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Discussion Starter · #1 ·
My son and I had his 350 Mercruiser engine rebuilt in June. We are having trouble with it. It starts fine, idles, and runs perfect for about 30 minutes. The temp gauge goes about 3/4 of the way to the edge of the green and red zone. (We haven't changed it for a numbered readout.) At high speed, it goes right to the edge of the red. After slowing down it cools off back to the 3/4 position. However, the idle starts to act up and it usually stalls. We cannot get it to restart for at least 4-6 hours when it cools off. I've had 2 shops look at it and talked to the engine rebuilder. (He's coming out to look it over next week.) We've changed the thermostat, points, plugs, wires, condenser, carbs been rebuilt, checked the timing, dwell, and run water thru all the exhaust manifolds and hoses. We rebuilt the lower unit water pump. We've pulled the drive and checked for obstructions in the exhaust. We pulled the water inlet hose after the ouil cooler; it's pumping plenty of water. The port side seems to be running a little cooler than the starboard. The starboard side of the manifold is at 205-210 degrees. While trying to restart, it seems as if it's not turning over fast enough. We have 2 new batteries and a new starter. Does anyone have any ideas?
 

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Why did you have it rebuilt (what was it like before the rebuild?)

Did it run right at anytime AFTER the rebuild?

I would check hose routing and make sure the cooling water is being circulated correctly.

Did you check temps with an IR thermometer.. those things are worth every penny at giving you GOOD data to help track down temp problems...

Another thought...

Did you check vacuum? I know one friend who had an older merc that had a problem with a cracked intake manifold... when it heated up, it would leak air into the system...while it was running, it would just run lean (and hotter), but you couldn't restart because it was too lean.

The crack was on the underside of the manifold, right under the carb. When it was cool, there was so little leaking through that it started and ran OK. Once warmed up, it must have warped enough to open the crack.
 

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btw.. I think 205-210 is WAY too HOT for a marine engine, unless you're measuring on a part that doesn't have a water jacket. What were you measuring with? Where and when was this measurement taken?
 

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Discussion Starter · #4 ·
We're using a Ryobi infrared thermometer. 205-210 is too hot. I can't remove the intake until the engine rebuilder looks it over. (It has a 3 year warranty)

We had it rebuilt because the heads were rusted out when we bought it and it had water in the oil. You might be onto something with the cracked intake. Vacuum gauge reads around 16 and bounces around alot. It might be leaking and too lean. It sure idles and runs good before it gets too hot.

2 different mechanics and multiple neighbors have checked the hose routing and we even compared it to my boat with the identical engine. It's a 1979 350 with all the hoses running from the thermostat housing.

Thanks for the reply. We've been very frustrated about this. I just hope the rebuilder makes it right.
 

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Long shot. Have you checked the freshwater intakes to ensure they are clear. Also the intake hoses for mud/sand etc. I agree with the comment it is running too hot. Watched a guy clean his intake hoses after running in shallow water. They were packed with crud. Just a thought.
 

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Discussion Starter · #7 ·
Found the problem!!!!!

The mechanic took the boat out of the water again and looked it over. Finally spotted the water hose in the outdrive was pinched right at the top where it attaches to the transom. We put the boat on the trailer and will be doing a complete bellows job shortly. Thaks for everyone's ideas. This has been a frustrating Summer for my son.
 
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