Question regarding oil consumption.

[LA DMAX]

I've had my truck almost 2 years now and from the start I never had to add any oil between oil changes, ever. Now I have 25,000 miles on it, came back from Laughlin last week and had to add a quart. Before I left on the trip I checked the level and it was at the top block of the dip stick, where it always is. 600 miles later it was down to the 2nd notch. For the trip I had 6 people inside, towing 2 Sea doo's and all our junk. Alot less than most haul everyday. The outside temp ranged from 75 to 109 and I used the AC all the time, driving between 65 and 75. I also noticed about a month ago that my oil pressure gauge was fluctuating quite a bit. It was usually between 50 and 80 all the time. Now it drops down to 25 at idle when it's warmed up. I checked the oil when I first noticed the fluctuation and everything was fine. I smelled the oil to check for any fuel in the oil but didn't notice any diesel smell(don't know if it would be faint or strong). Sorry for the long post but does anyone have any input? Is there any correlation between oil pressure fluctuating and now having increased oil consumption? Am I just stressing out over nothing? Thanks guys.

LA DMAX

[DmaxMaverick]

I'm probably opening up a can of worms here, but I'll give it a go anyway....

Don't believe what you see on the gage. They are generally not very accurate or consistent. They are more useful to "gage" trends. They do fade over time and fail due to time and wear. If you are consuming an abnormal amount of oil and the pressure drops more than what your trend has shown you, then you might have a problem that needs attention. I would suspect that the conditions you are operating in are more in play than mechanical failure. Hot southwestern summer temps will cause the engine to use up the useful life of oil at a much higher rate than moderate climates. It is the nature of the oil. You may have better results by changing to a different brand or grade of oil to suit your needs. The conditions you describe may dissappear with different oil, or a switch to synthetic. Synthetic oil is more resilient and durable under harsh conditions. You may also consider more frequent changes during heavy use. If you have a problem with fuel getting in the oil, it will usually increase in volume. Usually, the fuel will enter the system at a higher rate than the oil is consumed.

Do you fill the crankcase by the dipstick, or volume. The Duramax engine, by design, has a 10 qt. crankcase, including the oil filter and cooler. Unless it has been modified to hold more or less, the crankcase is 10 qts. I have read several times here, and other places, as well as shop talk, of people filling the crankcase until it's "right" by the dipstick. Well, the dipstick is the most inconsistent (from truck to truck) method of determining how much oil is in the crankcase, and ours isn't the easiest to read. This is true of most all vehicles. If you need 11 qts. to get the oil level up to what you believe (or have been informed) is "right", you have put in 1 qt. too much. The engine, as with most engines, by design, will not retain this excess. It will burn off, blow out through the vent or PCV/CDR or leak out.

The best method of ensuring that you have the correct amount of oil in the crankcase is to drain it when hot, and let it drain until all the oil possible is removed (oil filter removed). Add 10 qts. of oil and note the level on the dipstick after the engine has run up to temp and has sat, turned off for at least 15 min's. That will be the correct oil level to maintain. The level may be at the lowest notch or the highest, but it doesn't matter. If you prefer, you can adjust the length of the dipstick tube, or dipstick to get the level to indicate where you want it on the dipstick.

I would suggest you (or anyone worried about oil consumption) follow this, then record an oil consumption trend. You should also include regular oil sampling to get the full report on the condition of the oil. You will be more able, and prepared, to address any problems that may be present. It will also make it easier (or should, anyway) to convince the dealer of a problem when you've already done your homework. GM's method of determining excessive oil consumption is kinda' scary. It is determined by oil used per gallons of fuel burned. Not determined by mileage. This is better than using the qt/mile because it will reflect the use of the engine. Last I heard, it was about 1 qt. per 100 gallons. At 10 MPG, that would be 1000 miles. To me, that seems about double what is acceptible for a motor that is broken in with low miles. These engines are still too new to benchmark trends. If the engine uses 1 qt. of oil every 1000 miles, and does it for 400,000 miles, I can live with that. My '85 Blazer uses about a qt. between changes. Granted most of it leaks out, but it's been doing that for over 536,000 miles.

[a64pilot]

How old was the oil? The fluctuating gauge(if it can be believed) may well indicate viscosity breakdown. I second the notion of changing it and seeing if it is still using oil.
One thing I'm sure of, you can't change the oil too frequently.

[LA DMAX]

DmaxMaverick and a64pilot,
Thanks for your input, I greatly appreciate any knowledge/ideas you may have. The oil is about 2500 miles old, I've made this trip at least 5 other times with temps hotter than this last trip. I understand about the 10 qts in the crankcase and I believe that is the best bet, however, the dealer has been changing my oil from the beginning so I have to rely on them for accuracy. For the past 3 oil changes they have used Mobile delvac 1300 15W 40. The reason for my concern in that this oil consumption has not happened in the past, even towing approximatly the same load, similar or worse temps and the same distance. The pressure gauge I've heard is not the most accurate but it has never gone below 50 until now. I'm due for another oil change in about 1000 miles at which time I'll let the dealer know of my concern. Until then I'll monitor things and get an oil analysis done before next change. Thanks again guys.

LA DMAX

[a64pilot]

LA DMAX,
I just had a thought, change your oil now IMHO, you'd never get him to admit it, but I bet they put in the wrong oil. Thinner gas engine oil would do exactly what your describing. I doubt it would harm anything, but change your oil yourself or at a jiffy lube or something. Oil is cheap compared to this engine, can't hurt to change it.

[LA DMAX]

a64,
That is possible, even though on my receipt it says 15W 40. Are you sure that if they put in 10W 30 or something else it won't hurt my engine? I'm thinking what I will do is order an oil analysis kit from AV lube, take a sample and send it in. If it comes back with something other than 15W 40 I'll be POed. I did add 1qt of 15W 40 on Monday but if there is something else in there the analysis will pick that up right? I'll change the oil as soon as I can get a sample. Thanks again.

LA DMAX