RPM Variation / Knocking

[Lakeside]

The last couple of times I had the boat out, the RPM's would vary by about 500 RPM in the 4,000 RPM range (i.e. it would bounce between 4,000 and, say, 4,500, with the throttle in one position).  This would last for about the first 5 minutes and then the engine ran perfectly.

Also, it seemed that the engine knocked a little at idle speed.  This only occurred one time and stopped when the engine was warm.

Any ideas what's going on and what I need to do to fix it? 

[Water Sports]

How many hours on the engine?

[Rambling Wreck]

You may have one plug that was firing intermittantly. How old is your fuel? Are you using a fuel stabilizer? Cool nights produce high humidity. These are some simple things to consider.

[brad]

I agree, spark plugs may need to be replaced if you haven't done it recently (I replace mine each year when I winterize).  Another possibility that I've had is the Mass Air Flow Sensor may need cleaning.  Mine boat was running sluggish until I cleaning the sensor.  Night and day difference after a cleaning.

[ktdust]

+1 on the mass air flow sensor and plugs. If the maf is dirty it will make your rpms dip. Pull the maf, spray with sensor kleen or a similar product, let it dry and reinstall. I also had a knock once. Mine turned out to be a short at the fuel pump. It happened same time as the maf so it wasn't easy to figure out. I can't remember which fuel pump is primary or secondary but make sure the are both humming when the boat is on.

[Lakeside]

About 440 hours.  I changed the plugs 2 years ago.  Gas is from July. 

[ATXChris]

+1 on the spark plugs.  I switched to older style copper spark plugs and no more stumbling at idle or surging at speed.

Bosch Super Plus part #7927, $2 each

[Lakeside]

Thanks.  I'll be changing the plugs.

[Supraman]

Wakejunky strongly reccomended only using Denso plugs in these motors... I seem to remember him being a Lexus tech. I'll see if I can find the thread.

[Supraman]

Here it is:

http://www.epicmarine.com/forum4/index.php?topic=2301.0

[Rambling Wreck]

Tons of threads discussing which type of plugs folks prefer but don't forget to get rid of the gas you now have in the tank. You said it was from July. These days gas is only good for about 60 days and ethynol has made things even worse. It is too late to stabilize what you have so you need to either run it out or drain it and add some fresh fuel. Stabilize it when new.

[Capt Rick]

I just had the same complaint and here is what I found:

Run engine at 1000 rpm , Remove one coil pack clip at a time: If no change mark down the cylinder. Continue until all have been checked. The remove all spark plugs and roll engine over place a towel over each cylinder bank to catch any ( Water that might be pushed out)
If water is pushed on then you remove both exhaust manifold and install the garden hose to the front water intake with a clamp;

Look inside the exhaust chamber for where the water is leaking.

Order new both sides
1710450130 Manifold,Exhaust,SS,Starboard     738.88  $30.00   
1710550130 Manifold,Exhaust,SS,Port     743.73 

Do not attempt to repair after 10 years it is normal to replace exhaust manifolds. Install with same gasket and run. Then change oil and filter.
I also recommend doing a compression check at this time

Capt Rick

also clean MAF( inside the air filter ) mark the direction it comes out so you dont install it upside down.

Install one quart of Duralube for any knock concerns