Hot vs. cold oil levels

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Preeeus, Apr 23, 2019.

  1. Preeeus

    Preeeus Member

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    Has anyone checked the oil level of their Prius when it is hot and then checked it again (without driving it) when it was cold?

    If so, how large was the difference on the dipstick?
     
  2. xerox6135

    xerox6135 Member

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    Have really never seen much difference . I always check mine after the car has been sitting about 10 to 15 mins

    Posted via the PriusChat mobile app.
     
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  3. Pluggo

    Pluggo Active Member

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    I found this: Oil expands 0.7% for every 5ºC that it heats up. So the difference between checking at 20ºC and 90ºC is 9.8%. In an engine with 4 liters of oil, that is about 400 mi (13 ounces).

    So, if you top-off while the engine is cold you can easily over-fill and cause foaming oil to circulate through the engine. Check it hot, but after the car has been sitting for at least 10 minutes so the oil drains back down into the pan.
     
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  4. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    I haven't seen any noticeable difference, hot vs cold. That said, I mostly check it cold. I would guesstimate it'd be less than 1/8" variation at most.
     
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  5. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    I checked it this morning, in the garage (level slab), stone-cold, method being to pull the dipstick, wait about 5 minutes (checked tire pressures to kill time), then do the check: the level was just covering the upper pip. (The 5 minute delay is to compensate for the Prius disptick propensity to pull oil up the guide tube, give it time to drain back down.)

    Will try to remember to do a hot-check in the next few days, say stopped in the same spot, 10 minutes after a drive, otherwise the same drill. I suspect any temp expansion will be cancelled out by oil not fully drained down, level ending up about the same. But will give it a try.
     
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  6. Preeeus

    Preeeus Member

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    Hmmm... if Pluggo is correct (and he/she seems to have the most scientific data), it should be about .42qt difference, which should be very noticeable on the dipstick (we've measured the distance between the bottom mark and the top mark on the dipstick to be 1.5qts).

    But xerox6135 and Mendel are reporting no noticeable differences. As typing this, Mendel is offering to do a more precise check... thanks Mendel!

    Hopefully more people can chime in with their observations.

    BTW, our Prius recently passed it's official Toyota oil consumption test, which doesn't exactly make sense because we've been tracking the oil consumption like hawks, and it should have failed. The only difference is that Toyota showed us the dipstick while the engine was hot, whereas we always measure the oil level when it's cold. Hence this question is important to figure out if Toyota accidentally added more than 1qt of oil to measure oil consumption at the end of the test.
     
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  7. Grit

    Grit Senior Member

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    Then factor in oil weight ex. 0w, 5w or even 10w.
     
  8. The Critic

    The Critic Resident Critic

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    If I set the oil level 1/4" below the full mark when cold, it will be at or a hair above the full mark after a long highway drive.

    For this reason I only fill the engine with 4.2-4.3 qt of oil.
     
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  9. Preeeus

    Preeeus Member

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    I wonder when Toyota says to fill with 4.4qt of oil, if they take into account that it will expand when it is hot.
     
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  10. The Critic

    The Critic Resident Critic

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    If you fill with 4.4 qt, it is almost guaranteed to be overfilled, especially when the oil level is checked at normal engine operating temperature.
     
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  11. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    I'd disagree, for a couple of reasons:

    1. If Toyota specs an amount of oil to add in the course of an oil change, and you put in that amount, you've done it right. It's way overthinking it to say you must consider future oil expansion.

    2. I'm very sceptical of this oil expansion factor. Will do a second, hot level check, probably tomorrow, and post any variation from previous cold check.
     
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  12. The Critic

    The Critic Resident Critic

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    There are a lot of variables. If a tech reinstalls the drain plug when there is still a stream of oil coming out, adding 4.4 qt will result in an overfill.

    This is why the capacities are a suggested quantity and the repair manual always directs you to the dipstick (or the appropriate level check procedure) to confirm.
     
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  13. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    I find I usually need a bit of top up, to get it at the upper mark. But then I'm in the "leave it dripping, go in and have dinner" school.

    I'll just put in the spec'd amount, leave it thus for a few days (to settle), then check and top up. I like it right at the top mark (when cold), to have a good reference point, if it were to start dropping.
     
  14. The Critic

    The Critic Resident Critic

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    Change it on jack stands also adds another variable - more oil drains out because the oil pan is tilted in favor of where the drain plug is located. I change mine on a lift so the car remains level.
     
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  15. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    Yeah I have the front-only raised. Last time, just for giggles, I played a heat gun over the oil pan. FWIW, "dry fill" quantity is 5 US quarts.

    upload_2019-4-24_10-0-20.png
     
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  16. The Critic

    The Critic Resident Critic

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    That sounds correct. After the head gasket job, I think I refilled with about 5 quarts.
     
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  17. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    Did a cold oil level check this morning. Then this afternoon, after a 8km drive, let it sit for 10 minutes, checked again. No difference. If anything the level was slightly lower, maybe still draining down.

    Morning check, a couple of mm above top mark (on left):

    IMG_0389.JPG

    Afternoon check, a little hard to see, but barely above top mark:

    IMG_0390.JPG
     
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  18. The Critic

    The Critic Resident Critic

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    Drive for 50-100 miles at highway speeds, then check it.
     
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  19. Slider2732

    Slider2732 Junior Member

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    This all interests me, because of what Mendel said above. The way the oil tends to catch on the sides of the stick on removal.
    I can't really tell where it is on the stick!
    So, I can let the stick sit for a couple of minutes with the oil on, presumably at a few degrees downward angle and it should collect at the bottom to give a 'near enough' correct reading?

    I added half a quart to ours the other day, because we've done 4K miles and at 88K miles I presumed it will have used some. Couldn't see where the level was so played things cautiously.
    After owning dozens of cars since the early 1990's, this is the first time a simple dipstick has made me feel like one!
     
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  20. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    I pull the stick, then stall around for at least 5 minutes. The oil tends to get drawn up the guide tube (poor design), and this wait time allows that drawn-up oil to drain back down, giving you a semi-clear read. Also, I find the face of the dipstick nearest the front is clearest.

    Speaking of "clear", the oil was just changed, about a week back, so doubly hard to see.
     
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