to get the 45 degree pipe fitting out of the port side riser (it broke) to transfer to the new riser since it didn't come with a fitting. I have a 90 degree pipe fitting, would it be alright to use that in its place? I can't see why not but I thought I would ask.
 
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