Gas in the Oil

[rkikenda]

I have a 2004 Mercury Mercruiser 3.0 L 4Cyl carbureted engine, which recently overheated (~260 degrees) due to no cooling seawater circulating. Once I noticed the problem, I immediately shut down the engine to prevent further damage. I corrected the problem and have since replaced damage parts such as exhaust/water piping and gaskets/o-rings and everything is running fine. The only other problem I see is that after changing the oil, I find that the oil smells of gas and also looks like it has some foreign liquid (i.e. gas) in it. I was wondering if anyone has ever seen this problem and if it is due to piston-ring/cylinder damage or if it may be from something else (i.e. valve seals.) I didn't notice the problem before overheating and this engine has only been running since June 2003. I am capable of changing the rings and valve seals, but I wanted to see if anyone had any other ideas. Here is a link to a parts website that shows diagrams of the engine:<br /><br /> Mercruiser Parts Website

 

[Walt T]

Re: Gas in the Oil

have you changed the oil more than once? Check the fuel pump, make sure it's not pumping gas into the cranckase.

 

[crazy charlie]

Re: Gas in the Oil

Did it overheat momentarily and you shut it down or did it overheat while under full power and you shut it down as soon as you noticed?????How severe an overheat may be key here.Charlie

 

[imported_Mike_M2]

Re: Gas in the Oil

The most common cause of gas in the oil is a brokem diaphram in the fuel pump. Not sure if your 3.0 has a manual pump or not....

 

[rkikenda]

Re: Gas in the Oil

I was running at open throtle when it overheated and shut it down immediately. The exhaust pipe was smoking hot and the entire engine compartment was filled with smoke. So it sounds as if the fuel pump may be the culprit? The enginer runs just fine with now problems other than a little hesitation getting started. I am assuming that this may be due to it sitting for quite a while (1 month or so) as I repaired it. I changed the oil once after the overheat and noticed it contained gas. Once my repairs were complete I changed the oil again before testing. After testing I checked the oil and it smelled of gas. The fuel pump is NOT a manual.

 

[Walt T]

Re: Gas in the Oil

If you have an electric pump, thats not the problem. If it is a pump on the side of the engine near the front on starboard side then thats most likely it. It could also be a malfunctioning choke.

 

[rkikenda]

Re: Gas in the Oil

The pump is a mechanical one on the side of the engine. I am going to check the sight tube tomorrow to see if it shows any fuel. Thanks for the quick replies/suggestions.

 

[bomar76]

Re: Gas in the Oil

As Dieslwalt sais, check the fuel pump.<br />A pinhole in the rubber diaphram can pump gasoline into the crankcase.

 

[rkikenda]

Re: Gas in the Oil

Two more questions. Does damage to the fuel pump diaphram usually occur because of overheating? I bought a compression test gauge and am going to check each cylinder's compression. My question is, what should the normal pressure be? I basically want to see if, in addition to the suspected fuel pump damage, there is any piston o-ring damage. Many thanks for everyone's immediate responses.

 

[rkikenda]

Re: Gas in the Oil

Two more questions. Does damage to the fuel pump diaphram usually occur because of overheating? I bought a compression test gauge and am going to check each cylinder's compression. My question is, what should the normal pressure be? I basically want to see if, in addition to the suspected fuel pump damage, there is any piston o-ring damage. Many thanks for everyone's immediate responses.

 

[bomar76]

Re: Gas in the Oil

Q#1: No<br /><br />Q#2: Pistons don't have O rings. They have compression and oil rings. <br /><br />Overheating is more likely to crack a head,but you could have scored a cylinder or something.<br />If engine runs ok now and doesn't overheat or smoke it should be just fine.

 

[rkikenda]

Re: Gas in the Oil

Ok. I ran the engine tonight and checked the sight tube coming out of the fuel pump and there was no gas in it. The manual says that if there is gas in the sight tube then the diaphram is bad. Therefore, I can rule out the fuel pump, right? Are there any other thoughts? If not, I guess I am going to have to overhaul.

 

[fireship1]

Re: Gas in the Oil

Hello, You don't have to overhaul for those reasons! Fuel can only be getting into the bottom end from two places.(1) The fuel pump diaphragm is leaking internally somehow. If you have a pinhole it might not be enough to fill the overflow tube and make it to the carb.(2) You have a stuck choke or a carb that is flooding itself and the excess gas is washing down the cylinder walls and finding it's way into the crankcase. Check to see that the choke is open when the engine is warm and make sure that the carb does not leak into the intake when the engine is at rest when shut down. My guess is the fuel pump. You will be OK if you fix it now. Don't wait and run it like this for long periods. Excessive crankcase dilution reduces the lubricating properties of the oil. It's not good for engine bearings and the like. Check it over carefully, you CAN fix this! Good luck!

 

[rkikenda]

Re: Gas in the Oil

Fireship1, I am leaning more toward the carburetor problem since this has to have occurred from the overheating. I just don't see how the diaphram could get damaged. The carburetor sits on top of the exhaust manifold and would have taken a lot of heat. Is there anything jumps out at anyone here? I am going to do further investigation on the fuel pump, but I want to check everything out. When I initially start the engine it takes a while to get it going, as if flooded...It starts a little rough and then runs smooth after a couple of seconds of increased throttle.

 

[fireship1]

Re: Gas in the Oil

Hello, The fact that it seems to be flooded makes good sense. That was the final piece of the puzzle. The carb is flooding the engine. Rebuild or replace the carb and you will be all set. Good luck!

 

[rkikenda]

Re: Gas in the Oil

Could the carb have gotten damaged by the overheating? If so, what parts would likely fail? I will take a look at it this weekend, but I just want to make sure that I am looking at the right parts for failure.

 

[rkikenda]

Re: Gas in the Oil

Could the carb have gotten damaged by the overheating? If so, what parts would likely fail? I will take a look at it this weekend, but I just want to make sure that I am looking at the right parts for failure.

 

[fireship1]

Re: Gas in the Oil

The carb would not have sustained damage due to overheating. Your overheat and carb problems sound like isolated incedents. At this point your carb is causing your oil dilution problem.

 

[Bhamil]

Re: Gas in the Oil

Fireship is right on the money here. Your source of gas in the oil can come from only two places (if all else is correct), the fuel pump and cylinder wall wash-down. The fuel pump is pretty well ruled out but here's where Fireship is just slightly wrong; when the engine overheated it is likely steam came back through the manifold and carb. That would cause the moisture to be pulled into the fuel bowl opposite the venturi effect (which draws gas out). So, there is the likelyhood that the overheating caused the carb to be fouled. A simple rebuild by a carb shop will take care of the problem. There is a slight chance that when the engine overheated it could have scored the cylinder wall (they will take on a slight hourglass shape that causes the rings to scratch the cylinder wall) but that is not likely. It never hurts to take a compression test to get a base line reading but I really don't believe it's going to be necessary. Just follow Fireships' advice and you're going to be good-to-go

 

[rodbolt]

Re: Gas in the Oil

hello<br /> I agree with fireship but take exception to the fantasy about a reverse venturi effect. cant happen<br /> a venturi only works one way. you cant tell me that we are going to pull water from the exhaust and somehow through the exhaust valve then through an intake valve on a diffent cylinder then back out the intake port then through the manifold and past the throttle shutter and now the venturi is upside down so it is not really a venturi and somehow maybe osmosis into the fuel bowl? mmmmmm I am thinking not.<br /> logic dictates there are 2 points of fuel entry. bad pump or a carb malfunction. remember blowby is enough to make a gasoline smell in the oil. you posted you can SEE something in the oil? it aint gas. trust me you could add 1/2 gallon to the crankcase and still not ever ever see gas. if you see something visible odds are its water. water turns oil anything from a grayish to bright yellow milky looking. <br /> after an overheat under power that went on so long as to melt the exhaust hose I would truck on down and have a leak down test done. skip the compression cause we are really after cylinder sealing and leakage detection. leakage past the rings is a secondary concern we are looking for cracked heads and blown head gaskets.<br /> good luck and keep posting