Overheating 7.4L Mercruiser-gctid403963

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Today I had a 'why do I bother' moment, as my (raw water cooled) engine continues to overheat on the muffs in my final boating days before I have to travel for the summer. I posted a thread before, and I thought I had the problem solved by expanding the springs of the checkballs in the thermostat housing (see image).

I tried the trick again. I tried removing the checkballs. I tried removing the thermostat. When I remove a hose to check flow, water appears to be circulating just fine - everywhere. [Edit: background: The boat had this problem on my first run, then 'fixed itself', and ran fine the rest of the day. After fiddling with the checkballs before, I launched and the engine ran fine for two days. I have replaced the impeller and housing]

I did notice that when I disconnect the hose going from the checkballs to the starboard elbow, water flows back toward the thermostat housing under pressure from the exhaust - I don't know if that is normal.

The engine was rebuilt 14 hours ago. They are the original manifolds, but the boat lived its whole life in fresh water. If I run out of ideas, I'll have to remove the manifolds. Could it be the temperature sender? Is there a way to test it, or should I get an IR temperature gun? How could the engine be overheating when water is flowing through the block?

Attached files [img]/media/kunena/attachments/vb/707697=30052-mercruiser_87-89_454_thousing_new.jpg[/img]

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The image attached shows the cooling system (Note: the cold / hot legend on the whole cooling system is for the first image which is a detail of the thermo housing)

Attached files [img]/media/kunena/attachments/vb/707698=30053-cooling_merc454_fresh_ 001.jpg[/img] [img]/media/kunena/attachments/vb/707698=30054-cooling_merc454_fresh_ 005.jpg[/img]

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An easy and cheap way to find out if the temp sensor is bad I to replace it and see if you are getting the same reading. Look for Autometer part number 2252. It runs about $18 and is available at most auto parts shops.

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I'm with ya brother, but I'm just overheating at high rpm---stuck harbor cruising since April, and Catalina's calling!

Have replaced: impeller(twice) circ pump, thermo, tried no thermo, manifolds/risers, did the clear hose testing, ir heat gun tests--whew!

Have concluded a bad water flow from drive S tube, pulling boat this week to punch a hole in the bottom and install a thru hull pickup!

[itsabowtime2 Moderator Emeritus Find all posts View Profile Close itsabowtime2]

IR gun is nice to have. Less than $30 at Harbor Freight.

You said you are getting backflow. Could the flappers be the cause?

[captharv Founding Member Find all posts View Profile Close captharv]

Two thoughts, as these actually happened to friends boats.

1. When an alpha pump impeller breaks apart, the "fingers fall into the intake cavity imediately below the imperrer. The cavity has a plastic grate and is easily removable.

2. The exhaust elnows have a small opening where the water mixes into the exhaust stream. It can rust and restrict the flow. If this is so, replace it. Don't even think of trying to fix it.

I would try #1 first, its easier

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captharv wrote:
Two thoughts, as these actually happened to friends boats.

1. When an alpha pump impeller breaks apart, the "fingers fall into the intake cavity imediately below the imperrer....

2. The exhaust elnows have a small opening where the water mixes into the exhaust stream. It can rust and restrict the flow...

Thanks Capt. Harv. The broken impeller bits would explain an intermittent problem like I have. As far as the outdrive housing goes, I have replaced the impeller and housing. The old impeller was in good shape, and I saw no signs of bits anywhere in the lower drive. Water supply seems to be adequate. I don't think there are too many small passages in my raw water cooled engine block before the exhaust system.

Here is where I am perplexed: blockages in the exhaust passages, or backpressure caused by a stuck flapper, could impede water exit, and - I gather - affect the water flow throughout the system (if a creek is dammed then water flows into the pond, but there is no flow through the pond) . However, by removing the hoses from thermo housing to exhaust for a test, I eliminate that part of the system, as the water flow dumps into the bilge.

As you can see I am feeling a bit desperate creating a model in my mind of a beaver building a dam. But am I missing something here?

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itsabowtime2 wrote:
...You said you are getting backflow. Could the flappers be the cause?

Yes, and you have given me a thought. Although I had disconnected the hose from the T connection to the elbows, the hoses from the thermostat housing to the manifold was still in place. If the flapper were stuck, backpressure could have been routed back through this line, stopping flow.

Wouldn't one way to test this would be to 1) warming up to 'hot', 2) temporarily disconnecting all four output hoses from the thermostat housing to the exhaust system. I am concerned that if it works, then cool water will start flowing through the block and possibly crack it. Maybe I should just check the flapper.

As far as the IR gun goes, that would show me the temperature of the metal at the surface, as opposed to the sensor, which is taking the water temperature. I suppose there would be a lag in water temp vs. the temperature at the surface of the metal.

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wildman wrote:
... background: The boat had this problem on my first run, then 'fixed itself', and ran fine the rest of the day. After fiddling with the checkballs before, I launched and the engine ran fine for two days....

In fact, the problem has mostly occurred after the boat was not run for several days. Seems to support theory that the problem may be a sticking flapper, which is actually called a water shutter.

Attached files http://baylinerownersclub.org/media/....jpg[/img]

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Well, I checked the exhaust water shutter on the starboard side and it was fine. I started it up and it was cooling fine today. The only difference is I may have bumped up the water pressure a little on the outdrive muffs. I don't know if it is solved but there seems to be long periods of normal operation punctuated by complete dysfunction. :surr

I did get one of those IR tools from Harbor Freight. It seemed to work very good. The temperature reading at the base of the thermostat housing - where the temp sensor is - seemed to increase at the same rate as the reading on the temperature gauge.