Sporatic overheating questions.

Rusty Shackleford

New Member
Jul 3, 2017
6
Kaufman, Texas
Boat Info
1978 SRV 200 Sun Runner
Engines
305 Mercruiser
I bought a '78 SRV 200 (305/Merc) a few months back, and have been catching up on the typical neglected maintenance issues that are typical of too many old boats out there. Anyway, after cleaning up hacked wiring, replacing a few small soft spots in the floor, and some new carpet and upholstery here and there I took her out for a test run.

As expected I found a few more issues to deal with, but mechanically she did pretty good over the three hours I was out. I did notice that the temperature fluctuated between 130-175 degrees, but never hotter. The next weekend I take the wife and kid out, and after about an hour of moderate use the engine started getting hotter, I shut her down when the gauge showed 200. I pulled the engine cover and the manifolds were pretty darned hot, enough to sizzle just a bit when I spit on them. After about 45 minutes of sitting, and swimming for the kid, I fired it back up. It went back to 200 pretty quick as I headed toward the launch, but after about a minute of running there the temp dropped quickly back to 135ish. We loaded up and came home.

Got home and took the t-stat housing apart, and no t-stat present. So I went and got a 165 to put in it, and buttoned it all back up.

I have ran the boat a couple more times since then usually less than 3-4hrs per trip, and have not had it get hot yet. It now warms up to 165 and stays there, as I think it should with the stat in it.

In doing research i read where the old manifolds/risers will get rusty inside and flakes of rust can block passages, and I've read multiple other reasons one would get hot, but no other explanation lean toward an intermittent overheat. My reasoning there is, if a pump or impeller went bad, it would still be getting hot.

I haven't pulled the manifolds/risers yet to inspect them, wanted to make sure I'm barking up the right tree before I do. I have no idea how old they are. The stat housing did have some surface rust showing inside, but no flakes or really nasty looking stuff.


Anyway, is my thinking correct? Should I be looking elsewhere first?

Thanks in advance for any advice. I'm a long time boater, and hotrodder, so working on stuff doesn't scare me. I'm just not as familiar with this type of cooling system so I thought I'd seek advice from those who are.

FYI, I'm inland and doubt the boat has ever seen saltwater, if that makes a difference.
 
Ist thing I do when I get a new boat is change the T stat, and the water pump, backflush the system, and change all fluids. If the manifolds and elbows look good on the outside, I'll wait till fall to remove and inspect, to determine their age.
 
I think you're on the right track... sounds like the t-stat helped, and as you say, could be some blockage in the risers. Don't discount the impeller if you don't know the history... it could be on the verge of destruction.

Welcome to CSR!
 
I just had an overheat issue with my 5.7 mercruiser/alpha one. After changing the impeller and thermostat, it would still overheat sometimes. Finally found the water hose in the outdrive was kinked and the more it was trimmed, the hotter it would get. Runs perfectly at 140 with a 140 degree t-stat now.
 
Thanks for the replies.

I will be pulling the whole drive off this fall to go through it anyway. If the rest of the boat is any indication, I have no doubt I'll find plenty of neglect in there as well. I did check the lower unit lube before I bought it, no moisture in it and it looked pretty new, which coincided with the owner's claim that the lower unit and water pump had been serviced this spring. But the trim and limit sensors are disconnected (likely bad), and I think the lower shift cable is either stretched and/or out of adjustment because it doesn't like engaging reverse and wants to throttle up as it engages forward if you don't go super slow on the shifter.

I figure on replacing bellows and other typical wear parts while in there, and just make sure everything is as it should be. At that point, I'll probably pull and inspect the manifolds/risers to have peace of mind there as well.

I knew a lot of this type of stuff would be needed when I bought the boat, and refused to pay much for it at the time because of that. The seller tried convincing me of the boat's worth, and I agreed the hull was worth something, but the mechanical maintenance needed would far outweigh the worth of the fiberglass.

Until I get that done, I'll make sure I have a buddy boat out on the water when I take it out.

Thanks again fellas!
 
They may, or may not, be the cause of your current problem, but if you run in salt water and don't know the age of your manifolds and risers, then replacing them would be a good idea. Could very likely save your motor.
 
I'm struggling with an over heating issue as well. Only on the starboard Merc 5.7 with Alpha one Gen two will overheat while on plane for a few minutes. Had my marina mechanic change impeller, thermostat and checked exhaust risers for obstruction. Still overheating. I went ahead and connected a clear hose to the raw water inlet and noticed significant air only while at higher rpm 3000 - WOT.
I am getting a ton of clean water when at at idle with no air present. What could be the possible source of the air infiltration be coming from?
 
I'm struggling with an over heating issue as well. Only on the starboard Merc 5.7 with Alpha one Gen two will overheat while on plane for a few minutes. Had my marina mechanic change impeller, thermostat and checked exhaust risers for obstruction. Still overheating. I went ahead and connected a clear hose to the raw water inlet and noticed significant air only while at higher rpm 3000 - WOT.
I am getting a ton of clean water when at at idle with no air present. What could be the possible source of the air infiltration be coming from?

It might not be air infiltration. If something is restricting the water intake, the bubbles could be the result of the impeller causing cavitation. Just some food for thought.
 
I have the same issue, temp starts creeping up over 3000rpm. Below that it is solid. changed impeller same issue. Interested in seeing some replies on this one and will keep searching old posts to see if anyone else solved issue.
 
I have the same issue, temp starts creeping up over 3000rpm. Below that it is solid. changed impeller same issue. Interested in seeing some replies on this one and will keep searching old posts to see if anyone else solved issue.
The problem you run into here is there are any number of reasons why an overheat may occur. There is no one-size-fits-all answer. Amongst other things, it can range from sensor issues (meaning no "real" overheat) to intake issues to circulation issues and/or clogs to output issues.

However, in this thread, quite a few things were mentioned that can cause an overheat. If you go through the thread, and note the various things, what things have you tackled to this point? Have you done a raw water pump test?
 
I have twin 470's and the stbd engine is running hotter than I like and the port a little over normal. I have a closed cooling type system and my first fix is going to be a reservior cap change, the thermostat is next,and then off comes the cooler where I will flush it out and also back flush all the hoses. When I saw the boat on Craigs list the previous owner had it parked nose forward on the beach. Which could mean he might have sucked up some sand into the system and plugged it up, just a food for thought. If all this fails I will look at risers,manifolds,and compression tests to check head gaskets. Changing a water pump on the motor will be my last fix and I will leave that to a pro as the the installation tool itself is about 150.00 bucks.
 
Just a update on my overheating issue. I have replaced the thermostats, coolant, cleaned the heat exchanger, and replaced the exhaust riser on the stbd engine and replaced the thermostat,coolant,and flushed the system on the port engine. Now both engines are running alot cooler and I have yet to do a seatest at the lake to see if they both stay cool. The exhaust riser on the starboard side was partually clogged and the hose was burned at the riser neck. I cleaned it up but replaced it with a clean used unit from Ebay.
 
From the sounds of things overheating was gappening before you got it hence the absent thermostat. I would in addition to things youve already done replace the exhaust tubes they are compromised if they got that hot that the hose to the riser melted. Also replace the butterfly valves in the y pipe. I got a kit that included 4 exhaust tubes and 2 butterfly valves for around 150 bucks . Piece of mind knowing you have protection from water intrusion . I would also take off and carefully inspect and replace as needed every rubber hose in the cooling line soft hoses collapse and restrict water flow . All the molded hoses for my 454 where about 300 bucks.ive seen old hoses and exhaust tubes that although looking fine from exterior inspection had agedvto the point of imminent failure . One of my lower exhaust tube/boot had a pinhole leak when I pulled it off I was surprised it easily split along the seam . Good luck with the new boat . Let us know if you find obstructions in the cooling circuit and where they were .
Happy boating
 
Both engines had thermostats and all the hoses are new or seem to be. I don't think the 470 has butterflys as I dont see where they could go or any imfo that mentions them.
 
My reply was for the op I don't know what exhaust is on yours .in the 305 he has the butterfly valves are in the y pipe that connects the 2 lower exhaust boots and the drive via the bellows
 
Apparently my 470's have butterflys down in the straight exhaust pipe and most likely are gone since they are made of rubber but since they are there to prevent backwash from going up into the manifold I am going to do this later and just be careful when slowing down for now.
 
They are rubber bonded to a thin metal plate. If they are blocking the exhaust your overheat issue will continue if they are absent you are risking water intrusion . Why risk your motors over 100 bucks worth of parts and a few hours labour
 
Your right it needs to be looked at and fixed, I need to find a schematic of where they are located in the exhaust so I know what is involved and what I need to order. I'm going to wait for it to cool down here in Vegas still in the upper 90's, doesn't hurt anything setting in front of the house and still have other stuff to do. Thanks for the push.
 
Your right it needs to be looked at and fixed, I need to find a schematic of where they are located in the exhaust so I know what is involved and what I need to order. I'm going to wait for it to cool down here in Vegas still in the upper 90's, doesn't hurt anything setting in front of the house and still have other stuff to do. Thanks for the push.
A little jealous of your climate 4 weeks till I have to pull my boat out up here . Good luck with it and enjoy yourextended season
 
I think I have narrowed down the shutter part# to 45586 now just figure the least expensive purchase and how they are installed. Actually real tired of the heat hard to get stuff done outside just to darn hot. I'm a transplanted 23 yr. resident of Alaska so the heat gets to me.
 

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