Hello - Overheating problem on Mercruiser

S

Soundside

Member
Joined
Jun 30, 2014
RO Number
33672
Messages
5
Hi all, I'm new here, and thought I'd jump right in with a longstanding problem. Here's the background.
I have an old 23 foot fiberglass cruiser. On its 2nd stern drive system and 4th engine. Boat came with MC 150 (1966) six cyl. The whole power pack was replaced with MC165 (not sure what year). Later, I believe an upgrade to the lower unit was made. I *think* it currently has a 1-R drive (if there's some way to verify the external hardware, I'd appreciate help with that). The 165 was removed, and upgraded to a Chevy 350 around 1984. This was a custom built, originally automotive engine that was built with marine grade gaskets, plugs and seals. Later, the exhaust manifolds were replaced with Stainless Marine units. At some point, the water pump was also removed and replaced with a crossover pipe (replaces circulating pump) from Stainless Marine. Eventually, that engine corroded internally and we replaced it with a rebuilt long block crate motor (I know, it's not marine, but the boat gets very little use, and it was just not worth it). I also realize that the gear ratio in the drive is not the right one for a 350, but we worked with propellers and found a good combination (a vented stainless unit) that gives appropriate top end RPM and pulls nicely. The boat is used a handful of times each season. It is operated in salt water, but I flush it after every run.
Here's the problem (and all the crazy questions I've been asking myself): The temperature creeps up when running steady at cruising RPM (2500 rpm). Also, it heats significantly when coming off plane and dropping RPM (keeping revs above 1200 or so, and some forward motion helps). It has always done this to some degree, but seems to have gotten worse over the years. I am not currently running a thermostat. I've changed the lower unit pump multiple times and it has no effect. Is the cooling crossover a bad idea? When running that kind of aftermarket product, is there some augmentation that must be done to compensate for the lack of water circulation? I'm not crazy about the water flow system in the sterndrive anyway...should I dispense with that and get a thru-hull with a crank-driven pump? Are my headers just clogged? If I wanted to attempt to establish something like a stock cooling system flow path in this motor, what is the simplest version of the standard combinations that would work using the Stainless exhaust manifolds? There are a hundred permutations of thermostat housings and routing of coolant lines. Is there a generic, aftermarket cooling system setup that is known to work well with SB Chevy? What about stern drive coolers? Would that help lower temps overall? Thanks much for your advice. By the way, I don't know actual temps, but temp gauge is showing hot, and the water coming out of the sterndrive is hot to the touch.
 
Sounds like you're a little new to marine systems but you learned on your own. This is a good place to learn.

I'd help if you gave year of boat and what you can of your drive system. There's some difference between the drives you mentioned but the main thing, the raw water pumps are the same. They have the short vanes that apply more pressure and cause more wear on the housing than the newer types. It's very important to replace the housing along with the impellers. That could be your problem right there because it sounds like you just ain't gettin' good flow. May even be a restriction in the hoses. Gonna need a lot more info and pix to see what you got there.
 
Actually, I've been around boats all my life, but I admit ignorance on the specific details of these cooling systems. This one was a hybrid from the start, so there are lots of possible places for things to go a little wrong. I agree it sounds like I'm just not getting enough raw water from the pump (the heating when I slow down would indicate the pump is not providing enough flow), but I realize a restriction somewhere can behave like a lack of pump effectiveness.

I always change the pump housing with impeller.

What should I take pictures of? What other info?
 
I wanted to see what the plumbing looked like. Very little of my boat and drive system is as it came out of the factory too.

I got rid of the stock thermostat housing and replaced it with a home made housing that works exactly like a cars. You surely don't need all that plumbing from merc. Mine is FWC (Fresh Water Cooled) which means it has a heat exchanger that functions like a radiator. You probably have a Raw water cooling system in that it's cooled by the water it runs in.

First of all, the outdrive needs water running thru it to cool it down. Those gears get real hot. You can add an engine driven raw water pump. I have what is called a bravo style pump. Belt driven.

Old hoses can start to delaminate internally, there by causing a restriction. Another place that a hose can affect the raw water flow is the hose between the drive and the outer transom plate. Because the drive is lifted and lowered, the hose gets old faster than others. The length on this hose is very critical. 1/4" too long can make it kink causing anther restriction. Sometimes it causes a restriction in only one drive position or has free flow in one drive position.

Going to the other end of the cooling system is the exhaust system that also carries the raw water exiting the engine. Blockage of the exhaust ie the flaps blocking the exhaust or impeller vanes that broke off and block the system somewhere between the pump and all the way thru the system and back out the drive.

What you should do at this point is back flush all the hoses from the riser back down thru the engine thru the power steering or other oil coolers...which are other possible points of clogging or restriction.

Let us know what happened after you backflush.

It may seem that I'm getting into things that you already know, but I answer along the lines of the original mechanical/electrical expert (GVP) who explained in great detail because he knew many people were interested in the problems that pop up in marine systems and the different fixes.
 
No problem on the "too much" explanation. Sometimes it takes a while to find out how much has been done, what are the knowns, what is still in question.
A little more background.
Yes, it is raw water cooled.
I mentioned in first post, the boat is 1966 vintage (23' Slickcraft). My father bought this boat when I was 6 years old. It left the family for period in the mid 70's (second owner had entire new power package installed - Merc 165 - somewhere around 1978). That fellow sold the boat back to my dad in early/mid 80's. At that point, we swapped in the 350 engine. I think somewhere around 1990, he put a new gear housing/lower unit on it, and my understanding is that it was a newer design (Alpha?), but was compatible with the transom housing. That first V8 engine was replaced in the mid 2000's. My dad passed a few years ago, so it's now mine to deal with.

I will get some pics of the sterndrive and engine, and let you folks help me date everything.
I also started with a homemade t-stat housing when we first swapped to the V8. Later got the Stainless marine housing when we added those manifolds/crossover.

That little transom hose is the primary thing I was referring to when I mentioned that I wasn't crazy about how the cooling water flow comes into the engine. I have changed that hose before, and it is always a suspect area for me. That's one reason I wonder if it would be best to dispense with the sterndrive pump altogether. My brother has a number of racing outboards, and I have seen some modifications they make to these lower units that improves water pickup (half the time the lower unit is out of the water, and they modify them so the pickup is more effective). But before doing that, I think I need to do some careful troubleshooting. I like the idea of a backflush. For that matter, I think I may just change all the hoses inside the boat (most have been there for at least 10 years). Most of them have SS braided jacket, but they are still rubber inside, so deterioration is possible. Then, I think I might as well pull the manifolds and thoroughly clean/inspect them. I can't imagine they'd be clogged, but stranger things have happened (and since it tends to run hot, salt buildup will be faster than normal).

Yes, the sterndrive heat generation is significant, and that's one reason I asked about a cooler. I've looked at aftermarket drive coolers (over-priced plumbing that scoops up water and squirts it on the drive). I think my drive runs a little hotter than most, I've noticed it's hotter than comfort level to the touch. I've heard that depending on gear clearance and other specifics, some drives run much hotter than others.

Last dumb question for the day: Does the sterndrive rely on the internal water flow from the pump to help cool it? If I abandoned the water pickup and pump from the sterndrive and went to thru-hull, does this cause even more heating in the drive?
 
I think not running a thermostat is generally not a good idea. I'll leave it up to Charlie or Sandy to explain why. This is a common misconception and it can cause higher than normal engine temperatures.
I doubt this is the entire problem but it could be contributing to the over heating issue.
Bill
 
I thought running without a thermostat was more likely to cause an engine to run too cool, never reaching optimum temp???? But, agree with you--not a good idea.
 
First, running in salt water greatly increases corrosion but it sounds like converting to SS manifolds should prevent some of the corrosion blockage problem. Not being familiar with Stainless marine manifolds and risers and how they're plumbed. My rememberance was the manifolds were aluminum and risers were SS. The small water passages in the risers are what get blocked easily. Rust scale can still come from the Block so you wouldn't be completely safe from rust blockage. Without a free flowing cooling system to carry away the heat. the engine will over heat.

As I mentioned before, the outdrive needs water flowing thru it to help cool. A drive shower is an excellent way to keep the drive cool when under way. If you add an engine mounted raw water pump and thru hull pick-up. Make sure you include the outdrive in the inlet side along with the thru hull. That's exactly how my system is plumbed and I have a home made drive shower.

If you have power steering, the power steering cooler is a prime point of blockage from debris getting sucked up from the drive. Fact is, fresh water/lake boats are a prime source of parts as they often sucked up sand and burned up engines.

If I remember right, all the drives from the alphas back to the typeI were pretty much the same mounting pattern to the transom. The bravo and its predecessors, the TR series has slightly different bolt pattern but same hole cut-out in the transom. And a thought, being as old as your boat is, It might be a good idea to test the transom for water intrusion or wood rot. Drill random 5/16" holes on the inside of the transom to check the wood shavings. I say 5/16" or even larger holes because they're actually easier to fill than small holes that catch air and never fill completely.

We'll get to your other concerns as you go.

Oh yeah...my boat is a 1978 24' searay but doesn't look like any searay on the outside. Neither is anything in the drive train so it has gone thru many changes kinda like yours. One thing to consider, my boat had so much rot and water in the wood, I ended up removing all the wood from the transom to to helm and redesigning the whole cockpit to make it more fishing friendly.
 
That sounds familiar - I had a late 70's SeaRay that I replaced everything from transom forward. I also had to replace the transom in another old fiberglass boat I had. This Slickcraft is a very well built boat. I'm pretty sure it has balsa core hull. Some people rant about balsa core boats, but after seeing the rotted innards of two plywood core boats, I am convinced that whatever they did in this boat, it was the right way to do it. There are some spots in the transom and stringers that have some pockets, but overall it is very solid (specially for its age). My hat's off to you for all the work involved in that re-do of your cockpit.

I think all the Alphas (and even pre-alpha) are interchangeable, as you mentioned, but I've read somewhere that the drive couplers for six versus eight cylinder engines would not allow a six to be swapped for an eight. But we managed it just fine. I don't have power steering, but embarrassed to say I can't remember if there's an oil cooler. It does have remote oil filters, but the lines go straight to the filter mount.

I think the Stainless Marine manifolds are actually SS on the inside and have a formed aluminum case. Not sure about that. Risers are all stainless. In any event, they're just as likely to get clogged as any other manifolds. Checking and cleaning is due.

I also have a little leak into the boat coming from somewhere in the guts of the transom and I think it might be that the exhaust Y pipe is leaking (possibly where it joins to the gimbal housing). Only way to really dope it out is to pull the motor, and I plan on doing that this fall. Are there flappers in the Y pipe? I didn't think so, but can't remember.
Thanks!
 
The alphas I had definitely had flappers in the Y exhaust.
 
Most of those older drive flappers are probably gone from corrosion. They're known for getting stuck down where the two sides meet on V6/V8 engines. The connection of the Y pipe to the transom sometimes is a 3 piece and sometimes mounts directly to the transom. They have been known to rust out the mounting bolts or even corrode right thru the Y pipe or at the connection.

Come to think of it, I have a set of late model Y pipes with rubber connectors and flaps. I got it in case something went wrong but I ended up making my own SS exhaust system and thru transom exhaust. It's for sale if you're interested.
 
I don't even know what year my Y pipe is. Swapped it in when we put in the V8 in early 80's (at that time it was a "late model" used unit). So long ago, I can't remember details - I know we changed the inner transom plate, but I think the interface to the exhaust port on the gimbal housing was a direct bolt-in).

Thanks for the offer - if it turns out to be bad, will keep in mind, but I'm fortunate to have some pretty good used marine supply outfits in the vicinity. Would love to see some pics of your SS system. Hopefully today I'll get some pics of my old girl, once Arthur clears out.
 
You must log in or register to reply here.
Back
Top