Mercruiser 5.7 engine runs for 15 minutes then runs really bad ?

[chiefm]

I just bought a new to me 1986 Bayliner Arriva with a Chevy 5.7 I have 2 known issues. When it's cold it runs like a champ, after about 10 -15 minutes it starts to run really bad and then wants to cut off / die and hard to restart. I replaced the plugs and they were really dirty like it is getting too much fuel, rotor, cap and wires look OK.

Idle oil pressure was real low so I changed the oil/ filter and found that it had about 3 qts of fuel in the oil. The boat has not been on the water for at least a year so I was thinking that maybe the float was sticking and that was my run bad when hot and oil in the fuel issue. Carb looked great on the inside and the choke is not sticking, the rebuild did not change anything.

With fresh oil I have 50-60 psi of oil pressure but it still seems to be getting fuel in the oil so I am going to change the fuel pump. The fuel tank vent is not plugged and I did open the fuel door when it started running bad but no change.

Not sure what the engine temp is suppose to be but it does not seem to be overheating.

When it dies if I let it sit for 10 minutes or so with the flame arrestor off and pull one of the PVC looking things out of the valve cover I can get it to run but just enough to limp it back to the dock.

I have taken it out twice, first time had to get pulled to the dock, second time I was able to limp it back.

I want to check the timing and maybe adjust the carb but don't want to blow the engine if fuel is still getting into the engine oil.

Coil maybe?

Any advice would be greatly appreciated.

Stephen

 

[alldodge]

Howdy Stephen

Normally it would sound like a fuel issue because of the 10 to 15 minutes. The fuel pump was a major issue until it was changed out. When the diaphragm starts leaking, it will dump excess into the motor.

Getting it to run by removing one of the valve cover vent lines, that's different.

When was the gas filter replaced, and what did the contents of it look like?

I would start with checking compression.
When pulling the plugs note what they look like and which cylinders they came from.
If all look good, remove the fuel line off the tank and stick the end into a gas can with fresh gas, and give it a try.

 

[chiefm]

The carb inlet filter was good, no stuff in it. The water fuel separator is the only thing I have not replaced yet, the fuel does not smell bad. I pulled the new plugs to look and see what they look like after the last outing, Shows signs of running rich but none were fouled out. I think I am going to check the timing and carb adjustments.

 

[alldodge]

Take the separator filter off and dump the contents into a glass jar or ziplock bag and see what's in there. If it comes out all gas, cool, but might have all kinds of crud in there and that's what' causing your issue

 

[Maclin]

That is a lot of fuel in the oil.

Fuel in oil can come from a carb that is leaking fuel directly from the fuel bowl into the bottom of the intake and on into the cylinders and then past the rings. Carb is almost always in a flooded condition.

Do you know what kind of carb is on there? A Quadrajet and others have welch plugs in the bottom of the fuel bowls, and those can begin to leak and need to be re-epoxied in.

A Holley carb has a power valve than can fail and rupture, and the fuel from the bowl runs out directly into the plenum chamber in the intake.

Another contributing component can be a failing fuel pump diaphragm leaking directly into the crankcase. Marine fuel pumps have double diaphragms, with a vent tube in between them that runs up to the flame arrestor. It is a clear (may be yellowing) and should never have fuel in it. The idea is that the inside diaphragm will begin to fail and sent fuel up the tube. A non-Marine pump will only have one diaphragm and no vent tube.


With a carb flooding problem the plugs will foul easily and it will be hard to start without the throttle being opened up all the way to clear the flooded conditions. Sounds like what you have.

 

[Bondo]

and maybe adjust the carb

Ayuh,..... Welcome Aboard,..?? What carb,..?? 2bbl,..?? 4bbl,..??

I agree, it sounds like carb/ fuel pump issues,.....

'86, that's a mechanical fuel pump I believe,.....
Is there gas in the fuel pump vent sight tube,..??

 

[chiefm]

4 Barrel Quadrajet
The Clear tube has fuel in it, guessing that's not good, If I rev up the engine I can see fuel inching up towards the carb, My tube is connected to the carb. I will order a new fuel pump tomorrow. I sure do appreciate all the help !!! I will keep you all updated on my progress.

 

[thumpar]

Yep, That is a bad fuel pump.

 

[chiefm]

Just thought I would give an update. Today I changed the oil and oil filter. I sucked out about 9 qts of an oil and fuel mixture. Looked down the carb and the intake was dry. I changed the fuel pump out with a new one. I run her on the muffs for about 30 minutes, checked the timing, did a little adjust to the carb and she runs like a top. I now keep 40-45 psi oil pressure at idle and around 50ish on throttle, she never got over 150 degrees, lots of oil pressure now and the run bad after about 15 minutes is gone. Plan on taking her out for a test ride tomorrow, hope it goes better than the last 2

It seems to NOT be pumping fuel into the crankcase anymore either!!!!

 

[chiefm]

Update,
I want to thank everyone on here for the help, I finally took her out yesterday for an on the water test, MAN she runs great!!! She lots of power, no fuel in the crankcase, restarts and idles when warmed up. All the issues I was having are gone and it's a huge relief!! I let the kids tube for a while and we did a little cruising. I was beginning to think that I had made a really bad mistake. Once boating season is over I am going to repair the floor and redo the interior.