Overheating after freshwater conversion

[66pony]

I took the boat out Sunday (8/22) for a short spin (all I can really do right now). It’s the second time running with the bypass flow reducer installed that Monitor Products (heat exchanger manufacture) sent me (see details early on in this thread), and wanted to collect more data on how the temp behaves with it, and prior to the raw water thru-hull is installed.

I warmed up the engine to about 150deg and headed out of the marina. As it was very low tide I didn't open her up till I was near the channel about a 10 min ride at 2000RPMish, and the temp stayed at 154deg with a bay water temp of 74deg. I jumped up on plane and settled in at 3800RPM. The engine temp kept climbing, but slowed down as it hit the low 180’s. At 184deg after about 7 min, I could tell as before it wasn’t done heating up. I didn’t want to wait for the temp to get any higher so I slowly pulled back the throttle; first to 3000 then 2500, it held on to 184deg for what seemed like a couple of minutes and then it finally dropped to 174deg. At that point I know I had it under control and could reduce to idle speed. If I had dropped down to anything near idle speed the temp would’ve shot right past the195deg overheat alarm threshold.

My mechanic is installing the thru hulls this week. After he does, I’m going to take it out and pretty much run the same test as above.

The silver lining is that one of our favorite anchorages is close by, so after the quick test and of jotting notes we putted over to it, dropped a hook and enjoyed the afternoon.

For what it’s worth and because SCORPIO asked, I took some water pressure readings while I was at it:
RPM Water Pressure
600 12.2
1330 23.9
2000 32.8
3800 40.7

Looking at this I would wonder if you have something plugging the water circulation. Looking at the pressures, and knowing that my 496 only produces 20 psi, leads me to believe that you have a restriction somewhere and not enough freshwater flow is available to cool the closed system.

 

[Hornblower]

Looking at this I would wonder if you have something plugging the water circulation. Looking at the pressures, and knowing that my 496 only produces 20 psi, leads me to believe that you have a restriction somewhere and not enough freshwater flow is available to cool the closed system.

66Pony,

You obtain 20PSI at what RPM?

By freshwater flow you do mean the coolant correct, as opposed to raw water which is the seawater?

Do I assume correctly that the water pressure sensor measures the closed system/freshwater side?

 

[66pony]

I meant raw water, thanks for the correction. I get 20 psi at 3300 to 4000 rpm. My coolant is at most 15 psi (controlled by the cap). I wonder if the water flow on the raw water side is somewhat constricted. I would think that you should only get around 15-20 psi at cruise speed as that pump assembly was used prior to the conversion to cool the entire engine. What is the temp of the elbows?

 

[Hornblower]

I meant raw water, thanks for the correction. I get 20 psi at 3300 to 4000 rpm. My coolant is at most 15 psi (controlled by the cap). I wonder if the water flow on the raw water side is somewhat constricted. I would think that you should only get around 15-20 psi at cruise speed as that pump assembly was used prior to the conversion to cool the entire engine. What is the temp of the elbows?

The temp at the elbows is 110deg, even if we let the engine temp get up to 195deg. My mechanics tell me the raw water flowed like a fire hose when they visually checked it last week.

 

[66pony]

Did they check the flow before or after the heat exchanger?

 

[Hornblower]

Did they check the flow before or after the heat exchanger?

After. I know its a long thread but please read it maybe you can find something we haven't and should try.:smt001

Thank you for your interest and questions.

Sincerely,

 

[Hornblower]

I spoke to Monitor Product again today. Here's the email they sent me:

"Steve, I’ve attached a drawing showing the correct divider orientation on both ends of the heat exchanger. Have your mechanics remove the end covers and check."

Could anyone blame if this is the last thing we checked?

 

[4sure]

Just an FYI... I am dealing with similar issue with overheating. I did all the water flow tests as per the Mercruiser manual with a raw water cooled engine, all tested fine. After checking all hoses, components & impeller for obstructions and not finding anything wrong, I decided it was time to view raw water flow from a similar pump to compare. What a difference! The other boat pumped the same on idle; on higher RPMs it was much more. I installed a through hull fitting, overheat problem solved.

The next issue: Does the Bravo III drive require water pick-up cooling. I called Mercruiser they said it does not. I challenged them on this, they said again it does not. I am not sure I believe this answer. I will be speaking to my Mercruiser technician about this tomorrow to be sure the drive has adequate cooling. The posts on this site have helped me quite a bit. I thank you all .......

 

[Hornblower]

156 Degrees at 4000RPM! Details to follow.

 

[Westie]

156 Degrees at 4000RPM! Details to follow.

:smt038:smt038:smt038:smt038:smt038:smt038:smt038:thumbsup::thumbsup::thumbsup:

 

[Hornblower]

I have to hand it to my mechanics Joe and Charlie, because of their determination I now have a workable solution.

Today they checked the end dividers on each side of the heat exchanger as suggested by Monitor Products, but their orientation matched the drawings they supplied us with. So rather than call it quits and yank out the exchanger Joe had the idea to replumb and eliminate the exhaust manifolds and riser block from freshwater water side to reduce the amount of heat the exchanger had to remove.

Joe rerouted the raw water outlets from the exchanger to the exhaust manifolds rather than the elbows and since he had previously installed divider plates between the riser blocks and the elbows as part of the initial installation he had to route a hose from the riser up to the elbows. Since the in and out fittings on the riser and elbow are so close together he routed the starboard riser hose to the port elbow and the port to the starboard so the hoses wouldn’t kink. Unless you look close and scrutinize the setup you can’t tell there’s something different going on here. At this time he normalized the raw water pickup back to the outdrive too.

So after routing the hoses and testing it at the dock, Joe took the boat for a spin and called me with the results. He ran it at 4500RPM and it stayed at 158 Degrees. When he was sure it wasn’t going to climb higher, he dropped the throttle quickly down to 600RPM, and watched the temp rise slowly to 172 degrees, stay there for a while and then drop back down. Needless to say that this was good news, as I had encouraged them to yank out the exchanger when I stopped by this morning for a visit.

It didn’t require too much encouragement from Joe to convince me to come down and try it out myself, so I left work about an hour early to put it to the test. I let the engine warm up at the doc until it stabilized at 154 degrees. As soon as I got into Hart Cove I slowly brought in up to full throttle (4800 RPM). In no time it hit 159 degrees and that’s where it stayed. I headed towards Moriches inlet and tempting fate headed out into the Atlantic. It was a little sloppy out there, so after getting a face full of salt water I decided to see how she would recover. Like Joe I quickly brought it down to idle (600RPM) and it only spiked to 172. Next it ran at 156 degrees at 3600 and 4000 RPM and only spiked to 168. BTW the sea temp was 68 degrees.

So what I have here is a heat exchanger engineered to handle both the block and exhaust, now FWC the block only. This wasn’t the plan and not what I paid for but it’s a lot better than pulling it off all together. I do have a freshwater flush connector for the exhaust located at the transom and I plan on using it.

Next thing to do would be to replace the separating plates between the risers and elbows to eliminate the crossing hoses, but there’s no rush on that. I may wait until it’s time to replace the exhaust in a few years.

I left a message for James at Monitor Products to get his take on this. I’ll let you know what I find out when we talk.

For now I’m happy and I’m looking at the calendar for a weekend that we can take a cruise to Greenport or maybe Block Island, possibly Liberty Landing … how far is Bermuda? :grin:

Thank you all for you interest, advice and good will.

 

[EricW]

That is really good news. Finally, some results for you. It's not the ideal situation, but now you can enjoy your boat and not worry about it. It's very interesting that removing the manifolds from the FWC circuit dropped the temps that much.

 

[magster65]

Go boating!!

 

[66pony]

Great! glad to see you got to the bottom of it. Just for fun, what do your pressures run now?

 

[Hornblower]

Thanks Fellas.

66Pony, I'll collect pressure data and post it next time I take the boat out.

I spoke to James from Monitor Products on Friday, they don't have an explanation as to why exchanger is not working as a full FWC system (exhaust and block). He told me to call again on Monday afternoon. He wants to look into sending me some parts so it can be plumbed up as a true half system. I'll let you know what they do.

BTW, my mechanics Joe and Charlie have installed many Monitor and San Juan exchangers before, but they tell me this is the first time they ran into this problem.

So we were able to get out this afternoon, after filling up with gas in Seatuck Cove we headed to Davis park on Fire Island, almost an hour cruise at 4000RPM (30MPH). The engine temp stayed solid 158 degrees the whole way there. What an absolute pleasure it is was to be able to actually go somewhere without having to stare at the temp gauge. Not that I didn’t, just not as often and when I did it made me smile rather than grimace :smt001.

Here's a picture of the sunset on our way back.

 

[magster65]

:smt038

 

[annie]

Pleased you have a solution Steve, refer post 31. These forums are a great way to get answers.

 

[Hornblower]

Pleased you have a solution Steve, refer post 31. These forums are a great way to get answers.

Annie, yes that was a good suggestion. But it would've been nice to have made it work as a full system.

I’ve been talking to Monitor Products to understand exactly why the system runs so cool after moving the exhaust manifold and riser block off of the freshwater side. My Monitor Products contact James seems to think there is a restriction in the exhaust side, my mechanics think it’s the exchanger.

Here’s our email exchange:

Me: “As per our [phone] conversation yesterday, you said there are two types of plates/gaskets that can be used on the riser blocks, and that one type had a very small opening compared the other. Can you please provide the part numbers of those plates for our future reference?

I was also considering your idea that the reason the exchanger is doing it's job now is because there is a possible restriction in the exhaust manifolds or riser blocks, and now that they are no longer on the freshwater side the freshwater can move though the exchanger more freely. But as you've stated in the past that all the cooling really takes place on the raw water side and since that's the case wouldn't a restriction in a raw water cooled exhaust manifold or riser invariably cause the engine to overheat, or at least run hot?”

James: “The part numbers on the gaskets are:

Full Flow gasket # 27-864547A02
Restricted flow gasket #27-864850A02
Block-off gasket #27-864549A02

You have to remember the raw water pump has a capacity of about 30 GPM and most circulating pumps run at 50-60 GPM. So a restriction that cut the jacket water flow rate in half would still leave the raw water flow unaffected.

A restricted flow gasket would do that.”
​

So I checked with my Mechanic Joe, and he said that I did have the restricted flow gaskets but he changed them out when he initially installed the conversion kit.

James also says that Monitor now recommends that for full FWC systems that the separation plates (Block-off gaskets) be installed below the riser block so the exhaust manifolds are fresh water cooled but not the riser. This would apparently eliminate the possibility that a restricted flow gasket could cause problems for the freshwater side. BTW, they haven’t updated their installation manual with this information yet.

I’d really like to make this a true half system now (using the existing exchanger), we’ll see if Monitor is forthcoming with the parts.

 

[CJ Sea Ray]

My San Juan full system on a 7.4 MPI had the same problem until I put in a through-hull pickup. Problem solved. MerCruiser will only vouch for their half-system. Apparently, the restricted raw water flow from the Bravo lower unit cannot keep up with the block+manifolds+riser requirement. If you want a full system, I think you'll have to add the through-hull pickup. The other benefit is having a seacock (safety) and strainer (keeps stuff out of your oil and power steering coolers as well as your raw water side of your heat exchanger).

 

[cp_71]

EDIT - Just saw note regarding the thru hull pickup, noticed that the install manual for my 5.0 B3 mentioned that it is the preferred Merc setup as noted below.