MPG decrease with OLM decrease. Any one else notice this?

Discussion in 'Maintenance & Upkeep' started by Pikey, Oct 30, 2013.

  1. Pikey

    Pikey Moderator

    I noticed over my last two oil changes that when the oil life monitor gets down around 25% my mpg begins to decrease. From averaging 17.4 in the city to around 16.4. On the highway it drops from 21 to 18mpg. I changed my oil today (olm at 30%, I changed it because I wanted to see if mpg increased like last time) and my mpg went back up to 17.5mpg city and 22mpg highway. I use Mobile 1 5w-30 high millage oil and ac delco pf 46 filters. Yes, the oil does look dirty when it comes out. The millage is from the DIC and hand calculated. I don't think that the oil would be broken down that much considering that the OLM is set for conventional not synthetic. But, I have noticed the change over the last 2 oil changes. When I change it, it goes up instantly. Any ideas? Anyone else notice this trend?
  2. BRB46

    BRB46 New Member

    That's quite interesting. I haven't noticed that but wasn't looking either. I am at 83% now and will monitor it as it gets lower. Will report my findings later.
  3. Pikey

    Pikey Moderator

    Thanks! I am just interested to see if others notice the same thing. Like I said around 30%-25% I notice the loss in mpg. What oil are you using?
  4. BRB46

    BRB46 New Member

    I use Pennzoil Platinum 5W30 full synthetic. I had previously used Mobile 1 but for some reason i had to add a quart every 2500 miles. Now I add one every 5500 or so.
  5. dpeter

    dpeter New Member

    Not intending to hi jack the thread but.... I change my oil when the DIC says it's time and that is normally around 5,800 to 6,500 miles. So the question is ; What is this Oil Life Monitor you all speak of and does my 2000 Yukon have it? How does one access it?
  6. BRB46

    BRB46 New Member

    It is part of your DIC. Mine shows oil life in %, hence the life of the oil. I keep track of the mileage with my tripometer A. It starts at 100% and as you drive the % drops based on driving habits and outside air temp I believe has an effect on it as well. In the summer months I can go 8000 miles before it hits zero percent. In the colder months 5500 is all it gets.
  7. Pikey

    Pikey Moderator

    Mine is the same and I do the same thing as described above. My 2002 just had a light that would come on and say change oil, no percentage.
  8. buckmeister2

    buckmeister2 New Member

    Pikey, you are a great addition to this forum, and you really have me stumped here. There is nothing in the breakdown of lubricity or viscosity that could have that much impact on your MPG. Obviously, as oil gets dirty and thickens, it loses some of it's lubricity, which creates a bit more friction (or "drag") on internal components. But, I think we are talking about an effect well below 1/10 of a mpg, versus one or two full mpg. What I would rather suspect is that the bypass of dirty oil through the pcv and back into the intake may affect combustion, hence MPG. As the oil gets older/dirtier/contaminated, it is still passed back through the intake. Even clean oil injected into a fuel mixture reduces octane, and dirty oil would be that much worse. Only reason I can think of. Does this make any sense?
  9. Pikey

    Pikey Moderator

    Thank you for the compliment.
    that does make sense. I am considering making a catch can. I think that for under $25 I can get a high quality watts filter
    Last edited: Jan 3, 2014
  10. 99'HEARTBEAT

    99'HEARTBEAT Moderator

    Mike....just as the other Member's have mentioned, that's interesting......:neutral:
  11. RayVoy

    RayVoy Active Member

    I wonder if the PCM might be doing things like changing the fuel ratio.

    Do you know what might be an interesting test, wait until the change light comes on, check the mpg, then reset the light (without changing the oil), see if the mpg goes back up. You can then change the oil for real and reset the light.
  12. BRB46

    BRB46 New Member

    I'm down to 45% on my OLM and MPG doesn't seem to be any different than it was at 70%. My last fill up the range said 415 miles which for this time of year is higher than I previously would see. It wasn't uncommon to see it drop to 380. i will keep watch as it gets closer to 0%.
  13. BRB46

    BRB46 New Member

    I am now down to 32% and my MPG's seem to be dropping off a bit. Upon a fill up now my range shows 391 miles. The oil doesn't appear to be excessively dark/dirty but as buckmeister2 stated, the oil back through the PCV valve to the intake with dirty oil may not be helping matters. I always seem to get to this point in OLM in the colder months when winter blend gas is all that is available. At this rate i will be due to change oil in late April. will have to see how much worse it gets.
  14. BRB46

    BRB46 New Member

    I am now down to 4% on OLM. MPG's are in the 380 range now. Will likely be changing oil next weekend so I will see if after the reset, they improve.
  15. Pikey

    Pikey Moderator

    I have been chasing my drop in mpg for some time now. I was reading around and came across a thread somewhere where the poster was discussing a continued rough idle after a spark plug change. Someone suggested having a crank sensor relearn done. The guy did and claimed that the idle had smoothed out and his mpg increased. I have a friend that owns a shop and a tech II. I bought him a cheap case of beer and he did the relearn for me. After driving it about 700 miles, my mpg has returned to where it was when I bought the truck. I will see what happens when why olm gets low. On another note, before I changed my oil last time I ran 3 treatments of RxP gas kicker thru my truck. The company claims that it "de-carborizes" the motor. I am not one to believe any of the crap companies claim, but my buddy gave me three bottles for free so I tried it. I have to say, before I ran it thru when my olm said 80% my oil started looking "dirty". I am now around 60% on this oil change and the oil still looks really clean.
  16. BRB46

    BRB46 New Member

    Can I ask what a relearn might cost and what type of shop might do something like that. I don't have any mechanic friends so a shop would have to do mine. And what is RxP? I think i want to try a catch can too as I have a pool of oil inside the intake. I have cleaned it out twice so far and it keeps coming back. This can't help the mpg's mixing oil in the combustion air.
  17. Pikey

    Pikey Moderator

    The shop must have an advanced scan tool. The dealer wanted $150. It literally took 2 minutes. Most of the time was setting up the scan tool. Then he just put the gas pedal to the floor until it red lined, that was it. Rxp is a fuel additive. I am also looking at a catch can.

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