Octane specs, Miles/gallon, confusion!

Home  \  Repairs & Maintenance  \  Octane specs, Miles/gallon, confusion!

Hey all,

First off, I'm perfectly prepared for anyone who thinks I'm being an over-achiever with trying to get the most out of my 120,000-mile 3.4L 1995 Pontiac. (Auto, standard package, 2-dr hatchback) My fiancee gets mad because I constantly strive for better mileage when I'm already beating her 2000 Taurus by 5mpg. (actually, I need to look at her car too, it should be doing better than that)

Having said that, here's the dilly. I've been observing 20-22 mpg in the city, and 26 mpg highway, consistently with 87 octane fuel, which the owner's manual specifies as the correct octane for this engine / model. However, when I put in 91 octane, the mileage bumps up quite noticeably to around 24-26 city, 32 freeway (depending on speed -- sometimes as much as 36.) If this were a distributor-driven ignition system, I would clearly start suspecting timing and spark, but since this system is driven by ignition coil packs and computerized timing, what could I be looking at here?

Again, if any of you think i'm being too anal about getting more mileage than I deserve from this end-of-standard-lifetime Pontiac, feel free to throw some tomatos :)

posted by  pugsley


Yes, you are being anal about this... I whish I would get anywhere your figures with my '91 Cherokee 4.0 ...

Never the less you caught my attention as you should not notice any difference.. Did you measure this only once or did you do multiple tests?

Start at the basics... Your mileage would not lead to suspect a totally worn engine with low compression, leaking valves and worn valve seats... but check the compression ratio. How about valve timing, are your (hydrolic?)lifters OK? did you change oil to a differen grade? Have you changed the tires to a larger diameter? Transmission overhauls with a new converter?

If nothing has changed and you can replicate the test with the same results, I would start thinking about sensors. Thinking only, not replacing... Somehow your motor management system thinks that it needs to put in more fuel if you run on low octane: is your car equipped with a knuckle sensor? How do(es) the O2 sensor(s) look? Are they the right colour or are they black and soaked in oil? What colour do your spark plugs have? All the same?

tell me if you find something... I'm curious!

Good luck,

posted by  SamJaarsma

Hey Sam,

The funny thing is, it's been like this for quite awhile, since I started using 87 -- at least three years. I used to put in 91 all the time before that, so I don't know, this condition may have existed all this time.

As for colors - light tan O2's, actually just replaced them before my last trip to Vegas (and yes I tested it once again on that trip) .. just changed plugs too, the ones coming out were again a nice dusty tan color. As far as I can tell, good colors in all regards. When lined up next to each other, they all look the same.

Oh, and yes, I should have mentioned that the problem is recreatable. It's hard to observe distinctly unless I'm going on a road trip, but this past weekend to Vegas and back cleared up the fact that it is, in fact, still happening despite my recent tune-up. I better tell you that I did the wires too, even though they were only at 30K.

Let's see - stock rims (alloy,) stock tires, SUMITOMO brand actually. They're really nice. I recommend them to anyone. They stick really good in the icy slush, esp. for a sports car. Ooh I better charge them for the advertising now.

No trans changes, no oil deviation (10w30,) but I can't be sure about the compression and valves. There's no valve noise, at least. But since you brought it up, it may be time for me to invest in a compression tester. I better do that while I'm still engaged, *before* I lose control of the cash flow, if you know what I mean..

A knuckle sensor is a new one on me - didn't see anything in the book under that name. What's that?


posted by  pugsley

hmmm, still think the difference is too big... so nothing changes between tests and the road to las Vegas is not uphill one way (87) and downhill the other way (92)....

Knock sensor might be the word in English. It detects whether the mixture is ignited by the spark plug or ignites spontaneously -before its time-, which produces also a diesel-alike sound. When it occurs, the sensor tells the management unit, which then retards ignition a bit to avoid the problem.

I would not buy the compression tester yet. O2 sensors OK, spark plugs perfect does not indicate a worn engine... compression sounds healty from that.

Still, the mixture needs the spark only to ignite. A huge spark or a small spark does not matter: starting at low revs is the biggest problem. The spark it self has no influence on gas milage if not very weak, irregular or so. It must be the timing (retarded). The question is only why.

I would search the internet on your motor management system and try to find info on different fuels. Somehow there must be something designed to adjust the engine settings to the fuel. I would be surprized if that came from the O2 sensors alone (not enough parameters). If the engine was designed for 92 and motor management adjusts it to accept 87, that might explain why "safe settings" give higher consumption. However, normally those things are engineered to the smallest detail and no difference can be noticed..

Good luck,

posted by  SamJaarsma

Interesting. I do recall reading about a knock sensor, that's something to check out for sure. I will also see if I can find a way to verify the timing with a timing light, although I don't think the crank has any markings on this engine.

As for Vegas, yes it is downhill one way and uphill the other. However, the good mileage actually happened uphill, *and* at 10mph faster. I took this into account with my mileage quote in my original note, the actual mileage measured was 26mpg@75mph avg speed, dropping 2000 ft over 450 miles. Coming back, with 91 octane, 30mpg@85mph avg speed (this time *rising* 2000 ft over 450 miles.) Go figure!

I will check into the timing/knock sensor direction. I will definitely let you know when/if I discover the source of the problem!

Thanks much.

posted by  pugsley

Did you change the sir filter lol as i see neither of you sai danything about the airfilter also the PCV VALVE is another thing to look at and also the type of plugs you are useing and the gap on then all that make a big differents on gas also i will tell ya as far as the air filter goes i would get rid of that paper filter im sure that you have in there spend a little more money but a K&N air filter and the recharge kit it is well worth the money and you never replace the filter again just recharge it as it tells and it gives you more horse power better gas milage as it gives you better air flow then a cheap paper air filer also the K&N oil filter in great to as it keeps your oil pressure up and filters the oil better try that and see the differents thats all i use in my cars i am an auto mech of 15 and a half yrs.......................... :laughing:

posted by  Tom34

3 month old thread here buddy

posted by  99integra

Your Message