Fuel Dilution - Some of the causes are?

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Sep 29, 2003
Depends on your driving habits. If you take a lot of short trips(5500 miles for 2yr old car?) the car never gets out of open loop and will run rich. Do you make short trips?

If you drive long trips, the car gets into closed loop, (uses the O2 feed back to adjust the mixure) and the oil will reach operating temp and any fuel will be "cooked" off from the oil.

Assuming you have no mods.

I have scene modded cars with new cals, larger injectors, aux injectors, 160 degree thermostats, etc which can cause the fuel dilution, again with poor driving habits(short trips).

I have one turbo car with +20% injectors, 20 PSI boost calibration. I can watch my EGT(Exhaust gas temp) go from 1400 to 1475 at part throttle and at WOT the temp drops back down to 1400. The cal is slightly rich at WOT to prevent detonation. I can probably raise the boost a bit and still be on the safe side.

I have seen one of the two injectors stuck open on a throttle body Camaro. Let me tell you, after a 7 mile drive the crank case was over 2 qts high!

Oh yea, on an older GM v8, a bad diaphram in the mechanical fuel pump can fill it up too.
Very interesting. In my case, 100% stock 3.8L Chrysler minivan. Guilty of short trips, though just before oil change ran 2,200 miles over a week so plenty of time to evaporate out fuel.

Van has oil cooler, (towing package) but it's a coolant to oil style so one would think it helps to heat up the oil quicker as well as to limit it's upper temp. No excessive idling though gas mileage stinks....

Could a leaking intake gasket cause this symptom also? Silicon tends to be high in the UOAs...

Ron, did this issue just start or is ongoing? You also mentioned the terrible fuel economy, again is that something recent?

Higher Si: if you're getting excessive fuel dilution, especially a lot of cylinder/ring wash, there will be a lot of metal-to-metal contact.

A 2002 shouldn't be acting up like that, though if has been a city driver all its short life, you could already have severe deposits in the throttle body, idle air control system, intake valves, injectors, combustion chambers, crown land of pistons, etc. This will cause very inefficient combustion and increase fuel dilution.

If a vehicle is driven primarily Interstate, like some delivery vehicles and commmuters are, you can run the cheapest oil and poorest grade gas you can find, and the motor seems perfect at +100,000 miles. The constant high speed keeps everything clean.

At stop-n-go driving, parts of the motor run too hot, and parts of the motor run too cold, especially in very cold ambients. The motor is usually below 1,500 RPM, so poor turbulance in the combustion chambers. Plugs foul quicker. Lot's of sludge issues.

Sometimes when a fuel injector clogs or begins to plug, it will always leak, even when shut down. That puts a lot of raw gas into the motor.

I would check the obvious things first, maybe do a MotorVac on the thing to clean things out and change the oil again. How are the O2 sensors? With that much unburned hydrocarbons in the oil, there will be a lot of HC out the pipe. It should set the MIL, unless the O2's are so badly plugged they've given up.

A quick tailpipe analysis will tell you a lot about how the motor is running. If it's still on warranty, and you have to do yearly I/M in your state, this might be worth considering.
What are some of the reasons a modern (2002 m/y) fuel injected, computer controlled engine could experience significant fuel dilution issues (1.5% ~ 2.0%) within 5,500 miles?

I know winter temperatures play a role, but enough to cause this level of fuel in the oil?

Doesn't the fuel evaporate out when the oil heats up or once it's in the oil does it just mix in?

Is mileage a factor?

Excessive idling in trying to warm the car up in the morning. A lot of idle in general. Leaking fuel injector.
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