Duramax Diesels Forum Truck of the Week
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Old 03-19-2020, 10:20 PM
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Default Fuel in coolant

I recently met a guy who is having fuel in coolant issues. He has an 01, stock fuel and air, fass, and a ppe box tune. He's thinking it's the cups, but I'm having a hard time understanding how that could be the issue. If a cup leaks, its only coolant, combustion, and oil. Or am I wrong? If a head gasket leaks, it should only be coolant, combustion, and oil as well, that have a possibility of being affected. In the past at work, I've witnessed a fuel cooler popping and letting fuel into the coolant, but from what I know, his 01 should have the same fuel cooler as my 03 and they don't have coolant to them. Or am I wrong again? I'm at a loss here guys, I can't picture an area where the coolant and fuel could mix or even be within one cavity of each other. Is there something I'm missing? The guy said there are droplets of fuel in the overflow tank and it smells like fuel big time. He sent me a picture of it and I can see the droplets in the coolant. I made sure he didn't accidentally contaminate the system and it wasn't some sort of user error haha. Has anyone else ran across this issue or have any ideas on which direction to go? Thanks
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Last edited by CHEZELTINE; 03-19-2020 at 10:26 PM.
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Old 03-20-2020, 05:59 AM
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On a stock LB7, there's no interaction between the fuel system and the cooling system. The fuel cooler is to the air.

My only guess is his oil is super diluted with diesel, and it is actually mixing oil and coolant, but his oil is mostly diesel?

Now if this was a PowerStroke, it would 100% be cups. They use a very different injector setup and it can mix diesel and coolant if a cup fails. But not a Duramax.
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Old 03-20-2020, 07:05 AM
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Originally Posted by DAVe3283 View Post
On a stock LB7, there's no interaction between the fuel system and the cooling system. The fuel cooler is to the air.

My only guess is his oil is super diluted with diesel, and it is actually mixing oil and coolant, but his oil is mostly diesel?

Now if this was a PowerStroke, it would 100% be cups. They use a very different injector setup and it can mix diesel and coolant if a cup fails. But not a Duramax.
Unless for some reason the stars and moons aligned and he has bad headgaskets along with bad injectors leaking fuel into the oil
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Old 03-20-2020, 08:38 AM
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Originally Posted by DAVe3283 View Post
On a stock LB7, there's no interaction between the fuel system and the cooling system. The fuel cooler is to the air.

My only guess is his oil is super diluted with diesel, and it is actually mixing oil and coolant, but his oil is mostly diesel?

Now if this was a PowerStroke, it would 100% be cups. They use a very different injector setup and it can mix diesel and coolant if a cup fails. But not a Duramax.
Thanks, Dave. That's what I thought. He said oil was fine but I'll have him drain some out to verify.

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Old 03-20-2020, 08:43 AM
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Unless for some reason the stars and moons aligned and he has bad headgaskets along with bad injectors leaking fuel into the oil
It's a possibility, injectors are 4 years old and engine has 309k on it. I'm as confused as everyone else. Without being able to get my hands on it, I'm going to have him send in a coolant sample to confirm what's all in it. It's probably what Dave is thinking and his oil is diluted with a popped HG.

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Old 03-20-2020, 09:11 AM
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Thanks, Dave. That's what I thought. He said oil was fine but I'll have him drain some out to verify.

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just pull the dipstick and smell it and/or put a drop on a clean white paper towel (i know, paper towels are scarce right now ). if there is a big halo around the oil drop, thats fuel and thats no bueno.

if he really has that much fuel in the oil and been running it for a while, id be really worried about the bearings in that engine. they do NOT like fuel as a lube lol

out of curiosity, what makes him think its fuel in the coolant?
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Old 03-20-2020, 09:23 AM
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Sure it's not transmission fluid in the coolant?
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Old 03-20-2020, 09:26 AM
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Quote:
Originally Posted by WVRigrat05 View Post
Sure it's not transmission fluid in the coolant?
X2, that's a much more likely possibility.

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Originally Posted by Chevy1925 View Post
just pull the dipstick and smell it and/or put a drop on a clean white paper towel (i know, paper towels are scarce right now ). if there is a big halo around the oil drop, thats fuel and thats no bueno.

if he really has that much fuel in the oil and been running it for a while, id be really worried about the bearings in that engine. they do NOT like fuel as a lube lol

out of curiosity, what makes him think its fuel in the coolant?
I've seen a truck that was so diluted the paper towel trick didn't work. It was basically pure diesel in the crankcase, so no halo because it was all diesel. Owner just thought it was really clean oil LOL

That truck is still on the road after new injectors, but I agree, it must have done some bad things to the bearings in the process.
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Old 03-20-2020, 09:35 AM
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Originally Posted by WVRigrat05 View Post
Sure it's not transmission fluid in the coolant?
This! I wasn't even thinking of the cooler! Good call. This is why I come here!

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Old 03-20-2020, 09:37 AM
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Originally Posted by DAVe3283 View Post
X2, that's a much more likely possibility.


I've seen a truck that was so diluted the paper towel trick didn't work. It was basically pure diesel in the crankcase, so no halo because it was all diesel. Owner just thought it was really clean oil LOL

That truck is still on the road after new injectors, but I agree, it must have done some bad things to the bearings in the process.
This is the shape I bought my truck in before I knew anything about Duramax. Learned up real quick haha. Can't believe that's been 14 years ago.

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Old 03-21-2020, 08:21 AM
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Originally Posted by Chevy1925 View Post
just pull the dipstick and smell it and/or put a drop on a clean white paper towel (i know, paper towels are scarce right now ). if there is a big halo around the oil drop, thats fuel and thats no bueno.

if he really has that much fuel in the oil and been running it for a while, id be really worried about the bearings in that engine. they do NOT like fuel as a lube lol

out of curiosity, what makes him think its fuel in the coolant?
Because he says it smells like fuel big time with the overflow tank cap off.

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Old 03-21-2020, 08:23 AM
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Here's the pic he sent me. I'm really thinking it's trans fluid now. I'll update what he finds with fluid analysis. I'm thinking he might be smelling the diesel from outside the tank, but without having hands on, I just don't know. Thanks guys.

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Old 03-21-2020, 08:32 AM
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Quote:
Originally Posted by CHEZELTINE View Post
Here's the pic he sent me. I'm really thinking it's trans fluid now. I'll update what he finds with fluid analysis. I'm thinking he might be smelling the diesel from outside the tank, but without having hands on, I just don't know. Thanks guys.
Oh, there are actual droplets of whatever it is floating on top, that makes it easier. Grab a white paper towel or tissue or napkin (that might be hard right now LOL) and carefully dip the corner into one of the droplets. It should absorb up a bit, then pull it out and examine it. Might be able to tell what kind of fluid it is when it's not surrounded by red coolant.

But the real way is wait for the fluid analysis, so if that's already in process, might as well wait and see. It sure does visually look like diesel, though. Gotta admit that.
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