fuel dilution

Messages
24
Location
California
if you do a lot of short stop and go city drivng and then do a hiway trip of 50 miles or more at hiway speeds does that "burn off" or heat up and evaporate the fuel contamination in the oil. i assume that the fuel is not fully cooked off though, am i right? also if you were on an exptended drain interval with syn and a big oil capacity (8qt) and wanted to "freshen the oil" by either changing the filter OR replacing the oil, which would be better? i am planning a UOA at 5k on my '04 MB CLK first but want to plan ahead in case the fuel contamination is on the high side.
 
Messages
9,427
Location
Pensacola & Vero Beach FL
quote:
Originally posted by Brat57: if you do a lot of short stop and go city drivng and then do a hiway trip of 50 miles or more at hiway speeds does that "burn off" or heat up and evaporate the fuel contamination in the oil. i assume that the fuel is not fully cooked off though, am i right? also if you were on an exptended drain interval with syn and a big oil capacity (8qt) and wanted to "freshen the oil" by either changing the filter OR replacing the oil, which would be better? i am planning a UOA at 5k on my '04 MB CLK first but want to plan ahead in case the fuel contamination is on the high side.
I may be out to lunch on this, but my understanding is that gross fuel dilution, of the sort that made that a major issue, is really only an issue on older carbureted engines, which compared to today's cars, did a miserable job controlling fuel. Choked up (literally) for starting, they ran horribly rich. Then, the designers would have to set the carb to provide enough richness for even the leanest of the cylinders (usually the one most distant from the carb). The flipside, of course, was that all the other cylinders were running with too much gas. Nowadays, we have multiport fuel injection, adjudted to optimum mixture a zillion times a second by a computer that's literally a few thousand times more powerful than the first computers installed on the Space Shuttle. My thought is that if you have a 2004 Meredes that's dumping bulk fuel into your oil, something is awfully, awfully wrong and you'd better be at your dealership demanding a bit more than an oil change. Oh, here's a simple answer to you question: Yes, a good long highway run should, in an otherwise properly running engine, burn off (perhaps boil off would be a better term) any volatile contaminants in your oil, including fuel.
 

Patman

Staff member
Messages
22,012
Location
Guelph, Ontario
I sure hope that long highway drives can burn off most of the fuel in the oil, because just 2400km into my current interval with GC, I am pretty sure I got a real boatload of fuel into my oil this morning! [Frown] My car would not start, and so there were at least 2 dozen attempts (or more) of me trying to start it, so I'm sure the engine eventually ended up being flooded. But I hate the thought of ending my current OCI so soon, especially since it would put my oil change schedule right out of whack. It's lined up perfectly right now, I change it every 10,000km and my last change was at precisedly 160,000.0km (yes, I even lined up the tenth on the odo!) Eventually I did get it started, and drove the 25 mile highway trip to work (actually probably 35 miles today, as I made a detour to a GM dealer to see if they had any ignition coils in stock)
 
Messages
2,768
Location
Tn
Fuel dilution should not really occur with your car period. Even with short trips, it won't be significant. Your oil should look fine @ 5K and I would not expect to see any fuel dilution. In your climate, I don't think it's a concern at all. Yes, water vapor and fuel will cook off to some degree, but with the modern computer control and injectors, it shouldn't run overly rich anyway. With an 8 qt. sump, I think your oil will surprize you at how well it looks at 5K in this car. I wouldn't worry about what probably won't happen.
 
Messages
3,334
Location
Bolivia
fuel dilution is more of a problem in cold climates and carburetors, or dirty injectors. A highway drive will usually burn it off. Another little known cause of fuel dilution is reving the engine prior to shut off. Fills the chamber with rich mixture just when you turn it off without burning it.
 
Messages
897
Location
owatonna, mn
2.5% fuel after 4053 miles. =( The only other UOA on the same engine as mine showed 2.8% fuel in it. I guess it's just something that happens to us or my rings are really worn and so were his.
 
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