# Merc 9.9 problems



## DHower08 (Nov 20, 2015)

This one has me stumped as I'm typically very quick to diagnose and fix mechanical issues. 2002 Merc 9.9 4stroke won't go past 2000 rpm in gear or neutral. Sunday it ran fine then yesterday as soon as I started it it wouldnt run correctly. It idles fine and fires fine just like always and up to 2000 it's fine. 

Once you hit 2k it misses spits pops and just won't go any higher. Good solid spark tore the carb apart and cleaned it's got new fuel lines and filters plugs are good compression on top cylinder is 90 bottom cylinder is 91 or 92 psi. Would like to see higher compression but I don't suspect any internal issues with both cylinders being the same. Also don't suspect it to be a head gasket. 

What the heck else should I look at. Also went over all the wiring and nothing looks suspect and yes the primer bulb is functioning

Took a short video but cannot get it to post


----------



## KaGee (Sep 8, 2006)

Does it have a power pack?


----------



## DHower08 (Nov 20, 2015)

Just a regular 2 wire coil. Good solid spark on both plugs when tested


----------



## CaneCorsoDad (Jun 11, 2017)

Fuel pump or screen ?


----------



## DHower08 (Nov 20, 2015)

Fuel pump is operating fine. Spark seemed good to me but both plugs are black with just a touch of moisture on the number 2 plug. Could a weak coil be the culprit possibly.


----------



## 0utwest (Mar 21, 2017)

First thing that popped into my mind is a rev limiter of some sort . Like it has to be in neutral to be put into gear so it is not like 2500 rpm . Is this tiller handle or separate controls ? hope your getting me its Like a io that has a switch so you cant slam it into gear at high rpm or you can only get low rpm while your in reverse . hope this helps and go on merc. website with your issue . could also have low oil sensor issue ? feel free to text me your no. and all help you dig for prob. 440 417 3359 I'm in Geneva name is mike .


----------



## firemanmike2127 (Mar 17, 2013)

Try doing a cylinder drop test if you don't have a spark output tester. The spark on any modern engine w/ capacitive discharge ignition should be 1/2", bright blue, & strong when tested @ the plug boot connection. You should be able to make an improvised single cylinder spark tester using a 1/4" bolt inserted into the plug boot & a pair of vice grips clamped onto the power head. How about the fuel side ? Does the engine pick up any speed when you gradually cover the carb inlet @ the 2000 RPM point where it hit's the wall ?


----------



## Will_S (Jan 19, 2010)

Fresh fuel? Air cleaner/intake clean and clear?


----------



## bustedrod (May 13, 2015)

does that stator have a high and low output to the box,


----------



## fastwater (Apr 1, 2014)

If plugs are good and gapped correctly, wires are good and not arcing under load, fuel is fresh and a mud dauber has not built nest as to restrict air flow...with the plugs being black, highly suspect that the coil is breaking down under load.
Would check wiring and connections to coil and 'load' test coil.


----------



## Kwall (Feb 12, 2014)

I would replace the coil


----------



## bob forster (Mar 22, 2018)

DHower08 said:


> This one has me stumped as I'm typically very quick to diagnose and fix mechanical issues. 2002 Merc 9.9 4stroke won't go past 2000 rpm in gear or neutral. Sunday it ran fine then yesterday as soon as I started it it wouldnt run correctly. It idles fine and fires fine just like always and up to 2000 it's fine.
> 
> Once you hit 2k it misses spits pops and just won't go any higher. Good solid spark tore the carb apart and cleaned it's got new fuel lines and filters plugs are good compression on top cylinder is 90 bottom cylinder is 91 or 92 psi. Would like to see higher compression but I don't suspect any internal issues with both cylinders being the same. Also don't suspect it to be a head gasket.
> 
> ...


----------



## bob forster (Mar 22, 2018)

when you want more gas its not getting it. check fuel supply


----------



## DHower08 (Nov 20, 2015)

Fuel supply is strong fuel is fresh all lines and hoses new


----------



## DHower08 (Nov 20, 2015)

fastwater said:


> If plugs are good and gapped correctly, wires are good and not arcing under load, fuel is fresh and a mud dauber has not built nest as to restrict air flow...with the plugs being black, highly suspect that the coil is breaking down under load.
> Would check wiring and connections to coil and 'load' test coil.


How do you load test a coil. Talked to Mike a little bit ago he gave me a few ideas to try for testing the coil wanted to see what your idea was


----------



## Kwall (Feb 12, 2014)

I would start the motor and spray it with water i, if it’s leaking you will see it arc or hear a snapping noise, use a windex bottle


----------



## Kwall (Feb 12, 2014)

Kwall said:


> I would start the motor and spray it with water i, if it’s leaking you will see it arc or hear a snapping noise, use a windex bottle


( spray just the coil )


----------



## G.lock (Apr 1, 2014)

Do you have access to a different tank and hose to try. That alone will eliminate about halve the possibilities.


----------



## Shortdrift (Apr 5, 2004)

Will it rev above 2000 in neutral? If not, my guess would be the RPM limiter. My 2005 Merc 9.9 did the same thing and it was the limiter.


----------



## DHower08 (Nov 20, 2015)

Will not rev above in neutral like stated above it will not advance past 2000 rpm then start to act like it's missing.


----------



## Eyegagger (Jan 13, 2018)

I had a very similar problem with a Merc 9.9 that I own. It would only idle every time you tried to put it in gear it would stall. There was water in the bottom of the gas tank from sitting too long. My motor ran fine as well until it got down to the gas with the water in it at the bottom of the tank.The water got into my float Bowls on the carburetors , and into the small clear holding Reservoir that fed the carburetors. I drained the float bowls , the small clear Reservoir and put the gas from the tank into my lawn mower . The Merc runs like it's supposed to again. I'm sure you probably checked all that stuff already but that's what was wrong with mine, hope it helps.


----------



## Eyegagger (Jan 13, 2018)

I had a similar problem with a Merc 99 that I own although it was a two stroke the configurations might be different but it sounds like the same fuel problem to me. Motor ran fine first couple times out once the gas got down to the lower portion of the tank it would only idle. If you try to put it in gear it would stall. Had water in the bottom of my gas tank from letting it sit too long and not draining it and did not run the motor dry either after the last fishing trip. Water had also got into a small clear holding Reservoir that feeds the carburetors fuel, and both float bowls. After cleaning all three of those things out ,the tank the reservoir and the bowls The motor fired right up and ran like it should, you said one of your plugs looked moist??? Now I try to drain my tank every winter or I just wait till spring and put it in my lawn mower... Anyway that's just what was wrong with mine hope this helps!


----------



## Shortdrift (Apr 5, 2004)

Shortdrift said:


> Will it rev above 2000 in neutral? If not, my guess would be the RPM limiter. My 2005 Merc 9.9 did the same thing and it was the limiter.


"Will not rev above in neutral like stated above it will not advance past 2000 rpm then start to act like it's missing."

_*Limiter *_


----------



## firemanmike2127 (Mar 17, 2013)

Sounds to me like Shortdrift might be on to something here. I didn't know that this small Mercs had a limiter that was set with such a low trigger point. I'd probably try to verify the upper RPM that your engine will run & compare it to the spec listed in the service manual for your application before I'd run out & buy a non-returnable CDI box though. Mike


----------



## DHower08 (Nov 20, 2015)

Everything was fine on Sunday then next trip out this starts happening. Rev limiter typically is about 6000 to 6200 on my motor. Where is the limiter at on these things


----------



## firemanmike2127 (Mar 17, 2013)

I'm not sure right off where the limiter is set to kick in on your motor. I have a Merc 15 HP 4 stroke but have no idea what the present point is on that either. Some engines will have a speed limiting feature that kicks in when the operating temperature gets too high. They will still run in a 'limp mode' but not rev all the way up.


----------



## fastwater (Apr 1, 2014)

firemanmike2127 said:


> I'm not sure right off where the limiter is set to kick in on your motor. I have a Merc 15 HP 4 stroke but have no idea what the present point is on that either. Some engines will have a speed limiting feature that kicks in when the operating temperature gets too high. They will still run in a 'limp mode' but not rev all the way up.


Yep...good call on possibility of the limiter by Shortdrift. Didn't think of it either.

Shortdrift or Mike, you guys are more experienced then I on these Mtrs. so I need to ask...in a situation such as this, for testing purposes only, can the limiter be temporarily unplugged and eng. ran for a short period over problem rpm range to see if it's the cause.


----------



## DHower08 (Nov 20, 2015)

Motor does same thing cold or warm not an overheat issue


----------



## firemanmike2127 (Mar 17, 2013)

Have you checked your spark output (1/2") or tried the cylinder drop test ? Also, what about trying to partially block the carb air inlet to see if the motor is running lean ? Spark & compression are essential cranking tests that should be done before anything else. It's like when you go to see the doctor...the first thing his nurse does is check your pulse & B/P. The cylinder drop test will let you know of a particular cylinder is weak, misfiring, or dead. I'll use my spark tester on some engines while the motor is running to see if it the spark output has gotten weak or intermittent. Also, when you gradually block off the incoming air through the carb this will help you to determine if a lean condition is present. This should be done @ the 2000 RPM (or so) where your engine seems to top out at right now. I'm not sure if there is a procedure to check the limiter that's built into the CDI box on your Merc. There isn't an external lead that you can disconnect to my knowledge.


----------



## DHower08 (Nov 20, 2015)

Done all carb and spark testing I can think of fuel is good I believe it may be limiter kicking in early instead of the normal 6000 rpm or coil is cutting out at higher rpm


----------



## man164 (Sep 21, 2014)

If it is in limp mode the temperature of the motor will not matter. If it is a bad temperature sensor it will fail high......


----------



## fastwater (Apr 1, 2014)

DHower08 said:


> Motor does same thing cold or warm not an overheat issue





man164 said:


> If it is in limp mode the temperature of the motor will not matter. If it is a bad temperature sensor it will fail high......


Like man164 stated, your eng. may not be physically overheating. But if the temp. sensor is bad, the sensor may be telling the CDI the eng. is overheating regardless and causing engine to go into limp mode.
May be possible to bypass temp sending unit and see if eng. comes out of limp mode if that is the issue.
Is there a temp alarm/buzzer on the unit?
Do you have a repair manual for the engine?
What color are the wires going into the CDI box?


----------



## DHower08 (Nov 20, 2015)

Will look at wires tomorrow. And no buzzers or alarms on that engine


----------



## bustedrod (May 13, 2015)

oil sensor maybe, putting it in limp mode


----------



## DHower08 (Nov 20, 2015)

Busted rod is the winner of the day!! Going over entire engine again unplugged the oil sensor and Bam she runs perfect again. Alot of stress for a $15 part!! Thanks to everyone for the input and suggestions!


----------



## firemanmike2127 (Mar 17, 2013)

Cool !! 4 days, 35 posts, & a $ 15.00 part.....sounds like a success story to me. Good to hear that your Mercury is back to normal. Mike


----------



## fastwater (Apr 1, 2014)

Good deal!
Glad you got her back up and healthy again.


----------



## DHower08 (Nov 20, 2015)

I'll call my normal boat guy that I order parts from tomorrow and order the part . Good news is even if I don't have it by the weekend I'll be able to run it with the sensor unhooked for the day


----------



## fastwater (Apr 1, 2014)

That's great!
Know it goes without saying but just make sure oil level is up to snuff.
Have fun!


----------



## DHower08 (Nov 20, 2015)

Oil level is good very first thing I checked when I started having the issue with it


----------



## moondog5814 (Apr 8, 2007)

Threads like this are one of the things that are great about this site. Glad that you found your problem.


----------



## Shortdrift (Apr 5, 2004)

Well, I guess it was a limiter of sorts,,,,,,,,,,,,, Just kidding, glad you have it fixed.


----------



## bustedrod (May 13, 2015)

that's a good thing glad it helped. I just be sure you do have good oil pressure . fish on lol


----------



## DHower08 (Nov 20, 2015)

Oil pressures good. I ran it enough trying to diagnose it that it woulda blew if the oil pressure actually dropped that low


----------



## DHower08 (Nov 20, 2015)

$50 pressure switch. Lawd they be proud of that thing


----------



## bustedrod (May 13, 2015)

dag gone expensive part heh ?


----------



## humpty dumpty (Nov 30, 2013)

if mine I would check as stated before the rev limiter and if it has a low oil sensor that may have gone bad. good luck..tom


----------

