05 F250 Excessive blow-by.... why?

[tunedportcj5]

Hey fellas, long time lurker here... amazing wealth of knowledge here!

I have a 28 CC with twin '05 F250s that run perfect. 450hrs and have no problems running to 6k rpm. Long story short, if I run the boat hard for 20+ seconds above 5k rpm, the port motor will start smoking and you'll see smoke rise up from the wake 50+ yards behind the boat. You'll know instantly when it does this because the RPM will drop ~300 rpm (and the obvious smoke screen behind the boat).

Compression is in check and all of the plugs look great. Leakdown shows 6-7% loss across all cylinders. Seems acceptable to me?

The crankcase vent tube is oily and will build pressure if you hold your thumb over the tube (while running at idle and during leakdown test). I'm pretty sure this is the source of the smoke screen and RPM drop... engine is sucking oil through the intake... the intake is also oily.

I'm at a loss here... any suggestions from the pros?

Thanks!

[99yam40]

could you have too much oil in that motor causing it to foam up?
Just a wild guess, as the leak down did not show much blow/leak by

[boscoe99]

Blow-by is usually the result of excessive clearance between the piston rings and the cylinder wall. Or maybe a scored cylinder wall.

A differential pressure test is performed in exactly the one spot that the cylinder wall will have the least amount of wear. At top dead center. It also won't indicate a problem with a cylinder wall down below the point of top dead center.

Did you really do a compression test as well as a differential pressure test?

Might want to send a bore scope down into the cylinders and take a look.

[TownsendsFJR1300]

Sounds like even thou the leak down test is not bad, it is bad enough that the compression rings are allowing compression past them into the crankcase (somewhat carboned up) at the higher RPM's.

Once the crankcase is pressurized (much more than normal @ plus 5k RPM's), the oil gets blown out into the intake/eventually burned.

Rodbolt has mentioned a dealer ONLY engine de-carbonizer that is extremely successful and has raised leakdown #'s WAY LOWER than yours, to higher than yours.

As your seeing BLUE burned smoke / oil, the spark plugs IF CHECKED IMMEDIATELY, SHOULD show oily, carboned, etc.. Idling awhile to get to port is apparently cleaning them up (and the issue is ONLY occurring @ 5k +).

[pstephens46]

I would think the op should perform his testing directly in front of the marina. Back and forth at 5800rpm so that he can pull directly over to check the plugs.

[TownsendsFJR1300]

I would think the op should perform his testing directly in front of the marina. Back and forth at 5800rpm so that he can pull directly over to check the plugs.

Being there's usually idle zones way before you get to land, I'd simply run it
to say 5,800 in the appropriate area, (maybe get out of any channel for safety) then drop the RPM's down QUICKLY and then turn off.

Throw anchor, let the engine cool down, trim it up, then pull the plugs and see which plugs are oily (if not all)

[tunedportcj5]

could you have too much oil in that motor causing it to foam up?
Just a wild guess, as the leak down did not show much blow/leak by

Nope, I wish that was the case!

[tunedportcj5]

Blow-by is usually the result of excessive clearance between the piston rings and the cylinder wall. Or maybe a scored cylinder wall.

Did you really do a compression test as well as a differential pressure test?

Might want to send a bore scope down into the cylinders and take a look.

Yes sir... I promise

[tunedportcj5]

I would think the op should perform his testing directly in front of the marina. Back and forth at 5800rpm so that he can pull directly over to check the plugs.

Being there's usually idle zones way before you get to land, I'd simply run it
to say 5,800 in the appropriate area, (maybe get out of any channel for safety) then drop the RPM's down QUICKLY and then turn off.

Throw anchor, let the engine cool down, trim it up, then pull the plugs and see which plugs are oily (if not all)

Good info, I can give it a shot. Fortunately there are not any no wake zones at my dock... I can shut down the motor after the fogging and idle back in the stbd motor.

[TownsendsFJR1300]

Good info, I can give it a shot. Fortunately there are not any no wake zones at my dock... I can shut down the motor after the fogging and idle back in the stbd motor.

That'd work!

Forgot about the second motor...

**Also, what may help, (may not as bad as it sounds), try running a shock treatment of Yamaha's RingFree: https://www.shopyamaha.com/product/d...?b=Search&d=34

Can't hurt (just your wallet) and only help... If one cylinder is the culprit, perhaps soak the cylinder over night with it (should it be a carbon issue). It does help clean up the fuel system as well so it's not a complete waste of $..


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[tunedportcj5]

The leakdown was tested today with the motor at operating temp. Brand new OTC tester.

@90psi

1=84
2=85
3=84
4=84
5=85
6=85

According to OTC's leakdown chart. 84=7%, 85=6%

[boscoe99]

On the other hand, if the problem only manifests itself above 5000 RPM, and everything else is hunky dory, I might suggest not to run the motor above 5000 RPM.

This assumes that the motor is propped so that it can turn at or near 6000 RPM when the boat is loaded and the throttle is wide open.

[tunedportcj5]

On the other hand, if the problem only manifests itself above 5000 RPM, and everything else is hunky dory, I might suggest not to run the motor above 5000 RPM.

This assumes that the motor is propped so that it can turn at or near 6000 RPM when the boat is loaded and the throttle is wide open.

My thoughts too... I have no problems hitting 5800 with the 21p 4 blades. 6k with the 21p 3 blades. Its propped right.

My normal operating RPM is 3500-4800... I just hate the fact that something is wrong.

[tunedportcj5]

That'd work!

Forgot about the second motor...

**Also, what may help, (may not as bad as it sounds), try running a shock treatment of Yamaha's RingFree: https://www.shopyamaha.com/product/d...?b=Search&d=34

Can't hurt (just your wallet) and only help... If one cylinder is the culprit, perhaps soak the cylinder over night with it (should it be a carbon issue). It does help clean up the fuel system as well so it's not a complete waste of $..


.

Definitely good stuff, I used it religiously on my OX66s. I've run a healthy dose though these motors... Next resort is dunks method. I do have a local dealer that will sell me YIEC but after seeing these leakdown tests I'm not sure what the heck to think. I wish (I think) that one cylinder was out of whack so I can focus on that one.

[TownsendsFJR1300]

Definitely good stuff, I used it religiously on my OX66s. I've run a healthy dose though these motors... Next resort is dunks method. I do have a local dealer that will sell me YIEC but after seeing these leakdown tests I'm not sure what the heck to think. I wish (I think) that one cylinder was out of whack so I can focus on that one.

Yep, I use it religiously, others call it my "snake oil"..

Anyway, is this product the same as the "dealer ONLY" stuff?:
ACC-CLEAN-D1-00 INTERNAL ENG CLNR 1Q



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