Specific Gravity of Diesel Fuel VS Motor Oil

[faucet1]

The reason for the question is....in the event of an injector not closing properly and letting fuel leak into the cylinder will diesel fuel float on motor oil or is it reverse?

[Handbasket]

Doubt it'd be an issue here, but some larger Navy diesels back in the day (50's-70's, anyway) had spring-loaded flaps on the block sides. These were one each side per cylinder. They could be removed to improve access during rebuilds, but their real purpose (AIUI) was to vent crankcase explosions caused by excess fueling. The intent was to save the engine block.

I saw one brief training film... quite impressive to see foot-long tongues of fire shoot out of the side of the block. Nothing was said about what would have happened to the engineroom crew, of course.

Jim, "The Navy! It's not just an adventure, it's a dang dirty, greasy job!"

[westend]

Again, this would be a good time to drain and replace oil. The last engine I saw that had a fuel->oil problem like yours shot a piston/rod through the block. Oil is cheap compared to engine replacement.

[faucet1]

Well let's hope I find the source/problem before I lose an engine. Thanks for all of your comments.

[MEXICOWANDERER]

Sort of like wondering why the smoke detector didn't sound when flames are leaping up into the sky.

There will be a noticeable loss of oil pressure. This is a death knell signal.

Crankcase oil DOES NOT SMELL LIKE DIESEL FUEL.

If you smell diesel fuel on the dipstick, it's time for the pickup to go to the diesel hospital.

[faucet1]

So, when you get dilution, am I to understand that there is no smoke to be seen coming out the exhaust? One would think as the fuel leaks in, this extra fuel would cause some smoke.

[MEXICOWANDERER]

Any dilution of lube oil with diesel fuel is dangerous and WILL lead to total engine destruction.

Diesel fuel and lube oil mix wonderfully, like vinegar and water.

[faucet1]

In response to Westend, I don't smell diesel in the oil, but there probably is not enough to smell as yet. Just changed the oil in January, and have driven less than 2,000 miles. The oil level is only about 1 to 1.5 quarts above the level right after I changed it(I generally check oil before going on an outing). So, if something has changed/happened, it did so since the 1st of May.

[carringb]

Yup. It mixes. If your motor start "making" motor oil, time to dig deeper. It can tolerate some dilution, but not much. On out '08 Cummins 6.7L trucks, we had to switch to 4,000 mile oil changes due to oil dilution. On our case, the excess fuel was from the trucks being frequently in Regen mode. Cummins uses the regular injectors to dump extra fuel into the cylinders so it burns off in the DPF.

[faucet1]

If there is a leak into the crankcase it may/could be bad or deteriorating seals in the injector pump(as I understand). When an engine is shut down the pressure at the pump is quite high, and if an injector were open at the point of shut down I suppose the fuel pressure might push the remaining fuel in the line into the cylinder and down past the rings. Any of you diesel mech ever seen this happen? I did start the engine and didn't see any smoke from the exhaust. This is a 2002 engine and is electronic ignition. Will an injector remain open with the ignition switch off?

[Handbasket]

I don't know about newer emission-controlled diesels, but from 4 years of USN experience with diesels up to 2031 CID 'back when', I'd expect a stuck-open injector to result in lots of black smoke, especially at full throttle / under load.

Jim, "Mo' coffee!"

[mpierce]

Send oil in to get it tested. They will tell you if fuel diluted.

[markchengr]

Diesel fuel is quite soluble in lube oil. If your crankcase level is increasing with no indication of water present then what you have is known as fuel dilution. It is quite common in the large marine diesels I have worked with. You need to not only change your oil but find out where the fuel is leaking into the engine. Many diesels have been ruined by fuel dilution. Usually the only indication is rising crankcase level and possibly but not necessarily a decrease in oil pressure. I'm sorry. I know this is bad and probably expensive news. Good luck. -Mark.

[naturist]

Motor oil and diesel fuel are mutually soluble, so very unlikely to form separate layers. Between suck-&-mix of oil pump and oil spray coming off the bearings and bottom edges of cylinder walls, you got some strong mixing going on. If you are getting diesel in the crankcase, you gotta change the oil before low lubricity ruins expensive pieces.

[westend]

faucet1 wrote:
I suppose I was thinking about a cold engine and checking the dip stick when I posed the question. Crankcase seems to show above full reading since the last outing(maybe a qt). No milky substance. So, probably not antifreeze. My question is, what is it and where did it come from. Was not there before last trip.

Good time to change oil. Have you given it the sniff test??