Oil Analysis: Fuel Dilution

Started by Rosebud, December 20, 2018, 03:59:28 PM

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Rosebud

I got the results of an oil analysis today and everything looks good except that it shows an excessive amount of fuel in the oil. The testing lab suggests the high gasoline content is from blow-by. It's a 1275 A+ bored to 1380 w/ 10:1 CR. A compression test shows 195 to 198 psi across the 4 cylinders, so I'm confident I don't have significant blow-by. The car's driven almost every day and always reaches operating temp. What else might cause oil/gas dilution? My twin SUs are in good order and my mass O2 guage and spark plugs confirm I'm neither running too lean or too rich.


The analysis also shows the oil viscosity is on the low side @ 100°C, but the lab says that's caused by the oil/gas dilution. Also, boron is as on the high side. Anyone know if boron is used in any critical engine/transmission parts? I use a carbon reducer every couple of tank fulls. I'll check the label to see if boron is an additive. Thanks.
Rosebud
...the sled, not the flower
https://www.facebook.com/PoserMotorSports

MPlayle

Are you still using the original mechanical fuel pump?  If so, the diaphragm may be leaking slightly and that will drain directly into the case and oil sump.

Also, too high of fuel pressure if using an electric pump can cause slight overflow of the jet on shut-off and that fuel will leak down the cylinders and seep past the rings into the case and sump even if the compression is good (leaks down the rings of cylinders that are down the bores where the rings can be a bit looser).


Rosebud

#2
Quote from: MPlayle on December 20, 2018, 04:37:13 PM
Are you still using the original mechanical fuel pump?  If so, the diaphragm may be leaking slightly and that will drain directly into the case and oil sump.

Also, too high of fuel pressure if using an electric pump can cause slight overflow of the jet on shut-off and that fuel will leak down the cylinders and seep past the rings into the case and sump even if the compression is good (leaks down the rings of cylinders that are down the bores where the rings can be a bit looser).


I have an new SU electronic fuel pump that I had originally set to 2.5 psi. After reading your post, I checked the fuel pressure last night and it was reading almost 4 psi. I'll dial it back to 2.5 today and get another oil analysis in another 1K miles or so. I also noticed that it takes almost an hour for the pressure to bleed off to zero after shutting the engine off.

Do you think 4 psi was enough to allow fuel to get by the diaphram and contaminate the oil? Thanks.
Rosebud
...the sled, not the flower
https://www.facebook.com/PoserMotorSports

MPlayle

Does the line from the electronic fuel pump go to the original mechanical pump before going to the carb?

If using an electronic fuel pump, you should bypass the mechanical pump on the back of the block if it is still there.

Yes, 4 psi is higher than it should be and is enough to push extra fuel out the jet as well a the diaphragm of the mechanical pump (especially if it has a pinhole leak in the diaphragm).


Rosebud

#4
Quote from: MPlayle on December 21, 2018, 12:46:17 PM
Does the line from the electronic fuel pump go to the original mechanical pump before going to the carb?


The mechanical pump has been removed. I think I'll take a look at my diaphrams and see if I can spot any pinholes. I'm hoping the lower pressure will solve the problem. Thanks for the advice!

[EDIT] Just realized that my electric pump dosen't have a diaphram. So, I'll take a look at the float bowl jets and see if I can determine if they're leaking. I also discovered that I cannot dial my fuel regulator below 3 psi. If there's nothing wrong with my jets, I guess I'll need to get a new pressure regulator that I can dial down to 1.5 - 2[/size].0 psi[/size].
Rosebud
...the sled, not the flower
https://www.facebook.com/PoserMotorSports

jeff10049

#5
Boron is a normal oil additive.

If I can read that right looks fine for the oil you run here is a sample of that oil when new.
Silicon = 6 ppm (anti-foaming agent in new oil, but in used oil, certain gasket materials and dirt can also add to this number)
Boron = 191 ppm (detergent/dispersant, anti-deposit buildup/anti-sludge)
Magnesium = 18 ppm (detergent/dispersant, anti-deposit buildup/anti-sludge)
Calcium = 3354 ppm (detergent/dispersant, anti-deposit buildup/anti-sludge)

Rosebud

Quote from: jeff10049 on December 29, 2018, 09:11:47 PM
Boron is a normal oil additive.


Thanks, Jeff. I guess the lab doesn't see much boron, but the numbers are spot on for Amzoil. Everything's good except for the fuel dilution. I pulled my fuel floats off and drew a vacuum on the check-valves. Doesn't appear to be any leaks. My fuel pressure regulator to had crept up to 4 psi so I dialed it back to 2.5 psi. I'll put a couple thousand miles on the oil and send a sample in for retesting. Will post the results here[size=78%].[/size]
Rosebud
...the sled, not the flower
https://www.facebook.com/PoserMotorSports