External pump fresh water cooling system flush

[Captain Kirk]

Hello fellow Afourians, looking to you for corroboration or further advice on some advice from a marine mechanic who has since retired. After installing a new double acting thermostat and still having the motor overheat, I’m thinking that the cooling system needs to be flushed. What do you think of this method:
After removing the thermostat (for the fourth time) and putting the housing back on, I plan on running muriatic acid through the anti freeze cooling system by using an external pump hooked up to the exit hose from the Oberdorfer fresh water pump up through the diverter valve on the side of the manifold, through the thermostat housing, to the exhaust manifold and out the exit port on the exhaust manifold which will flow back into the diluted acid bucket where it can be picked up again by the external pump and recirculated. I will close off the crossover valve (which I left on when installing the new thermostat) to make sure the acid will go back through the diverter valve on the side of the manifold. After a few cycles I'll run fresh water through from the dock hose to clean out the remaining guck. This way I won't be starting the motor at all. How does that sound to you all. Any advice will be greatly appreciated! Thanks Kirk
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[Dave Neptune]

The plan is OK but removing the t'stat and closing the bypass, then filling with the acid solution and start the engine. This would work fine but flushing the gunk and acid out will be a bit problematic to do a good job.

Since the engine is fresh water cooled (running anti-freeze through the block) I seriously doubt the problem would be in the block as the reason for using anti-freeze is it doesn't rust the block up.

I would bet the problem is on the raw water side of the HX as it will get the krud build up that the block won't! Have you determined that the HX is actually removing heat or just flowing water. A heat gun will give a lot of answers. You can even feel the exit fittings of the raw water side of the water exiting the HX, if it is cool bingo no heat is being removed, very common.

A good way to flush the block or the HX is to use straight vinegar or industrial grade if you can find it. It does take a couple days of soaking and no real stinky mess. The worst part is the boat will smell like a salad instead of an out-house. Most HX's can be easily removed and serviced away from the boat.

Note vinegar does take longer but is not nearly as caustic to work with. If vinegar won't do it then go to the acid.

Dave Neptune

[Captain Kirk]

Thanks Dave, Good suggestion about checking the temperature of the raw water leaving the hx. I've been using a heat gun to check various parts of the motor while running and to see if my deck guage which was pinned at 250 was accurate. The temperature near the sender at the back of my v-drive motor and near the thermostat was around 150-170 but the front near the exhaust manifold exit port on the metal was close to 300 as was the adjacent part of the cooling manifold. As it was evening and I had to get back to port, I ran in at lower revs to get home which cooled it a little but not within the specs of under 180.
As far as having 'gunk' in the passages, there was a considerable amount showing in the thermostat hole in the head when I first took off the housing and can be seen in the picture if I can get it to upload. One of the other Afourians suggested that the gunk that you can see in the head was left over from when it was salt water cooled and never properly flushed after the heat exchanger was installed.
If I use an external pump to flush the acid or vinegar through, is it really necessary to start the engine? As the heat exchanger was removed and boiled and flushed a mere 60 hours ago, I've been hesitant to consider this a problem. I'll check the temperatures on the heat exchanger while running it at the dock late this afternoon or tomorrow. Gotta do an oil change anyway.

[Dave Neptune]

Captn, that does look funky. It also looks really oily too. Have you been finding oil in the expansion tank or indications of water in your oil?

I would flush it with a good strong detergent by running the engine with the detergent and water after draining the coolant. You want most of the oil removed. Then you'll be ready for an acid flush. Vinegar won't cut the oily residue. Run the engine with detergent and water then drain if it looks better try an acid wash.

The engine should of been thoroughly flushed before adding the HX. That's a poor installation of an HX by who ever did it.

I'd also check the diverter in the side plate since the back of the engine is so much hotter than the front.

After flushing the engine with the acid flush and drain a couple of times until most all of the loose scale is removed. The scale will tend to pile up at the exit fitting of the block. I'd also unhook the HX after the detergent flush so it does not get clogged with scale.

Dave Neptune

[Captain Kirk]

Hi Dave, Thanks for your quick response! If you're referring the the lack of cleaning the cooling chambers when installing the HX, as a 'poor installation', I agree. I actually don't know who did it. On top of that the Hx is down below the side of the engine instead of in a normal configuration, higher up. The The way it is now the 'radiator cap' on the hx is the only thing preventing hx from leaking. See pic..
Can you recommend a detergent to use and what concentration?
The last time I checked the diverter, a few months ago, it was running clear. More investigation needed obviously. Thanks again for your responses.

[Dave Neptune]

Captn, get a length of hose and plumb your exit from the manifold directly to the pump intake. This will make a closed loop. Fill the block with fresh water and a couple of tablespoons of dish soap (Dawn or Palmolive Etc) then put the t-stat housing back on. Leave the bypass almost completely closed just so it will flow a tiny bit too. Start the engine and let it run around 12~1500 RPM until it gets warm then shut it down and drain. Take a look inside and if there is still a lot of oil repeat. The HX will still supply water to the exhaust so that the exhaust will not overheat.
When the oil is gone you will be able to see the actual block and then it will be time to acid flush. If you really break a lot of debris loose you may want to replace the pump impeller. Especially if you run the engine with the acid to stir things up. I usually use a circulating pump in a bucket and go around the pump as I have a machine coolant pump. Warming the engine (running it) really helps get the acid everywhere. The bucket will gather most of the debris so keep the pump off the bottom.

Dave Neptune

[Captain Kirk]

Thanks again Dave for these clear instructions. Will do when I get down to the boat next.
What do you think of me using your recirculating method of just connecting the manifold exit to the intake on the fresh water pump with the soapy water first and then using a small AC transfer pump, a "Star 360 HPP", for circulating the acid, thereby bypassing the Oberdorfer fresh water pump?
Do you run the motor while drawing acid from the bucket with an external pump?
I was planning to use a small 'Star HPP 360" transfer pump with a little trepidation, as it says on the side of it, "not to be used with temperatures above 130 degrees F". Thanks again for your much appreciated advice!

[zellerj]

My A4 was running at about 210 F for a number of years. I could not figure out why for a long time. Then I took off the side plate and found 25 years worth of crude surrounding the cylinder walls. Cleaned that out using a dental pick, replace the side plate, and suddenly my engine was running at 140 F. I would give that a try. Remove the starter and alternator and the side plate does not take too long. One hour worth of dental pick work, and reinstall. Worth the effort in my mind.

[Captain Kirk]

Thanks Zeller, I'll keep that in mind. I'm sorting hoses right now to flush the gunk out of the head first. It's a challenge to know which troubleshooting attack to try first. I'll keep you posted after I get going on it.