Haxby!
You 'da man!! Thank you very much. That was just what I was looking for! Have you used this yet? I think you are right about saving the time.

Has anybody out there tried it?

Thank you again, man!

exactly. the answer for sure

Well, there ya go. Plug 'n play, call it a day.
:cool:

Here is a MUCH cheaper and simpler "plug and play" solution.
http://www.afterthoughtsauto.com/o2sim.html

I would not mess with a plug in computer that adjusts fueling curves by modifying the O2 sensor readings...this simple solution above simply tricks the ECM that the Cat is still there.

The ad for the above site says that it does not work with the later vehicles (only up to 2003), so I doubt it works with the Mercruiser stuff. Also, I don't know if it will plug in.

I like the one from CP Performance because it is made for marine products.

Drivehard - How do you know that it adjusts the fueling? I don't see why it would need to.

Looks like the CPPerformance Maxvolt system may be the part for bypassing the system if it works with the Mercury ECM programming. CP can tell you that.

However, be careful here. check your engines warranty carefully. The new catalyst marine engine emmission systems are required by law to carry a three year warranty and it also has a bearing on the standard engine warranty. Not sure how yours is affected but if you have a warranty issue. If you have modified your exhaust and engine in any way your warranty is out the WINDOW!. Just a thought.

As for additional power I would like to see an actual dyno report on the 10-15HP gained from this system, what size engine etc. If you really get 10-15 HP on a small block that might represent 1 mph on your boats performance? NOt a lot so consider that before you go just changing exhaust or disconnecting this system.

You might be better of just considering the addition off muffler bearings or the alternator phase shift capacitors !?

Best Regards,
Ray @ Raylar

I'm not sure if it's changed but when I went to Merc school a couple years ago for the catalyst engines they were not running closed loop. There is a separate computer monitoring the exhaust /emissions system and it can talk to the engine ecm but it is not altering the fuel curve based on lambda readings. I would guess the MaxVolt deal just tricks the ecm into thinking the O2's are still there and doing their thing.

The rest of the story
 

Hi Ray,
I have tried polishing out muffler bearings before, but they just don't shine like a nice set of headers! :) You have helped me realize that a few horsepower may be nice, but what I really want is a better looking exhaust system. Thank you for making me think!

Haxby - What I have since learned about the Mercruiser catalyst system is that all of them run closed loop and it is self contained in the one engine controller. The part from Afterthought Auto will NOT work on vehicles later than 2003 because the engine computer does a test on the catalyst. I don't no what the test is, but the part from CP Performance runs the test as well as some other things that I did not pick up on.

Who is going to be the first to report on this device? Me?

It seems in the past that anything in a boat runs about 10 years behind the auto industry...figured that the marine injection system would be what the auto injection system was back about 2003. What does the second O2 sensor look like "post cat"? Is it a 4-wire?

Why would you have an O2 sensor pre-cat and not run closed loop? That seems odd to me...unless it is there only to compare values pre and post cat. Wow...that is a lot of cost just to monitor that the cat is there and functioning.

Maybe I am way of base here...but if I understand how the system works, this is what I believe is happening.

To answer the question about fueling...if I read the directions correctly, it is only making three connections PER SIDE of the engine.

One to the ECM side of the second O2 sensor
One to the sensor side of the first O2
One to the ECM side of the first 02 sensor

So...the connection to the ECM side of the second O2 sensor is simply to trick the ECM into thinking the second O2 sensor is getting the correct readings that the cat is still there. That sensor is no longer needed though...

The second two connections basically "condition" what signal from the primary O2 sensor that the ECM is seeing. The ONLY thing that little box can send to the ECM on that wire is the air to fuel ratio. It can not change anything else. So what I believe is happening is that the box is telling the ECM that the engine is running slightly lean from where it is really running...which makes the ECM dump slightly more fuel into the mix.

If the primary O2 sensor is not used for closed loop operation...I would call total BS that the box could effect operation AT ALL...other than to keep it from throwing a code.

If this is the case (which I totally believe is possible)...how could they possible have three different settings that would change the power output of the engine? I'm sure the main ECM would not change fueling or spark timing based on what the "cat computer" was telling it...so I would for sure call "BS" on this little box making any difference other than to keep that little dash light from popping on.