Gas pump theory/thought/woes??

On my daily I generally only put couple gallons of gas. Not that I'm cheap but it only gets driven maybe 20 miles a week. Car is tuned for 93 octane, that being said

Lately  my car has been feeling down on power at wot. I chopped it up to the high temps outside. But couple days ago I put lil over half tank in and no it's running like it should.

So my thought/theory.

Is it possible that only putting couple gallons at a time I'm not getting enough 93? Been wondering if there's enough 87 still in the gas pumps lines I'm getting a not getting enough 93 and it's messing with the tune.

Thoughts??

I'd say a plausible theory. 

Gas has a shelf life. If you're driving the car that little, is it possible that you're slowly accumulating more and more old gas?

In reply to Floating Doc (Forum Supporter) :

Doubt it, my daily commute is approx 1.5 miles, sometimes food runs. I'm usually 90% full throttle at all times. I run low on gas atleast once a week. 

I forget the exact standard, but not a plausible theory unless your gas station is violating the law.

The octane number listed on the pump is a minimum octane number.  There is a standard (I think it's 1/3 of a gallon) that they have to conform to.  The hoses look huge and look like they hold a lot of gasoline, but they are just thick-walled.  They're usually only 5/8" ID, so the amount that is in the hose is minimal.

The 1/3 gallon rule (I think that's what it is) says that a tester runs X gallons of the lowest octane through the hose, then switches to 93 octane and pumps 1/3 gal.  The test on that 1/3 gallon must have a result of 93 or higher.  If it doesn't, they either have to reconfigure the pump, or raise the octane.  I also think there is a new standard in which they have to have separate tanks for each grade.  It used to be they kept a tank of 87 and a tank of 93 and the pump mixed them to get 89 or 91.  In that way, if you selected 89, the pump would shoot straight 93 for the first couple seconds to average out what you're getting, then start mixing in 87 to get 89.  At least that's how I understood it from a gas station where I used to work in college.

I never have trouble filling my motorcycle tank with 3 gallons of 93.  If you use 89 or 91 it pings audibly.

Can someone chime in and correct me if I'm wrong?

A good test might be to find a gas station with the separate nozzles for each grade.  Then you know what you're getting is all 93.  If the problem goes away, your theory might be plausible.

Ok, somebody correct me if I'm wrong. But the higher octane is only supposed mean resistance to detonation. So if the problem was you were getting too low octane of gas the only symptom you should have is pinging/ spark knock. There shouldn't be a power difference simply from lower octane unless it's causing predetonation. 

Im not saying gas can't be the problem. Stale gas will cut power, or maybe the last time you filled up the station got a slightly weak batch. I don't know, but if it seems better now I would wait and see if the problem comes back.

In reply to Curtis73 (Forum Supporter) :

That is the way I heard it, too.

i have also heard plenty of stories where the truck driver says "Hey i have an extra 300 gallons of 87 that I don't want to take back" and the station operator has him drop it in the 93 tank. Hearing this from a former station employee.

gearheadmb said:

Ok, somebody correct me if I'm wrong. But the higher octane is only supposed mean resistance to detonation. So if the problem was you were getting too low octane of gas the only symptom you should have is pinging/ spark knock. There shouldn't be a power difference simply from lower octane unless it's causing predetonation. 

Im not saying gas can't be the problem. Stale gas will cut power, or maybe the last time you filled up the station got a slightly weak batch. I don't know, but if it seems better now I would wait and see if the problem comes back.

Yes the amount of energy is the same in different octane fuel and yes the octane rating is wat will determine a fuels resistance to knock.   It also determines the speed at witch the energy in the fuel is released. I was told by a fuel sails person the knock resistance of fuel is really the speed at which fuel burns. In other words both octagon ratings will ignite at the same level of compression but the higher octane fuel releases its energy over a longer period of time thus over all lower max cylinder pressures. It is also why you can run higher static compression with higher octane fuel. 
 

In modern cars that have good knock detection you will not hear the knock but the car will sense it. Typically called silent knock. As knock is detected the ECU will compensate usually pulling timing. This reduces power and increases heat.  This will happen long before you actually hear knock in fact with modern engin controls hearing knock at all is rare. But power loss due to poor fuel is not.  I suspect the the op just got a bad couple gallons.  
 

I can ask my friend exactly how his pumps work. He has two tanks. Only the 89 is a blend. It would be interesting to know the volume of fuel that is after the blend valve in the pump. I suspect it could be up to a gallon. His pumps have longer hoses so he can pump gas to either side of a car no matter how the car comes up to the pump. 

Heat in the fuel pump? If you're constantly running your tank that low your pump might not be able to dump heat to the fuel and isn't able to keep up fuel pressure as the pump heat soaks. Just a WAG, I am not an expert.

gearheadmb said:

Ok, somebody correct me if I'm wrong. But the higher octane is only supposed mean resistance to detonation. So if the problem was you were getting too low octane of gas the only symptom you should have is pinging/ spark knock. There shouldn't be a power difference simply from lower octane unless it's causing predetonation. 

Im not saying gas can't be the problem. Stale gas will cut power, or maybe the last time you filled up the station got a slightly weak batch. I don't know, but if it seems better now I would wait and see if the problem comes back.

Yup.  Same energy content, just more resistance to detonation.  As Dean said, knock sensors would sense detonation and retard the timing.  In an engine that is truly tuned for needing 93, anything less would cause a reduction in timing advance and therefore reduction in power.

Conversely, in an engine tuned for 87, you're right.  93 offers no benefit.

Error404 said:

Heat in the fuel pump? If you're constantly running your tank that low your pump might not be able to dump heat to the fuel and isn't able to keep up fuel pressure as the pump heat soaks. Just a WAG, I am not an expert.

That's what I was thinking. Running low fuel levels all the time may not be letting the pump cool properly.  Can cause premature pump failure.  We had a technician who would run his service van until the low fuel light came on.  We had to replace the fuel pump twice in 3 years. 

In reply to dean1484 :

That's my thought about plumbing/pump. Am I getting one gallon 87 and one 93.

Car is nothing special, just JY swapped ECU. Upper power band felt like it  "flattening" out before.
 

Curtis73 (Forum Supporter) said:

gearheadmb said:

Ok, somebody correct me if I'm wrong. But the higher octane is only supposed mean resistance to detonation. So if the problem was you were getting too low octane of gas the only symptom you should have is pinging/ spark knock. There shouldn't be a power difference simply from lower octane unless it's causing predetonation. 

Im not saying gas can't be the problem. Stale gas will cut power, or maybe the last time you filled up the station got a slightly weak batch. I don't know, but if it seems better now I would wait and see if the problem comes back.

Yup.  Same energy content, just more resistance to detonation.  As Dean said, knock sensors would sense detonation and retard the timing.  In an engine that is truly tuned for needing 93, anything less would cause a reduction in timing advance and therefore reduction in power.

Conversely, in an engine tuned for 87, you're right.  93 offers no benefit.

I won't go completely down the rabbit hole but I do know that the "good"  fuel at my friends station has a much more comprehensive additive package that is specific to shell where as the regular 87 is just standard swill. As such I run a tank of the 93 in some of my cars every now and then. It seemed to clean the fuel system up a bit and particularly my truck it runs better after.  The truck does run better with the better gas but there is no added power that I can tell.  Oh. I will run the good stuf when towing. My logic is that with the added load and heat from towing I could be getting more knock so why not up the octane and slow it to run more timing. Seems to work as the truck seemed much happier when on 93 and towing somthing heavy. 
 

or it could all be in my head. LOL. But the truck has about 350k mikes so I figure it can not hurt. The PO did the same with respect to using 93 when towing so I figuared why not just keep doing it. 

I don't know the full set of facts but its possible that moisture is accumulating in the ethanol because the partially full tank is  allowing the introduction of humid air into it. 

I have read multiple times over the years that a partially full tank attracts moisture because of the  introduction of humid air.  In the past, with real gasoline, the water just accumulated in the bottom of the tank.  But, with ethanol involved, the water is absorbed.

In reply to jharry3 :

Air should not be getting into a modern emissions controlled fuel tank.

Pete. (l33t FS) said:

In reply to jharry3 :

Air should not be getting into a modern emissions controlled fuel tank.

So let's say I fill my tank right up the filler neck, put the cap on tightly, and drive until I have one gallon left in the tank.  I guarantee that when my tank is empty, it's got air in it.  The tank has evaporative controls, but it's not sealed from the atmosphere.

In reply to 1988RedT2 :

Well, granted it isn't hermetically sealed, but it's also not going to be "breathing" to the point that the ethanol is going to attract moisture, in the low amounts of ethanol in gasoline.  If you are running the engine to where the evap system is pulling fresh air in, you're also using the fuel and it won't be in the tank very long.

This is a good chunk of why newer cars can sit for a few years and still have decent fuel in them.

Well from my googling research on this answer varies. Some say only 1/3 of a gallon, some say up to 1 whole gallon. Really depends on the pump configuration and hose length. So if I am only putting 2 gallons at a time I am getting less then 93.

So from now on I'll be adding much more then couple gallons at a time.

So, even if it's one whole gallon, you could put 1 gallon at a time and still be getting the full 93.  If one gallon is 93, then two gallons is 93 or more.

I still contend, that the pump and how it dispenses is not the issue unless that gas station is cheating.

Pete. (l33t FS) said:

In reply to jharry3 :

Air should not be getting into a modern emissions controlled fuel tank.

Disagree.  Air should not be getting out.  Gas caps have a valve that prevents vapors from escaping, but anything over about 1/2 psi it does let vapors out.  Conversely, it almost freely accepts incoming air.to prevent collapsing the tank when it cools.

Subscriber-unavailabile said:

Well from my googling research on this answer varies. Some say only 1/3 of a gallon, some say up to 1 whole gallon. Really depends on the pump configuration and hose length. So if I am only putting 2 gallons at a time I am getting less then 93.

So from now on I'll be adding much more then couple gallons at a time.

I was told the lines hold about a cup or two.

Either way, yeah, always run it down and then do a full fill.  There's a benefit to that: The fuel composition is seasonal.  Winter fuel in the summer and summer fuel in the winter can cause weird issues with the evap sysem, at best.

Curtis73 (Forum Supporter) said:

Pete. (l33t FS) said:

In reply to jharry3 :

Air should not be getting into a modern emissions controlled fuel tank.

Disagree.  Air should not be getting out.  Gas caps have a valve that prevents vapors from escaping, but anything over about 1/2 psi it does let vapors out.  Conversely, it almost freely accepts incoming air.to prevent collapsing the tank when it cools.

But the canister vent valve is generally open.  The fuel tank can "breathe" albeit through the charcoal canister.

Ever notice that fuel caps no longer warn about possible pressure in the system?  I didn't until recently.  Evap systems have changed a bit.

I have often wondered about this as well. 
Do you always fill up at the same station? Is it possible they haven't had fresh fuel in a while and it's not so good anymore? 
Twice now on road trips I've gotten a bad tank of fuel in two different cars. Both times it was down on power and fuel economy dropped. I thought something was wrong with the car. A fresh tank brought it back. 
Maybe try a different station (a busy one) and pump a little more. 

So my friend that owns a gas station and I figured out that you will get .21 of a gallon MAX of what is in the line between the mixing valve and the dispenser nozzle.  On his pumps it is 17 feet between the mixing valve and the nozzle,  The ID of the pipe that runs from the valve to the top of the pump where the hose connects is 1/2" ID.  The hose is about 7/16"  For easy math we used 1/2" and 20 feet when in reality we measured 17' of total length.  Anyway at 20' and 1/2" ID pipe it is only .21 gallons and in reality it will be less as the length is actually a little shorter and the hose ID is slightly smaller.  Also he has the much longer hoses so he can reach the far side of a car if some one pulls up with the gas cap in the far side.  If you go to a more common station the hose is much shorter.  So I am betting it is much more common for the amount in the hose / pipe after the mixing valve to be closer to .15 of a gallon.  The short of it is that if you put two gallons in you are probably only are getting between 7 and 8 percent lower grade gas before you get the full 93.  That really should not effect anything.

My bet it you just got bad gas and it is not a mixing issue. 

Unless you're having other symptoms, my guess is that your car isn't down on power and you're just getting used to the power that's there.  HP is addictive like that.  The cheapest troubleshooting is to fill it up at the newest, busiest gas station in town.  After that, get a dyno run and compare it to one from when it was tuned for 93?