New VS old Mercruiser V6 & V8

Mercruiser sterndrive motors, improvements and observations after having been introduced about 8 years ago.

New Mercruiser 300hp V8

Change and evolution (sometimes revolution) are inevitable, new designs, new materials and new rules and regulations dictate cause it. The humble sterndrive motor is no different, so after many years of using the venerable small block V8 (GM based), in about 2015 or 2016 Mercruiser rolled out the replacement motor.

Having delivered numerous boats with these new motors (V6 & V8 in differing power outputs) and been involved with warranty claims, repairs, service bulletins and sea trials over the last eight years or so, I feel comfortable saying that Mercruiser have produced the best marinized small blocks available. All of this post is based on my experience and my own observations.

The motors

V6 4.5L available is 200 and 250hp configurations, couple to Alpha or Bravo series (usually Bravo 3) drives, with and without DTS (Digital throttle & shift), from memory only seen DTS on the 250hp model. These V6 models replaced the old 4.3 V6, the 5.0 V8 and the low output 5.7 V8. Both power outputs are available with closed cooling (separate fresh water cooling system with heat exchanger).

The V8 models are 6.2L and have outputs of 300 and 350hp in sterndrive configuration and up to 370hp for tow sports inboard models. The V8 motors replace the old 350 MAG MPI (300hp) and 377 6.2L 320hp model. In my humble opinion, unless you have a larger boat with a large engine room, the 350 model makes the big block 8.2 380hp model almost obsolete, with its higher weight, fuel consumption and larger physical size. The 383 Mag stroker is another motor with a 350hp output, based upon the older motors and not the new ones, there may be some reasons why you would want to stick with the older motors, I will go through this in the following paragraphs.

Parts catalogue and GM VS Mercruiser

For anybody that doesn't know, Mercury publishes its parts catalogue (minus prices) online, its completely free, here is the link (opens in a new window), if you are getting parts for these new motors I would strongly recommend you use the official parts catalogue and not any other, in fact I would recommend using this method full stop, so many new references are published, old parts changed etc. This is the definitive and updated source. I would also recommend buying genuine parts, I am all for saving money but the number of aftermarket parts that just straight up don´t work, don´t last and fail is incredible.

The older motors are referred to as GM motors, the base engine being the legendary small block GM (Chevrolet V8), these motors should be hailed as a design classic, produced for six plus decades, powering everything from a boat to a truck to a racing car, it is basically an icon of engineering.

The new motors are referred to as 6.2L V8 Mercury. I have heard from a very reliable source that the base motors are still built by General Motors, BUT, they are only used by Mercury Mercruiser and not in any other form, hence why they no longer have the GM designation. This is really a match made in heaven, Mercury get a steady supply of engines from a known source and can use the expertise in making them water ready, GM get a steady income from selling their motors.

Minor improvements

Mercruiser have consolidated parts from their outboard motors and used them in these new motors, this makes sense economically and also helps with parts and spares. A couple of examples of these common parts are;

New fuel filter location, starboard side, protective red cap in place

Fuel filters, these are now the same as those found on a lot of outboards.

Water pressure sensors, these were a fairly common failure on previous motors, the new sensors (which also fit the older motors) are common to outboards, you can tell them apart from the older sensors by looking at them, the older sensors were polish brass and used a 1" socket to remove them from the steering cooler, the new ones have a 11/8" head and look like they are made of stainless steel. I do not know if these new sensors have any internal differences or a different sort of calibration but they have a much lower failure rate than the original ones.

Engine specific minor improvements

Larger drive oil reservoir, now on starboard side, the brown cap to the left is for power steering fluid, also now moved to starboard.

Oil reservoir, the drive oil level reservoir has been enlarged considerably and relocated from the port side of the motor to the starboard side, there is still an alarm for when running low on drive oil.

Quieter, to my ears at least, these new motors seem a lot quieter and a lot smoother than the previous motors, even when giving it full gas, the motors never seem to become "throbby". One thing of note, it could be different in other markets, I have never seen any of these new motors with thru hull exhausts, they have not been for sale in new boats in Europe for years, I do not know if the new exhaust design robs the motors of that roaring thunder soundtrack. This could be one of the reasons why the 383 and the older motors are still so popular, a lot of people will not want to fill the holes in their hulls and go thru drive exhausts.

Alternator now in centre, Mercathode new location above, thermostat housing new design to bottom right of alternator, much easier to access.

Alternator, the alternator has been relocated more towards the centre of the motor, the old style alternators were mostly supplied by Delco and I think put out around 60 or 65 amps, the new alternators put out a considerably higher 90 amps, this is good news, faster charging times for larger battery groups, this is especially important in new boats which have higher electrical demands.

Old style alternator fuse, bottom; located right on the starter motor, note how difficult the access is, below, replacement fuse being fitted.

Alternator fuse, this should really go in the section of major improvements, ask any tech who has to change one of the old ones that sits old the starter motor and protects alternator output, the hardest part is getting to it, the second hardest part is removing it, they can get very corroded. The new "cube" type fuse has been installed on top of the motor at the back, so easy to check and replace.

New fuse location, top of the motor, viewed from the rear

Engine covers, there are three of them, one over the front of the motor, two on top, none require any tools to remove, they are certainly more attractive than the old black flame arrestor cover.

Engine covers, top left, all 3 covers in place, top right, one top cover removed showing fuse box and red resistor/diagnostic port, note the green light on the hole of the front cover for the Mercathode. Bottom left, close up of engine oil fill and dipstick, easier than old angled filler neck into rocker cover, bottom right, view from above, one cover removed.

The new covers protect a lot more of the top of the motor than the old style, useful if water is splashing about and more so for when someone has to lean over the motor to reach something at the back.

Major improvements and differences

Intake and admission system

Quite a lot of noticeable changes here, fewer parts than before, this is good from the point of view of there are fewer points of failure, I suppose the downside is the replacement part instead of being just a sensor, can be a large part of the intake system and be costly, that said, of all the motors, I know of only one that had a serious issue and need the whole throttle body assembly replacing.

Very important change is now the absence of the IAC (Idle air controller), they were one of a handful of annoying things on the previous model MPI motors, the tell tale signs of a failed or failing IAC is the motor starts, then stops very quickly or idles roughly for a short while then stalls, fortunately an IAC is not expensive and can be replaced very quickly. The new motors have idle control via the engine ECM.

The TPS (throttle position sensor) is also no more, the ECM controls the throttle body to determine engine demand. The intake temperature and pressure sensor (MAP) still is fitted.

Air intake location

Left, view from rear of motor, new flame arrestor design, behind that is the resonator. Right, view from the top, behind the resonator is the throttle body, the two relays on top are for trim up and down, the ECM behind.

Instead of having the throttle body and air intake on top of the motor in the middle of the "V", its now located at the back of the motor (for a sterndrive installation). The flame arrestor sits straight onto a resonator, this is then fitted to the throttle body assembly, there is no more TPS (throttle position sensor), this function is now handled by the ECM. The whole intake system is considerably quieter than before. The intake manifold looks nothing like the previous models either.

Ignition system

Now has more in common with the larger 8.2L (502 CI) V8, there is now no separate distributor and coil, instead 4 coil packs are used, each one supplying spark to two cylinders, this system use a wasted spark ignition, meaning that sparks are fired in pairs, when one cylinder is on compression another is on exhaust but still gets a spark. Wasted spark is not really a new or revolutionary system, it is however pretty robust, some personal watercraft (jet skis), motorcycles and vehicles have been using this system for a long while, the previous 496 Big block from Mercruiser also used coil packs.

At time of writing I do not know of any coil packs having to be replaced, this is just as well as, depending upon how the motor has been installed and the space available in the aft end of the engine room, they could be difficult to get at. The extra sparks do not seem to waste the spark plugs any faster either.

This system is altogether a massive improvement, the previous "crab" style of distributor caps, they were not problematic as such, but they did seem to build up carbon and also if people were not careful putting them on, they would crack, forums are full of posts about these, although all looks okay, a tiny near invisible crack can cause moisture to get in and cause poor running. The coils were expensive to replace and I have seen many ruined just because water got onto them from above, they will put up with a certain amount of water but in a poorly designed drain or air vent, washing the boat or heavy rain will soak them.

Fuel system

Top, new location and type of fuel filter, starboard side, bottom, new Fuel System Module (FSM), now located port side, raised nearer top of motor

The older motors had a fuel cooler, either mounted behind the port side exhaust manifold, which could be difficult enough to reach that the manifold would need to to be removed to gain access (earlier models) or on the starboard side where two fuel pumps (low pressure and high pressure) would sit in the cooler.

The original coolers had a metal body, over time deposits of salt, scale and so forth had a tendency to stick to the inner walls of the coolers, especially the newer designs, no matter how much you flushed the motor with fresh water, if you did not periodically inspect the cooler for blockage you could end up with fuel vaporization, or heat soak. The tell tale signs of this are the motor fires right up when cold, gets to temperature, then after being stopped, it becomes very hard to restart. A lot of people make the mistake of replacing the fuel pumps (expensive) and not solving the problem because the cooler is still blocked or partially obstructed so does not get the water flow, all the coolers have raw water flow, not part of any closed cooling system.

The new fuel system has been renamed as a fuel supply module, it has been relocated to the port side top and front of the motor, plastic (composite) bodied, it still contains both fuel pumps, the access is so much easier than before. I would have to double check its still the case, but it is more cost effective to replace the whole module than remove it, open it and replace fuel pumps. This is not the first time for this type of practice, I am not a massive fan as it makes for waste.

The fuel filter has been changed also, now a common filter to a lot of the outboard motors, its located on the starboard side of the motor and has a distinctive castellated red head to it, you need a Mercury specific tool to remove them, these tools are cheap and click into a 3/8" drive, no more canister filters.

Exhaust system

New exhaust manifold and one piece riser/elbow, view from the starboard side, note the red cap on bottom left for the fuel filter.

This is probably the most significant change. The new exhaust manifolds do not look anything like their previous generations. The exhaust system was divided into three parts, manifold, riser and elbow, the new system has just manifold and elbow, the elbows are supplied at certain heights, negating the need for a separate intermediate riser, the elbow and riser now being one part.

Exhaust manifold removed from motor

Emissions systems

The last of the old style motors did have emissions systems fitted, catalyzers in the exhausts and oxygen sensors, these were not obligatory for all markets. The first of the new motors that arrived in Europe only had the wideband pre catalyst oxygen sensor fitted, the idea being that these sensors control fuel trimming, leaning off or richening the mixture as needed. You may have heard the term "open loop", that's what these first motors are, no post catalyst oxygen sensor, the pre catalyst oxygen sensors rarely get changed, they are reliable.

Exhaust manifold and elbow showing pre and post catalyst oxygen sensors

Later models have both pre and post catalyst oxygen sensors, the original post catalyst sensors were problematic, they did not last long and did not take very long to start throwing errors. The errors relating to emissions systems do not appear to cause actual running problems, the engine will not go into a guardian mode because of emissions, but the alarms are a nuisance. Pre catalyst oxygen sensors are fitted in the exhaust manifold, post catalyst ones are fitted in the elbow/riser assembly.

Closeup view of the oxygen sensors, the pre cat one is installed in the manifold, post cat in the riser, note the protective shroud.

The first generation of post catalyst oxygen sensors were being replaced so often, it caused sufficient problems for Mercruiser that they rolled out a replacement sensor, the problem with the original sensor was determined to be that they were failing due to thermal shock, catalysts are very hot and so are exhaust gases, so when the motor had been running a while the sensor would get hot, engine stopped for a bit, then restarted, the sudden stream of cooling water that hit the sensor caused the thermal shock, so the replacement sensors are thermally shock protected, these sensors can be distinguished from the original ones by a white band on the electrical connection where it plugs into the loom, they have a different part number also.

Original ECMs supplied to work with the original post catalyst can be be flash upgraded to be compatible with the new thermally protected sensors, it depends on engine serial number, you will need a Mercruiser dealer to do this for you, in fact, all emissions related problems require official Mercruiser software to eliminate them, just changing the sensors will not erase the active errors even though the condition does not exist (at time of writing).

Emissions errors

Emissions errors are in two different groups, ones that can possibly be dealt with by nothing more than plugging in the diagnostic computer and others that require parts replacements.

There is an error along the lines of "too much oxygen for the amount of fuel", I am paraphrasing as I cannot remember the exact wording but basically its caused by going too slow for too long, to avoid this, don´t be afraid to get the motor up to cruising speed and beyond. These new motors seem to have the best rev range at 3500 to maybe 3800 rpm, after quite a few tests, it seems you can go a bit faster and not use any more fuel according to the computer, I do not know if this is deliberate because of burning off more emissions or a combination of the oxygen sensors and the engine ECM create a condition that allows for better economy.

Replacement of oxygen sensors, almost always, I would say only one in every hundred is a pre catalyst sensor that needs replacing. It always seems to be the starboard side post catalyst one that fails first, I have been told some workshops replace both post catalyst sensors when one fails, I understand why, but unless viewing live data indicates the other sensor is failing, I am not sure the validity.

I think Mercruiser were probably hamstringed a bit into having to install emissions controls, recreational boating makes only a small amount of pollution, what about all those replacement sensors and plastic packaging they come in? There is an interesting report here about how much of a contribution recreational boating makes to greenhouse gases.

I will be posting in the future of how to deal with the post catalyst oxygen sensors, a simple solution and does not involve removing or deleting any of the emissions sensors.

Blocked exhausts

Blocked exhaust elbow, causing exhaust temp alarm

Another issue with the new exhausts has caused quite a few problems with overheating in the exhaust elbows in the area of the exhaust temperature sensors. I cannot be certain of the root cause, it could be from running slowly for extended periods, causing carbon deposits and reducing cooling water flow, it could be an area that is just difficult to flush through well so salty deposits are not fully removed. My advice for dealing with this is say at no more than three years or every third season, remove the risers for inspection, the exhaust system is still made of cast iron like the old manifolds and do not have an infinite life expectancy, especially in salt water, it also gives you the opportunity of inspection.

The replacement exhaust manifolds are expensive but a vital part of engine longevity, the catalysts do not usually require replacement, but inspection is the key, if they do need to be replaced please know they are very expensive.

Its another reason why I understand people prefer to rebuild their existing motors or replace them with the previous range, just to avoid emissions issues, I am not aware of actually being able to buy the new motors without the full emissions systems factory installed.

Conclusion

For all the emissions problems and replacement sensors, Mercruiser really has knocked it out the park. I actually do not know what the next stage of evolution would be. In a world where outboards on new boats and conversions to outboards from sterndrives is getting more popular, I say, long live the V8 sterndrive, right now it just doesn't get better than these. I would quite happily have these motors in my own boat.

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