Emissions controls, errors, problems and solutions to deal with them on Mercruiser motors

The dreaded oxygen sensor alarm, the nautical equivalent of the check engine light on a vehicle. Note something important, (service engine soon), not return to port immediately.

So it finally happened a few years ago, emissions systems on recreational inboard and sterndrive gasoline engines became a thing. The process has been far from easy, I am not completely sure of the timeline or who got them first, my understanding is that California got them first, it was mandatory, then other parts of the USA, the finally the rest of the world, that said, Europe got varying versions as at first they were not mandatory.

My personal feeling, some based on bias and some based on the years of having to deal with emissions control systems on recreational marine engines is, they are largely unnecessary and cause more problems and possibly more pollution and waste than if they were not fitted.

I would say right now the number one marine engine fault is emissions errors, the number of oxygen sensors replaced in any one calendar year is incredible. There is a very interesting independent study, reporting that recreational boating is responsible for less than 0.1% of global greenhouse gases, you can read the full report here.

Basic emissions system

Three basic components make up the emissions system, two oxygen sensors, one pre catalytic converter and one post, then the catalytic converter itself in between the two, fitted into the exhaust manifold.

Left: Pre catalyst oxygen sensor on the starboard side. Right: Post catalyst oxygen sensor, also on the starboard side.

Pre catalyst oxygen sensor, a broadband sensor, this is because it monitors more data or does more work. Its major job is to monitor the air and fuel ratio (AFR). The AFR setting gets sent to the engine black box or ECU, this setting controls a factor called fuel trimming. Trimming the fuel means to either enrich the fuel and air ratio or reduce the fuel flow or lean the mixture.

Closed and open loop, If you are viewing the catalyst data on a diagnostic computer you can see the pre cat oxygen sensor has two states, open and closed loop. Open loop is where the sensor is not trimming and fuel, closed loop is where the sensor now influences the ratio of the air and fuel mixture, leaning off or richening up by adding more fuel. The pre cat sensors tend to give many fewer problems, they are in the path of exhaust gases only.

Post catalyst oxygen sensor, these are the problem sensors, they are also the reason why I don't think the full emissions system is well implemented or thought out. With the exception of a few motors, marine engines suck up cooling water from outside the boat, either fresh or salt water and sometimes a combination of both, this water circulates around the engine, it is the expelled via the exhaust system, the system has not changed much over the years.

That fundamental design is what is killing the post catalyst oxygen sensors, they are in the direct path of the cooling water being expelled via the exhausts, salt water makes problems worse, due to added corrosion.

Failed post catalyst oxygen sensor, apart from the carbon type deposits, the tip of the sensor has been eaten away by saltwater.

Catalytic converters run at extreme high temps, somewhere in the 600 to 700 Celsius range, imagine shutting down the engine, anchoring for a bit, then when you first start the motor again, a jet of cold water passes straight across the face of the oxygen sensor, this cold water damages  the sensors due to thermal shock from rapid cooling. The original sensors were updated to help cope with this, they have a special coating and are identified by a white band on the cable that plugs into the wiring loom. Depending on the serial number of the motor, the ECM can be reflashed to work with the new thermally shock protected sensors but only a Mercruiser dealer with the right program can do this update.

Post catalyst oxygen sensors really have only one job, they are there to ensure the catalyst is working correctly, in a perfect world they should hardly be doing very much at all.

Systems according to age and market

The earlier motors, such as the 350MAG MPI had all four sensors (both post and pre cat) but, the pre cat sensors were not the broadband type, all four sensors were the same.

The next that came along (in Europe anyway) were the new generation of motors with only the pre catalyst oxygen sensors installed, the newer broadband type, so they were performing fuel trimming but the post catalyst oxygen sensors were absent. This particular set up was short lived, probably a couple of years but is also the least troublesome.

Finally we have the full installation, broadband pre cat sensors and the narrowband post cat ones, these are the ones that cause the most calls, emails and messages about errors and alarms popping up on screen.

Things I would like to point out;

Big block motors do not seem to suffer nearly as much, I am not sure why, it could be because their exhaust system is a different design or maybe the fact that there are far fewer big block motors in circulation and so by the law of averages the smaller motor is bound to have more errors.

Errors on the emissions system do not seem to cause the engine to into guardian mode (a reduced power mode) at least over the short term, I do not know any motors that have been left for considerable operating hours with active emissions alarms, but reading the operating history of the engines using diagnostic software I have never seen any but 100% of engine power still available with active emissions alarms.

Unlike a road vehicle where failures of emissions controls can cause the limp home mode with reduced power, Mercruiser doesn't seem to have included failed emissions components as a reason to reduce power, this is an excellent strategy, unlike a car, where at least you can pull over, on a boat its rather more of an issue, you cannot have people in potentially dangerous situations with reduced engine power due to emissions systems failures.

Volvo Penta also install oxygen sensors and use emissions systems, I am not very familiar with their strategy or their oxygen sensor failure rate as there are not nearly as many Volvo gas motors around. I would hope they would take the same common sense approach and not limit the power of the motors on the water due to emissions system alarms.

The exhausts and elbows are still made of cast iron, that is they have a service life, they should be checked and replaced as necessary, same as the non emission control motors. If the exhausts get to a point where they rot away and damage the catalyst to a point where is has to be replaced, be prepared for a heart stopping invoice.

Main emissions alarms and errors

I do not know of any software except the dealer G3 that can clear the emissions error codes, also, the codes are "sticky", meaning that even when the issue causing the error has been resolved it still requires the software to erase the active error codes, replacing the parts alone will not stop the error code.

Pre catalyst sensor failure, this is the least common of the errors, solution is to replace the sensor, erase the codes, test the boat with the computer connected, maybe record the data to make sure that there is nothing more than a failed sensor.

Post catalyst sensor failure, by far and away the most common problem. The sensors either are rotted or blocked solid, solution is to replace sensor(s), erase errors & run boat, whilst you're at it, install the fix for this problem (see below).

Lastly, something in between, I cannot remember the exact wording but its something along the lines of  "too much air for the amount of fuel" or something similar, this is often triggered by using the motor for prolonged periods. The solution can vary, if you are lucky, the post catalyst oxygen sensors can be cleaned, reinstalled and the errors cleared, otherwise its replacement of whichever sensors are throwing the code.

Avoiding and fixing the issue

Not having the emissions errors occur is better than fixing problems, but, with even the best practices the emissions system can throw error codes. Using the motor at a higher rev range seems to help, lots of time spent idling along or going slowly seems to increase the likelihood of error codes. I can only imagine it must be similar to buying a high performance car and driving round the city centre day in and day out, eventually the emissions system will not longer be able to cope with the constant tuning of the fuel and air ratio, resulting in poor combustion and the inevitable error codes. If you find that this is the way you will be using your motor, you may have bought the wrong boat, something suited to a slower pace might be better.

Legalities

Messing with the emissions system to do anything other than repair it to correct working order is probably illegal in a lot of places. I cannot condone or support any deleting, modifying or alterations of the emissions system. The "fix" is not a cure, it is simply a way of extending the life of the post cat oxygen sensors and still have everything working exactly the same as from the factory, the readings given by the sensors are the same, the only thing we are doing is moving the sensor further away from the stream of exhaust water, the gases will still be read by the sensor, this is not altering the way the system works or doing any special "tricks".

The pre catalyst (broadband) oxygen sensor is not touched at all, as stated previously these sensors have a very low failure rate, I would guess at about 1/100th the failure of post catalyst sensors. What causes the failure of the post catalyst sensors is corrosion, not an actual fault as such of the sensor.

Where to begin?

Oxygen sensor tool, in order to remove oxygen sensors, you need a specific kind of socket, there are several types, I personally would recommend the shorter one from the photos below, the walls are much thicker, this is really helpful if you need to remove a sensor that is quite stuck in place, the longer socket has a tendency to open a bit when stressed, hence the hose clamp. Also worth noting is the long socket is no good for removing the port side bank pre catalyst sensor, its too long. These sockets are made to be used with a 1/2" ratchet or breaker bar.

Two types of oxygen sensor socket, the shorter style at the top is better, the longer ones can start to open up a bit when the sensor is really stuck, hence the hose clamp.

Anti seize, possibly not strictly necessary, the new sensors come with a smear of anti seize compound on them so I always apply some. Follow this link to the product I use, suited for use in salt water. I doubt any problems would be caused, but as is always the way, the problems that do occur never happen at a convenient time. The compound can be used elsewhere too.

Oxygen sensor adapters, the most important part, in reality they are not very big, they do not displace the sensors outwards by much at all. They will often come with a mesh or filter on the end, my advice is to but ones without or remove them, there is a  possibility of salt forming over the mesh and stopping the sensor from working, it would also worry me that if the mesh fell off, where it might end up.

One thing that obviously goes without saying, make sure the adapters you install are stainless steel, 316 grade is best, 304 is okay.

Oxygen sensor adapters in stainless steel, the mesh covers are removed before installation.

Process

Remove the bracket around the oxygen sensor, its held in place with a bolt into the exhaust elbow, the factory use a bit of thread locker so they might be a be tight, make sure you use a good fitting socket to not round of the head of the bolt.

Removing the bracket around the oxygen sensor

Unplug oxygen sensor from wiring loom. The pre and post catalyst sensors have different plugs.

Remove oxygen sensor from exhaust elbow, this is more comfortable when the motor is cold.

New and old oxygen sensors, this one was in desperate need of replacement, not the anti seize compound on the threads of the new sensor.

Install adapter. You can use the same socket for removing the oxygen sensor, I always add a bit of anti seize in the threads of the adapter, even though they are not expensive, when the exhaust elbows need replacing the adapters can be reused. The oxygen sensors can now be reinstalled in the adapters.

New oxygen sensor adapters installed.

Reconnect oxygen sensor and install bracket, repeat on the other side. I rarely attack my tasks with vigor or alacrity, preferring the "slow and steady wins the race" approach. I did this installation in about 20 minutes. So much depends on access, in this case the access was basically perfect, another reason why this installation is easier with the motor cool.

At this point I always start the motor, check for leaks and usually have the diagnostic computer connected to make sure all is working correctly. The visual difference with and without the adapters installed is barely noticeable.

Summary

This "upgrade" or change is not the ultimate solution, it is a way of keeping the emissions system working and extending the life of the post catalyst oxygen sensors. I would advise doing the installation whilst everything is working correctly and there are no errors if possible.

We need to take a step back, look at the bigger picture. Recreational boats do no add considerably to global emissions. I have know engines that were having these sensors replaced yearly, that means more plastic, more shipping and, more scrap or recycled metal, all of which are almost certainly larger contributors to global emissions. These are just my thoughts.

So far, at least from what I have seen diesel motors on recreational boats yet, commercial diesels have had them for a while. Given that when taken care of, both gasoline and diesel motors in recreational boats can last a very long time, will we see a shift in boating? Pure electric, at least for the moment is not viable for many boaters, we will see a trend running backwards and, go back to fixing and reconditioning rather than replacement?

Finally, I would like to say that I actually feel for Mercruiser on this issue, I suspect they were forced into installing these systems, I would imagine the amount of money it must cost in warranty claims for failed sensors is absurdly high.

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