I have a 1967 Ford Mustang with a 289 small block V8. It had been sitting in the garage for a while (years, actually), so I did a fluid refresh, changed the fuel, changed the oil, and changed the coolant. After fixing an air pocket in the fuel line I was able to get it running and driving and everything seemed fine.

The Problem

However, when I got home I checked the oil level and found it was very high. I thought perhaps I'd added way too much oil on accident, so I changed the oil again, drove the car again, and found the oil level had risen again.


As far as I know there can only be two places additional fluid could be coming from. Either coolant is entering the oil, or fuel is entering the oil. The liquid in the oil pan is not milky. I've smelled it, and it smells like motor oil, perhaps tinged with the smell of gasoline. I tried burning some, but it didn't ignite, so if the additional fluid is gasoline there isn't enough to burn well. I have to conclude it's fuel in the oil, but I'm not sure how to confirm this.

More importantly, how could gasoline be getting into the oil? Besides sitting for a long time, the engine was essentially brand new (it was totally rebuilt in 2010), so there can't be any real damage besides whatever stagnation can cause. I've wondered if perhaps the piston rings are stuck closed and fuel is bypassing the rings as it enters the cylinders while it's running, but the amount of rise in the oil level is significant and the engine doesn't act like it's starved for fuel.

The Ultimate Question

Could there be any other fluids that could be contaminating my oil? If it is fuel, how is it getting into the oil? And most importantly, what do I do to prevent it from happening in the future?

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    Welcome to Motor Vehicle Maintenance & Repair! Commented May 29 at 19:24
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    A nice photo of your rebuilt engine/vehicle could help give some inspiration here, try adding one with edit
    – Criggie
    Commented May 30 at 3:33
  • Monitor coolant levels. Coolant system is generally easy to check for leaks with a pressure tester. Or just let it get hot and feel the upper rad hose. It should have pressure. Shut it down and check the pressure on the upper hose every 5 min. It should hold pressure for a while, and drop as it cools. If the drops quick, coolant is leaking somewhere.
    – rpmerf
    Commented May 30 at 12:58
  • Electric or mechanical fuel pump? If it has an electric pump, turn the key to run, but don't start the engine. This should turn on the fuel pump. Look around the carb, look down the bores for leaking fuel. If it's a mechanical pump, you will need to run the engine and look. Try reving it up a bit to simulate on throttle / off throttle of driving.
    – rpmerf
    Commented May 30 at 13:01
  • In my experience, coolant in the oil will emulsify, and an opaque foam condenses at the highest, coolest point of the engine (generally under the inside of the filler cap). It looks just like the inside of an Aero milk chocolate bar. Commented May 31 at 8:02

5 Answers 5


Used motor oil should smell like oil, or oil and exhaust. If you can smell gasoline, you have fuel in the oil.

You said the car sat for a long time. If this engine has not been modernized with fuel injection and it has a carb, the fuel in the bowl may have become thick and sticky as it sat, or turned into a solid. This could be preventing the float valve from fully cutting off fuel flow when the bowl is full. Fuel can then overflow into the intake manifold and find its way into the crankcase through the cylinders and rings. It will be in the liquid state and not atomized, so it's hard to burn and will seek a low point with the motor oil.

Suggest you open the carb and see if the fuel bowl is over-full. If so, you may have to clean the carb to stop the leaking.


And by the way, you can identify fuel in the oil by placing a drop of oil on filter paper (like a coffee filter) or even a paper towel. Observe the drop for a few minutes and see how it spreads. If it spreads into a uniform circle, it's pure oil. If it spreads with a light outer band surrounding a dark circle, it's got fuel in it.

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    While this is a good thought, there's no place for the fuel to be dumping into the crankcase from the carb. It only dumps into the intake manifold which is separate. If there was as much of a rise in oil level as the OP is talking about and sneaking past the rings, it wouldn't run well if at all as it'd be flooded. Commented May 29 at 19:31
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    @Pᴀᴜʟsᴛᴇʀ2 Yep, through the cylinders, past the rings as a liquid is my thought. Let's see how it turns out if the OP follows up. (Good thought on the fuel pump by the way!)
    – MTA
    Commented May 29 at 19:39
  • I had wondered if the carb might be gunky, but when I drove the car it really did run quite well. There was some smoke and roughness at first but after a couple miles it rumbled away like clockwork, no matter what throttle level or RPM was involved. This is especially telling in my experience because the engine has always been a touch sensitive to fuel flow. That being said, when I investigate the fuel pump I'll be sure to check out the carbs as well, just to be thorough. Thanks! Commented May 29 at 19:59

If the car has a mechanical fuel pump (which the 289 did have), please check that the diaphragm in the fuel pump is not leaking. When the pump is not used for a long spell, the rubber diaphragm can dry rot and crack, letting fuel into the crankcase. On a car that old, a new pump would probably be a good investment even if it isn't the cause of the leak.


More than likely, there's coolant getting into your motor oil versus fuel. And, there's no other fluids which could be getting into your oil.

Regardless if it looked milky when you changed the oil, what did the first part which drained out look like (or did you notice)? Oil is lighter than water/coolant, which means water would come out first when draining the oil.

As for fuel, I don't know of an easy way to tell other than what you've described ... however, a likely spot for fuel to be entering the engine on your 289 is at the fuel pump. If there's a problem with where the diaphragm is at, it could pump fuel right into the engine where it attaches (where the fuel pump arm goes up into the block to meet the cam). You could possibly take it off of the engine and check for signs of it leaking, or cobble something together to actuate it by hand to see if it's pushing fuel by there.

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    When I removed the contaminated oil it all basically just seemed like oil. The first fluid to come out of the pan did seem to be regular oil, although I admit I wasn't watching particularly close. I'll see if I can find the time this weekend to swap the fluids again and check. Your thought about the fuel pump is intriguing. I had issues with it before I discovered the air pocket in the fuel line, so maybe there's more to that issue I didn't notice because I assumed the air was the entire culprit. I'll follow up when I can! Thank you! Commented May 29 at 19:55
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    @fectin - Water is heavier than oil, so it will come out first if there is water in the pan. Rereading, I see how it can be confusing. I'll update. Commented May 30 at 13:37

If an engine sits for a long time, some parts tend to take a memory or a "set"

Tyres can develop a slight flat spot, piston rings can get a bit stuck in their groove, and valves like the PCV system can stick a bit.

This may be a heisenproblem which goes away with a bit of time and some driving, along with the associated heat cycling.

If problems persist it may be worth taking an oil sample while its warm and stirred up, and sending that to an oil lab for an analysis.

Its also worth checking the radiator coolant height and seeing if that is going down faster than expected.

  • It may be worth popping a plug out and slipping a borescope camera inside a cylinder - if there's fuel going down beside the pistons, it might have a "washed" look. A photo of your plug tips might help indicate something useful too.
    – Criggie
    Commented May 30 at 3:37

A probable reason: inaccurate measurement, either when filling or when checking later.

In an engine, oil has a great deal of cavities and surfaces to reside at for a while before going to the pan. This is even more pronounced for the fresh, cold oil from the factory bottle.

Because of this, it is rather easy to overfill the engine in the first place.

If a coolant gets mixed in the oil as much as visibly changing the oil level, you will see the coolant level getting down (beware when checking: the coolant level drpends on the temperature way more than the oil level)

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