MPG change, where to start looking? Options

[914Bryan]

I know this isn't an exciting high performance question, but a question just the same.

I'm having an issue with the mileage my 74 2.0 DJet and am wondering where to start looking for the problem. I commute 60 miles each way to work (120 total). My driving is 98% freeway, non rush hours so a steady 75 mph, motor is turning 3200.

I could always count on 32mpg, day in, day out. Over the last two months, it has slipped to 28mpg. I burn a tank a week, so this has been about 8 tanks of gas that I have noticed this.

Engine is running fine, no change in performance.

Would a tardy valve adjustment cause this? I'm a little over 6k since my last one.

Any ideas?

[jonvo4591]

I was in an autotive training class last week and we were on the subject of alcohol content in fuel. Apparently they just passed a law that allows up to a 15% average alcohol content in fuel over the course of a month. Here's the catch: gas stations don't have to change the label to tell you. There have been reports of as much as 30% alcohol content according to my instructor, and there are people he's talked to that have seen a consistent 3 or 4 mpg decrease.

I don't know if what applies in MI applies in CA, but it's just a thought.

[somd914]

Have you noticed any other issues with the engine?

Valves can cause a slew of issues.

Check the usual culprits also:

- Pointless distributor or old-school points? If points, check them.
- Air cleaner
- Vacuum leaks
- Plugs - clean/fouled?
- Etc.

[rick 918-S]

Fuel was my thought also. There is no good reason for fuel to have alcohol in it.

[tscrihfield]

Fuel was my thought also. There is no good reason for fuel to have alcohol in it.

Agreed!

Those cheap bastards are already making record profits! Why save pennies to give crappy fuel!


(IMG:style_emoticons/default/blowup.gif)

[3d914]

Ditto on the points. And when you check them, don't just check the contacts. Check the little plastic (phenolic) rib that rides the cam. I've had numerous points where those just deteriorated in a matter of 20K miles.

[JamesM]

I was in an autotive training class last week and we were on the subject of alcohol content in fuel. Apparently they just passed a law that allows up to a 15% average alcohol content in fuel over the course of a month. Here's the catch: gas stations don't have to change the label to tell you. There have been reports of as much as 30% alcohol content according to my instructor, and there are people he's talked to that have seen a consistent 3 or 4 mpg decrease.

I don't know if what applies in MI applies in CA, but it's just a thought.

(IMG:style_emoticons/default/agree.gif) Step 1 should be find an ethanol free station.

Though you will wind up paying for for it anyways, so in the end its (IMG:style_emoticons/default/sheeplove.gif) either way.

[Cupomeat]

With everyone else's advice, I'd go over the basics;
1. plugs
2. points
3. valve clearance

Now, do you have your cooling flaps and thermostat still working on the engine? If it is running too cold, the mixture will be enriched.

Now, strangely my audi 1.8t motor seems to give me the best gas mileage around 50-70deg ambient. You would think a colder temp air charge would be the most efficient, but I lose 2 MPG running in the winter, and that is a modern watercooled car.

Good luck

[VaccaRabite]

Now, strangely my audi 1.8t motor seems to give me the best gas mileage around 50-70deg ambient. You would think a colder temp air charge would be the most efficient, but I lose 2 MPG running in the winter, and that is a modern watercooled car.

Good luck

In the winter you are running winter fuel and it takes longer for your engine to reach operating temps. There could be the difference.

[Cupomeat]

Now, strangely my audi 1.8t motor seems to give me the best gas mileage around 50-70deg ambient. You would think a colder temp air charge would be the most efficient, but I lose 2 MPG running in the winter, and that is a modern watercooled car.

Good luck

In the winter you are running winter fuel and it takes longer for your engine to reach operating temps. There could be the difference.

Good points! (IMG:style_emoticons/default/agree.gif)

[rick 918-S]

Fast check for the points is ignition timing will change. Smaller gap will have less of an effect on mileage as it will increase the time to coil builds energy. (which is actually a benefit) The change is in efficiency will be in the timing change by the shift in dwell angle.

[Tom_T]

I know this isn't an exciting high performance question, but a question just the same.

I'm having an issue with the mileage my 74 2.0 DJet and am wondering where to start looking for the problem. I commute 60 miles each way to work (120 total). My driving is 98% freeway, non rush hours so a steady 75 mph, motor is turning 3200.

I could always count on 32mpg, day in, day out. Over the last two months, it has slipped to 28mpg. I burn a tank a week, so this has been about 8 tanks of gas that I have noticed this.

Engine is running fine, no change in performance.

Would a tardy valve adjustment cause this? I'm a little over 6k since my last one.

Any ideas?

Yes on the valve adj. & the other things mentioned above Bryan.

Also, we've switched the winter/summer blends here, so that could be some of it.

Also, if you use ARCO or TOTAL (new name they're going to since BP sold-out), they over-oxygenate their gas, so it gets slightly less mpg than the majors & Valero - according to some out here I know.

I don't know of any stations selling 100% gas around OC nor SoCal in general, but you might find some race fuel or avgas to use, but they may have other problems if not unleaded & proper octane & whatever additives CA makes them add. There was another topic on the 100% gas stations topic on here a week or so ago, but SLITS only found a couple up in NoCal, & I'd think he'd know of any down here...if there were any. (IMG:style_emoticons/default/sad.gif)

Lastly, inspect your right foot....carefully - have you changed driving habits any in that time? (IMG:style_emoticons/default/biggrin.gif)

Also, if you want to max mpg in a 2L Djet, IIRC around 2800 & 65-67 will get you best at perhaps 36-39 on100% gas in the old days - less than today with 10-15% ethanol. That's from my personal experience steady cruising SoCal to SFO area on I-5 back in the late 70's & early 80's - when CHP still pulled you over too regularly over 55 there back then - so 65-67 was pushing your luck.

BTW - 55 is NOT the best mpg on our 914s & esp not for the 2L Djet in stock form, even though mandated back then! (IMG:style_emoticons/default/dry.gif)

Rick - CA switched from MTBE for smog reduction additive in gas about 10 years ago, over to the ethanol instead - after they found out it was a possible carcinogen that went through the soils on spills/leaks like greased lightening & into the water table. Ethanol is the lesser of 2 evils for CA gas additives in this case. (IMG:style_emoticons/default/dry.gif)

Of course for years before that they ignored complaints that MTBE ate thru rubber hoses/ect. & caused more leaks than the emissions of VOCs it saved, & we were replacing the gas tank seals/hoses on our 88 Westy every other year at $200+ a pop back then! (IMG:style_emoticons/default/mad.gif)

[Dave_Darling]

It is normal for your fuel economy to suffer when the weather gets colder, for a variety of reasons.

- It takes more energy to get the engine, transmission, and tires up to temperature. That energy comes from your fuel.
- Your tire pressures will drop somewhat, and that gives them more rolling resistance. Check the pressures.
- Colder air is slightly denser, so it takes more energy to push through it.
- The denser air means the engine spends a little bit more energy to pull that air past the mostly-closed throttle, which costs a tiny bit extra fuel.
- Here in CA, colder weather generally comes with rain, which takes even more energy to push through (in the air) or move aside (with your tires).

This year, I believe they changed over to "winter gas" in October due to some supply issues. So the change in fuel is probably not a cause for your particular drop, unless you noticed it starting in October.

That said, it can't hurt to check the stuff above, and also the tire pressures.

--DD

[bulitt]

Rear brakes dragging?

[bandjoey]

IS THERE A WAY to test the gasoline to see the alcohol content...and then pick one gas station and stay with that one?

[VaccaRabite]

sure. run to flaps and get some test kits. they will tell you the approx % of ethanol (or so the packaging says). I have never used one.

Zach

[914Bryan]

Thanks everyone, this is all great advice! Most of it is an easy fix, other than the valves, but I know they need to be done..Now I have somthing to do on Monday. I never consdered that the rear brakes might be dragging, guess I will check that when the ass is off the ground.

The fuel thing is beyond our control. Sad that it has come to this, but what are we to do? Buy 55 gallon drums of unleaded racing fuel? My friends that are into sandrails and off roading always buy their gas this way. Big 55 gallon drum of gas with a hand pump. I wonder if its cheaper and if it can be had without alcohol. I also wonder about storing that in my garage? Some fireman may not appreciate it if my house goes up in flames.

Anyway, I will let you know what I find out after getting my hands dirty!

[euro911]

Having the same issue with my Toyota trucks. Been loosing 2~3 mpg over the past couple of month on both of them. Driving habits haven't changed, so I'm suspecting it's the cheap-ass gas they're forcing on us (IMG:style_emoticons/default/mad.gif)

[Katmanken]

Winter gas mix has more aromatic volatiles in the mix than summer gas, and the volatiles, such as butane, acetones, etc, have a lower energy density. The addition of volatiles makes the winter gas easier to ignite when everything is cold.

So not only do you get an energy drop from the addition of alcohol, you get another one through the addition of winter aromatics to the gas.

The aromatics are pulled out of the summer gas to reduce emissions as they tend to boil out of the gas in your tank during hot weather.

So yeah, expect worse mileage.

Hope that helps.

[jonvo4591]

To do a down and drity alcohol content test, you'll need a few things.

A water bottle, a sharpie, a fuel sample and a little water.

Make 10 marks in equal increments on the side of the bottle with the sharpie. Then pour some fuel in the water bottle till it reaches the 9th mark. Take the amount of water it would take to fill the bottle up to the last mark, and pour it in. you should have the bottle full up to the last mark.

Now, shake the bottle up for a bit, and then let it sit for 5 minutes or so. The water will combine with the alcohol and the parting line should raise between the fuel and the water/alcohol mixture. This will give you an idea of how much alcohol the fuel contains.

This test is probably best used as a comparison test between different fuels.