High Idle (2900rpm) - 1988 l6 115hp Mercury

[mrcrunchybeans]

Hey! Long time lurker - and it's finally time to ask for help.

Pictures:

https://imgur.com/a/RoGnEns

Video of Engine Idling Fast:

Serial Number: 0b300098
Model Number: 1115452BD

I've got a (new to me) 1988 Mercury 115hp Oil-Injected [tower of power?] engine. The previous owner did not have any maintenance records on it, which led me to do a complete tune-up before getting too comfortable with it.

I bought 3x carburetor rebuild kits and rebuilt the carbs. I also replaced the fuel filter, added seafoam, tuned the timing, replaced the spark-plugs. Replaced the water pressure gauge, added a trim sensor, drained and replaced the lower unit oil, lubricated the linkages, added a second battery to isolate the starter from the accessories and the BlueSea Add-A-Battery kit.

This is my first boat, and quite honestly, I'm not sure 100% what it is "supposed" to sound like. My Seloc manual says that idle speed is supposed to be at 650rpm in forward gear at idle, but my tach read closer to 3400rpm! It seems like a vacuum leak, so I used my propane torch trying to find a leak somewhere, but failed. I then rebuilt the carbs again, to see what I missed (and actually I rebuilt them all 5 times at this point). Still, same thing.

The only "symptom" I have noticed is that it is idling fast. To my untrained ear, the engine sounds to pur nicely, and it operates well on the water. My fear is that if it is truly idling at such high rpm, that I will destroy my lower unit.

I decided that maybe my tach was wrong, so I checked the back of it and it had a dial to calibrate, which I moved and no longer trust the onboard tach. So I bought an inductive tach that wraps around the spark plug wire. That tach reads that my current idle speed is 2900rpm!

I'm at a lost at what else to do to isolate the problem. Any help would be appreciated.

Compression Test:

C1 - 107 107 107
C2 - 106 106 106
C3 - 105 105 105
C4 - 107 107 107
C5 - 103 103 103
C6 - 103 103 103

-Brian

 

[racerone]

Step #1 ----It is an absolute MUST DO. to remove that oil injection unit and heave it over the fence into the land fill.-----Your tachometer shows 2800 but that is totally wrong !!

 

[mrcrunchybeans]

Step #1 ----It is an absolute MUST DO. to remove that oil injection unit and heave it over the fence into the land fill.-----Your tachometer shows 2800 but that is totally wrong !!

Tell me more. I've read about disconnecting the idle stabilizer (so I did), but I haven't read about removing the oil injection unit. Would I just then mix the oil directly in the fuel tank? What goes wrong with the oil injection and why do people get rid of it? Does that contribute to the high idle?

Both my tach's say it's idling high. What are you basing your perspective on that the tachs are wrong? - I'm just trying to learn and understand. Thanks for the help!

 

[racerone]

Some will argue.-----Throw the oil injection unit away.----Your little tachometer is way off.---Not even close to reading correctly.

 

[demarko210]

Just from the sound of the motor in the video, it does not sound like the engine is idling at 2900. It sounds to me to be idling just fine. I would agree that the tach is off. I had a tach like this when i had my inline 6. I would add a adhesive sticky that came with the tach to the fly wheel and aim the laser at the sticky to see how fast the engine is turning.

 

[Chris1956]

You should probably do a link and synch, since the PO was clueless. Of at least check the idle pickup timing and max spark advance.

A OEM service manual is in order, as it will be hard to find someone to work on those older engines.

I do agree the idle speed is fine. Likely 800RPM or so.

 

[Texasmark]

Step #1 ----It is an absolute MUST DO. to remove that oil injection unit and heave it over the fence into the land fill.-----Your tachometer shows 2800 but that is totally wrong !!

Get some 50:1 premix in your gas, circulated through your engine prior to doing that.