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Where is the oil going in my 2003 Avalanche?

My 2003 Avalanche (5.3L) suddenly started losing oil, 2 quarts since my last oil change (3,000 mi). There is no evidence of it leaking and no apparent smoking, so when I went to get my oil changed today I was telling my mechanic about it. They couldn’t find any evidence of a leak either. Then when the pulled the fill plug on the transfer case fluid poured out, a lot of fluid. They are the only ones that service this vehicle so we were puzzled as to how it could be overfilled, especially by so much. Then I got to thinking, the engine has lost about two quarts of oil and the transfer case has mysteriously gained a bunch, possibly two quarts. So the question is, is it possible that the engine oil is somehow moving into the transfer case?

Impossible. There is no direct connection between the engine and the T-case…The TRANSMISSION and transfer case ARE connected however…How many miles on your Avalanche? it’s 10 years old…A quart of oil every 1500 miles is considered normal for a NEW car…

I don’t know if there is a way for oil to move from the crankcase to the transfer case in your car. In my Ford 4x4 truck, no, as the xfer case uses gear oil, not motor oil. And there’s no passage. But you need someone experience w/your car to say for certain.

No harm if you check the radiator fluid for signs of a brownish oily film, that would indicate oil may be leaking into the water jacket. And checking the oil after it is drained for any signs of coolant is a good idea on the next oil change. Also, checking visually for oil leaking while the engine is running is a good idea. It might only leak when running.

1500 miles per quart is a little high for oil consumption, but isn’t extreme. You may just have to live with it, and keep an eye on the oil level to make sure it never goes below 1 quart low. When , if, the consumption ever gets to 500 miles/qt, then the problem starts to become something that has to be dealt with in the not so distant future. Best of luck.

If it’s not leaking oil then it’s burning it and more than likely past the piston rings.

It would be interesting to know how often you have the oil changed (miles or time-wise) and why your mechanic has not suggested a dry and wet compression test.

No leaks - oil consumption - compression test a given.

You really need to get in the habit of checking the oil level on a regular basis and topping it off as needed. Running 2 quarts down is not good and contributes to the problem.

@Caddyman, perhaps the transmission cooler (which should be in the radiator, I believe) is leaking.

Couldn’t the engine oil and the ATF then mix, thus winding up in the transfer case?

@kinetix, did the fluid in the transfer case seem to be a strange color?

The key here is that somehow the engine oil is making it’s way into the transfer case, I don’t think there is any doubt about this because is was way overfull with some sort of fluid that had to come from somewhere. These guys have done all of my service and most repairs (few as there have been) for a long time and are mechanics not just oil changers. I was downstairs looking that the bottom of the rig when the pulled the fill plug on the transfer case and fluid poured out. They did the last service on the transfer case and have checked it many times since, it was not overfilled during servicing and I doubt someone sneeked under my rig and pumped a bunch of oil into it.
I check all fluids regularly and when I found it to be low, added oil several times which totaled 2 qts over the course of 3,000 miles, This happened suddenly and I’m trying to determine the cause.BTW, the rig has over 236k miles, but please don’t start telling me how that’s a lot of miles and to be expected, it has never “used” any oil before.

Please try to hold the condescending tone. I appreciate suggestions, but please read the entire post and focus on the fact that oil is “disappearing” from the engine and additional fluid is “appearing” in the transfer case, on this there is no doubt. The number one suspect right now is something to do with the cooler lines and if they somehow have been breached allowing the engine oil to get into the transfer case. If you don’t believe please me don’t reply to this post.

thank you

Frist there is no way for engine oil to to get into a transfer case. There are no cooling lines for the transfer case. If there is to much oil in the transfer case then there would be to much in the trans. Did anyone check the trans to see if its low or over filled As for were the oil went from the engine if its not leaking it and its not in showing up in the radatior, then its burning it, I see you have the 5.3L, I have that in my 06 GMC with about the same milage. I started using oil a Qt in 2000 miles. I ran a can of Sea Foam thru it and changed the oil. Its helped some. Also check and see if you have a PCV vavle, some of 5.3L dont have them. If you do replace it. It on the drivers side vavle cover.

All of these suggestions about compression checks, etc., would make sense if not for the fact that somehow, a whole lot of extra fluid of some kind ended up in the transfer case, it didn’t get their by magic, there is someplace on this engine where the oil was able to get from the engine to the transfer case, I just have to figure out where that is. This all happened since the previous oil change, it was not using any oil before that, zero, zip, none, nada and the transfer case fluid level at all previous oil changes was normal. I myself go down below every time it gets serviced to watch and inspect the underside of the vehicle.

@kinetix, I do believe that possibly that extra fluid in the transfer case wasn’t ATF.

But did anyone actually save it long enough to determine what it might have been?

Like I said, if engine oil got in the transfer case, the fluid drained from the transfer case would have been a strange color.

Please post back with more information about exactly what came out of the transfer case.

We didn’t save or check it because the hold scenario was just unfolding and it wasn’t until afterward that I started to put two and two together. I plan to go back on Monday when there is another more experienced mechanic there and we can take a sample then. By then there may be “extra” fluid in the transfer case again.

You need to drain the oil from the transfer case and I think you will find your answer, water. If the seal around the shift linkage fails, water will get into the transfer case and settle at the bottom, the oil will rise so that will be the first thing to come out when the fill plug is pulled.

You haven’t mentioned whether or not someone checked the transmission fluid but since you have good mechanics, I am assuming that they did and there wasn’t a problem.

As for the engine, I believe that is a separate issue. I think you are leaking oil, just because everything appears to be dry does not mean that it is not leaking. It could be burning the oil, but my experience is that oil burning starts and becomes worse over time. A sudden loss of oil after an oil change makes me thing that there is a leak around the oil filter or the drain plug.

I’m more inclined to think oil filter because the oil would spray out under pressure and might not leave a trail. The oil issue might just clear up with the next oil change. If not, the oil pressure sending unit would be my next guess, they can crack and again, they spray the oil out only when the engine is running, so there might not be a trail, at least not in the areas that you are looking.


Those are some good ideas. We did not pull the drain plug but may try that tomorrow, but where could the water come from? There was a lot of extra fluid but it didn’t appear muddy as if water was in there. We stood under the rig looking all around the engine area with flashlight and there just wasn’t a sign of oil anywhere including the frame and surrounding area. It seems if there was that much oil leaking it would show up somewhere.


It still boils down to the fact that if the engine is not leaking oil then it’s burning it; pure and simple.

You state this truck has 236k miles on it and used no oil until recently. At some point an engine is going to go downhill and may go downhill quickly. The mechanical fact of life is that a compression test should be done.

I had referenced earlier about not allowing an engine to run 2 quarts down as that contributes to the problem and I might ask how often you raise the hood to check the oil level?
If you do not check the oil level, then may be the engine has been consuming oil for a long time and it’s just now that you’ve been made aware of it.

Engine oil is not getting into the transfer case and to be bluntly honest, I’m a bit skeptical about the story you were given that the transfer case was 2 quarts over full.

kinetix, water will splash up all over it when you are driving in the rain. It can get into the transfer case anywhere there is a breach that is above the oil level. It would most likely be on the top or else you would see oil leaking from the transfer case.

"I check all fluids regularly and when I found it to be low, added oil several times which totaled 2 qts over the course of 3,000 miles, This happened suddenly and I’m trying to determine the cause.BTW, the rig has over 236k miles, but please don’t start telling me how that’s a lot of miles and to be expected, it has never “used” any oil before. "

OK4450, lets give him the benefit of the doubt here.

Thanks Keith. I’ll be back at the shop tomorrow and we’ll pull the drain plug. I know it sounds improbable but despite the high mileage, this rig never used more than a few ounces between changes (@ 3,000m). The theory is mine not the mechanics. I have personally witnessed every service there (from below the vehicle) including this one when I watched a lot of liquid that looked like some sort of oil come out of the fill (not the drain) plug.

If anyone has any new ideas please offer them up. Please remember, there are two things going on here. They may or may not be related. If not, it’s a heck of a coincidence. So if you just want to tell me that I should “raise the hood once in a while” or that I should expect a vehicle with this many miles to use some oil, or it must be (suddenly) burning it, don’t bother to post.

I’m offering you sound mechanical advice. If you choose to never raise the hood and check the oil level and refuse to believe that an engine with going on a quarter million miles may not use any oil then have at it. You’re certainly not alone in this world.

@keith may have the sol’n. It’s water. If I drive my 4x4 truck though deep water, the owner’s manual says I have to check my xfer case fluid to make sure water hasn’t leaked in.

Has this truck been driven (or parked) in water deep enough so that it could enter the xfer case?

Have you ever had to top off the transmission fluid? Since you seem to monitor the engine oil quite closely I’m curious if the transmission fluid has ever been added to? If so, that would certainly explain your added transfer case fluid.

George and Keith, I live in a very dry climate, we have had little to no rain in the period this has happened and I don’t drive through rivers, puddles or any other water.

asemaster, I check the transmission regularly and it was right on when I had it serviced yesterday.

ok4450, you don’t seem to want to read the details of my post and continue to make condescending comments. I would appreciate it if you would stay out if this discussion.

I asked about the transmission fluid level because the seal at the rear of the transmission may fail, allowing fluid to migrate from the transmission into the transfer case. This is the only way that I can think of that transmissio fluid can end up in the transfer case. There is no possible way for engine oil to find its way into the xfer case.

I don’t mean to be rude or condescending either, but as for the motor oil consumption, have the PCV system serviced, have your mechanic search for any Tech Service Bulletins related to your engine and complaint, and then just add oil as needed. Given the age and mileage of your truck you should be happy it’s not burning more oil. That level of oil consumption isn’t out of the ordinary for a brand new car, much less yours.

One other thought, are you 100% certain that your service department is using the exact same oil they have been previously?