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496 Mag Heat Exchanger

Old 05-18-2016, 11:38 AM
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Default 496 Mag Heat Exchanger

Trying to replace the thermostat on a 496 mag, I have no room to see and I have no shop manual.
Does the hear exchanger just pop off upwards enough to remove thermostat,
Or is this thing hard mounted?
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Old 05-18-2016, 12:37 PM
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After you remove the flame arrestor, just remove all of the hose clamps off of the heat exchanger fittings, including the two large bands that are holding the H.E. to the crossover. Then it just lifts right off.

Here is a picture of just the Heat Exchanger so you know what to look for:

[ATTACH=CONFIG]555693[/ATTACH]

Notice, there are two hose fittings on the bottom, on one side of the Heat Exchanger, one behind the other. (Right side of picture above.)


You want to replace the large round grommets between the heat exchanger and the crossover, as they are known problem areas (for leaks). Also, while you have the heat exchanger off, go ahead and buy two new end gaskets for the heat exchanger, take the ends off, and clean the ends out. The ends are about 2" deep, and this is where all the debris ends up, which can and does impact cooling. This is not a hard job at all, but BE VERY CAREFUL with the heat exchanger once you have it out.
1) It's heavier than you expect.
2) They are a bit "fragile" so if you drop it, it's likely done.
3) They are ridiculously expensive to replace. I paid $1200 each for new ones last year.


Take note of the "clock position" of the collar mounted over the thermostat, You need to return it to the same position when you reinstall. You will be able to tell how it re-installs, but if you are not paying attention, you can install it backwards. Ask me how I know that.

Your Mercury dealer or anyone of the parts houses can get the grommets and gaskets for you in no time. Again, I strongly recommend doing this if you are pulling your heat exchanger.
Attached Thumbnails 496 Mag Heat Exchanger-542-1500.jpg  

Last edited by Sydwayz; 05-18-2016 at 12:46 PM.
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Old 05-18-2016, 01:28 PM
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out of curiosity, can you clean out the ends of the heat exchanger without removing it?
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Old 05-18-2016, 01:35 PM
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Originally Posted by Jims406rs
out of curiosity, can you clean out the ends of the heat exchanger without removing it?
Yes, as long as you have good access. But being able to run water through it, and SEE through it is important to make sure none of the longitudinal tubes are clogged is helpful.
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Old 05-18-2016, 02:19 PM
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These heat exchangers can be serviced by the local radiator shop if you discover any issues, for me it was small amount of antifreeze seen in the exhaust when starting.
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Old 05-18-2016, 03:41 PM
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Yeah, I was merely swapping thermostats due to a whipple stage 2 reprogram.
Unfortunately I didn't have all new of the above said parts, so now I have a little drip drip around where the exchanger meets the thermostat housing. I assume that's where it's coming from.
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Old 05-18-2016, 03:48 PM
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the rubber grommets that the heat exchanger mounts into are very important. I would replace those for the reinstall.

first you have to remove the 2 hose clamps and all the hoses that are connected to the heat exchanger. The pull it up and out.

T stat rest in the port side of the crossover .

We pressure test these all the time because people think that the air actuator system drains all the water completely from all water sides of the engine for winterizations - 100 % for a fact I know - it does not.

Also I highly recommend buying a OEM service manual. The manual will tell you how to pressure test the whole cooling system. Very hard for me to understand how a person works on their own engines without a service manual at the very least. I am just saying as the service manual has all the specs and proper service procedures along with other detailed info and diagrams plus wire diagrams. Alot easier than hopefully getting the correct info from where ever ie: a friend or the net or winging it as they go - and so on.

Most single service manuals brand new are 100 to 150 dollars about equal to 1 hour marine shop labor rates. Used can range fro 40 to 80 bucks depending.

If needed I have most OEM service manuals for sale. New and used.

Also for correct parts and service procedures it all goes by engine serial or if working on the drives that serial number as well. Very good idea to know these serial numbers when looking for info.

Last edited by BUP; 05-18-2016 at 03:57 PM.
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Old 05-18-2016, 07:46 PM
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Originally Posted by BUP
the rubber grommets that the heat exchanger mounts into are very important. I would replace those for the reinstall.

first you have to remove the 2 hose clamps and all the hoses that are connected to the heat exchanger. The pull it up and out.

T stat rest in the port side of the crossover .

We pressure test these all the time because people think that the air actuator system drains all the water completely from all water sides of the engine for winterizations - 100 % for a fact I know - it does not.

Also I highly recommend buying a OEM service manual. The manual will tell you how to pressure test the whole cooling system. Very hard for me to understand how a person works on their own engines without a service manual at the very least. I am just saying as the service manual has all the specs and proper service procedures along with other detailed info and diagrams plus wire diagrams. Alot easier than hopefully getting the correct info from where ever ie: a friend or the net or winging it as they go - and so on.

Most single service manuals brand new are 100 to 150 dollars about equal to 1 hour marine shop labor rates. Used can range fro 40 to 80 bucks depending.

If needed I have most OEM service manuals for sale. New and used.

Also for correct parts and service procedures it all goes by engine serial or if working on the drives that serial number as well. Very good idea to know these serial numbers when looking for info.
BUP/John is right. I have a manual, and it has come in handy.
I also have to say a HUGE thanks to John who went out of his way to help me track down a T-bar clamp for the manual drive trim indicators. We tore one up last week replacing indicator cables, and John was a HUGE help. Thanks John!!!
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Old 05-18-2016, 09:30 PM
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No problem anytime Brian and thanks for the thanks.

The tip for reinstalling the heat exchanger into the crossover grommets is to spray them with soapy water or glycerin like what Volvo Penta uses for impeller installs. I am a huge fan of the water base glycerin even for impeller installs and for some hose installs. Do not install the heat exchanger with dry grommets

What I have done is used a thin even coat of perfect seal on the new grommets outer and inner for the install. The heat exchanger slips right in and I have never had a leak afterwards even years down the road.

Do not use the old grommets you asking for a leak especailly if they are 10 years old or older. They are very due for new installed ones.

Last edited by BUP; 05-18-2016 at 09:32 PM.
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Old 05-19-2016, 11:14 AM
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BUP, what is the number on the heat exchangers...
...as in how many tubes can you plug (braise shut due to individual leaks) and the heat exchanger still be called serviceable by Mercury standards?
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