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Author Topic:   Mercury Optimax 200-HP 2005 Problem
rocket posted 09-30-2004 07:01 PM ET (US)   Profile for rocket   Send Email to rocket  
I am looking for some help trouble shooting an intermittent but repeatable engine performance problem on my 2001 CONQUEST 26 with twin 200-HP Merc Optimax 2005 powerheads. I have 2005 powerheads because Mercury was very gracious and replaced both 2001 powerheads due to plug fouling problems. After breaking in the 2005 engines for 50 hours on a two week vaction up the St Lawrence River, where they ran flawlessly, I took the boat to its permanent slip at Fair Haven on Lake Ontario. After a few weekends, two things happened to engine performance.

First, the starboard engine gradually lost upper RPM, trailing about 400-RPM behind the port engine at full throttle. I pulled the plugs and #2 was badly fouled. This brought things back to normal.Second, the port engine began displaying unusual performance which seems initiated by a hard impact with a wave and again leaves upon a hard impact. Lake Ontario is frequently considerably rougher than the St Lawrence, so I think it is possible this problem was present since day-one, after the new dressers were put on. When cruising at about 4700 RPM and trimmed for optimum speed (bow up slightly - say 3 degrees of engine tilt) a wave impact makes the port engine jump from 4700- to about 5100-RPM while at the same time dropping fuel consumption from about 16-GPH to 11-GPH. The throttle handles themselves are unmoved and still side by side in alignment. The port engine speed oscillates about 100-RPM around the 5100-RPM nominal at a period of about 1 second. The engine is in essence in an "altered state" and stays that way until I either pull the throttles all the way back to minimum/neutral, or, if I smack another wave hard enough, it instantly goes back to 4700-RPM and 16 GPH fuel consumption. Pulling the throttle controls in parallel back to a mid throttle position then upwards to the original 4700-RPM position does not stop the problem.

Another behavior in this altered state is that if I bring the other engine's throttle up to get it to about 5100-RPM the osciallation in the port engine stops and the boat seems to run great, except the port fuel consumption remains way below the starboard engine's consumption. If I push the throttles (out of phase but at the same time) forward, I find that the starboard engine hits its usual upper limit of about 5500-RPM but the port engine in its altered state begins to easily over rev past 5750-RPM and the port throttle is not even all the way forward yet. In its normal full throttle mode the port engine reaches about 5600-RPM.

And the final symptom is that with the engines trimmed fully under (maximum bow down position) I can not initiate the problem at all. So far I have crawled thru the controls and linkages, replaced the throttle position sensor and MAP vacuum sensor to no effect. I also replaced the spark plugs (none were fouled). At this point I decided to look into the air plenum for something that might be literally bouncing around with the impacts and to my astonishment I found a fairly large yellow plastic cap about 1.5 inches in dia and 3/4 inch tall. It has since been confirmed that this is supposed to live in the counterbore in the flywheel to protect the nut on the end of the crank from rust. I eventually found its twin loose in the other engine's air plenum stuck/resting in one of the rectangular slots of the plenum casting feeding the reed valves. Removal of the yellow plastic caps has not solved my port engine's odd performance.

Corporate Mercury Service was brought in and has been very supportive trying to find the root problem, but, after several visits to the boat trying different things, has not yet effected a cure. They did confirm the reed valves are undamaged from the yellow caps and the caps did not appear to be missing any matter, so I do not believe anything went into the pistons. The yellow caps are just somewhat beat/scratched up as one might expect from such a situation.

The current theory being pursued is RFI. I was told (correctly) my batteries were not to spec (1000 Marine Cranking Amps)--I had 875-MCA Deep Cycle style batteries, and that this might be driving the RFI problem by making the alternators do extra work. So I spent money and put in brand new batteries that met spec. Still had the problem.

As the Mercury field rep dug further he found a redundant ground on the sending unit of the fuel tanks that he told had, on occasion in the past, been found to cause a ground loop (more RFI), so I disconnected the redundant ground. Still no luck. The next item spotted was the actual harness between the engines and the fuel sending units at the engine end; it seemed not to reflect the expected wiring diagrams (it's all bone stock right from a Whaler dealer to me the first time owner), so now I have been requested to take the boat to a reputable Optimax dealer where at Mercury's cost they will swap out the harness as the next step in the process of elimination. By the way, the laptop computer check done by the Mercury field rep found no meaningful faults with any engine components.

As I said, the Mercury team seems responsive but not to any positive effect yet. Also, winter is moving in so I am hot to not go into my 3rd summer in a row with engine problems.
Hence my visit here to this web site.

My own thinking is to swap ECUs next between the engines (I know they are programmed as either port or starboard but Mercury says it's OK to give this a whirl) to see if the problem moves to the starboard engine. If that bears no fruit I plan on placing an oscilliscope probe (dc powered and portable) on the leads at the ECU where the throttle position sensor and MAP vacuum sensor pins enter the ECU. I am hoping to be able to induce the problem with a wave strike and perhaps find one of these signals gone crazy with either AC hash on the DC signal or something. I might literally track it to a loose wire or poor connection althugh I have already plugged and unplugged the ECU harness and other wiring.

It seems to me there are two distinct possibilites that would drive up the RPM while driving down the fuel consumption.

--Case One: the ECU suddenly sees (or thinks it sees) a MAP signal representing a low engine load so it cuts back on the gas which leans out the engine (starts to melt my pistons too) and of course this will drive up the RPM.

--Case Two: the ECU suddenly sees (or thinks it sees) a reduced throttle position signal so based on its fuel map it cuts back the gas but with the actual throttle plate still wide 90% open this causes a lean condition (starts to melt my pistons too) and of course will drive up the RPM.

That is why my plan is to hang the scope probe on these two sensors and then go for a ride, induce the problem and see if I can catch the culprit in the act.

What I just can't get my head around is why the trim down position avoids the problem. I can go out in 4- to 6-footers and literally run the boat at 35 MPH taking enormous impacts and not set off the port engine problem. Yet with trim partially up it's not too many good whacks before the problem shows up-- and if the whacks continue for another minute it will eventually catch one to put it back to normal.

I managed to induced the problem on smooth water by repeatedly running across my own wake until I managed to finally build up some pretty nasty waves and finally crack one pretty solid. After that if I continue in a straight lne away from the self created waves the engine stays in its altered state until I pull it off plane and put the thottles back to minimum.

It never happens on smooth water by itself no matter how long the run.

OK - anybody else been thru this yet with an Optimax that can shed some light on it?


jimp posted 09-30-2004 08:04 PM ET (US)     Profile for jimp  Send Email to jimp     
Rocke -

I'm no expert. Just a guess based on my 2003 225 Opti on my 1990 Revenge 22 WT. Prop ventilation when tilted up?

Not sure of your fouled plugs.

My 225 will run fine at lets say 3700 rpm, tilted up a bit (2.5-3.0 on the SmartCrraft gauge), then the gremlin hits (never in smooth water) and she jumps to 4000 RPM and speed drops off several knots (I didn't check the gph). So I slow down, stop, and then come back up to speed again and she runs fine, until the next wave or gremilin does it again.

I'm using a 17" stainless Revolution-4 4 blade prop in the middle hole of the engine mounting bracket. My old prop, 19" aluminum 3-blade Black Max, NEVER did this.

What props are you running? Can you get some loaners and try something different?

Its inconvenient to say the least. Maybe Larry Goltz can shed some light. I'll email him to make sure he sees this.


LHG posted 09-30-2004 08:50 PM ET (US)     Profile for LHG    
My inclination is to say that we can be of little help here, since we have no resident Optimax experts. Since the engines are rebuilt by a delaership, the people at Mercury who arranged for this re-installation should be contacted. We are not talking factory produced new motors here, but rather powerhead replacements by a dealership.
rtk posted 09-30-2004 10:37 PM ET (US)     Profile for rtk  Send Email to rtk     
Nice to know that Mercury took care of you.

You mention the engine rpms exceeded 5750-RPM. How high did the engine speed go according to your tachometer?

When the engine speed goes from 4700 to 5100, can you hear a difference in the motor, or is this according to your tachometer? I have a hard time believing that a 200-HP motor will only burn 10-GPH at 5100-RPM.

As far as I know, if that motor has the Guardian System that the recent Mercury Optimax and EFI motors have, the engine speed will not increase over 5750-RPM. The computer just will not allow that motor to do it.


rocket posted 10-01-2004 11:58 AM ET (US)     Profile for rocket  Send Email to rocket     
The motors are not dealer rebuilds. They are brand new, taken off the assembly line in mid June '04 and were shipped overnight. As I understand it the 2005 power heads were only coming off the assembly line for about two weeks at the time they pulled this pair and sent them to me. As I said, I am very pleased with Mercury's efforts; it just seems like a tough nut to crack. Maybe in the end it will turn up to be some esoteric RFI issue. I will pass along an outcome in the hopes of saving someone else a little trouble.

When the engine jumps speed from 4700 to 5100-RPM its easily heard by the ear as an upswing in pitch and this matches the upward swing of the tach. The scary part is the drop off in fuel consumption which indicates a lean condition is in play.

As for ventilation, good point. I thought I might have somehow partially spun a prop hub. But if that was the case I would sure see a huge difference whenever I tried to get up on plane from a standing start. I am running the bone stock Mirage props that came with the boat from the dealer and on flat water with full tanks of gas I can trim her out to about 46 MPH at about 5600-RPM on the port and 5500-RPM on the starboard engine so the props seem a very good match for the situation.

As for outright ventilation, it seems if it's truly that (say because the motors are too high or trimmed incorrectly) the Starboard engine ought to do it too once in a while. Both props are in like new condition, no nicks or dents. Also, I can back the engines off to about 1600-RPM where I am plowing thru the water (not on plane) so I am not ventilating and the altered state is not exited as evident by the continued reduced gas consupmtion. I recently got some input that perhaps the float in the vapor separator tank might be getting jammed and unjammed by the impacts. I think this would cause true surging and a loss of average RPM, not an increase but having never experienced it I can't say. In regards to the 5750-RPM, yes the ECU will not allow the engine to run over that limit in a sustained continual manner but the warning system does come on (the loud continuous alert horn), and I have been chicken (prudent?) and not tried to keep it up there, especially when the max RPM is reached 10 or 15 degrees before the full throttle position. I do not try and run the engine over 5750-RPM and merely did so in an exploratory fashion to try and figure out the total picture. When the field rep dumped the ECU faults to study them, this over-rev condition was indeed in the memory, but only had a duration of one second. I was really caught flat footed and pulled the throttle back the instant I realized it happened.

Tnx to the guys who read thru this long problem statement and tried to help.

John from Madison CT posted 10-03-2004 06:47 AM ET (US)     Profile for John from Madison CT  Send Email to John from Madison CT     
If you go to: you will find lots of resident Merc experts who could probably help.


jimh posted 10-03-2004 08:24 AM ET (US)     Profile for jimh  Send Email to jimh     
Because this problem never occurs when the engines are trimmed down, I would suggest that perhaps there is something in this situation that is different. Two things come to mind:

--the exhaust back pressure will be higher;

--the propellers will have better "bite" and less chance for ventilation.

Higher exhaust back pressure could affect the engine speed and behavior.

With regard to propeller ventilaton, I had a similar recent experience. I bought a new boat, and on initial use I was very concerned that the motor had problems. The engine speed was jumping around. I later discovered there was nothing wrong with the motor. The speed jumps were all in response to propeller ventilation. Different propellers and different trim eliminated all of this behavior. You could test this by changing propellers. Perhaps Mercury could arrange to loan you a set of different propellers for a trial. (I am assuming your engines are counter-rotating; if not, just swap propellers between port and starboard engine.)

Good luck. And please keep us posted on the results.

rocket posted 10-04-2004 01:51 PM ET (US)     Profile for rocket  Send Email to rocket     
JimP and JimH, I was thinking about the prop issue again. I did a lot of testing yesterday (beautiful day out of Fair Haven, NY on Ontario) as there was a nice set of 2 to 3 foot swells to play around with, sunshine and 70 F. When the RPM goes up on the port engine my speed does not "seem" to drop off but I never actually took a hard look at the speedo to confirm. Speed dropping off would indeed be a sure sign of ventilation. I will confirm this next time out. I dropped the boat off at a Marina in Rochester last night to have the fuel tank wiring harnesses upgraded per the Merc field rep's directions. Its all under warranty and I am just praying it solves the problem. When I was out trying to debug things yesterday on Ontario's waves I confirmed the throttle position sensor output is stable at the input to the ECU at around 4 volts although I do see some AC "hash" of about 1.5 volt peak to peak at 100KC or so. This hash is also unchanged when the event happens. I also checked out the output from the MAP sensor and its well behaved too. I also unplugged various DC electronics such as on board fridge, depth finder, search light etc in hopes of coming up with something that might impact an RF issue (if it is RF as the field rep thinks it might be). No luck. I will bring up the prop ventilation possibility with the field rep and see what he says. Tnx again for the great input.
davej14 posted 10-04-2004 02:29 PM ET (US)     Profile for davej14  Send Email to davej14     
This problem is way beyond my expertise but is there any chance you could run the boat as an experiment with just one motor and then the other to see if there is some feedback between them? Just a thought.
Barry posted 10-04-2004 03:19 PM ET (US)     Profile for Barry  Send Email to Barry     
Since the problem does not occur when the engines are trimmed down or in, have you tried trimming in during the anomaly?

What is the condition of your oil tanks?

bsmotril posted 10-05-2004 03:41 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Your motors have dual throttle position indicator sensors on each powerhead. If the output from both of them does not match each other, a check engine fault and code are generated. So I think you can rule out your #2 hypothesis. Both sensors would have to see that problem simultaneously for the ECU to interpret it as fact.

On thing that can play havoc with troubleshooting Optis is battery voltage. A weak, or intermittent shorted battery will cause all kinds of strange problems underway. On a choppy lake, a battery can have intermittent cells short out and drop voltage. Try switching the batteries between motors to rule that one out. Should be able to do that with the battery switches if the dealer rigged them correctly. mine were not rigged right when new and I had to get the wire diagram from Whaler to prove it to them.

rocket posted 10-06-2004 01:18 PM ET (US)     Profile for rocket  Send Email to rocket     
Tnx for the additional thoughts gang.

DaveJ14 - The boat will not plane with only one engine so I can not functionaly test for the phenomenon that way. However, at idle I did verify that with either engine running the AC hash exists on the entire DC system (al three batteries).

Bsmotril - I think you have an excellent point and it ties in with Davej14's comment. I verified that the Perko battery switches do indeed control only 1 engine at a time by unhooking battery cables and observing which engine started. Everything seems hooked up correctly. Still, how am I seeing a common AC hash on all the batteries even when only 1 engine is actually running? Maybe its the battery charger. Even though its "off" (not on shore power when mobil) it still is the only common component I see that connects all the positive posts of the batteries when the Perko switches are set correctly to keep them separate. I have had the phenomenon with two completely differnt sets of batteries. After the Merc field rep found my original batteries were not up to spec (they were deep cycle/cranking hybrids) I ran out that night and put in a brand new pair of cranking batteries. The problem is still there.

Barry - I have not tried to trim down the engines during the problem while on plane. I will. Thats a great idea and may shed some light on the issue.

To all - tnx again for the ideas. I took my oscilloscope last night and checked the DC system on my 1989 Chevy Suburban to form a basis of comparison to what I saw on my Whaler. At idle the Suburban has only .07V AC hash peak to peak and as the RPM goes up the AC hash actually drops down to maybe .03V AC. Its very "clean" and repeatable. On the Whaler on idle I had around .03V AC but as the RPM climbed the hash climed also all the way up to 1.5 Volt. So the boat has about 20 times more AC hash on the DC system than my old suburban and it seems to "hop around" a lot more on the scope.

The boat is at the Merc dealer in Rochester since early Monday getting the new harnesses between gas tank sending units and the Motorola ECUs on the engines. I will update here as soon as I can try out the boat with the new harnesses, hopefully this weekend if the weather permits.

Tnx again.

rocket posted 10-14-2004 05:55 PM ET (US)     Profile for rocket  Send Email to rocket     
Here is the latest. The fuel electrical harnesses were replaced between the gas tank sending units and the engine ECUs. A test ride last night still finds the problem alive and well. Worse, the problems seems to be happening more frequently and with less impact needed to start it, plus it has even happend now with the bow trimmed all the way down in a light 1 to 2 foot chop. Plus one new behavior arose. On smooth water while at full speed the port engine lost 2000 RPM inside a few seconds and the boat went off plane. It idled quite well. The various sensors indicated no problems, eng temp is good, water cooling pressure good, telltale flow is good etc. After a wait of a minute or two I tried to put the boat on plane but the port engine would not climb much over 2Krpm. I shut down the engine for a few minutes, checked the primer bulb (seemed OK) and then started it up and tried again to plane the boat. Still no luck. I kept it idling and every few minutes would try again to get on plane. Eventually it did get back on plane just like a perfectly fine engine. I ran about 3 miles at speed and it ran smoothly. I was hoping it would stay "broke" so the troubleshoot would be simple. My vote now is for fuel system issue. Perhaps inside the Vapor Separator tank itself where there is a needle and seat assembly along with a float (like in a carb). I think this weird change that starts with a wave impact is the needle/seat in the VST and the engine RPM jumps up because AIR is being sucked into the fuel pumps inside the VST and in essence feeding the engine a lean mix. I don't think there is a fuel pressure sensor on the output of the hi pressure fuel pump on these optimax engines. A shortcoming. I have already tried swapping the engines to the opposite fuel tanks to no avail. But the primer bulb is past the heavy fuel tank 3 way manual switches whaler installed in the transom so its also still possible I guess there is an issue with the primer bulb check valves. I will take 5 minutes and swap the lines after the bulbs so the engines see the other's fuel system 100%. If all this is from a stupid primer bulb I will kick myself. After that its on to the VST or simply adding my own fuel pressure gage (remote read of course)and seeing what happens with/without the problem present.

Comments appreciate

rocket posted 11-05-2004 06:20 PM ET (US)     Profile for rocket  Send Email to rocket     
I did more testing to isolate the problem. I fed both engines from two new virgin 6 gallon fuel tanks thru clear fuel hose to elimiate fuel delivery as an issue. I also installed a vacuum gage and confirmed the engine does not have to suck too hard to get fuel. The problem still happens. We took a Mercury mechanic out from our marina to see whats up and confirm the issue. It happend a dozen times in 2 to 3 foot chop, about 5 or 6 times on each engine in an hour or so. His conclusion was that we have a common engine issue (probably the fuel flow inside the Vapor separator tanks)as he did not notice any obvious prop ventillation when he looked over the transom. I noticed that as the end of the season grew near and I let the gas tanks go light the starboard engine started acting up with this RPM swing as frequently as the port. I also managed to observe the hi output fuel pump pressure thru a remote gage setup and it remains a solid 95 PSI or so when the problem is and is not present. The corporate Merc people are pretty sure its "prop slip"; its just not true "ventilation" in the obvious sense. Based on the hi pressure fuel output being unchanged when the problem comes and goes I have to conclude the Merc corporate guys are right. I seem to have a boat that can literally leap out of the water in 5 and 6 footers, yet have the props bite immediately upon re-entry yet in these 2 to 3 footers on Ontario start venting/slipping in such a way that I am forced to drop the boat off plane to stop the issue. Whoever said I had prop slip - you get the "you got it right" award. I will consult with Whaler and see whats up. I can't believe I am the only person with this 2001 and twin 200s that runs a boat on Ontario. None of the guys with the inboards have to deal with this type of issue in 2 to 3 foot chop......

My thanks to all who gave input.

MarkCz posted 11-08-2004 03:23 PM ET (US)     Profile for MarkCz  Send Email to MarkCz     
I saw this in another web site and it sounds a little like your situation. Did you ever hit anything that would cause the prop hub to have been damaged/spun?
Earier in the season, while running at 4000 RPM's and above the engine would jump up a 100-200 RPM's every few seconds. Below 4000 RPM's it would run fine. 2 Weeks ago I brought the throttle up to 3k RPM's and the engine would rev high but the boat wouldn't move. Backed off the throttle to idle and pushed up again, everything seemed fine. This weekend, the boat wouldn't go over 1800 RPM's without high rev/loss of power. Boat goes in and out of gear with no abnormal noises from the lower unit.

Does this sound like a spun prop hub?

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