EOC made me lose 3 MPG
 

This is my 3rd tank since purchasing the car and I am confused.

If you look at my fuel log...

First tank was 24 mpg. Was still really cold (30 and lower) and snowy so I had to let it warm up before driving to keep from fogging up windows etc...had to use 4WD a couple times.

2nd tank was 28 MPG not as cold (40's) so no need to let warm up, no 4WD and just drove it like a grandma.

3rd tank I was 25 MPG, (temps mid 40's-mid50's), I was EOC down hills and approaching lights. Left the engine off until traffic started rolling again.


Is it possible that my carb when restarting the engine uses a large gulp of fuel to do so? I am not totally familiar with carb's so is this a normal thing or is it possibly more to do with the fact that the carb is 25 years old and may not control fuel as well as it should?

Could it be that the carb and ignition is in a seasonal tune that makes it run more rich as the temps come up a bit?

I am pretty mechanically inclined but this is only my second car that I have ever owned that has a carb so I am lacking a bit in knowledge.

The fuel filter is in the engine bay and it is a clear type that looks pretty good inside.

Thanks for any input...I want to have consistent numbers before I start doing any serious aero mods.

It could be the carb/timing, it could be something else like driving a different route, a different mix of city/highway, or the even the same routes w/ different conditions (change in wind direction or something). W/o a lot more info it's hard to come to a comparison.

My trips to work are the same route...and going to the grocery store is the same as well.

I only had one 10 mile trip on the hwy last tank but I doubt that would average out to a 3mpg difference.

I often get 2-3mpg of "noise" between fill-ups, just from pump / nozzle shutoff variability, or from filling up in the mountains vs. filling up at sea level.

I can't help much with the carb questions, but I do remember my '83 2WD Hatch got 30 when I drove it like an idiot in high school!

I would not EOC a carbed vehicle. The key just shuts off the spark. The momentum of the engine pulls it through a couple more intake strokes, sucking fuel (the same amount of fuel as a couple intake strokes while idling)into the engine. This can put some extra fuel into the cat, and dilute the oil on the rings. Others dissagre and feel the heat in the engine will evaporate the fuel and it will not be a solvent to the oil :confused:. I do not know which way for sure but my fuel injected cars get EOC and my carbed vehicles get neutral coasted to 15mph and brakes the rest of the stop (ideally)

When you start it do you have to pump the throttle? If so you are squirting a bit of fuel in to get the thing running again. As much as you would have used idling? Depends on duration of the stop.

The gain or loss of 3 MPG in the first tank you try it in is not necessarily indicative of it's success. If you are willing to try it for a few more tanks it may pay off. I know In my case EOC is good for about 5-10mpg improvement, but I am fairly aggressive with the EOC.

Asking just because I don't know the specifics of your vehicle, but are you sure the odometer is still ticking off miles when you key off to do the EOC? Do you key back to the ON position and bump start or keep it off and key start when still rolling? Curious minds want to know.

I don't do EOC, so take this for what it's worth, but I read about another member on this board with a certain model Nissan that would enter open loop operation for a chunk of time after doing EOC. This rendered the tactic basically useless - a net loss in fact.

I noticed you're filling up your tanks pretty quick, with only an average of 162 miles. That magnifies errors caused by topping off or automatic shut off at the pump. The longer you go in a tank, the more those fractional gallon differences will matter. Yes, I know it means less feedback, but if your feedback is filled with noise, it's hard to know what changes are meaningful and what aren't.

As far as the possibility that your carb'ed engine is getting worse MPG from shutting off and restarting, it's definitely possible. The accelerator pump is designed to prevent hesitation, but it also squirts a big (up to 50cc with a V8 engine) shot of gas everytime you depress the gas. With that said, there are guys that have used EOC with carbs and improved their mpg. As bestclimb mentioned, there is a risk that shutting off the engine, particularly at any speed higher than an idle, will allow gas to continue to be pulled through the carb what the pistons finish cycling on momentum and that gas can dilute and wash down your oil from the cylinders. If you turn the engine off at 2500rpm, I can see a problem, but at an idle I see it as far less of a concern.

I always push in the clutch and let it drop to idle before shutting it down.

The sucking fuel through after shut down makes sense. But I honestly don't see how it could be enough to make a 3 mpg difference.

I would like to try and fill up when its closer to empty...however my hours are greatly reduced at work presently and I have to buy gas when I get paid every week.

The weather is much nicer now and when I get a chance I need to replace the plugs and check the cap and rotor and look through my service manual on adjusting the carb I smelled it when I started it up after work tonight and it smells a bit rich...so I think it needs a spring time adjustment.

I will keep doing EOC this tank and see how it comes out. If the numbers are under 28 again I will go back to not EOC and see if they go back up.

I fill up at the same station every time and actually use the same pump because I go when nobody else is there.

You can always try coasting with the engine running.