(referencing a 2005 Yamaha AR230HO)

I solved an intermittent overheat that resulted in limp mode on one of my engines due to a bad thermosensor, but was still getting overheat warnings on both engines during WOT runs over 4-5 miles. Monitoring with YDS (Yamaha Diagnostic Software), both engines were doing the same thing at about the same rate, so I ruled out electrical issues. I then suspected that the issue was scale build-up inside the water jackets, particularly calcium carbonates, likely due to very poor flush practices from previous owners. I devised this closed loop flush to remove the scale.

I always flushed the engines thoroughly post-salt with Salt-Away during wash down at the house and I would "pre-flush" with fresh water from a spigot before I left the dock area, when available. I am convinced that there is just no way to completely prevent the accumulation of build up, especially from salt water. I've also concluded that the small-diameter factory flush inlet is inadequate to fully fill the waterway components. With that in mind, I imagine issues can worsen dramatically if proper post-salt flushing is not done after each outing. I experimented with solutions and tried a few different approaches to closed-loop flushing and it improved my temperature readings each time. This write-up only explains what I found did the best to remove the crud while minimizing the risk of damaging the engine. Check here for the full experiment, including results of different acid solutions and my thoughts on the shortcomings of the Yamaha flush system.

SULFAMIC SOLUTION: This is what I used to flush the system and is not to be confused with sulfuric acid. I did a few impressive experiments with it on parts prior to coming up with this flushing technique. It works very well on calcium that builds up from salt water. I tried different concentrations and found 4% to be adequate without being too harsh. You can buy Sulfamic Acid crystals at Home Depot in the tile cleaning section. To prevent damage to the aluminum parts, I mixed in some Thiourea which can be found on eBay. The formula is explained below and each engine got a fresh batch of solution. Originally, I followed the sulfamic flush with a solution of Soda Ash to neutralize the acid but I no longer recommend this step as future experiments showed me how reactive the Soda Ash can be with aluminum. Using the Thiourea and then doing an extended soapy flush should suffice in rinsing away the acid.

- Submersible Water Pump (Northern Tool 42358).
- Garden Hose Remnant (Northern Tool 500011).
- 2" PVC Pipe cut to a length of 6" (1).
- 2" PVC Cap (2).
- 3" PVC Pipe cut to a length of 2" (3).
- 0.50" x 2" PVC Riser (in the grass sprinkler section) (4).
- 3" to 1.50" PVC Reducer (5).
- 1.50"-to-1" PVC Reducer Bushing (6).
- 0.75" Slip x FHT PVC Hose Fitting (in the grass sprinkler section) (7).
- Male end of a Garden Hose with about 48" of hose attached (8).
- All Purpose PVC Cement.
- 3/8" ID Tubing about 10' in length.
- 3/4" ID Tubing about 2' to 2.5' needed.
- 1/2" OD Tubing about 2' in length.
- Garden Hose Repair End (Female 5/8" to 3/4"; see pic further down ).
- 5 gallon bucket.
- Roll of Electrical Tape.
- (2x) Radiator Hose Clamp Pinch-Off Pliers.

BUILD THE FLUSH ASSEMBLY: Parts 1 and 2 or not part of the flush assembly. Using PVC cement, assemble parts 3 thru 7 in the above pic just as they are laid out. Be mindful not to allow the FHT Fitting (7) to seat completely into the back of the 1" Reducer Bushing's (6) recess, otherwise the collar will not spin. Drill a hole near the 3" inlet (this hole will go through combined parts 5 and 3) - the largest regular bit I had at the time was 5/8" but I think an 11/16" drill might've been better. I shaved the threads off smooth on one end of the 0.50" Riser (4) with a bench grinder so that it would fit into the 5/8" hole (I had to grind it fairly thin for it to do so). Apply cement to the shaved end of the Riser and tap it into the new hole with a mallet.

DISASSEMBLE THE EXHAUST:  I flushed one engine at a time and started by removing the clean-out plug tray to gain access to each water box and remove their straps. Loosen the four hose clamps that secure the aluminum joint with "UP" embossed on it (fiche part #45; referred here-after as the "UP Pipe") between the muffler (fiche #36) and the water box. Remove the UP Pipe and both rubber collars (fiche #46 & 47) that are attached to either side. Unbolt and remove the tail pipe and gasket (fiche #43 & 42) to allow for larger debris to flow out of the system quicker.

MOVE THE WATER BOXES: After removing the clean out tray, I unclamped and popped off the large hoses coming off the top of both water boxes. I removed the starboard water box up & out through the tray hole and then slid the port water box aftward a few inches. I finally unclamped and removed the long, white drain tube that runs from the interior deck to the scupper.

MAKE THE LOOP: Work one engine at a time. With the UP Pipe (fiche #45) removed from the engine and on a workbench, I slipped the Flush Assembly (3-7) over the smaller end of the UP Pipe - I had to use very course grit sandpaper (40grit) to open the inner diameter of the 3" PVC (3) enough to get it to slide over it snugly (I was advised that I could have heated the PVC up to get it to stretch over the pipe). With the embossed letters "UP" considered 12 o'clock, I affixed it back onto the muffler (fiche #36) so the Riser (4) pointed between 9 and 10 o'clock. I used Electrical Tape to secure the Flush Assembly (3-7) to the UP Pipe and make it all water tight. I then added the 3/4" ID Tubing to the threaded-side of the Riser (4). The Garden Hose (8) should now be attached.

Mallet the 2" PVC Cap (2) onto the short length of 2" PVC Pipe (1) and insert this into the open end of the muffler (fiche #36) - this plug greatly slows the solution from making its way up the exhaust and to the pistons. The pipe portion will betoo loose and would fall out, so wrap PVC Pipe a few times with electrical tape, just below the Cap edge. I wrapped mine until it was just thick enough that it snuggly seated inside the muffler. It doesn't have to be absolutely air tight, it just has to stay in place.

I slipped the larger rubber exhaust collar (fiche #46) back onto the end of the muffler (fiche #36). I took the entire Flush Assembly (now consisting of parts 3-8, the UP Pipe, and the 3/4" Tubing) and inserted the Garden Hose (8) through the exhaust hole in the stern, ran it over the first water box support bracket, then through the hole in the second support bracket, and then to the outside through the scupper. I then plugged this entire Flush Assembly back into the rubber muffler collar (fiche #46). While supporting the assembly so it was level, I tightened the two hose clamps (fiche #48 & 49).

I removed the two hoses coming from the top and bottom of the thermostat housing. I cut the 3/4" ID Tubing coming off the Flush Assembly's Riser (4) so it was long enough to reach the larger/lower nipple of the thermostat housing. With the correct length measured/cut, I unbolted the housing to remove the thermostat and attached the Tubing to the lower nipple, then remounted the thermostat housing. I attached the 3/8" ID Tubing to the upper nipple on the thermostat housing and ran it up and out to a 5 gallon bucket sitting outside below the scupper. The purpose for this 3/8" Tubing is to give any excess pressure and solution a route to escape back to the bucket - not including this will likely cause/worsen back flow into the exhaust and up to the pistons.

Place the Submersible Pump in a 5-gallon bucket that is sitting outside the boat, below the scupper outlet. Attach one end of the Hose Remnant to the Submersible Pump and attach the Female Garden Hose Repair End to the the other end, then place this end in through the tray area. Disconnect the hose that delivers raw cooling water from the impeller duct to the starboard side of the engine - you will find this hose by looking down through the tray area and seeing this hose coming from the engine bay and plugging into the back of the pump plate (on the port side of each plate). Plug the Female Repair End (originating from the submersible pump) into this raw water hose - see middle pic below and note the plate nipple for the raw water intlet. This connection will feed the flush solution from bucket to the engine via the Submersible Pump. Back inside the engine bay, use one of the Hose Clamp Pinch-Off Pliers to clamp off the red Yamaha flush inlet hose above where it and the raw water inlet hose meet at the y-coupler (rear, lower, starboard side of each motor).

Outside at the impeller housing, I inserted the purcahsed 1/2" OD Hose (the red-orange hose in the next pic) into the upper starboard outlet located on the outer-side of the pump plate - this connects to the small hose (fiche #5) coming off the muffler (fiche #36) and is one of the returns that brings solution back to the bucket. Finally, using the second pair of Pinch-Off Pliers, I clamped the other pee hose (fiche #4) that runs out from the huge 90* rubber elbow (fiche #33) out to the starboard peehole. The system is now closed-loop and ready for pumping.

In the pic above: the hose through the scupper connects to the back of the flush assembly; the clear hose connects to the top of the thermostat housing; the green hose with yellow stripe connects to the raw water inlet hose; the red-orange hose plugs into the plate where the small hose from the muffle connects.

HOW IT WORKS : The engine is not running during this procedure. The pump delivers solution up the remnant hose and into the raw water inlet hose that the boat's jet pump utilizes. This flushes throughout the engine and is returned via the flush assembly along with the hose plugged into the outside of the jet pump plate. Some excess solution should make its way up to the thermostat housing. I originally tried using the factory flush inlet but found I did not get nearly as much pressure or solution through the system due to that hose's much smaller inner diameter. This was probably also not allowing me to fill the entirety of the exhaust jackets during my routine post-outing flushes. To me, this small diameter hose is a flaw in Yamaha's design which prevents a thorough flush with the average garden spigot.

MIXING SOLUTIONS: I opted to use a 4% acid solution with about 1% inhibitor (that's 1lb of sulfamic crystals and 1.5oz of thiourea in 3 gallons of water). I mixed up the sulfamic solution with boiling water and mixed the solution in a 5 gallon bucket after the flush system was installed. The hot water is required to fully dissolve the crystals and also allowed me to feel that different areas of the engine were getting the solution. I would flush for 15 minutes and allowed it to drain. I then removed the flush assembly (including the UP pipe) and fired the engine for two seconds. I followed this with a flush of some soapy water then several clean flushes of fresh water with an additional garden hose feeding the Yamaha flush inlet until sediment became minimal in the bucket. Adding the second hose helps push larger bits of sand out that were broken up during the acid flush or lying along the floor of the muffler.

1. Sulfamic solution (closed loop) - 15mins
2. Clear system
3. Soapy rinse (closed loop) - 7mins
4. Clean rinse w/ additional hose at Yamaha flush adapter (open system) - 3mins
5. Clear System.
6. Reassemble.

closed loop = pumping, with return hoses feeding back to the bucket for recirculation.
clear system = drain, remove "UP" pipe and PVC Plug, then fire engine for a few seconds.

open system = pumping, but return water will spill up out of bucket and onto driveway since we are also adding the flush adapter.

Three gallons of water will be adequate to fill the system and leave enough in the bucket so that the pump doesn't cavitate. The thermostats can be placed into the bucket of sulfamic acid to clean them while the flush is being done - the acid will not harm the rubber. You can also throw the tail pipes in as well. I always replaced my tail pipe gaskets, but this probably isn't necessary. I advise flushing for 15mins because this should allow enough time to dissolve a decent amount of calcium without the solution backing up into the cylinders. I would recommend repeating Steps 1 through 5 before reassbembly if you are having overheat issues. I did three flushes, one with a retail descaler followed by one with the acid in the spring of 2016 then another acid flush Spring 2017, and was still getting plenty of junk out.

RESULTS: During the fresh water rinses, I eventually added another garden hose hooked to a spigot with the Yamaha flush adapter to the standard flush inlet under the hatch. Feeding both inlets simultaneously, the greater volume helped to push additional bits of larger debris along. I did a few cycles with clean water, catching the water in the bucket and letting the sediment settle. I then skimmed the water off and collected the sediment in a jar from all phases. Unbelievably, the jars shown here contain only a portion of what I flushed out of each engine on the third flush! There is a bottom layer of sand, a middle layer of very fine silt (dissolved calcium?), and the top layer of solution. I am also showing how well the thermostat housing was cleaned. Following my flushes, I had zero issues with overheating in the remaining years I owned the boat.

FUTURE CONSIDERATIONS: This is something that should probably be done every one or two years, depending on the type of water frequented and the number of outings. I would have liked to modify the flush assembly for a larger output hose diameter and connect directly to the barb on the back of the scupper (rather than run the short garden hose through it), allowing increased system flow. My hopes for this would be to increase the current to move debris along and to reduce solution backing-up into the exhaust. The downside to this could be that the system empties faster than it can be filled, preventing the sulfamic solution from cleaning the upper surfaces. However, a plastic ball valve could be installed on the return hose to restrict flow during acid flushes (calcium) and open it fully during clean water flushes (debris). In either case, I have proven that this system does what it was intended to do.

THERMOSTAT MOD: I decided to drill an additional hole in my thermostats. I am not the original owner and the manual does not state how to orient the hole in the thermostats - my guess is the holes sit at 12-o'clock to allow air to pass out of the system when the thermostat is closed. Unfortunately, this can allow calcium to settle in the thermostat housing during lower temperature driveway flushing. I believe these boats were designed/intended for freshwater use (due to the make-up of the factory anodes and the inadequate flush system), so calcium may never have been a big R&D issue. The new hole is 1/16" and I opted to place the larger, original hole at 6-o'clock to help evacuate any minerals during routine flushing.

Calcium content of waters (PPM = mg/L):
Seawater: 400 PPM
Freshwater: ~40 PPM*
* A very rough average as waters vary greatly across the US . The range seems to be as low as 0.50 PPM and can be as high as 90 PPM.

Trail Tech TTO: For added peace of mind, I later installed sensors (part # 732-EH2) near each thermostat to monitor temps from helm gauges. These are battery-powered gauges that can be hooked up to an outside power source, are programmable, have clocks, and are backlit. The 22mm adapter is a very tight fit - I could probably get away without the clamps! There is a 19mm option, but that could be too much of an exact fit to be water tight. Both engines consistantly showed around 121 degrees Fahrenheit at WOT, which matched readings from another boater with the same sensors.

CrankyGypsy (established 2001)