Ville123

Greetings, I have a question regarding PCV system.


I have a 1987 twin pre-vortec merc 5.7 chris craft stinger, and I have a problem with one engine. After a normal steady ride (5-15min 20+knots), when revving down, with still in gear (like arriving in harbor etc.), after about 10-20 seconds one engine bogs down, dies and will not start for a while. The symptoms were just like the needle in the carb would not close or float level is wrong (flooding the engine), but I found out that it is actually fumes from the PCV system that bogs down the engine.

When I pulled the PCV valve from the cover, everything works.


There is a steady stream of steam coming from the valve cover PCV hole. This is the same with the other engine. The oil is clean and not foaming, so I presume this is condensation. The boat is used occasionally and weather here is cold and air is moist. On the engine that has the problem, the previous owner has installed an edelbrock breather on one of the valve covers. The other engine is without that.


So it seems that the fumes or the vapour from the PCV are high after riding on load and that is bogging the engine down in idle enough for it to die.

I can keep the engine running by revving a bit and the problem goes away, also the idle is perfect under any other conditions.


Has anyone had similar issues and can comment or offer a solution? I understand that the PCV should suck from one valve cover, and take air from another. Both PCV hoses from the covers end at the flame arrestor. Could the breather cause this somehow? Any other ideas?


Thanks,

Ville

SB

interesting issue.

There is no pcv on those engines. P= positive, which means intake manifold vacuum drawing from it, and using s valve (ususlly spring loaded) to meter that air.

There are crankcase vents on these engines though, that go right to the flame arrestor (no vacuum) .

Pull the dipstick and smell the oil. Smell like gas ? Oil very black ? White?

Can you take pic of top if engine and share ?

Compression test too.

Ville123

Hi,

Thanks for the reply. Sorry for the confusion about PVC terminology.
The oil looks dark, but not extremely black. Like an old engine after some use. The dipstick smells of exhaust fumes a bit. So does the condensation vapor that tends to pool on top of the intake manifold.
I cannot make the compression test now, but I believe that it would not be too good. The other one, that works, had a bad leak test on some cylinders, but since it worked, I decided to give it a season or two more. The problematic engine was rebuilt by the previous owner but the quality is unknown.

The pictures are from last season, new risers and manifolds are installed after that, also had the carbs rebuilt, thermostats replaced etc. But I havent touched the insides of the engines yet.

But thanks for helping to understand the crankcase vent system. My question is, where does this type of system take in fresh air normally when the vents both are for exhaust? Was the breather installed on it to create more ventilation in the cranckase? For reminder, the other engine does not have it, just a normal oil cap.

Dragracer_Art

If there is no breather on the engine with the PCV in use (sealed)... it will pull a vacuum in the crankcase. It's not designed to work that way.

You need a breather in one valve cover and PCV in the other if you want to use a "positive crankcase ventilation" system.

My factory Mercruiser had vent tubes from each valve cover that ended in front of the flame arrestor. Any crankcase fumes would be gently drawn through the flame arrestor and burned in the engine. There was no vacuum leak/PCV valve used at all. I tried running a PCV valve and the resulting vacuum leak from it made the engine run like crap. I didnt feel like messing with jetting, so I went with the Mercruiser way of doing it.

You need to have one system or another... or the engine compartment will smell like oil/gas fumes.

You can troubleshoot it by plugging off the PCV hose and put a breather in both valve covers. See if that helps the engine run clean/smooth. If that works, leave the PCV system out entirely and fab up a system like Mercruiser uses by running 1/2" tubes from each valve cover up to the flame arrestor. That way the fumes wont stay in the engine compartment they will get pulled into the flame arrestor instead.

The problem with running an automotive style PCV system is that it pulls vacuum from below the throttle blades. It will inherently draw more oil into the combustion chambers as a result... fouling plugs and promoting detonation. The Mercruiser dual breather hose system is the best way to go in my opinion.

See pics below. This is what I did with my 5.7 Mercruiser earlier this year. It worked perfectly.
Not easily seen in the photos is the breather hose on the starboard side. It's identical to the port side, just barely visible in pic.
There is no PCV valve in this system. Just breather hose/fittings/tubes.

SB

Check the fittings in the valve cover. See if they are actually pcv valves or hollowed fittings. Shoukd be hollowed fittings.

if pcv valves, there us no vacuum at the flames arrestors to open them. Which means, they’ll stay closed. Need to be replaced with open futtings and test again.

Edit in: Just saw dragracerArt’s post above . He has it correct.

Ville123

Hi, thanks again for replies!

It seems that I have the normal "mercruiser" type system. Meaning that I do not have PVC system and no valves, just open fittings that draw air into the flame arrestor. Basically the same system that dragracer described and has.
But as an addition, the previous owner had installed a breather (without PVC valves being used).

My original question remains, where does it take air in with this system? If at all? With dragracers comments, that it draws exhausts gently, I get the idea that it does not get fresh air in at all?

Could it be that because of my breather on that engine (fresh air gets in and vent increases), it vents too much air/condensation in too short time and therefore bogs the engine with water/exhaust?

Thanks also for explaining the difference between automotive and merc system. I am familiar with the automotive system, and was confused by this.

All the best,
Ville

Dragracer_Art

It will not suck fresh air in through the breathers... it only goes one direction with the Mercruiser way of doing things. The blowby from the combustion process creates enough crankcase pressure to push the fumes out of the breathers/hoses. It will actually push out the gaskets and leak oil if there are no breathers at all and the crankcase is sealed.

If you want your engine compartment to smell fresh again... you need to do away with the push-in breathers in the valve covers and pipe a tube from the valve cover to the flame arrestor like on the port side of the engines. All fumes that escape the crankcase need to be directed to the flame arrestor where they will get pulled into and burnt in the engine.

Your bogging issue is likely unrelated to the breather situation.
Probably a stuck float in the carb, bad fuel, clogged fuel filter, water in gas or bad fuel pump.

Also be sure your flame arrestor is clean and not plugged with oil/dirt deposits. I've seen some older ones that got really cruddy and restrictive.

SB

Exhaust gases don’t reburn. If you have that much ‘blowby’ , theoretically it could cause your issue.
can you video the blowby thru valve cover you speak of ? Will have to post youtube or etc and provide link here.
you need to do compression test and post here too.

Dragracer_Art

I noted some things in this pic...

1st... the breather is useless. Needs to be a oil fill plug/cap in that hole. It's just making the engine dirty and venting fumes into the engine compartment.

2nd... the port side looks like a PCV valve in the valve cover. It needs to be an open tube... not a PCV. The tubes can be found online/ebay if you dont have any.

3rd... it looks like you have the two hoses going to the flame arrestor correctly... so at least that.

Cant tell because of the plastic cover... but would be helpful to see how the hoses are routed and if that port side hose goes to flame arrestor or a PCV vacuum port on carb.

F-2 Speedy

In a factory valve cover with a pcv hole on the underneath side it is shrouded so that oil and what not cant be sucked in, if the previous owner just drilled a hole and plugged it in it may be picking up stuff that is shouldn't and putting it through the carb.