Jeff F

iTrader: (1)

Vehicle:
98 Camaro Z28
6 speed
140,000 miles

Mods
MTI cold air box
Borla cat back
No extensive tuning (CAGS eliminated and EGR gear turned off)


Recently changed in the last 400 miles
All 02's

Previously changed in the last 6,000 miles
Plugs
Wires
Fuel filter
Fuel injectors
New fuel pump (old one died)
Air filter

Alright, car was getting in the single digits for fuel mileage, not good period. Could watch the fuel gauge over the span of 25 miles go down a 1/4 tank. Everything was checking out fine, other than the 02's were changing at a slower rate than usually seen. So swapped out the O2's and now mileage is up to 18 mpg for the first half of the tank. This is very repeatable and accurate. Top the car off with 3-6 gallons and the average is almost dead nuts 18 mpg. Once it gets below half a tank, the mileage will drop off to below 12-14 mpg. Driving style is the same.

This has definitely got me stumped. Not sure which direction to go. TIA!

newton

iTrader: (4)

I may sound like a rookie for this, but possibly defective MAF?

fortmyerspolice

iTrader: (7)

fuel filter, seafoam it too

Jeff F

iTrader: (1)

Forgot to add the fuel filter was replaced same time the pump was done. May try the seafoam. Cheap.


No worries. I'm not seeing anything that would indicate a bad MAF, but I have cleaned the MAF.

Adam013085

i second the seafoam. It helped me gain 2-4 mpg or so

1/4milerunnerZ28

iTrader: (2)

can anyone tell me exactly how to use seasalt? maybe a clip or step by step directions? i have never used it and my 02z has 38,000 on it.

wssix99

iTrader: (5)

What part of the country do you live in and what is your true gas mileage?

The behavior of the fuel gauge is non-linear, so you can not calculate mileage by looking at it! (The needle will go down faster over the last half of the gauge than it does over the first half - so the half tank mark on the gauge is probably not really 1/2 of a tank. Its something less.)

You need to calculate your mileage by fuel fills. (Take your mileage between full fill-ups and divide it by the gallons required to fill back up.)

Besides the natural mileage your car is getting, the composition of your local gas formula can also impact your numbers during the winter. (You'll get methanol in higher concentrations in some areas, which can lower your mileage a small bit.)

Jeff F

iTrader: (1)

Location is in Washington State.

Using a tech2 to see the fuel remaining in the tank, and then when filling up, it's the same amount added each time. So around 3/4 of a tank it's always 3.5-4 gallons and so on.

cool whip

iTrader: (1)

That's exactly right. Filling up your tank and driving until the gauge reads half empty does not mean you've used up half a tank of gas. Nor does it mean that if you've driven 140 miles, you should be able to drive 280 miles with the rest of the tank. You're not going to get the same mileage out of the "top half" of a tank of gas as the "bottom half" because they are not even.

The only way to find out your true MPG is to fill up the tank, reset the tripometer, drive it 'til it's near empty then divide miles driven by how many gallons it took to fill the tank again. And no topping it off by putting in a few extra ounces after the pump has shut off. You can't get an accurate fill each time that way.

Joshiedoom

iTrader: (4)

Fuel Pressure Regulator? MAF? IAT? do you still have the AIR pump? Change the pcv valve? Air Filter?

I guess the worst it could be is the rear end going out.

The best I have ever got is 25.5 with my a4, 145,000 miles at the time.

wssix99

iTrader: (5)

The Tech 2 will be giving you the same info the gauge does.

The "gauge" really isn't what shows the off reading. Its the sending unit in the gas tank. The Tech2 or any other scan tool will just tell you what the sending unit is giving as an output (just like the gauge does). The fuel sending unit reads the level of the tank. Since the tank isn't perfectly symmetrical, the level (and fuel reading) is not proportional to the volume inside.

Crimsonnaire

iTrader: (1)

Jeff F,

This is a serious loss of MPG. If the gas is really going down that fast and there's no obvious gas leak you can detect from the outside, the first place of suspicion would be the engine. Do you think any of your cylinders might be misfiring? If not, then the next thing to check are the cylinder valves. If they're worn out or have contaminents built up, then the gas could be corrupted or there might be insufficient intake of air for the combustion.

Have you performed a compression check on the cylinders?

Jeff F

iTrader: (1)

I haven't even wanted to start going down that road. The car actually runs fine at WOT and at part throttle as well. No gas smell building up in the garage when the car sits overnight, so no leaking lines or tank. I have no vibration in the engine to make me suspect a misfire, the plugs do not indicate a misfire either.

The tech 2 is actually fairly accurate, gives me an idea of where the float is, I put x amount of fuel in it. Next time it gets to that point on the tech 2, I put in almost the same amount. I'm not going nuts here, there is a discernible drop in gas mileage. I may just strap it down to the dyno and run it at 60 mph until it runs out of fuel and see what it does. Fricking waste of full tank of gas though.