78 Mercruiser Alpha One 5.7 runs cool on muffs but overheats quickly on raw water?

[warshark25]

Hello all, first time posting here. I have a 1978 Sea Ray with a 5.7 alpha one outdrive. The issue I have come across is not being able to keep this boat from overheating on the water. It has been the issue since day one of getting it. I can run the boat on muffs and the temperature stays normal being at idle or bringing the rpms up 2500-3500. The engine never gets lose to running hot. Now I am not 100% sure if I have the thermostat housing that requires check balls, I have the log style exhaust manifolds. I have replaced the impeller and housing, cleaned out the out drive, replaced the hose running from the out drive to the hull, replaced the thermostat, water pump and manifolds and risers. When on the water I removed the plug on the side of the thermostat housing when starting the engine it shot about a 3 ft stream of water out if that info helps at all. I am at a loss at to what else I should look into? Also, I have removed all the cooling hoses and made sure there are no obstructions in them. Thank you for your time and advice, hopefully there is something obvious I have overlooked.

 

[Scott Danforth]

welcome aboard

my guess is that this is a new to you boat

first thing I would do is replace your raw water pump, however you have done that. have you checked the power steering cooler for obstruction?.

when on the muffs, you have citi water pressure pushing water into the motor.

when on the water, you dont. your relying on the raw water pump to push water.

while at it with the boat, I suggest removing the drive, checking alignment and replacing the bellows.

 

[sogood]

Have you checked the oil cooler for obstructions? On muff the water flow is being helped by being pumped into the engine, whereas on the water it's depending on the impeller to draw water in. Did you fit the little "key" ( can't remember the proper name ) on the shaft that locks the impeller in place? Otherwise the shaft spins but the impeller doesn't. Is your thermostat fitted properly and operating likewise?

 

[alldodge]

can run the boat on muffs and the temperature stays normal being at idle or bringing the rpms up 2500-3500

Running the motor on muffs at this rpm can burn your impeller up even on muffs. Max rpm should not exceed 1500

 

[warshark25]

Yes the boat is somewhat new to me i had purchased it with a bad engine. I have since installed a rebuilt engine that did not have any overheating issues prior to install, I have removed the oil cooler and inspected for any obstructions, i see you point about running in the muffs didn't really think about it actually pushing water up through the engine, as well I have removed the lower outdrive the impeller looks to be in good shape i can post pictures of it later today and the set up on the engine if that might help? Yes sir I have installed the key for the impeller, thank y'all for the advice. My understanding is that the previous owner was having similar issues and ended up blowing up the last engine because of this.

 

[alldodge]

Being a 78 model I think your cooling system is probably like in the pic, unless the exhaust has been converted to newer style.

 

[Scott Danforth]

one thing that may be an issue. the manifolds are 40 years old if they are original. there may be enough restriction in them preventing enough flow.

 

[Bondo]

as well I have removed the lower outdrive the impeller looks to be in good shape

Ayuh,..... Welcome Aboard,..... Drop the lower unit, 'n Replace the impeller,... Do the whole pump kit,....

It's probably older than 2, so isn't as flexible as it should be,....
The impeller is only expected to last 2 to 5 years, tops,...
The tiniest scorin' or polishin' ruins the pump liner pieces,....
A new to you boat, deserves a new pump kit,....

 

[warshark25]

Hello, apologies for the slow response time. That is the cooling system that this boat has, I replaced the impeller and housing a year ago, BUT that was a year ago and the boat has been pretty much setting since. I removed the lower half of the out drive last night and pulled the "old" impeller out, I am guessing since it sat so long in the same position it has caused 3 of the ribs to remain slightly curved even when the impeller is out of the housing. I have ordered a new impeller for it. But, I would not think that such a small bend on 3 of the ribs would cause such an issue. But what do I know! This is my first boat and experience working on these things. I can post pictures tomorrow of what the impeller and housing look like and see what y'all think of it.

 

[alldodge]

Its not unusual for the impeller to get a set in some of the blades over time. Normally if its not to old the blades will straighten out over time. I agree the impeller is probably not the current issue.

If the manifolds are original they may very well be the issue along with possible build up in the thermostat housing. How old are they?

 

[warshark25]

The manifolds themselves are about 6-7years old never ran in seawater, the thermostat is 2 years old. Is there a way I can test the manifolds? Also if the manifolds are clogged wouldn't she still run hot running on muffs? I ordered another impeller and will test again this weekend if not I'll look into saving up for some manifolds!

 

[warshark25]

Also something else to add when running in the water even though it was reading hot on the gauge the manifold and risers were cool to the touch

 

[alldodge]

The manifolds themselves are about 6-7years old never ran in seawater, the thermostat is 2 years old. Is there a way I can test the manifolds?

Yes, can do it with acetone, but only being 2 years they should be fine. Only issue is if there clogged. Might not be clogged due to you last comment.

Also if the manifolds are clogged wouldn't she still run hot running on muffs?

Maybe not, because in the water there is a lot more load and fuel being burned.

I ordered another impeller and will test again this weekend if not I'll look into saving up for some manifolds!

Might not need either

Also something else to add when running in the water even though it was reading hot on the gauge the manifold and risers were cool to the touch

You may have a gauge issue. Get an IR temp gun and take a measurement at the thermostat housing

 

[warshark25]

Ok I will get myself a temp gun and see how that reads, i can pull the thermostat and see how she looks in there. Hopefully it's the gauge or sending unit acting up! I hadn't really considered that, if manifolds are clogged is it possible to clean them? As well how can I use acetone to test them? Thank you for the info so far

 

[alldodge]

I wouldn't remove anything right now, not even the thermostat until you know you have an over heat issue. Manifolds can be flushed out with a garden hose. Acetone is used to find cracks, and yours being fairly new I don't see that as a possibility

 

[warshark25]

Hello all! I have replaced the thermostat and made myself a test trough to instead of dragging the old lady to the lake 30 min away! With the outdrive in the trough I'm getting no water flow out the exhaust, put it on muffs and wala runs cool. I removed the outdrive again the impeller looks to be in good shape alone with the housing I am kind of at a loss. I had ordered a new housing and impeller kit should be here monday

 

[alldodge]

Don't understand what you have the drive in but think its probably a tub of such. Remove the hose from the thermostat housing going to the drive. See if water comes out when started.

 

[kenny nunez]

One other thing to check with the lower unit off, connect a piece of 5/8" heater hose to a garden hose and clamp it to the copper tube going up in the upper case. With the hose turned on see if there is any leakage from under the plastic cover that the pipe is in. Some times they are melted from not enough water and distort.

 

[warshark25]

Hello friends! I replaced the impeller and housing, disconnected the inlet raw water hose feeding the thermostat. By far a strong feed of water. My gauge still says it's running hot to I installed another sending unit and temp gauge as a temporary which reads 160-170 at idle for 15 min running the engine st 1500 the gauge reads 150. Also used a IR gun on it manifold run around 140-150. Risers on the front side 110. Backs of them are 90-95. Sadly I am feeling my gauge or sending unit has been gin vying me bad reading now. Is this around the temp range they should be? I ran the boat for about an hour with no change in temp on the IR or different gauge. Also the outdrive inlet is 8" submerged in a 40 gallon tub. Well above the chance of it running dry. I do feel like a dummy if it was just the gauge/sending unit. Will text with a different IR tomorrow. Just wanted to give y'all an update!

 

[warshark25]

Update: took the boat to the lake today she is running at 160 at idle or plane. Ran her for about 30-40min before she started to bog down some, I believe my old carb is giving me issues but as for the over heating it's nothing like before, as well when replacing the gauge there were 3 wires to the signal one of which "purple" was causing the new gauge to read 240(max) as soo. As the key was turned on not sure what it went to but after disconnecting I was able to achieve
Accurate readings from the engine. Thank you all again will give more updates next run