8.1 mpg going down after oil change ?!?!

moo snutz · July 12, 2005

Not too sure what's going on. Changed out front / rear diffs (Mobil 1) and also engine oil (Mobil 1 5w-30). I immediately noticed that my mpg went from a solid 11.7 average to 10.5 on my current tank.

Has this happened to anyone else?

Oh yeah, changing out the oil filter on the 8.1 is a PITA! Anyone have any tips for avoiding dumping the hot oil from the filter all over the place? I know there's thread about a filter relocation kit but I don't want to go that direction.

496 CUBIC INCHES · July 12, 2005

When I change the filter on my 8.1 I just crack it loose and let it drain for a few minutes. Then loosen it a few more turns and let it drain some more. I use two pans so I can let the filter drain quite awhile while I let the oil drain from the engine.

The first few times it seemed like a pain,but after doing it for a couple years its like second nature now.

You could also use an ice pick and punch a hole in the bottom of the filter and let it drain that way.

silverado3500 · July 13, 2005

The ice pick/punch method is very effective.

BBSilverado · July 13, 2005

Not too sure what's going on. Changed out front / rear diffs (Mobil 1) and also engine oil (Mobil 1 5w-30). I immediately noticed that my mpg went from a solid 11.7 average to 10.5 on my current tank.

Has this happened to anyone else?

Oh yeah, changing out the oil filter on the 8.1 is a PITA! Anyone have any tips for avoiding dumping the hot oil from the filter all over the place? I know there's thread about a filter relocation kit but I don't want to go that direction.

<{POST_SNAPBACK}>

My mpg dropped also, it is now at 10.7. I too changed to Mobil 1 on the last oil change. I thought my mpg dropped because I started using the AC now that it has been hot but it might have been the Mobil 1. I'm interested in finding out too.

GMSVCMGR99801 · July 13, 2005

I drive a 2004 2500HD 8.1L w/3.73 and also run Mobil 1. I consistently get 9.8 to 10.2 and drive 6mi. each way to work. I switched to Mobil 1 at 4,000 miles and now have 7000. I do not recall any better mileage prior to switching to Mobil 1 but I live in Alaska and bought the truck in the fall. I did not track the mileage over the first several thousand miles due to very frequent use of 4 wheel drive. I noticed the 2 prior posts re: Mobil 1 are a 2005 and a 2004, how many miules are on these trucks and is the issue Mobil 1 or engine break-in ???

moo snutz · July 13, 2005

I've only got 1500 miles on the truck so far, so it is still pretty early to get an accurate average (so far it was 11.7ish). BUT, I would think with the engine starting to break in, that the mpgs would increse & NOT decrease (at least a little bit). I just find it strange that it happened immediately after switching to Mobil 1.

FWIW - I have been using the AC for a month now, so that has been a constant.

Haulin' 8.1 · July 13, 2005

Moo keep up with it for a few more tanks and let us know if anything changes. I'm curious myself, I've found the Mobil 1 WILL reduce friction enough to make a mileage improvement. I've used it 3 times and made note of the improvements. Once when I was a teen and used it in my 375 HP '75 Trans Am I put it in the diff and trans and gained 1.5 MPG. Another time in a '88 Honda V-Four 750 Magna with shaft drive I used it in the diff and got an extra 35 miles off of each tank (aprox. 2.5 gal.). And finally I have experimented with the stuff in regular 5 HP push mowers in every case the mower would start much easier, run much smoother, and use way less fuel. The only bad side in the mowers was that the engine spun so freely that if you were in full mow and hit a stump or rock the crank would almost certainly twist and render the engine a junk pile. With normal oil it would just stall.

Are you sure nothing else changed? You said you just started using the A/C a month ago so I'm assuming your at a high elevation of New Mexico (NM right?). If it just got hot around there then the gasoline companys will have recently changed the formulation for the higher average temps. Octane is very relative to temperature and air pressure. So the gasoline you get for cold weather at a high altitude is likely to be very different from one for hot weather down low, even from the same company. They only spend enough on aditives to obtain the advertised octane where you buy.

Has there been any change you can remember in the fueling station or truck that also may explain this?

Vernon

moo snutz · July 14, 2005

Moo keep up with it for a few more tanks and let us know if anything changes. I'm curious myself, I've found the Mobil 1 WILL reduce friction enough to make a mileage improvement. I've used it 3 times and made note of the improvements. Once when I was a teen and used it in my 375 HP '75 Trans Am I put it in the diff and trans and gained 1.5 MPG. Another time in a '88 Honda V-Four 750 Magna with shaft drive I used it in the diff and got an extra 35 miles off of each tank (aprox. 2.5 gal.). And finally I have experimented with the stuff in regular 5 HP push mowers in every case the mower would start much easier, run much smoother, and use way less fuel. The only bad side in the mowers was that the engine spun so freely that if you were in full mow and hit a stump or rock the crank would almost certainly twist and render the engine a junk pile. With normal oil it would just stall.

Are you sure nothing else changed? You said you just started using the A/C a month ago so I'm assuming your at a high elevation of New Mexico (NM right?). If it just got hot around there then the gasoline companys will have recently changed the formulation for the higher average temps. Octane is very relative to temperature and air pressure. So the gasoline you get for cold weather at a high altitude is likely to be very different from one for hot weather down low, even from the same company. They only spend enough on aditives to obtain the advertised octane where you buy.

Has there been any change you can remember in the fueling station or truck that also may explain this?

Vernon

<{POST_SNAPBACK}>

Vernon - I am in northern New Mexico (7000 ft). Sounds like you're on to something. We've had a pretty good hot spell lately around here with temps being in the 90s & high relative humidity. I usually pump 88 octane but I did use 86 on one of my last tanks. I'm driving up to Colorado Springs & then to the Florisant area on Friday. I'll keep a close eye on it. FWIW - I did notice a slight bump in my mpgs today, around 10.9 according to DIC.