I watched my raw-water-cooled 1982 Volvo MD7A engine run progressively
hotter over the past summer, until it got so bad I couldn't use the
boat anymore. After asking around about options, I decided to attempt
a chemical flush of the cooling circuit. Here's what I did, and the
outcome, for whomever it might benefit. This is rather long, but I
found myself wishing I could find more information before I did the
job, so maybe this will help someone.

OUTCOME:

It appears this was a major success. Before the flush, the engine ran
hot well into the red even at 1300 RPM. After the flush, there is
visibly much more water coming out the exhaust, and I can run it
continuously at the manufacturer's recommended maximum of 2500 RPM and
it stays nicely in the green.

PROCEDU

Searching the web led me first to a product called Marsolve, which is
no longer available (though the company still has their web site up.)
I found an equivalent product now being marketed as Rydlyme Marine
(http://www.rydlymemarine.com/). I was unable to find a local dealer
for this product, so I ordered some directly from the manufacturer
(Apex Engineering Products, Aurora IL). The woman who took my order
told me that Apex used to manufacture Marsolve, but has taken over
marketing the product themselves. Rydlyme's MSDS indicates it's made
with hydrochloric acid, but there are other additives in it that they
don't divulge.

Rydlyme has instructions for flushing a raw-water cooled marine engine
http://www.rydlymemarine.com/instructions.html. I followed these
instructions pretty closely, with a few exceptions. Here's what I did.

I borrowed from a friend a utility pump, Teel model 1P579F. This is an
impeller pump, almost the same as the raw-water pump on my engine,
powered by a small 115V electric motor. I bought a bunch of 5/8 inch
internal clear vinyl hose, and some plastic hose fittings. I connected
things up as follows:

1) Closed the raw-water intake seacock and disconnected the hose
connecting the seacock to the reverse gear. Attached a length of vinyl
hose from pump output to the reverse gear input.

2) Bypassed the seawater pump. I did this by disconnecting the copper
tubes that connect to it, removing the pump from the engine, and
connecting the copper tubes back together with a short length of
plastic 1/2 inch water pipe. This makes it sound simpler than it was;
I had to fuss around quite a bit before I got this secured and
water-tight.

3) Disconnected the upper (outlet) hose from the thermostat housing.

4) Removed the thermostat housing. I wanted to make sure that the
flush was going through the engine block, not bypassing around the
thermostat. To accomplish this, I determined that the bypass path was
through a single 1/4 inch hole in the thermostat housing. I plugged
this hole with a small plastic dowel I found at the hardware store. I
had an old thermostat, from which I removed the spring and plunger so
that it couldn't close, and I put the thermostat into the housing and
bolted the housing back on. It's necessary to have a thermostat in
place in order for the housing to seal, otherwise I would have left the
thermostat out.

5) Connected another length of vinyl tubing from the output of the
thermostat housing to a bucket, and a third length of tubing from the
pump input to the bucket.

6) Poured into the bucket one gallon of Rydlyme Marine, and about three
quarts of water, for a total volume of about seven quarts. Rydlyme
recommends diluting to 50%, but my solution was diluted a bit less than
that.

7) Started the pump and ran it for 3-1/2 hours, 1/2 hour less than the
maximum recommended.

8) Pumped out as much of the Rydlyme as possible, then flushed with
fresh water (from a garden hose) until it ran clear.


Now I was ready to test to determine if I had done any good. In order
to do this, I rigged up an inexpensive pressure gauge and a valve on
the garden hose attached to the input to the reverse gear. I opened
the valve. The tap water pressure was quite high, 120 psi, so I used
the valve to regulate the pressure into the engine. As I increased the
pressure, I still didn't get very much flow out. I was worried that I
hadn't done any good. Then I decided to try back-flushing, so I
reversed the hoses and applied the tap water into the thermostat
housing. Hooked up this way, when the intake pressure got up to about
80 psi, something inside the engine broke loose, a whole bunch of black
stuff washed out of the engine, and all of the sudden it flowed MUCH
more freely. Clearly I had cleared something out.

After that happened, I flushed it back and forth in both directions
until I got no more black stuff out.

At that point I reassembled the engine, and took it for a test drive.
It runs wonderfully cool now. I have run it at normal operating speed
(1500 to 2000 RPM) for over an hour, with intervals at 2500 RPM, and
the temperature stays right in the normal range. It's never run this
cool for me!

A few comments about this:

First, this job ruined the pump I used. It has a chrome-plated bronze
pump head, and when I took it apart to replace the impeller, I saw that
about half of the working surface inside the pump head had no more
chrome plating, and on these areas the underlying bronze was badly
etched away. I don't know what kind of shape it was in before I got
it, but I'm pretty sure at a minimum that the Rydlyme contributed to
etching the bronze. This makes me very glad that I removed the
engine's seawater pump from the circuit, as it's made of bronze too.

Possibly related to the erosion inside the pump head is erosion that I
saw of just about anything on the engine that's made of copper. On the
MD7A, the pipes that carry water into and out of the seawater pump are
made of copper. These had the appearance inside the ends of the tubes
of having been chemically etched... a rough surface with a bright
copper color. The solution that was pumped through the engine has a
light greenish yellow color after the flush; this suggests to me that
it has a fair amount of copper dissolved in it. To confirm this, I
dropped an iron nail into a bit of the solution: within a few minutes,
the nail was copper-colored. This is the same result you get if you
drop a nail into a solution of copper sulfate, so I conclude there's
quite a bit of copper in the solution. While I don't think this
damaged the engine in any significant way, I think it's worth noting.
Rydlyme claims that their product is safe for all metals, but for what
it's worth, something caused the copper to be etched.

One other evidence of chemical reaction is that there was considerable
effervescence of some gas (I would guess hydrogen) out of the engine
while it was being flushed. The effervescence slowed considerably
during the period of the flush.

I find myself wondering what would have happened if I attempted the
back-flush with tap water BEFORE I did the acid flush. I'll never
know.

And finally, I didn't remove the reverse gear from the circuit, as
Rydlyme's instructions recommend. It got flushed along with everything
else. I doubt if there's anything in it that would be damaged
significantly.

Maybe in a year or two I'll discover that this job did some serious
damage to something in the engine, but for now, I'm delighted to be
able to use my it again. It probably doesn't have that many more years
left in it, but I'm happy for now. If only I could get it to start
more easily...

Ordinary backflushing would probably have removed most of the crud. The
problem is that when the cooling system is clogged up it is typically in the
exhaust section. You should consider at least removing and inspecting the
exhaust manifold and riser or just going ahead and replacing them. A
failure of the exhaust system components will lead to water in the engine.
If you're lucky it will just hydrolock. If you're not lucky you will break
a rod.

wrote in message
oups.com...
I watched my raw-water-cooled 1982 Volvo MD7A engine run progressively
hotter over the past summer, until it got so bad I couldn't use the
boat anymore. After asking around about options, I decided to attempt
a chemical flush of the cooling circuit. Here's what I did, and the
outcome, for whomever it might benefit. This is rather long, but I
found myself wishing I could find more information before I did the
job, so maybe this will help someone.

OUTCOME:

It appears this was a major success. Before the flush, the engine ran
hot well into the red even at 1300 RPM. After the flush, there is
visibly much more water coming out the exhaust, and I can run it
continuously at the manufacturer's recommended maximum of 2500 RPM and
it stays nicely in the green.

PROCEDU

Searching the web led me first to a product called Marsolve, which is
no longer available (though the company still has their web site up.)
I found an equivalent product now being marketed as Rydlyme Marine
(http://www.rydlymemarine.com/). I was unable to find a local dealer
for this product, so I ordered some directly from the manufacturer
(Apex Engineering Products, Aurora IL). The woman who took my order
told me that Apex used to manufacture Marsolve, but has taken over
marketing the product themselves. Rydlyme's MSDS indicates it's made
with hydrochloric acid, but there are other additives in it that they
don't divulge.

Rydlyme has instructions for flushing a raw-water cooled marine engine
http://www.rydlymemarine.com/instructions.html. I followed these
instructions pretty closely, with a few exceptions. Here's what I did.

I borrowed from a friend a utility pump, Teel model 1P579F. This is an
impeller pump, almost the same as the raw-water pump on my engine,
powered by a small 115V electric motor. I bought a bunch of 5/8 inch
internal clear vinyl hose, and some plastic hose fittings. I connected
things up as follows:

1) Closed the raw-water intake seacock and disconnected the hose
connecting the seacock to the reverse gear. Attached a length of vinyl
hose from pump output to the reverse gear input.

2) Bypassed the seawater pump. I did this by disconnecting the copper
tubes that connect to it, removing the pump from the engine, and
connecting the copper tubes back together with a short length of
plastic 1/2 inch water pipe. This makes it sound simpler than it was;
I had to fuss around quite a bit before I got this secured and
water-tight.

3) Disconnected the upper (outlet) hose from the thermostat housing.

4) Removed the thermostat housing. I wanted to make sure that the
flush was going through the engine block, not bypassing around the
thermostat. To accomplish this, I determined that the bypass path was
through a single 1/4 inch hole in the thermostat housing. I plugged
this hole with a small plastic dowel I found at the hardware store. I
had an old thermostat, from which I removed the spring and plunger so
that it couldn't close, and I put the thermostat into the housing and
bolted the housing back on. It's necessary to have a thermostat in
place in order for the housing to seal, otherwise I would have left the
thermostat out.

5) Connected another length of vinyl tubing from the output of the
thermostat housing to a bucket, and a third length of tubing from the
pump input to the bucket.

6) Poured into the bucket one gallon of Rydlyme Marine, and about three
quarts of water, for a total volume of about seven quarts. Rydlyme
recommends diluting to 50%, but my solution was diluted a bit less than
that.

7) Started the pump and ran it for 3-1/2 hours, 1/2 hour less than the
maximum recommended.

8) Pumped out as much of the Rydlyme as possible, then flushed with
fresh water (from a garden hose) until it ran clear.


Now I was ready to test to determine if I had done any good. In order
to do this, I rigged up an inexpensive pressure gauge and a valve on
the garden hose attached to the input to the reverse gear. I opened
the valve. The tap water pressure was quite high, 120 psi, so I used
the valve to regulate the pressure into the engine. As I increased the
pressure, I still didn't get very much flow out. I was worried that I
hadn't done any good. Then I decided to try back-flushing, so I
reversed the hoses and applied the tap water into the thermostat
housing. Hooked up this way, when the intake pressure got up to about
80 psi, something inside the engine broke loose, a whole bunch of black
stuff washed out of the engine, and all of the sudden it flowed MUCH
more freely. Clearly I had cleared something out.

After that happened, I flushed it back and forth in both directions
until I got no more black stuff out.

At that point I reassembled the engine, and took it for a test drive.
It runs wonderfully cool now. I have run it at normal operating speed
(1500 to 2000 RPM) for over an hour, with intervals at 2500 RPM, and
the temperature stays right in the normal range. It's never run this
cool for me!

A few comments about this:

First, this job ruined the pump I used. It has a chrome-plated bronze
pump head, and when I took it apart to replace the impeller, I saw that
about half of the working surface inside the pump head had no more
chrome plating, and on these areas the underlying bronze was badly
etched away. I don't know what kind of shape it was in before I got
it, but I'm pretty sure at a minimum that the Rydlyme contributed to
etching the bronze. This makes me very glad that I removed the
engine's seawater pump from the circuit, as it's made of bronze too.

Possibly related to the erosion inside the pump head is erosion that I
saw of just about anything on the engine that's made of copper. On the
MD7A, the pipes that carry water into and out of the seawater pump are
made of copper. These had the appearance inside the ends of the tubes
of having been chemically etched... a rough surface with a bright
copper color. The solution that was pumped through the engine has a
light greenish yellow color after the flush; this suggests to me that
it has a fair amount of copper dissolved in it. To confirm this, I
dropped an iron nail into a bit of the solution: within a few minutes,
the nail was copper-colored. This is the same result you get if you
drop a nail into a solution of copper sulfate, so I conclude there's
quite a bit of copper in the solution. While I don't think this
damaged the engine in any significant way, I think it's worth noting.
Rydlyme claims that their product is safe for all metals, but for what
it's worth, something caused the copper to be etched.

One other evidence of chemical reaction is that there was considerable
effervescence of some gas (I would guess hydrogen) out of the engine
while it was being flushed. The effervescence slowed considerably
during the period of the flush.

I find myself wondering what would have happened if I attempted the
back-flush with tap water BEFORE I did the acid flush. I'll never
know.

And finally, I didn't remove the reverse gear from the circuit, as
Rydlyme's instructions recommend. It got flushed along with everything
else. I doubt if there's anything in it that would be damaged
significantly.

Maybe in a year or two I'll discover that this job did some serious
damage to something in the engine, but for now, I'm delighted to be
able to use my it again. It probably doesn't have that many more years
left in it, but I'm happy for now. If only I could get it to start
more easily...

wrote:
I watched my raw-water-cooled 1982 Volvo MD7A engine run progressively
hotter over the past summer, until it got so bad I couldn't use the
boat anymore. After asking around about options, I decided to attempt
a chemical flush of the cooling circuit. Here's what I did, and the
outcome, for whomever it might benefit. This is rather long, but I
found myself wishing I could find more information before I did the
job, so maybe this will help someone.

OUTCOME:

It appears this was a major success. Before the flush, the engine ran
hot well into the red even at 1300 RPM. After the flush, there is
visibly much more water coming out the exhaust, and I can run it
continuously at the manufacturer's recommended maximum of 2500 RPM and
it stays nicely in the green.

PROCEDU

Searching the web led me first to a product called Marsolve, which is
no longer available (though the company still has their web site up.)
I found an equivalent product now being marketed as Rydlyme Marine
(http://www.rydlymemarine.com/). I was unable to find a local dealer
for this product, so I ordered some directly from the manufacturer
(Apex Engineering Products, Aurora IL). The woman who took my order
told me that Apex used to manufacture Marsolve, but has taken over
marketing the product themselves. Rydlyme's MSDS indicates it's made
with hydrochloric acid, but there are other additives in it that they
don't divulge.

Rydlyme has instructions for flushing a raw-water cooled marine engine
http://www.rydlymemarine.com/instructions.html. I followed these
instructions pretty closely, with a few exceptions. Here's what I did.

I borrowed from a friend a utility pump, Teel model 1P579F. This is an
impeller pump, almost the same as the raw-water pump on my engine,
powered by a small 115V electric motor. I bought a bunch of 5/8 inch
internal clear vinyl hose, and some plastic hose fittings. I connected
things up as follows:

1) Closed the raw-water intake seacock and disconnected the hose
connecting the seacock to the reverse gear. Attached a length of vinyl
hose from pump output to the reverse gear input.

2) Bypassed the seawater pump. I did this by disconnecting the copper
tubes that connect to it, removing the pump from the engine, and
connecting the copper tubes back together with a short length of
plastic 1/2 inch water pipe. This makes it sound simpler than it was;
I had to fuss around quite a bit before I got this secured and
water-tight.

3) Disconnected the upper (outlet) hose from the thermostat housing.

4) Removed the thermostat housing. I wanted to make sure that the
flush was going through the engine block, not bypassing around the
thermostat. To accomplish this, I determined that the bypass path was
through a single 1/4 inch hole in the thermostat housing. I plugged
this hole with a small plastic dowel I found at the hardware store. I
had an old thermostat, from which I removed the spring and plunger so
that it couldn't close, and I put the thermostat into the housing and
bolted the housing back on. It's necessary to have a thermostat in
place in order for the housing to seal, otherwise I would have left the
thermostat out.

5) Connected another length of vinyl tubing from the output of the
thermostat housing to a bucket, and a third length of tubing from the
pump input to the bucket.

6) Poured into the bucket one gallon of Rydlyme Marine, and about three
quarts of water, for a total volume of about seven quarts. Rydlyme
recommends diluting to 50%, but my solution was diluted a bit less than
that.

7) Started the pump and ran it for 3-1/2 hours, 1/2 hour less than the
maximum recommended.

8) Pumped out as much of the Rydlyme as possible, then flushed with
fresh water (from a garden hose) until it ran clear.


Now I was ready to test to determine if I had done any good. In order
to do this, I rigged up an inexpensive pressure gauge and a valve on
the garden hose attached to the input to the reverse gear. I opened
the valve. The tap water pressure was quite high, 120 psi, so I used
the valve to regulate the pressure into the engine. As I increased the
pressure, I still didn't get very much flow out. I was worried that I
hadn't done any good. Then I decided to try back-flushing, so I
reversed the hoses and applied the tap water into the thermostat
housing. Hooked up this way, when the intake pressure got up to about
80 psi, something inside the engine broke loose, a whole bunch of black
stuff washed out of the engine, and all of the sudden it flowed MUCH
more freely. Clearly I had cleared something out.

After that happened, I flushed it back and forth in both directions
until I got no more black stuff out.

At that point I reassembled the engine, and took it for a test drive.
It runs wonderfully cool now. I have run it at normal operating speed
(1500 to 2000 RPM) for over an hour, with intervals at 2500 RPM, and
the temperature stays right in the normal range. It's never run this
cool for me!

A few comments about this:

First, this job ruined the pump I used. It has a chrome-plated bronze
pump head, and when I took it apart to replace the impeller, I saw that
about half of the working surface inside the pump head had no more
chrome plating, and on these areas the underlying bronze was badly
etched away. I don't know what kind of shape it was in before I got
it, but I'm pretty sure at a minimum that the Rydlyme contributed to
etching the bronze. This makes me very glad that I removed the
engine's seawater pump from the circuit, as it's made of bronze too.

Possibly related to the erosion inside the pump head is erosion that I
saw of just about anything on the engine that's made of copper. On the
MD7A, the pipes that carry water into and out of the seawater pump are
made of copper. These had the appearance inside the ends of the tubes
of having been chemically etched... a rough surface with a bright
copper color. The solution that was pumped through the engine has a
light greenish yellow color after the flush; this suggests to me that
it has a fair amount of copper dissolved in it. To confirm this, I
dropped an iron nail into a bit of the solution: within a few minutes,
the nail was copper-colored. This is the same result you get if you
drop a nail into a solution of copper sulfate, so I conclude there's
quite a bit of copper in the solution. While I don't think this
damaged the engine in any significant way, I think it's worth noting.
Rydlyme claims that their product is safe for all metals, but for what
it's worth, something caused the copper to be etched.

One other evidence of chemical reaction is that there was considerable
effervescence of some gas (I would guess hydrogen) out of the engine
while it was being flushed. The effervescence slowed considerably
during the period of the flush.

I find myself wondering what would have happened if I attempted the
back-flush with tap water BEFORE I did the acid flush. I'll never
know.

And finally, I didn't remove the reverse gear from the circuit, as
Rydlyme's instructions recommend. It got flushed along with everything
else. I doubt if there's anything in it that would be damaged
significantly.

Maybe in a year or two I'll discover that this job did some serious
damage to something in the engine, but for now, I'm delighted to be
able to use my it again. It probably doesn't have that many more years
left in it, but I'm happy for now. If only I could get it to start
more easily...