Overhauling a Mercuiser Bravo III Raw Water Pump – the Good, the Bad, and the Ugly


Even with the best preventative care, boat motors are notoriously subject to failure. For the typical inboard/outboard, the raw-water impeller is one of the #1 wear items and sooner or later you’ll have one go while out on the water. If you’re lucky, you’ll have 2 motors so you can get back home on the “spare” – otherwise you can plan on a tow. Once you’ve got the boat in a position to do some work, this is a fairly simple fix that can save you a lot of labor cost vs hiring a mechanic. Read on for some of our lessons learned and tips to make your water pump overhaul go smoothly.

Our number came up last month after a fun day with friends and fireworks. Luck did hold in the sense that the impeller failed just about the time we idled into our marina. The temperature warning horn sounded shortly thereafter and I quickly shut the engine down – noting the temp was up to about 200 degrees. With plenty of people aboard and some recent 1 engine docking under our belt (the subject of another post) we got “Southern Comfort” back into her slip without too much difficulty.


This is one of the easier problems to troubleshoot. If you have sky-high engine temps, no obvious cooling hose leaks or obstructions it’s likely the raw-water impeller. Depending on your situation, a quick test with the engine cooled down to check for water flow will confirm.

The Fix

Note I said above if you’re driving a single and the water pump fails – plan for a tow (you do have towing insurance don’t you?). Technically speaking, the Bravo III water pump can be serviced in the water as it is engine mounted. If you have an Alpha drive, you’ll need to remove the lower unit – not something that can be done in-water. Here are the conditions I would consider for when this is actually possible to in-water service the Bravo III while away from a dock or slip-

  • You have no small children or spouse on board who will be become impatient
  • You are boating in a temperate climate where the temperature is under 75 degrees
  • You have a “roomy” engine room or large hatch with good front access to the engine
  • Your engine room isn’t 120 degrees from running hard
  • You have a well stocked tool bag, rags, dish soap, and lots of patience on hand
  • and of course, you have an overhaul kit handy
  • Don’t be discouraged, this isn’t a big job – just a job that goes much easier with some preparation (and a little practice).

    Recommended Tools

  • Wide blade common screw driver for hose clamps
  • 1/4 or 1/2 inch socket set
  • English box end wrench set
  • Dish soap or other water soluble lubricant
  • Gloves
  • Correct overhaul kit
  • Getting Started

    The difficulty of this job is directly related to your accessibility to the water pump housing. In my case, I have the 4.3L Mercruiser V6 engines which mount the pump housing on the lower left side (facing) of the engine front. Had the failure occured on the port engine, my job would be easier since I have good access to the center of the engine room. As it was, the starboard impeller failed and the water pump was snuggly mounted just forward of a fuel filter, and just port and above the hull. Did I mention my engine room only had about 15″ of forward clearance in front of the engines?

    Removing the water pump housing

    My installation powers the pump from the serpentine belt so step 1 was to remove the belt. Before doing so, be sure you have a reference for how to route the belt back on (in my case I had engine #2 as a handy comparison). This is printed in the mercruiser installation and engine manuals which come with most boats – or you could take a digital picture. Removing the belt is simple, loosen the belt tensioner pully (upper right on my engines) using a pair of box end wrenches and remove.

    Once the belt is gone, the real fun begins. On the rear of the pump housing are the intake and outflow hoses. These should be anchored with hose clamps. Be sure you have a way to tell which hose went where. In my installation, there was so little room along the engine and hull that they couldn’t really get confused. Loosen the hose clamps and slide them back along the hoses. If your hoses have any slack (and you have working room), pull the hoses from the pump housing. If you are doing this service in the water …be ready with something to plug off the water intake hose as you may begin to take on water if the hose is below the waterline.

    I lost almost an hour working out how to get the hose clamps loose with my limited clearance. Once the hose clamps were loose, I decided to go ahead and loosen the pump housing from the mounting bracket – as there was insufficient room to back the hoses off the pump. My pump was secured with 3 bolts which thankfully turned pretty easy. The final challenge was negotiating the pump between the engine and hull to get it around the bracket. After another 15 minutes of experimentation, the pump came clear through a combination of pushing and tilting towards the hull. The hoses popped off as part of the process.

    Housing Overhaul

    Mercruiser recommends that the entire impeller housing be replaced whenever the impeller is serviced. Not surprisingly, everthing is sold as a kit. Once the pump is off the bracket, removing the housing requires spinning off a few more bolts and the metal portion (with pulley and drive shaft) will separate easily. At this point, you should take stock of what’s left or not left of your impeller. I found a severely damaged impeller and 2 or 3 of the original 8 blades. Set aside the impeller and pieces for later.

    Your next enjoyable task is to seat the new impeller in the replacement housing and get this back attached to the shaft and pulley. On the back of the housing, you’ll find a diagram for which way the impeller spins for your engine type (Left or Right), if you don’t seat the impeller with the blades set for this direction, you may be get more practice at pump overhauls than you really want. This is where the lubricant comes in to help in the process. We also employed some carefully set tie-wraps to “shape” the impeller as we eased it in. Once about 1/2 the impeller was seated, we removed the tie wraps and then mounted the pulley side and drive shaft. Don’t forget the new wear plate which mounts between to two pump halves. Finally rebolt the non-bracket bolts. Be careful not to over-tighten these as you can crack the plastic housing.

    Engine Forensics – Finding all the Pieces

    If a substantial part of the impeller is MIA when you get your pump housing apart, it’s not a bad idea to pull some more hoses in the cooling chain to collect up the strays. This is a much better option than creating a hot-spot in the engine somewhere. Fortunately, almost all the other hose connections should be in easier spots than the pump hoses. The 1st place to look is at the thermostat housing at the top, front of the engine. I pulled all 4 hoses and found 2 more large blade portions. I found 2 more at the intakes to each manifold. The picture at right shows my final “reconstruction” of the impeller.

    Put it all back

    Now it’s time to reverse the steps used to get the pump unmounted. Use the legend on the back of the housing to get the water intake and outlet in the proper location for your engine rotation. Rebolt the pump to the bracket. In my case, I need to also install the hoses as part of this step due to limited rear clearance (and reverse the push and tilt manuever). I lost about an hour for this step. It would have been faster, but I was getting pretty tired and I was about standing on my head to get my hands into position to make everything work. Reseat your hoses, clamp everything, and then re-install and tension your serpentine belt. Lastly, perform a test run on muffs or in-water and check for restored cooling and leaks.

    Tips for Longer-Lasting Impellers

    Unfortunately, I’m not aware of any golden bullets for long-lived impellers. The common rule of thumb is to change impellers every 2 years whether they need it or not (not that you’ll get much warning before an impeller lets go). Some change their impellers every year. On our previous Chaparral w/Merc Alpha 1, I serviced the impeller shortly after purchasing the boat in 2004. The boat was a ’96 but had seen relatively little use and I suspect still had the original impeller – it looked like new. Conversely, I have the maintenance logs for “Southern Comfort” which clearly show the impeller I replaced was less than 2 years old. What can make for a short-lived impeller? Here are some suggestions -

  • Running in shallow water – sand will tear up an impeller fast (as well as do other non-desireable things to your engine cooling)
  • Dry Running – some quote as little as 3-5 seconds that you can run “dry” without seriously damaging the impeller
  • Improper Installion – the initial shock of resetting the blade deflection will reduce impeller life (if not destroy it immediately).
  • Conclusion

    Hopefully, you’re not too scared to get started. I have to believe that most installations are not quite as difficult as my starboard engine. All told, it took about 4 hours. I expect that I can cut that to 90 minutes in the future based on having my technique and tools more refined. At $99/hr at my local shop this is a major savings – even if they only took 2 hours for the job. My 5 yr old helper Caleb summed it up best when he told grandma that we had to “yell at it a lot” when we fixed the boat.

    4 thoughts on “Overhauling a Mercuiser Bravo III Raw Water Pump – the Good, the Bad, and the Ugly

    1. thanks for the atricle, i have an evinrude outboard but the information provided has been the most thorough and decriptive of anything i have found in weeks of researching impellers and water pumps.
      thanks for the tips and insight.

    2. well! if anything thanks a lot for the insight!! it really help me to NOT do certain re building mystakes!

    3. My marina is telling me that it is necessary to remove the engine from my Rinker Fiesta Vee cruiser in order to change the raw water impeller. I thought this was routine maintenance? Is it necessary to remove the engine to perform this maintenance? The overheating alarm sounded shortly after launching but stopped after starting and stopping the engine a few times and then ran fine the rest of the weekend. However, the engine would not draw water from the ear muffs back on land. If it is not necessary to remove the engine, could you provide instructions on how to do this or point me to a source of information?

      Also, do you think it is a good idea to repower the boat with a new 5.7 MAG MPI SeaCore? The hull is in perfect condition but it seems like I am having lots of little problems with the motor. I am in my ninth season with this engine at about 550 hours. The boat is in rack storage but used mostly in salt water.


    4. Mike,
      Unless your engine is mounted extremely close to the hull, it should not require removal to service. Impellers are ‘consumable’ items so I would find it hard to believe Rinker would mount an engine which prevented normal servicing of the raw water pump. You could call Rinker to verify. That being said, it can be very difficult to get at the pump and connections depending on your engine room layout.

      The pump is typically mounted low on the left or right front of the engine and will be connected to a serpentine belt with a pulley. Two large water hoses will be connected at the rear of the pump housing.

      A typical engine removal is 4 hours (out and back in) or ~$400 in labor at current shop rates. Judge this against how long it would take to service the pump leaving the engine in. It took me ~3 hours the 1st time I serviced mine and only 45 minutes the 3rd time (figure at least an hour weather the engine is in or out).

      If you go the engine removal route, get a firm quote on the labor before they pull the engine. A novice mechanic can take well over 4 hours for this – but a reputable shop should flat-rate the cost based on the industry ‘norm’ of 4 hours.

      The repower question is mostly one of cost/benefit. I’m not in a financial position to contemplate engine replacement on my boat until dictated by fate or circumstance of a major failure.

      Keep in mind that new engines aren’t always trouble-free either – especially during break-in. If you’re pulling the engine for the raw-water pump, you would gain a small savings in the install fees for a new engine having already pulled the existing engine.

      Please comment back on what you decide to do.

      - Virgil

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