I did an oil and filter change last week, and it seems that since then, my mileage has dropped 3-5 mpg. Could this just be coincidental? What should I suspect?

We're talking the same roads, and same driving style.

The oil level is closer to the 'full' mark than before the change, but is not overfilled. Also, I only had to add 4 quarts of oil (and drained about that). The only thing I forgot to do during the change was to loosen the dipstick.

Any clues?

Thanks,

Jas.

 

Well the oil weight can effect the mileage a bit, but that seems pretty extreme. What oil did you use?

 

Castro Syntec 5w30. Same stuff that was in there.

 

Jas-
Out of curiosity (not sure it's relevant) how many miles did you have on the previous oil & filter? Is the oil filler cap tight? And the dipstick fully inserted? Probably wouldn't make any difference, but I had similar results on another car back in the day. A small air leak from a loose oil filler cap messed with the vacuum, and it made a big difference in performance. Good luck!

 

About 3400 miles (the first 3400). Cap is tight and filter screwed down properly with new oring. Same with dipstick.

I sucked about 8 oz of oil out so it is about in the middle between full and add marks (about where it was before change). Will see what that does, or if it helps.

Discovered a way to check oil, since it's so hard to read on dipstick with clean oil: Pull dipstick and lay on paper towel. Check towel to see where oil gets absorbed.

 

It's probably that cheap American Castrol you are using. If you'd been using European Castrol...

Is this the computer reading, or your calculations from fill-ups? If the latter, give it a few tanks and check the average.

 

It's computer reading. I reset my trip computer whenever I leave my driveway, and the variance is from the same ten mile round trip I take almost every day.

One problem could be if I try to increase my mileage, it usually decreases...

Could be a tank of bad gas.

But I wonder if the extra oil in the engine could be creating drag. Even the slightest touch on the throttle sends the instant MPG reading down below 10mpg at lower RPMs

 

I think that if the octane is low, it could reduce mileage.

How are you driving? Some people think that the way to get good mileage is to slowly accelerate. Turns out it is more economical to quickly get up to speed (I don't mean WOT) and then maintain that speed. That could account for a decrease when you try for good mileage. Try brisk acceleration. Then pay attention to holding the throttle steady.

Unless you have really over-filled it, I don't see how that would have any effect. The only way it would increase resistance is if the crank is dipping down into it. If that is the case, you've got problems. The oil could get whipped into a froth and your engine could be damaged. My understanding is that you would have to fill it quite a bit over the top marker for this to happen.

Perhaps there was a reason that oil was on sale.

I think you should drain the oil, replace it with MINI branded oil, and record your mpg. Then drain that and replace it with Castrol 0-40w full synthetic and record your mpg. Next replace it with Mobil 1 Extended 5-30w, then Redline, etc...

Report your findings.

 

Here are a few external factors which maybe involved in the MPG difference:

Cooler outside temps means slower engine warm up. This could mean more fuel consumption as the engine spends less time in the most efficient operating mode. This will be especially true if there are a lot of short frequent trips.

Cooler air temps means higher air density. This could mean more fuel usage per unit volume of air. Not sure if this would increase or decrease MPG.

Winter fuel. Have your gas stations switched to winter gas yet? Winter gas is more volatile and contains less energy. I can't recall but I think winter gas gets slightly less MPG than summer gas.

 

Rear swaybar should help the MPG because you can take corners with less braking... Hmm. I drive harder as the car gets older and the MPG goes down also. But if your driving style is the same, I don't know why it should go down.

 

Since we're tossing out lots of ideas, how about your tire pressures? Cooler weather results in lower pressures, which could affect mpg. Hope you figure it out soon!

 

Haven't checked my tires, but after removing about 8 oz of oil from the car, it seems to have helped. I also got a fresh tank of gas, but I think things got better after removing the oil. It's weird to think that a change in oil level that's still within range would make a big difference, but it seems to have.

I'm thinking of pulling more out so it's just above the 'add oil' mark and see if gets even better.

I do need to check my tire pressure to, because temps have dropped from the seventies to the forties and fifties here in just the last week. The other night it was 65 at 10pm. Tonight it's 44 and it's only 8pm. There's a freeze warning for tonight where I live (PA).

But with the colder temps, I should be getting higher mileage if I calculate it from the amount I pump into the tank. When temps were in the 90's, I was putting more than 14 gallons in a 13.2 gallon tank.

 

I'd like to have someone explain to me how there could possibly be a causal relation ship there. I suspect it was coincidence.

I think that is a very bad idea. The car is designed to run with oil at the full mark. With only 4.5 qts. of oil, reducing the quantity that much makes for a rather large % change. I suspect that could lead to higher oil temperatures, and require more frequent changes. On my Acura Integra, a mechanic once told me that running a quart down can starve the engine of oil. I don't know whether that is true or not, nor whether it applies to the MINI, but they hold the same amount of oil.

I think you are more likely to find the problem here.

The gas is stored underground. Temps underground don't change very quickly. Ever gone into Carlsbad Caverns? It can be 110 on the surface, but inside is a constant 56 F. The summer phenomena with gas is that when someone "tops-up" a tank in hot weather, it overflows after it warms up. Search the forum for tank capacity. Lots of people report that the MCS holds more than specified.

This also brings up the point of running it dry. You must be coming very close to doing that. IIRC, it can be harmful to do so. Others have reported that the OBC estimate of remaining gas can be 40 miles off -- it was still showing that amount when they ran dry.

 

I can only say that removing a few ounces of oil made a difference.

Then why would the car have been delivered with the oil level about a third of the way up between the 'add' and 'full' marks? It hasn't burned any oil in 3,500 miles.

Except for the fact that the problem was occurring when temps were in the 70's and went away when the temps dropped to the 40's. I don't think tire pressure has anything to do with it.

That might be true if the gas went straight from the underground tank to the car, but it has to get to the pump, and then to the car. Why would it be suggested not to fill your tank during the heat of the day, but wait till cooler evening or morning hours? I'm not making this stuff up. Pumps are calibrated for a specific ambient temperature, and if you fill up on a hot day, you get less gas than if you fill up on a cold day.

It may be harmful if you run out of gas in a fuel injected system, but other than that it doesn't matter much if you have 1/2 a gallon or 5 gallons in the tank. Actually it probably helps in preventing crap from building up in your tank.

I think you're just upset because I got Castrol oil for $4.60 a quart, and it's the same crap you paid more than $5 for at the dealer

 

Jas-
Thanks for sharing this process with us. It's gotta be frustrating for ya
Your car's situation with an oil change (possible quantity vs. mileage) is new territory, at least for me. I haven't been able to find a relevant hypothesis anywhere else, and even my personal mechanic is stumped
Anyway, hang in there, and keep us posted. Seems like the only thing that hasn't been offered as a factor so far is that it's been a full moon
Of course, maybe that explains some of our suggestions

 

I don't know how the oiling system works in the MINI, but in older cars didn't the crank sit in the oil and splash oil inside the crankcase or cylinder walls?

If that were the case, oil level could make a big difference. Imagine the difference between walking on the street or waist deep in a swimming pool.

I have to be in the neighborhood of my dealer this week, so maybe I'll stop in and see if they'll plug my fob in their computer to see if it registers some unusual event that could have caused this.

At least the MINI has returned to more normal mileage levels (27mpg on a recent 230 mile drive), so I'm happy for that.

If I figure it out, I'll let you know.

 

No. That would cause the oil to be whipped into a foam. Car engines have had oil pumps and filters for a very long time.

 

I guess my old VW bug was a frothy devil then. Because that's exactly how the cylinder walls under the pistons got lubricated. It did have an oil pump and a filter though.

Maybe there have been technological changes, but if what you say were true, why wouldn't the oil be in a separate tank instead of in a pan right under the engine? I haven't done it, but I bet if you removed that pan, you'd be able to see moving parts.

 

I believe the MINI, like most street cars, has a wet sump, i.e. the oil just sits in a pan at the bottom of the engine, underneath the crankshaft. The oil is sucked from the bottom by the oil pump and sent where it's needed, eventually returning to the oil pan. If the oil level is too high, the crankshaft can whip the oil into a froth which could get sucked into the oil pump. Froth doesn't really lubricate as well as liquid. If the level is high enough, it could create resistance against the crankshaft. Wet sumps are cheaper to manufacture.

Some cars have dry sumps i.e. a separate tank containing the oil. A lot of race cars have dry sumps because high G turns can make the oil slosh around a wet sump so much that the oil pump sucks air or it messes up the car's weight balance. It's more complicated and expensive to manufacture.

BTW, I am not a mechanic, but I did stay in a Holiday Inn Express last night

 

Here's what the scientist in me sees in this sentence. I conducted an experiment (oil change) and got unusual results (low MPG). I then modified two of the experimental conditions (oil fill level and gas) and got a "normal" result (normal MPG) therefore I conclude it has to do with the oil levels. To me that's a false assumption. Bad gas is just if not more likely the culprit here than you adding slightly too much oil.

Also the gas tank issue is likely due to the fact that MINI gives us the volume of the tank alone, not the volume of the tank, filler tube etc combined.

 

I might agree, but this all seemed to occur with the same tank of gas, and went away when I removed some oil, not when I refilled the tank.

As far as the tank issue you are correct, but it is a proven fact that if you fill your tank on a hot day and a cold day, the pump will tell you you are buying more gas on the hot day, even though the amount of gas is exactly the same. That's part of the reason people are reporting amounts near 15 gallons on fillups. Gas station pumps are calibrated to average ambient temperatures (I think 60 degrees here in PA). Buy when temps are lower and you get a better deal. Get gas when it's 90 outside, and you're not getting all the gas you're paying for.

Was that you in the next room? I was doing a heart transplant next door.

 

Oil's have anti foaming agents to stop this from happening, one of reasons we change oils in cars is how the polymers break down and create issues like foaming, and another reason not to add things to your oil like that LUCAS products cause they can promote foaming over time. The wet sump systems do not foam, and they all have oil that splashes around, most have baffle plates under the main bearing caps to stop the splashing that reduces efficiency of the motor, but alot of cheaper cars do not.

 

Not to be a **** but
You seem willing to attribute it to the oil because that is the cause that you want to blame but in reality gas is a more likely culprit as I fail to see how having extra oil in a sump (aside from a slight increase in pressure) would cause the car to consume more fuel to travel the same miles.

 

I'm willing to attribute it to the oil, because everything else remained the same. The other possibility is my trip computer is screwed up, which wouldn't surprise me.

It used to be I had to make an effort to get less than 25mpg, now it seems to be the opposite (although after the oil removal, I'm at about 27mpg in mixed driving.

 

The MINI manual and the manual for my 1990 Integra state that engine damage can result from over-filling the engine with oil. The Acura service manager told me that the cause would be the crank dipping into the oil and causing it to foam. Perhaps the anti-foaming additives in oil are to deal with normal causes of foaming, and the crank would overwhelm such additives? Or, is there another explanation for how too much oil would damage the engine?