Gas mileage fell 25% - Oil viscosity to blame? *Update*

My little Saturn runs like a top. Hardly a problem and never gets less than 30 mpg UNTIL last month.

The gas mileage has fallen to 23 mpg and I've done everything, including the spark plugs, air filter, PCV valve and cleaned the fuel injectors. With all of that, the mileage fell by another 1 mpg!

Today, I'm reading on the internet that people have complained that the wrong oil viscosity in their Saturn caused a dramatic drop in mileage. Well, I pulled out my Owner's Manual and the paperwork on my last oil change.

Whoa! They put in 10w-30 instead of the recommended 5w-30. Well, I headed out to the oil change place with my documentation and the guy looks at me like I'm crazy. I told him that I would like the oil changed to the recommended product and I'll even pay for it. "No problem", he said "but we are all out of 5w-30 until Monday." Well, if I rolled into the place today and asked for an oil change, they are just going to give me what every they have left. I won't be going back.

Should I just wait until my next regular oil change (about 2300 miles) or am I spinning my wheels?

Re: Gas mileage fell 25% - Oil viscosity to blame?

Unless there's a reasonable explanation that you'll be having some very cold weather before your next change, I wouldn't expect a problem in waiting. But then, there's no reason to accept my word as authoritative anyway; it's mostly intuitive.

Re: Gas mileage fell 25% - Oil viscosity to blame?

Empath said:

Unless there's a reasonable explanation that you'll be having some very cold weather before your next change, I wouldn't expect a problem in waiting. But then, there's no reason to accept my word as authoritative anyway; it's mostly intuitive.

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LOL. Ya, we're going to have a coldwave here in La Quinta. They expect it to fall below 100. :naughty:

Re: Gas mileage fell 25% - Oil viscosity to blame?

My standard disclaimer -- the advice you get on the internet, including mine, is worth exactly what you pay for it!!!

That said, I really don't see how that degree of difference on the bottom side of multi-viscosity oil could have that much effect, particularly at your location (you're pretty much in a warm, dry climate, right?).

I'd go to WalMart, get the recommended oil and change it out myself. No need to change the filter. Then check the milage.

That's a really big difference in performance. I'd look at something more basic -- catalytic converter beginning to clog, a faulty (not just dirty) fuel injector. Ignition control module dealing with ignition timing, injector control and things like that.

Might be worth it, at today's gas cost, to have a good shop run a computer analysis. Around here, that would cost less than a $100.

Kind of a long shot, but if you're using gasoline from a single source, you might try another source. I guess it's possible that substandard gas could be in the market these days.

Keep us posted.

Re: Gas mileage fell 25% - Oil viscosity to blame?

Yup, I'd say the oil has a fairly large part in your reduced gas mileage. My owners manual recommends 5w30, and I put 10w30 in one time to see if the hydrualic lifters would stay quiter and not have oil run out of them on startup. Well it was the reverse , took longer for the oil to get to them and quiten them down.

Anyways, I noticed my HP went out the window (even after the lifters got quite and was opening the valves all the way). So I'd say the oil difference is signifigant.

Does your car throw any SES codes? Perhaps a bad O2 sensor? I know mine probably needs changing (180,000miles on it) but I don't want to drop the $80 bucks on one.

Re: Gas mileage fell 25% - Oil viscosity to blame?

I wouldn't think the oil has much to do with your mileage at all. Maybe a vacuum line is cracked, broken or leaking. That'll pull your mileage down quite a bit. Are you using the same gasoline you had used in the past? Does the car perform as well as it used to? Does it have the same acceleration?
I'd say your best bet would be to have a qualified mechanic check it out. It may be something very simple to fix.

Re: Gas mileage fell 25% - Oil viscosity to blame?

In my area, the gasoline formulation is changed each spring and fall. Has something to do with pollution, ozone, whatever. There is a formula for warm months, and one for cool months. Anyway, I notice a clear difference in gas mileage when this happens.

When Katrina hit, W waived all of the restrictions that required certain gas formulations in certain areas of the country. This freed up supplies, and removed artificial barriers as to who could have what particular formulation. There are something like 40 different gas formulations. Fed or state regs limit each state or region to a certain flavor.

So after Katrina and W's actions, you may be receiving a different formulation, resulting in the mileage differential, or at least contributing to it. As I say, my mileage noticeably changes 2x a year when our formulation is changed.

best regards
me

Re: Gas mileage fell 25% - Oil viscosity to blame?

The performance seems the same and I vary the gas stations between 2 or three. I only use Chevron or MobilExxon from high volume stations.

I think that I'll change back to 5w-30 tomorrow and drive it for a tankfull or two. If things don't look up, I'll do the requisite tests for a "silent killer" like an O2 sensor or catalytic converter. The car only has 57,700 miles so I don't expect any of these items are defective.

Re: Gas mileage fell 25% - Oil viscosity to blame?

If this helps-


I had the same thing happen to my car. The mileage dropped about 7 miles per gallon. The cause- the calipers (not sure if that is spelled correctly) were not releasing fully. So in reality I was driving with my brakes on slightly. I got new calipers and gas mileage returned to normal.

Good luck,
rob

Re: Gas mileage fell 25% - Oil viscosity to blame?

fleegs said:

If this helps-


I had the same thing happen to my car. The mileage dropped about 7 miles per gallon. The cause- the calipers (not sure if that is spelled correctly) were not releasing fully. So in reality I was driving with my brakes on slightly. I got new calipers and gas mileage returned to normal.

Good luck,
rob

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Thanks Rob but I had my brakes looked at about 2 weeks ago. No problems in that area.

Re: Gas mileage fell 25% - Oil viscosity to blame?

Just a thought -- check you tranny fluid if auto and clutch if manual. A little slippage might not be readily noticible, but could have an effect on performance.

Re: Gas mileage fell 25% - Oil viscosity to blame?

BIGIRON said:

Just a thought -- check you tranny fluid if auto and clutch if manual. A little slippage might not be readily noticible, but could have an effect on performance.

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It's a manual and I don't feel any slippage. I had the clutch master cylinder replaced about a year ago and got my highest mileage around the same time.

Re: Gas mileage fell 25% - Oil viscosity to blame?

How about the pressure in the tires? Low air pressure can have a big effect on reducing milage.

Re: Gas mileage fell 25% - Oil viscosity to blame?

Powernoodle: ah good point. Gasoline that has ethanol in it will give you less MPG. It may be cheaper at the pump but it actually ends up costing you more. Where I live they are no longer required to say on the pump if it contianes Ethanol. While it's good for the farmers, it's not good on cars, fuel pumps, filters, injectors, stuck valves, ugh. Rusty gas tanks... Stations that convert over and don't flush the water out of their tanks, aluinium peices from nozzels.. yeah.. but anyways, I fill up with one station that I know doesn't have it (only one in town left) and my gas needle never seems to move. Get gas anywhere else and I can literally see it move while I'm driving!

Fleegs: I had the same thing happen but it was only after ~140,000 miles and I noticed it when I was going through brake pads, and smelt hot metal and one side of the car would lock down when I pressed the brakes. A quick fix of new front brake hoses and I was good to go! (the rubbers swells with age and restricts, causing pressure not to release from the calipers).

Doubt it's a clutch unless you live in a hilly place or are bad on clutches. Do you have a tach? You can tell if it's slipping usualy by that.

Re: Gas mileage fell 25% - Oil viscosity to blame?

California used MTBE in their fuels for years but it starts (or started) in October or November of each year. I didn't see any of the labels on the pumps at the gas stations lately so I don't know if they're still doing it.

In any event, they always claimed that gas mileage would drop by only 2-3%. At 30 mpg, that's less than 1 mpg.

Re: Gas mileage fell 25% - Oil viscosity to blame?

Try the old VW beetle clutch test -- set the parking brake, put the tranny in a top gear (3rd or 4th on a 5sp ?). Add about half throttle and release the clutch. If the engine stalls, the clutch is tight, if it keeps running at all you have some slippage.

Re: Gas mileage fell 25% - Oil viscosity to blame?

BIGIRON said:

Try the old VW beetle clutch test -- set the parking brake, put the tranny in a top gear (3rd or 4th on a 5sp ?). Add about half throttle and release the clutch. If the engine stalls, the clutch is tight, if it keeps running at all you have some slippage.

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I'll try it in the AM on the way to get my oil changed .

Re: Gas mileage fell 25% - Oil viscosity to blame?

Call me crazy if you wish, but the US gas supply companies have relaxed the quality guidelines on their product. I read an article on it and it made sense. The gas in the past month isn't lasting as long as it used to. I have a 2004 Grand Am GT2 with a V6, I can almost see the gas needle going from full to empty now. Never used to be that way - it would go forever on a full tank. Just a thought.
Rob

Re: Gas mileage fell 25% - Oil viscosity to blame?

Suggest you try it after you exit the garage. Parking brakes have been known to fail. Learned the hard way.

Re: Gas mileage fell 25% - Oil viscosity to blame?

My 2c -

WAY more likely to be the gasoline than the oil. After the car reaches operating temp, that first number (with the W after it) does NOTHING for you. That's the viscocity when it is cold. When warm, it is the other number.... which is apparently what you were supposed to be using anyway.

And since you didn't even pay the 2c for this, I agree with the others - consider what you paid for the advice!