My motor WOT changes day to day

[jrboatin]

Hello all,
I took my boat in for service because my WOT went down to 5300 from the 6000 ish it used to do. 2005 Sanpan 2500fe 155 4 stroke Mer. 13p prop, changed nothing from the day it was new, I was gettong 6000 rpms 29 mph, now I am getting 5300 rpm 25 mph.

The dealer did a minor service and gave back (plugs, filter changes) Sure you are saying just like I did wow thats going to get my 700ish rpms back? Well after calling Mer couple times, I checked per Mer fuel pressure (idle and full throttle) 40psi both, check metal screen over injectors (Mer said to remove if there) none there, Compression check should be between 180 - 210 lbs, all good from top 215 215 210 212, then they said to replace any black fuel line with gray w/red lettering cause over time the black may collapse from fuel pump sucking and line being weak.

Now they want me to check injectors again but like a relay, to see if on injector is acting up.

Now the funny part is over the weeks of checking the items Mer and dealer wanted me to check out, my RPMS have changed from day to day but for the most part stays lower. On one day I can get 5300 rpms next 5600 next 5800 and just last Saturday I was getting 6200 rpms (first time in a year) called dealer on from lake and advised him, he was shocked, now went out yesterday 5300rpms.



I am starting to think the ECM is going bad and not the motor like everyone is thinking, all motor items I have checked out seem great.


What do you all think? Or what should I do next?
Thanks Jrboatin

[HandymanHerb]

I with you on the ECM

[rockymax]

I have had the exact same problem. Dealer checked engine three times on and out of water with computer and found no errors each time. We raised the engine and that helped raise the rpm's and speed. However, the rpm's are still below the stated manufacturer optimum range and I am still seeing day to day fluctuation in speed and rpm's on my guages. We now believe the guages are the problem with the flucuations. I have ordered a new lower pitch prop to raise the rpm's and hopefully we can determine if the guages are the problem.

[jrboatin]

The dealer gave me a 10P cupped prop to try, cause he said my 13P was way wrong for my boat. So I did try it, hit 6200 rpms but still only ran 25 mph. he told me to keep useing it, I told him NO cause my 13P did the job befor. He also told me that Mer motors wear over time and you have to reprop to adjust to the wear, that running a 10P will relax the motor at top end, this will only cover the problem I told him. I returned prop to dealer and still have my 13P on, if it did it before it should do it now.
I never heard a of a motor that you had to reprop with in 4 years, so that means in 8 year I will be down 1400 rpms, can you think in 12 years I will be turning 4000 rpms and will be getting passed by 60 hp motors.. hahaha No way.......

Do not reprop, it will only cover the problem, I believe....

Just got of phone with Mercury in Wisconson, they told me to take to another dealer becuase the dealer I am dealing with has NEVER called Mer 1 time they said, dealer has told me they have called Mer several times.. If the original dealer does not want to look at then take it to another, sad to say I think I will have to get it to another dealer.

[jim454]

This may sound crazy but have you been playing around with your trim?


RPM'S would change quite a bit and it would explain the day to day random engine speed.

What is ECM?

[oldmn19]

I agree somethings rotten in Denmark here. No way is trim changing RPM's that much in my experience. I also can't buy the motor performance changing that much over 4 years unless you run it daily and have several thousand hrs on it. Your compression is right where it should be. I agree with Merc that you need a new Dealer who will take the time and has the equipment to really check it out.

[mikeherb]

Any way to know if your gauges are the problem? Is that possible? Just throwing shit on the wall here I guess.

[jrboatin]

I do use the trim all the time, It does change with the load on my boat. So I set trim just befor the motor free revs then tap down a bit. If I trim up past that point my motor does rev past 6000 with no load , I have tride that.
I cant see the gauges being off only becasue problem (wot rpms) change so much. If it stayed at 5300 wot all the time them I may concider
gauges. But at this point I am not.

I think I will tow it to another dealer, so sad....


ECM (electronic control Mod) I believe to be the computer brain for the motor...... Or it maybe called ECU (electronic Control Unit)

[rockymax]

Your engine needs to be checked by a dealer who has the Mercury computer software to determine the problem, if any, with your engine. Without doing this, it is speculation at best. As to props, they do wear over time, particulary aluminum. Slight nicks will affect their performance. That is why it is important to have your engine put on the computer as this will also determine your rpm's at WOT. If rpm's are above or below the manufacturer's optimum range, re-proping is the normal remedy unless other engine problems are determined.

[jrboatin]

If my RPMs change day to day do I reprop day to day then ? No, I look for the problem.

Isn't this board all speculation anyways?
It still helps me continue with people that understand and feel my concern.... Thanks all

I have had it to the dealer, they connected there computer to it and it came back clean. No hits in it, they said. But this does not mean
there is not a problem. As we all know taking a car to a dealer when you have a problem but they say "we cant find a problem"

Changed prop to new one also , old one had no nicks and still had paint on it.


I have reprop a boat, only for performace reasons never cause of a motors lose of rpms will I put on a lower pitch to gain rpms back. Thats just wrong to do, its like covering the problem. Find problem fix it and the motor will run like it did.....

[dockholiday]

Any chance there may be a problem with the throttle linkage?
doc

[GXPWeasel]

I agree with others thinking it may be the tachometer itself. Most tach's can have intermittent problems, especially if the wiring may have a loose connetion somewhere. With a boat at 4 years old, it is very possible that you have a loose wire going to the tach, causing skewed readings, or the tach itself is bad, causing a mis read in RPM's.

Can you actually HEAR the RPM difference from day to day? I know this sounds weird, but I can definitely tell when my boat looses a couple hundred RPM just by the sound, no guage needed. Maybe if it sounds the same all the time at WOT, no matter what the guages say, then this could support my above answer.

[mjlyn]

I'm with the last poster. Can you hear the rpm difference? You should be able to hear a 500 rpm difference.

Also you said in the original post that the RPM at WOT is different on different days. So does the RPM ever vary on the same day at all? Seems like if there was a motor problem the revs would vary some while you were running WOT. You are clearly not hearing any missing or varying rpm while you are running or you would have said so already.

Seems odd that all would be perfect on Monday at 5200 all day then on Friday shes running perfect at 6000 all day with you never hearing any variation in the sound or any missing.

I'd try a new tach. Unfortunately you probably wasted your money on that new prop (if you had to buy it) I agree with your previous statements regarding the prop. A solid chunk of metal (prop) cant cause the revs to change from day to day.

[jrboatin]

Hey ,
I would agree with most of you about the Tach but when it runs at 5300 rpms I can only do 25 mph but on a good day I 6000 29 mph.
So if the tach was wrong my MPH would be the same no matter what rpm gauge said, I do not believe its the tach.
When at dealer he told me there was a small miss but he fixed when he index the new plugs. No miss at all. But did not feel it befor either.

Yes I can also tell when its running good, the motor does tell me, then I look at tach and rpms are high, when low RPM I can hear the motor
working.

Now when I did run fuel pressure test, the cowl was off, running down the lake, that day top rpm was 5300 rpm, at 5000 motor running sweat but push the throttle full and well the only way I can describe it is when I was younger I used to flip the air filter cover over to get that loud noise. Well thats what it sounded like, I have run up and down lake with cover off many days, and that sucking noise
is always there, does change if wot change.

Thanks for all the ideas... it helps wideen my thoughts before I take back to dealer....

[HandymanHerb]

Well one thing about it sooner or later your going to be an hero to the next guy who has this problem.

Dam sure have all of us stumped