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What would cause diesel in the tank to be dark?

I am changing out a fuel tank and noticed the diesel is dark colored and smells like motor oil. Is this likely to be from blowby? The engine was recently rebuilt, but has used lots of oil ever since. I forgot to look in the tank before the shop closed and have never cut a filter apart so wouldn't know about the possibility of algae or asphaltene. The oily smell made me think of blowby, but wondered what else it could be.

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Thanks Blackwood. I called Polaris and the tech I talked to is suspecting asphaltene. I am waiting on a sample kit. Course, I could not believe how much a sample costs for fuel, but guess that is cheaper than to start throwing parts at it (I hope). Then, if it is, I will be very curious what you do about it. Thanks again.

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The price for customers with existing accounts at Polaris is $347 for the basic and $729 for the advanced. It's more if you don't have an account with them. I was shocked after paying $28 for oil samples. The tech explained that it was much more complicated than oil and tried to explain why. But I want to know, so I went for the basic sample. They said that if I still don't know anything, that I can select certain other tests which are included in the advanced for more money of course. I will most likely give up if I don't learn anything from the basic tests. The heck of it is that if they tell me it is asphaltene, I still won't have a clue what you do about it.

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It's a 1999 Cummins ISX 565. I have finally located some asphaltene conditioner. Now, I am wondering if I will need to use it from now on or just treat a few tanks.

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Algae

Check with a boat dealer for a cure

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Return fuel runs very hot and asphaltene could be a possibility for the dark fuel. In most all the places that oil and diesel cross the fuel pressure is higher than engine oil pressure so it would be highly unlikely that you have oil in your tank, but just to be on the safe side I would take a sample and send it off if you still have some of the fuel just to find out what is in it. If the inside of the tank is covered with a dark substance it very possibly is algae. If this is the case go to any Fleet Pride and buy some Killems and read the directions before you open the bottle. It is very poisonous and you absolutely do not want to get it on your skin. This will kill any and all algae in your fuel system.

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I live in Birmingham and the local CAT dealer has their own lab for fuel and oil I think theirs is around $20 and I also use Titan Labs through Gulf Coast Filters and theirs is $20 also......how much is Polaris?

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Finally got the results back from the sample and it is asphaltene. Before I submitted the sample, I mentioned this to my mechanic and the shop foremans at Peterbilt and Cummins. All three of them looked at me as if I had three heads. In other words, they had never heard of it. Now, what do I do about it?

Also, there was some phosphorous and calcium in the sample. It said this would indicate cross contamination with lube oil. Now, where in the world would this occur at?

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need to know what kind of engine this tank was connected to.

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tompetty69 will be eternally grateful.
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