# Rust deposits under oil fill cap?



## oops1 (Jan 9, 2015)

This is a two part question. Went to change oil in wife's car and found a lot of moister under the oil cap. Google helped me on that but why was it rust colored? The cap is plastic..Also.. Her coolant always weeps out until there's just about a half inch in the reservoir . If I refill it..it disappears . It never runs hot but I'm wondering if these two issues may be related? It's a 2010 Buick Enclave. Thanks for any tips or advice.

Ps..anybody want to buy a 2010 Buick Enclave?


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## GoldDot40 (Jan 9, 2015)

A natural byproduct of internal combustion is H2O. Most of it will burn off with the internal heat of the engine. What doesn't get burned off will become steam and get circulated through the PCV sytem. The tiny amount left will rise to the highest point...which is usually the oil fill cap. 

The color is from the residual moisture mixing with any oil that may simply splash up there while the engine is running. This is very common in any vehicle with no mechanical issues. However, if the buildup is excessive under the oil cap, then I'd suspect you could have coolant getting into the crankcase/oil somewhere.

The fact that you're losing coolant and not seeing an external leak is a red flag. How does the oil look on the stick? When you drained it, did you drain it hot? If there's coolant in the oil, the stream of oil coming out will visually have steam in it....IF you drain it while it's hot. Also look at the oil filter when you remove it. Any 'sludge' deposits would be another flag.

If you suspect you could have a system leak, the 1st thing to do would be to have the cooling system/radiator pressure checked to see if it holds pressure. If it won't, the leak needs to be located an repaired.

The picture of the coolant reservoir you posted is residue of (orange) Dex-Cool antifreeze. When it sits exposed to air for long periods, it crystallizes. It will leave behind a residual stain like you see under the cap of the reservoir.


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## oops1 (Jan 10, 2015)

The oil looked fine.. There wasn't any milky color and there was no sludge.I did not drain it while it was hot..it was warm so I dont recall seeing any steam.Makes sense that the deposits are the same color as the coolant..didnt even think of that.Thanks for the response. I will take it to have the radiator checked.


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## oops1 (Jan 10, 2015)

The picture of the coolant reservoir you posted is residue of (orange) Dex-Cool antifreeze. When it sits exposed to air for long periods, it crystallizes. It will leave behind a residual stain like you see under the cap of the reservoir.

I'm a little confused here^^^ that's the oil cap sitting on the coolant reservoir. You are saying I am getting coolant in the oil?


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## Bilge Rat LT 20 (Jan 10, 2015)

You may have a coolant leak in the heater hose connections at the heater valve in line.
It will leak near the tire and not very much.
 Head gasket leak. 
The Dex cool does bad things when it gets air due to a leak. Drain that crud out and use the green stuff.
Get the system pressure tested.

If the car is driven any distance the engine heat should evaporate any water and not be seen under the cap unless you have a coolant leak. A small leak will not turn the oil milky at first.


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## Artfuldodger (Jan 10, 2015)

I wouldn't bee too concerned about the rust in the oil cap as Bassquatch mentioned it's just water vapor.
My Chevy Astro uses coolant too and I haven't found a leak. The oil always looks normal. Could a head gasket leak coolant directly into the combustion chamber and not into the oil?


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## T-N-T (Jan 10, 2015)

Took me a couple months to find my last coolant leak.  Could smell it, but with so much stuff crammed under the hood, I just couldn't locate it.
Water pump gasket.  Seems easy enough to see, but I could not see the leak.


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## GoldDot40 (Jan 11, 2015)

oops1 said:


> The picture of the coolant reservoir you posted is residue of (orange) Dex-Cool antifreeze. When it sits exposed to air for long periods, it crystallizes. It will leave behind a residual stain like you see under the cap of the reservoir.
> 
> I'm a little confused here^^^ that's the oil cap sitting on the coolant reservoir. You are saying I am getting coolant in the oil?



My bad. I was looking at the pic from my phone. I originally thought that was the inside of your coolant reservoir. Now that I see it on my PC, I see it's the underside of the oil cap. Don't sweat it. That looks fairly normal. Keep and eye on the coolant level though. Let us know what the pressure leak test results are.


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## DAVE (Jan 11, 2015)

Artfuldodger said:


> I wouldn't bee too concerned about the rust in the oil cap as Bassquatch mentioned it's just water vapor.
> My Chevy Astro uses coolant too and I haven't found a leak. The oil always looks normal. Could a head gasket leak coolant directly into the combustion chamber and not into the oil?



Yes and on modern engines that is more likely than leaking into oil.


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