can PCV valve cause fuel dilution?

[1 FMF]

the past 2 oil analyses came back with around 4% fuel dilution, causing viscosity drop and higher wear. It's on a 2002 3.0L Mercruiser, which is a GM 181 cid, inline 4-cyl iron block/head, with 2-barrel merccarb.

I've rebuilt the carb, rejetted, and adjusted float level higher, which fixed a dead spot in acceleration from a lean out condition. I have OA before all this tinkering and dilution was high then. Plus, the spark plugs still look like before, clean burning.
There's only 2 things I can think of. One is the mechanical fuel pump, somehow seeping gas into the crankcase. The fix here would be to spend around $150 on a holley electric pump and run for a season to see if that cures the problem. Not something I really want to spend the money on.
The other is the PCV valve, or lack there of. The current oem design is just a 1/2" hole in the rocker cover with a free flowing 90 deg elbow, with a plastic tube going to the carb flame arrestor.

I'm not too knowledable about crankcase ventilation, and positive vs. negative crankcase pressures. Could adding a pcv valve, to slightly restrict ventilation out of the rocker cover, help with the high fuel dilution? Would the restriction in ventilation increase crankcase pressure, and help keep fuel from coming past the rings? That's if that is where it is coming from.

Other than this and the fuel pump, I can't think of anything else. Engine runs great, spends most of it's time at 3000+ rpms. I don't idle it much, especially after the 1st UOA showing high fuel %.

 

[Matt_S]

I don't think the addition of a PCV valve would help. Any increase in crankcase pressure you'd get would be insignificant compared to the pressure inside the combustion chamber during the power stroke. I can't imagine that blowby would be affected at all. It might even make the fuel dilution worse. PCV valves are supposed to open as vaccuum decreases, meaning it's fully open at WOT (when blowby is greatest) and barely open at idle. If you're spending a significant amount of time at idle (i.e. trolling), the valve would be closed most of the time and more of the fuel/vapors would remain trapped in the crankcase.

What type of operation does this motor see? Short trips? Trolling? Extended cruising?

 

[The_Jeff]

That hose sounds like the "fresh air hose" that is half of most PCV systems. The other half goes to (ported?) vacuum on the other side of the throttle plate. Fresh air on a "real" PCV system is usually going in the fresh air hose then out the PCV valve and getting combusted... sweeping clean any oily/gassy vapors.

You don't have this; the hose is there for blow-by at WOT. It basically keeps the fumes out of the bilge but does little for oil cleanliness.

You'd need a vacuum source below the throttle plate on the carb, to drill a hole in the valve cover, add a grommet and pcv valve and hose. I wouldn't do it; the Coast Guard or some other authority is really picky about messing with that sort of thing. Flammability in the bilge is their #1 concern. Someone who knows boats more will chime in. (I don't expect a proper PCV design to make that worse, but what do I know?

)

 

[mechtech]

It is winter.
Having a carburetor, you have a choke. It is on much more, now.
You have also raised the float level.
Seems about right, to me, that you'd have elevated fuel in the oil.

 

[1 FMF]

I'm not running the boat in the winter, I would if my drysuit was comfortable and could find a driver that can stand the cold.

Boat is used only from June to end of September, when water > 70F. I ski with the boat and that's it. After the 1st UOA, I made a best effort to minimize idle time to see if that was a factor in fuel dilution. It wasn't.

I start the motor, fast idle around 1500 rpms no more than 3 minutes, then it's in gear at 1500-3000 rpms to get where I want to. I ski at 4200-4400 rpms. When done, boat comes back to me at idle (less than a minute) then is shut off while waiting for next skier. I'd say 80% of the time the motor is at 3000 rpms or greater.

Also when starting the engine cold, I manually open the electric choke until the spring heats enough to hold it open. Motor doesn't need any choke, except a little to initially start, and I've adjusted that to be mostly open, choke plate is like 45 deg when cold. It is not fully closed.

The PCV vacuum idea has me interested. I will have to research it some. There is a 1/4 npt hole in the manifold, for where you'd take a vacuum reading with a guage. Think that would work? How much trouble would it be tuning the carb for the extra air from the PCV valve?

 

[1 FMF]

One more thing,
like was said, all that's happening is positive pressure from the crankcase (higher at higher throttle) is free flowing through open hose to the flame arrestor. Routes vapors to the carb instead of the bilge, and has some effect at keeping the oil clean (as opposed to no vent for blowby gases).

what's the chance that this way of doing it is the cause of high dilution? I don't know of any other late model 3.0L with no PCV valves like mine with used oil analysis that I can compare to.

By adding a real PCV valve, and routing it to intake vacuum so that vapors are constantly sucked out, think that would that help reduce the fuel dilution, maybe?

 

[dickwells]

If the motor does not need any choke except to start it probably is running way too rich leading to the oil contamination.

 

[1 FMF]

ok, but the 2 barrel mercarb is very simple.
There's not much I can do with it, short of getting a weber.

There is one idle fuel mixture screw, and procedure for setting it from the merc service manual #26 is this:

• engine warm, idle around 600-700 rpm, boat in the water;
• in gear preferably, I've done both ways and arrived at the same setting;
• turn screw in clockwise till motor just starts to stumble from being too lean, remember position;
• turn screw out counterclockwise till good idle, then continue till motor starts to stumble or idle drops from being too rich, remember
• position;
• turn mixture screw in clockwise to midpoint setting

I've also tried to do it with a vacuum gauge, really hard to get a steady reading at that low a rpm since it's a 4-cyl. But I've never had a dirty or sooty plug with any kind of buildup to make me think it's been running rich at idle. Not so much this year since I avoided idling as much as possible, but the previous two seasons the boat saw a fair amount of low rpm use, and the plugs have always looked the same.

 

[mechtech]

OK, you have a reasonable idle setting, but under load, your float, power valve, or jets , may be making it rich. There is normally no vacuum advance, either, which would help efficiency. Also, boats run cool, and have fuel wash , more than a car with a nice 195 deg thermostat.
What is the saying? A boat is a box in the water to put money into?

 

[1 FMF]

ok, you got me on the boat is a money pit.
But the problem I have is this, I have an old 1976 boat that I repowered in 1993 with a 351w marine engine, a drop in replacement for the old mercruiser model 233. It runs a holley 4-barrel, runs rich- and most people will say holleys run rich to begin with, and I can even confirm it because the spark plugs turn black, more than any I've ever seen. This engine also runs a 140 thermostat, although since I started doing UOA on it I've had a 160F thermostat in it. The used oil analysis never showed fuel.
On my 2003 boat with the 3.0L, the plugs are clean and tan, and I'm getting used oil analysis with > 3$ fuel, and it runs a 160F and temp gauge runs at 175F. My 351w temp gauge (with new gauge and sendor so I know it's accurate) always read 160F or less