Slipping gas mileage, any suggestions?

[Cicero17]

My son, the engineer, tells me gasoline is formulated out of two principal molecules. Octane is the 8 carbon molecule you are used to thinking of as gasoline. Heptane is the 7 carbon molecule also included. For reasons I don't understand, but having to do with the bonds in the molecules, heptane has more energy in it than octane. Therefore, the higher octane gasoline is going to have less energy in it overall. Naturally, you will see your mpg drop.

There is no logical reason to run an octane rating any higher than the lowest that works in your engine without knocking.

The only other difference from gasoline to gasoline is the amount of detergents, which are necessary to keep your injectors clean. My understanding is that its basically the same from grade to grade within each brand, although if someone really knows better I am listening. I like to add a bottle of injector cleaner about 500 miles before each oil change just to be more certain.

 

[surfstar]

Geez.


(back to the original question)
If you don't notice much change with the techron, try another tank with either Gumout Regane or Redline SI-1.

Afterwards to keep it clean you may want to run FP60 or Lucas UCL each tank - these keep your fuel system clean, lubricate the upper cylinders, and usually give a slight MPG boost that makes the product pay for itself.

 

[Drew99GT]

My bad on the hijack.

Back on topic: with 87 octane, maybe you were getting a hint of detonation and the PCM was pulling some timing. Has it been insanely hot in Tennessee like everywhere else? That would reduce mileage. If you have fresh plugs and tune up parts then it could just be a coincidence. Change in gasoline formulation perhaps.

 

[crw]

I've always thought that when O2 sensors get lazy this is the type of problem you'd see. Not lazy enough to throw a code, but enough to affect performance. I've just ordered a set for my 83K mile Mustang. It'll be a while before I start to get results back, but if I find out anything interesting I'll let you know.

 

[CBDFrontier06]

With an 8 year old engine, I would suspect dirty fuel injectors coupled with a slight loss of compression. Even the best cared-for engines can be expected to lose efficiency and power over time, and considering other similarly aged engines I've seen, I think you're doing pretty good.

 

[ebaker]

Summer gas frequently has ethanol, which contains less energy. Also in the summer there is more drag from the A/C compressor. Higher octane gas may give better mileage because there's less chance of the knock sensor retarding the timing.

 

[Shannow]

If the higher octane fuel has a lower energy density, then it may reduce mileage depending on the engine, I do agree.

Many of the higher octane fuels down here are higher density than regular (and bloody expensive).

Cicero, your son has described how octane "equivalence" is established. A test fuel is compared to the equivalent knock rating of iso-octane (2,2,4 trimethyl pentane by definition 100 octane), and normal heptane (by definition 0 octane).

Different engines respon differently, giving the Motor Octane No, Research Octane No, and the (R+M)/2 method.

 

[sprintman]

Auto-RX it and clean the rings. Increased compression means better mileage.

 

[merc80]

Clyde, why the maxlife? I'm not leaking/burning any oil.

Sprintman, my arx arrived yesterday. I changed out my filter and added about 8 oz last night.

I've used regane a couple times in the last 15k miles. I just wanted to try the techron and see if I notice anything. If I don't get any improvement in the next couple tanks, I'll throw in another bottle of regane. Thanks for all the good advice guys.

 

[mechtech]

Cicero - Ask your son how MUCH less potential energy there is in Premium vs. Regular gasoline.
It is very slight, and I don't think it could be measured by normal gas mileage testing.

 

[GreenMachine]

all depends on what your engine and its timings are speced for.

My 98 Deville for example has the high compression northstar. So when I use regular I see my MPG decrease and acceleration suffer. So I stick with premium.

The higher compression causes the lower octane fuels to combust randomly and out of time.

While an engine with a lower compression ratio will not be able to cause that same effect, nor will it have the faster timings, and pressure, to cause the fuel to combust randomly.

This also causes a vehicle to lose MPG simply because it will not be giving the premium fuel the envirement in which the faster timings are taking advantage of, and since the compression is lower it will not combust as easily.

good article here: http://contactmagazine.com/Issue54/EngineBasics.html

 

[bulwnkl]

quote:

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Originally posted by LarryL:
I can wait.

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I surely hope you're not waiting on me; I'm not digging. An interesting link that does work is this one. It doesn't discuss flame propagation rates like the white paper, but one of its sections does mention that premium fuel averages 0.7% higher heating value than regular, and there's a separate section showing a very nice, tight graph of how closely fuel economy is tied to a fuel's heating value.

 

[merc80]

Just a quick update. I'm nearing the end of my arx rinse cycle. MPG dropped to an all time low of 21.8 on my last tank of the clean cycle. My last tank was 25.8, my best in several months. My mileage has slowly increased every tank during the rinse phase. The only other maintenence I've performed was a new fuel filter.

I'm not sure which was a bigger factor, the arx or the temperatures cooling a little bit around here. It's dropped about 10 degrees during the daytime in the last week, so my a/c isn't working as hard.

 

[javacontour]

If the temperature has dropped, chech your tire pressure.

Your tires will lose pressure when the temp drops. It's dropped 20 degree here, and all my tires were below 30 last week. Aired them back to 34, and my mileage went from 31.x to 34 on the last tank. Yeah, that's only one tank, but it's worth checking.

PS, I still ran the A/C because it was still humid, so the difference is not all A/C reduction.