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Author Topic:   2004 Mercury 90-HP Two-Stroke Problems
MikeyLikesIt posted 10-09-2006 10:20 PM ET (US)   Profile for MikeyLikesIt   Send Email to MikeyLikesIt  
Please help. My wife and I were out boating, and the [2004 Mercury 90-HP two-stroke outboard motor] was running great, as usual. I let my wife off at the dock and was motoring around to the ramp to load the boat. When past the "no wake" marker I hit the throttle. The engine acellerated as normal, up to 4,000-RPM, then dropped to 3,200 as if I had backed off the throttle. I then backed off and gave it another try: up to 4,000 then back down to 3,200. I repeated this a few times all with the same result.

With the boat home, I did all the usual maintenance, thinking the problem may be the spark plugs. Long story short, it's still doing the same thing. The engine doesn't miss as best I can tell. I did use a cheaper brand of oil recently, which makes me wonder if it could be an oil related safety device to prevent engine damage. I hear no alarms, however.

Any similar experiences? Any ideas?

My compliments on a great site. I've been lurking here for quite some time.


an86carrera posted 10-09-2006 10:43 PM ET (US)     Profile for an86carrera  Send Email to an86carrera     
Fuel pump new?

If not rebuild it.

Could check for bad fuel line/bulb/pickup first. oh is your vent open on tank?

Would not run boat like that could burn a piston from lean condition.


MikeyLikesIt posted 10-09-2006 11:15 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
Thanks an86carrera, I've checked everything you mentioned except the fuel pump. What is a good way to check the pump?

Yep, don't want to run it till its right.


MikeyLikesIt posted 10-09-2006 11:20 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
By the way, I did try squeezing the bulb while underway, but it made no difference.


btb posted 10-09-2006 11:34 PM ET (US)     Profile for btb  Send Email to btb     
Does your motor have a "safe mode" when the electronics limit the revs due to some sort of condition like overheating, low oli pressure etc?
MikeyLikesIt posted 10-10-2006 06:17 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
That's what I was wondering. I don't remember seeing anything in the owners manual about that, but, I guess I'll re-read the manual and see.


rtk posted 10-10-2006 08:02 PM ET (US)     Profile for rtk  Send Email to rtk     
"The engine doesn't miss as best I can tell"

Is the boat slowing down or is there a change in how the engine runs (sounds) when this happens when running on the water?

I would think you would be able to sense a loss of performance when an engine drops 800 rpms. If the boat keeps moving the same speed and the engine sounds the same then I would look at the tachometer. You may be getting false readings.

As far as checking the fuel system, invest in a service manual for the engine. An outboard fuel system is usually checked with a vacuum gauge installed inline on the fuel line. The manual should outline the diagnostic procedure using this gauge.


BOB KEMMLER JR posted 10-10-2006 08:17 PM ET (US)     Profile for BOB KEMMLER JR    
Probably has a Yamaha or an Etec stuck in the intake ;o)
MikeyLikesIt posted 10-10-2006 08:48 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
Thanks Rich. When I say it doesn't miss, I mean it doesn't "cut out" or "miss fire". It is not just the tach. It runs up to 4000 without a missfire, then it immediately drops down 3200 without a missfire. It drops in power, not just the on the tach, just as if I had pulled back on the throttle.

The suggestion of a service manual is a good one. I'll do a search on the net, but I'm always looking for a deal. Any suggestions on a good place to buy one?

Thanks again,

MikeyLikesIt posted 10-10-2006 08:50 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
That's funny Bob......I think. : )

BOB KEMMLER JR posted 10-10-2006 09:00 PM ET (US)     Profile for BOB KEMMLER JR    
It was a feeble attempt at humor :o) Go through the entire fuel system from the tank to the carbs and clean every line and passage in the system,but First i'd pull a plug wire at a time while running and see if there's any change in the way it runs.I can't imagine it would run good on two cylinders,but it just might be a bad coil pack.
MikeyLikesIt posted 10-10-2006 09:16 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
Thanks Bob, I've done the fuel line thing, but will probably do it again this weekend - at a slower and more methodical pace. I have a Racor external fuel filter but I did not check the little factory inline filter yet. I'll also try the spark plug wire suggestion. These are the kinds of info and suggestions I was hoping to get by posting here. Keep the suggestions coming guys.


cmarques posted 10-10-2006 09:29 PM ET (US)     Profile for cmarques  Send Email to cmarques     
Check the left side of the engine for a burnt hose I believe it is called a "pulse fuel line" and is about a 2-3 inch elbow 3/8 line near the fuel pump. Mine burnt through and caused all kinds of running problems but made a cough or air suck type of noise and rpm's would drop. My engine is a 2002 90hp 2 stroke Merc. I replaced the hose 2x then found the threaded plastic fitting was partially melted inside and just cleaned out the passage and hose has been fine since.

Hope this helps


MikeyLikesIt posted 10-10-2006 10:02 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
Thanks Chris, will do.


btb posted 10-11-2006 09:37 AM ET (US)     Profile for btb  Send Email to btb     
Your description of the revs dropping "just like I throttled back" sounds exactly like the electronics reducing revs due to some emergency-like condition being detected (ask me how I know). Anyway, now I have mentioned it twice - its all I can do.....
BOB KEMMLER JR posted 10-11-2006 11:19 AM ET (US)     Profile for BOB KEMMLER JR    
3200 rpms sounds kind of high for a limp home mode,but i haven't run into that condition on mine,so i am not sure.If it is in limp home mode,it could be a faulty oil level sensor
btb posted 10-11-2006 05:35 PM ET (US)     Profile for btb  Send Email to btb     
See the article about clogged oil supply in a Yamaha Precision Blend system. That problem has been almost certainly traced to some sort of contamination of the oil tank filter, causing the oil -level warning system to kick in.
MikeyLikesIt posted 10-11-2006 06:55 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
Thanks guys. I've been suspecting something like a rev limiting mechanism. I'm going to drain and clean the oil tank and refill with the overpriced Mercury oil this weekend, if I don't find something else first. I'll also check out the other article later tonight when I'm back at my computer.


MikeyLikesIt posted 10-11-2006 07:11 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
Bill, I just read the other article. Sounds like that may be my problem. I've been using the same oil. It sounds more like the revs are being limited rather than just running bad, and, since you say you had no alarm it makes me even more suspitious. I've had no alarm either. I'll be getting oily this weekend for sure.


jimh posted 10-12-2006 09:27 AM ET (US)     Profile for jimh  Send Email to jimh     
A thorough reading of the owner's manual for your motor should inform you if there is any sort of engine speed reduction system built into the motor. If there is such a system, the conditions which cause it to take effect should be spelled out.

I am not familiar with how the lubricating oil is mixed with the fuel in that particular Mercury engine. It would be good to have a summary of how the system works in the 90-HP motor. Perhaps someone familiar with them can inform us.

I do not think the engine would necessarily reduce its operating speed if there were a reduction or loss of oil flow. A two-stroke engine will run just as well without oil in the fuel, until it self-destructs from lack of lubrication. Therefore I do not think the reduction in engine speed you describe is related to lack of oil. It is probably related more to lack of fuel or to an intentional limitation in speed by some supervisory system in the engine (if such a system exits in that model).

I agree with the comments of others that limiting the engine speed to 3,200-RPM is not particularly indicative of a safety system, so therefore I doubt that this is taking place.

This leaves lack of fuel supply as the problem. This also fits well with the description of the problem. At idle speeds the engine is supplied with enough fuel. When engine speed is increased, there initially is sufficient fuel to run at high speeds, but once the supply that built up during idle running is used, the fuel system is not able to keep up. Look for a problem with fuel delivery.

Because you have tried pumping the primer bulb and had no effect on the problem, it sounds like the obstruction in the fuel system is downstream of the fuel lift pump. Check the small fuel lines which distribute fuel to each carburetor from the fuel pump and gasoline/oil mixing pump.

jimh posted 10-12-2006 09:30 AM ET (US)     Profile for jimh  Send Email to jimh     
[This discussion about repairs will be moving to the REPAIRS/MODS area.]
MikeyLikesIt posted 10-12-2006 06:26 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
Thanks for the input jimh. I will do what you suggest. I'll also report back if and when the problem found.


MikeyLikesIt posted 10-12-2006 09:31 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
OK, I just reread the owners manual and it only spoke of a sound alarm for overheating and for low oil: no mention of any "rpm limiting" or "run safe" device. So, I'll start with another fuel system inspection and go from there.

Thanks for all the input everyone.

Clark Roberts posted 10-13-2006 07:23 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Could be a squeeze ball failure (loose check valve ball). I have had these problems with the Tempo and Attwood brands and now only use Quicksilver, OMC or Yamaha brand. Suggest you get a portable fuel tank complete with hose, squeeze ball and fitting and make a trial run. If all is fine then start diagnosing your on-board fuel delivery. Also it could be sucking air at a fitting/clamp or "O" ring in fitting... good luck... Clark... Spruce Creek Navy
Sal DiMercurio posted 10-13-2006 09:50 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Power pack.
My $.02 worth.
boatinpete posted 10-15-2006 09:31 PM ET (US)     Profile for boatinpete  Send Email to boatinpete     
Have a 99 Merc 90,could not get boat on plane but it would start good and idle smooth,try to get on plane and no go,Took ot to one dealer and he cleaned carbs,changed plugs and $250 later same problem.Took it to Silver Lake Marine and master machanic found problem less then half hour,bad coil.Problem fixed and been running smooth since.
Buckda posted 10-15-2006 10:31 PM ET (US)     Profile for Buckda  Send Email to Buckda     
High speed stator.
MikeyLikesIt posted 10-16-2006 08:44 PM ET (US)     Profile for MikeyLikesIt  Send Email to MikeyLikesIt     
Thanks guys,

I did some work on the boat this Saturday and found one problem with the fuel system. There was a leak where the fuel line, just after the bulb, connects to the line the comes out of the motor housing. It was clamped with some cable ties. When I squeezed the bulb after it was firm, a substantial amount of gas ran out. Looks like it was possible for it to be sucking air into the line at that point. I replaced the cable ties with some hose clamps. I hope that solves the problem, but we had stormy wheather and I was unable to test it out.

I was unable to find a service manual localy so I'll have to order one. Also, I don't know what a power pack or a stater is. I do know what a coil is. If the loose fuel line connection is not problem, then it looks like I'll be learning what these things are.

Thanks for all the info so far. Any other suggestions?


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