Engine gaining oil

I recently had my oil changed, and the mechanic changing my oil was very surprised when he checked the oil level before he changed it. The oil was reading a good 1/2 inch to 3/4 inch above the full line. Now this was a surprise to me because between changes, I didn’t add anything to the engine. Its almost as the engine was gaining oil. My car is a 1997 Subaru Legacy with 228,000 miles on it. It has a small oil leak and has been losing oil in the past. Does anyone know how or why this could happen? Could the small oil leak be letting something in?


The only way the engine oil level can rise is if another liquid is being introduced into the crankcase. This can be coolant or gasoline.

If the oil doesn’t smell like gasoline, then it must be coolant.


Any idea how coolant could be getting into the system? Leaky head gasket, or something of the sort? If it were coolant, then shouldn’t the coolant in the radiator overflow be dark / contaminated as well?

Because it’s a Suburu, my first suspect would be a head gasket breach adding coolant to the oil.


Just checked the coolant and the oil. The coolant level is fine, and there doesn’t appear to be any oil buildup in it. The oil does have a slight smell of gas. I know gas in the oil is bad, but how serious is that?

Gas is a solvent and will break down the oil. If the oil has a gas smell, then the first thing to check for is leaking fuel pressure regulator.


A second possibility is a leaking fuel injector. It will add gas to the oil when the engine is shut down.

The odor and the fact that you have not mentioned the oil looking milky/foamy both point to gasoline dillution. This generally happens due to injectors that leak after the car is turned off.

How serious? Oil diluted with gasoline has low viscosity and substandard lubricity, so the engine is wearing faster, though at 228k miles, that is like telling a 75 year old that he is aging faster than normal. The cylinder that is receiving the excess gas is getting excessive wear on startup due to the oil being washed off the cylinder wall. And I suspect that as the car warms and the gasoline tends to boil out of the oil, the excess fuel vapor coming in through the crankcase ventilation system may overwhelm the car’s ability to adjust the mixture, causing rich operation and overheating the catalyst.

Hm. Okay. Thanks for the suggestions. I’ve also been looking around and there are a plethora of other reasons that gas could be ending up in the oil. I guess I’ll be taking it to the mechanics soon to have him look over all the possibilities.

As for right now, Manolito, you mentioned that the car probably boils off the excess gas when it is running. Can I assume that during long drives the oil will return to a safe operating level, and that I don’t need to worry about the dangers of overfull oil? Or should I keep my eye on the oil levels looking out for foamy oil?

Thanks again.

Can I assume that during long drives the oil will return to a safe operating level, and that I don’t need to worry about the dangers of overfull oil?

No. I’d recommend no long drives at all in this condition. This needs fixed.

I recommend fixing it before it catches on fire. You won’t know it’s burning until it can’t be saved.

You might also check your brake booster unit. I had a VW bus once that “gained oil” and after several years I finally correlated that phenomenon to all the brake fluid I was adding with no obvious leak present.

I doubt the crankcase has gained enough gasoline to catch fire. But gasoline does undermine the ability of oil to lubricate, though it may clean up some of the sludge on your older engine. I would get the oil changed sooner rather than later and monitor it closely.

Also, it may be useful to have the fuel system subjected to a leak-down test to make sure it isn’t losing pressure–this will help diagnose a sticking fuel injector. It also would probably be a good idea to get the oil analyzed (Google “Blackstone”) to see what makes up the gained quantity–eg. is it fuel or partially coolant?

I would not drive your car further than somewhere that can change the oil if you haven’t already.

With 228k miles it could be worn rings letting blowby into the crankcase. Combined with cold weather it can condense gas into the oil. Definitely get it changed ASAP. Then monitor the oil level carefully.

Have you been making a lot of short trips in cold weather?

Senior, you need to get this properly diagnosed and fixed ASAP.

Oil is a lubricant AND a hydraulic fluid. It provides a slippery barrier between the parts AND it provides a pressurized fluid barrier between critical bearings and the surfaces that spin in them.

Gasoline washes away oil, essentially leaving metal to metal contact. And it dilutes oil, preventing it from maintaining that critical pressurized barrier. Gas in the oil will allow the engine to rapidly self destruct. Since gas can also get into the oil by being pushed past the rings on the compression stroke due to internal cylinder damage (including badly worn cylinders and tired rings), I’d probably start with a compression test before doing anything else, but that’s me. I would not criticise others for starting elsewhere.

Forget about checking for foaming. If you’re getting gas in the oil, you have much more serious problems.