Motor oil in carb/flame arrestor on 1990 Mercruiser 4.3

Recently I have noticed small amounts of oil pooling up on the intake, on the bottom of the inside ring of the spark arrestor and on the outside of the carb. The source of the oil is the clear fuel line coming from the fuel pump.

The oil only comes out when the motor is run at 3500 rpm's or higher which is rare since I don't run the boat hard very often.

My question is, what is causing the oil to come out of a clear fuel line (rebreather line, excess fuel line, whatever you call this line, etc.) that comes from the fuel pump?

No pressure is in this line, no fuel is coming from it so I am not sure the except term/description for the clear line.

Compression test was done this spring and consistant 160 on all cylinders except 145 on the #5 cylinder.

The motor has 1450 hours on it (1,025 babied hours on it from myself)

Boat has been meticulously maintained and I run synthetics throughout, notably mobil 1 for the motor oil. When I change the oil every 100 hours, there is no trace of fuel in it.

Any ideas out there on what could be causing this and what should I do to alleviate it?

Thanks,


Monto

Re: Motor oil in carb/flame arrestor on 1990 Mercruiser 4.3

time to replace fuel pump

Re: Motor oil in carb/flame arrestor on 1990 Mercruiser 4.3

I take it you have an mechanical fuel pump on the side of the block, I would guess you have a pin hole in the pumps diaphram and at high rpm it is pushing oil into the line.
are you sure the oil is not coming from the breather hoses on the valve covers?
rob

Re: Motor oil in carb/flame arrestor on 1990 Mercruiser 4.3

rbh said:

I take it you have an mechanical fuel pump on the side of the block, I would guess you have a pin hole in the pumps diaphram and at high rpm it is pushing oil into the line.
are you sure the oil is not coming from the breather hoses on the valve covers?
rob

Click to expand...

duh... good point. with that many hours and a low compression cylinder it is every bit as likely blow-by pushing oul up the breather hoses if that's how you're set up...

Re: Motor oil in carb/flame arrestor on 1990 Mercruiser 4.3

100% sure oil is not coming from the breather hoses. If I take the flame arrestor off, there is a small notch in the carb where the plastic fuel line connects to it and oil is building in the hollow, then slowly leaking out onto the carb, manifold and on the inside ring of the spark arrestor.


So people's thoughts are fuel pump? Straight OEM or any other recommendations?

Someone did mention a diaphram in the fuel pump in person to me last week.


Thanks,

Monto

Re: Motor oil in carb/flame arrestor on 1990 Mercruiser 4.3

quick question when you did your compression check how warm was your engine before you took your readings, as the engine should be warm
as well do you idle alot or run it at high throttle, as you could have a little carbon build up on you # 5 cyl valves? (possible old gas not burning good enough)
rob

Re: Motor oil in carb/flame arrestor on 1990 Mercruiser 4.3

In regards to the compression check, I had a mechanic perform it and do not know offhand if the motor was warm or not.

As for running the motor, 80% of the past 1000 hours were around 600 rpm's at a slow troll since this is a great lakes fishing boat.

I never run the motor full throttle and very seldom crack it above 3500 rpms.

The breather hoses are clean, no blow by coming out of them and plus you could eat off of the valve covers..... all areas of the motor are pristine other than where I mentioned where the fresh oil is accumulating.

Still think it could be the fuel pump diaphram? Please advise, I am already looking at go2 marine's website for ordering some parts.


Thanks,

Monto

Re: Motor oil in carb/flame arrestor on 1990 Mercruiser 4.3

yeah go for the new pump, it does make sense.
as well open up your motor once in awhile, wot! blow out some of the crud thats been building up.
rob