[steve_oToy]

I have a 93 Corolla DX, with 242K miles on it. Original owner.

There is a strange fuel consumption issue with this vehicle, and wanted to ask before starting to change anything out on it. Average commute is < 20 miles round trip each day. Used for mostly around town driving - no long trips, and in fact, seldom anything longer than 50 miles. Driving style is not significantly changed within the last 5 years.

I'm not using any super-accurate mileage calculation, but having driven the car for 23 years & putting at least $20,000 worth of fuel through it, I know what "normal" mileage should be. Routinely, on a trip or longer commute (> 25 miles), it has gotten around 30-31 mpg. Around town driving is usually in the 27 mpg range.

The fuel capacity says it's 13.2 gallons, but when I fill up (from E on the fuel gauge), I have generally put between 11 - 11.5 gallons (when the sensor clicks off). When I fill up from the fuel gauge indicating a half-tank, it takes about 5.5-6 gallons max. Putting 5.5 - 6 gallons from E will put it at a half-tank. So, the fuel gauge seems to be generally accurate (or close enough).

Here's the consumption issue.

• The car sips gas throughout the first half tank after a fill up. Routinely, I get 180 miles out of the first 5.5 - 6 gallons, around 30-32 mpg. If I fill up again when the gauge shows a half tank, it takes between 5.5 - 6 gallons, and it will average around the same mpg. It can range from 27-31 mpg, usually toward the higher number.
• On the other half tank, I can get 100 miles from it, maybe up to about 125 miles, an average of 20-22 mpg. Filling to a half tank will repeat similar mileage, with a range from 18-22 mpg. Anything over that amount (between 1/2 & 3/4 tank) will bump the mpg up slightly.
• To a tank (between 11 - 11.5 gal), it will average around 24 mpg. Not sure how long I'd say it's been happening, but I do not put a ton of miles on it each year (approx 2000 in the last 12 months).

It's due to have the timing belt replaced after the first of the year, and I'd like to figure out what would cause the odd consumption pattern I've noticed. Some info:

• The car uses a little bit of oil (maybe 1/2 quart over 3000 miles) & has been changed at 3,000-3,500 mile intervals its entire life.
• It idles fine and runs good & it starts every time, and has never had any issue like this.
• There is only a tiny puff of smoke when it cranks first thing after sitting overnight - it does not do it otherwise.
• Plugs have been changed since last timing belt (at 180,000 miles), but not recently. Best guess is 40,000 miles +/- on them. I cannot recall when/if I have changed plug wires, but it cannot be more than one time.
• The thermostat works fine. All belts and hoses get changed at the same time the timing belt (60,000 mile intervals).
• No known electrical issues.
• I think the O2 sensor has been changed, but cannot remember at what mileage. Best guess is between 120,000 and 150,000 miles.
• Decent (Hankook) all season tires, with approximately 30,000 miles on them, no unusual wear on them, rotated regularly and uniformly inflated.
• Fuel tank has the locking door on it & it works fine.
• There is no known gas leak anywhere, and no smell of gas.
• There is little to no change in the driving pattern as a tank of gas is consumed. Most often driven in the 30-45 mph range.
• Having put virtually every mile on it, and filled it thousands of times, I know the "normal" use mileage has been between 350-385 miles on a full tank (11 - 11.5 gal), with approximately 165-190 miles being a half-tank.
• Even though I'm only approximating the mileage I get, I do notice that I travel considerably fewer miles once the tank is half-empty.

The mileage overall has changed. Historically, I would get around 375 miles on a full tank of gas. It's now approximately 100 miles less on the same amount of fuel. Up to the half-tank mark, the mileage (180 miles) would project out to that same 350-380 mile total. On the second half of the tank, it gets about 10 mpg less.

If I had a leak, it seems like that I would be experiencing a mileage drop on the first half-tank, or a relatively constant mileage drop overall. This seems to be just the opposite of that.

Any ideas as to what might be causing this unusual consumption?

 

[RollaLover96]

Just my two cents, I could be wrong but worth looking into.

Clogged/Dirty fuel injectors? It may run well at near full bc of the gas to air ratio in the tank but as pressure diminishes
(wt bad fuel injectors) so will fuel economy. Im going through this now also in a '96, it's confirmed it is my Fuel injec.

 

[94RollaDad]

Sounds like you may have over 100k on the "new" O2 sensor - I'd consider replacing that again as they usually last only about 100k and will affect fuel mileage when they get "lazy" (they rarely just die).

Another option to look into is the fuel pressure regulator. The diaphragm can become brittle with age and not let enough fuel back through to the tank, causing rich condition and negatively affect mileage.

A third option is the thermostat. A working thermostat will let the temp gauge gradually build up to middle of gauge, and once it's reached operating temp it will stay in that position no matter what. A stuck open thermostat on the other hand will cause the temp needle to drop back again at highway driving due to overcooling, and then typically come back up again during idle/city driving. This will also negatively affect mileage.

If your plugs are "standard" / copper they are overdue now and could also contribute negatively. If your spark plug wires, distributor cap and rotor are original those are probably also worn at this point and could cause mileage issues.

I'd normally advise to diagnose before replacing parts to not over spend. You can diagnose the thermostat using what I referenced above. For the rest of the parts you would probably get benefits from replacing all of them if they are are indeed original and simply consider it part of standard maintenance to replace them.... Suggested sequence:

- Start with plugs, cheap and easy. Only Denso or NGK.
- Distributor cap and rotor next - also pretty cheap. Go Denso or OEM.
- O2 sensor next - cheap but depending on how stuck it is might be slightly tricky to get off. Since it's already been replaced through it hopefully shouldn't be too hard.
- Spark plug wires and fuel pressure sensor are the most expensive and I'd save those for last and potentially not do them at all if the above parts take care of your issue.

 

[DannoXYZ]

Even though I'm only approximating the mileage I get, I do notice that I travel considerably fewer miles once the tank is half-empty.

I noticed that the 1/2-way mark changed on my car with age. Probably due to the float not moving as smoothly. Try only using full-tank mileage and fill-up amounts to calculate MPG on the next 5-tanks and see what the numbers say.

 

[DrZ]

The needle doesn't drop uniformly. Others have noticed this.

You really need to accurately calculate the fuel mileage, otherwise your mind plays games with you. Record the exact amount of gas you put in and your odometer reading at every fill up. Do that over 10 fill ups. Then calculate your total miles driven divided by total fuel added. Try to fill up at the same exact pump each times.

Repeat this twice. Ten times filling from half tank to full. Then ten times from E to half, except go to full on the first and last fill up using the same pump.

Just get a receipt showing gallons each time and jot down your odometer reading on the receipt. It only takes a few seconds.

 

[Hiro Protagonist]

I'd say the style of the tank has a big part to play in it. No Toyota that I've owned with an under-the-floor fuel tank (3 Corollas, 1 Celica, 1 Camry) has had a perfectly linear fuel gauge - F to 1/2 is ok, 1/2-1/4 speeds up, and the last quarter almost disappears before your eyes (in the second half of the tank you're lucky to get 50% of the range you got in the first half). Of the two cars with vertical (behind the back seat) tanks (1 Corolla, 1 Soarer), the Soarer had a digital gauge with an LCD readout that was accurate down to the LITRE, but the Corolla one had a dodgy sender that made anything below ~3/4 read bone-dry empty.

Under-the-floor tanks are usually weird flat lumpy shapes to fit around suspension/exhaust etc, whereas behind-the-seat tanks are more often uniform shapes. It's much easier for a gauge to be linear when the tank has a uniform horizontal cross-section top to bottom (and the taller the tank, the greater the resolution)

 

[Pete Suhman]

Yep, the E-to-1/2 and 1/2-to-F 'burn rate' discrepancies are due to the shape of the tank.
If the tank holds 13 gallons, then I'll bet that if you pour 6.5 gallons into an empty tank, that the gasoline's surface will not be exactly at the halfway point, as measured from the top to the bottom. And the float for the sender might stay at the top of its range of movement until after a gallon or two have been consumed.

Maybe old gas is causing the actual MPG decrease?

 

[Hiro Protagonist]

Yep, the E-to-1/3 and 1/2-to-F 'burn rate' discrepancies are due to the shape of the tank.
If the tank holds 13 gallons, then I'll be that if you pour 6.5 gallons into an empty tank, that the gasoline's surface will not be exactly at the halfway point, as measured from the top to the bottom. And the float for the sender might stay at the top of its range of movement until after a gallon or two have been consumed.

Also, E on the gauge doesn't mean the tank itself is physically empty, nor does it mean that the level is below the pickup. I regularly take a tank below E well in to the warning light zone, but that's because I know my car and I know how far it will go once the light comes on solid (not counting the occasional flash on/off when it starts to get low around corners).

 

[94RollaDad]

I think you guys might be underestimating OPs ability to judge the issue though? He's the original car owner! Which means he's had it for over 20 years! If he's doing some approximation on the MPG calculation I'd still trust he knows his car well enough after 20 years of driving it that he can pick up on when mileage starts to drop. It also sounds like he actually does pay attention to gallons filled vs miles driven, not just relying on gauge reading.

Obviously doing a proper MPG calculation will further bring light on the issue. I'd usually be much more inclined to agree with you he should start looking at that first (I have given that same advice in a number of other threads) but this seems a bit different.

 

[Marshall2u]

I'm not 100% sure I understand the OP's issue. First 5.5 gallons? Second 6 gallons? I'm not sure what all that was about. If it's just that his mileage had gone down lately, overall, it might be because he's only driving the car 2000 miles per year (stale fuel, short trips, blah, blah, blah.). The whole guesstimating mileage based on the gauge being at the halfway point is pretty useless. You can't use that anywhere near accurately. You can only use miles traveled vs. gallons burned, regardless of gauge reading. The gauge isn't even needed to calculate mileage, just to keep those who don't from running out of petrol. As far as the gauge accuracy? As others have stated, the first half of the tank shows phenomenal mileage, but the second half, especially the last 1/4, shows horrible. Not lineal at all! Do I love that? No! But it's something I/we quickly become used to, and easily deal with.

 

[Apples555]

Use a service like Fuelly to accurately calculate mpg. It will confirm the issue is fuel consumption.

From what I read, it sounds like mpg is acceptable to OP until the tank is halfway empty.


Sent from AutoGuide.com Free App

 

[steve_oToy]

I think you guys might be underestimating OPs ability to judge the issue though? He's the original car owner! Which means he's had it for over 20 years! If he's doing some approximation on the MPG calculation I'd still trust he knows his car well enough after 20 years of driving it that he can pick up on when mileage starts to drop. It also sounds like he actually does pay attention to gallons filled vs miles driven, not just relying on gauge reading.

That was one of my points, 94RollaDad. Having driven this car the equivalent of almost 10 times the circumference of the planet & putting at least 8,000 gallons of gas through it, I can estimate it well enough (without tracking it) to be able to detect a mileage drop. Even by an approximate estimate, I'm not that far off.

I can count on 1 hand the number of times that I've put in 12 gallons since I've owned it. I also know when the fuel gauge is on "E" (aka warning light zone, as Hiro Protagonist said) how many more miles I can go (before I think I'll be walking) - and that works out to 11 gallons or a little more on a fill up.

I used to get between 350-375 miles to those 11-11.5 gal & now get 275 to the same. I know even if I don't fill it up approximately how far that'll take me, which upon a fill up afterward is going to come out to a little over 11 gallons.

It's just that once the gauge crosses 1/2, my mpg is going to be considerably lower than what I had up to that point.

Obviously doing a proper MPG calculation will further bring light on the issue. I'd usually be much more inclined to agree with you he should start looking at that first (I have given that same advice in a number of other threads) but this seems a bit different.

I had thought about plugs, and dirty fuel injectors, and the O2 sensor, but hadn't considered some of the other suggestions.

Question on the fuel pressure regulator - would that affect the entire tank, or would it vary/change once there was more air in the tank than gas?

I know the t-stat is working. Heat works good, rises to the middle & stays there. It (along with all the hoses) get changed along with belts (and timing belt) every 60,000 miles.

I'll probably add the O2 sensor to the list when I have the other work done & save myself a considerable amount of swearing (if memory serves).


As far as the dirty fuel injectors, what method(s) seem to work the best? If it's an additive, I'll try that first. If not, any recommendations/advice is appreciated.

Many thanks to anyone who responded!!

 

[94RollaDad]

Question on the fuel pressure regulator - would that affect the entire tank, or would it vary/change once there was more air in the tank than gas?

It would affect the entire tank. The fuel pressure regulator can be a bit hard to understand, but think of it this way: It's a valve blocking fuel from floating back into the tank through a return line. What happens when you have a fuel pump on one end of the line pushing hard to pump fuel into the line, and a fuel pressure regulator on the other end of the line refusing to let any of it circulate back to the tank? The fuel pressure builds up! What happens when the injectors then open up? Fuel gushes into them to relieve the pressure.

So far, so good. Now, the function of the fuel pressure regulator is to ease that pressure to allow a more appropriate supply of fuel to the injectors in any throttle condition. Which means to ease the pressure at situations less-than-wide-open-throttle, particularly in idle situations. That saves fuel, and also ensures a better fuel-to-air ratio. How does the fuel pressure regulator accomplish that? By being connected to a vacuum hose. Vacuum in the manifold is highest at - you guessed it - idle, and lowest at - you guessed that one too - wide open throttle. So the vacuum will "suck" the diaphragm more at idle, opening the regulator and letting more fuel back into the tank, while closing gradually as throttle increases and vacuum decreases.

When the diaphragm gets old and brittle two things can happen: You get a diaphragm leak (sucking fuel into the vacuum line) or the diaphragm simply hardens up, and it's not flexible enough to open properly at high vacuum. The former (leak) seems to be very common on old American cars. The latter seems to happen more to Toyotas (I'm generalizing here...). You could be in that situation which could explain low MPG.

I'll probably add the O2 sensor to the list when I have the other work done & save myself a considerable amount of swearing (if memory serves).

Get Denso brand or NTK (owned by NGK).

As far as the dirty fuel injectors, what method(s) seem to work the best? If it's an additive, I'll try that first. If not, any recommendations/advice is appreciated.

There are so many opinions and approaches. The best is going to be removing the injectors and shipping them to a company that does proper flow testing and ultrasonic cleaning and sends them back with new gaskets/o-rings. Second best would be a device that hooks up to the fuel line and gives the injectors a direct boost of cleaner mixed with gasoline.

Short of that I like the 3M 3-in-one kit that has tank additive, injector cleaner and throttle body cleaner. The additive isn't the strongest but the injector cleaner (applied through a tiny tube through the throttle body) is very decent.

For tank additive my personal preference is going to be 3M Max Strength Fuel System Cleaner (not any of the other 3M additives - they are weaker) or Red Line SI-1. You'll hear all sorts of opinions from the group here but those two are very good and contain a high concentrate of PEA - which is the good stuff you want.

 

[Pete Suhman]

diaphragm simply hardens up, and it's not flexible enough to open properly at high vacuum.

I've never heard of this phenomenon, I'm guessing that it occurs more on cars which have sat unused for long periods?

the injector cleaner (applied through a tiny tube through the throttle body) is very decent.

Does it just clean the tips? I guess that the tank additive cleans the injectors from the inside?

 

[94RollaDad]

I've never heard of this phenomenon, I'm guessing that it occurs more on cars which have sat unused for long periods?

Yeah, it was DannoXYZ who brought it up here a while back, noting how he'd improved his MPG by replacing his FPR and that the old one was brittle. I replaced mine too "just because it's old" (in the face of most of my advice to diagnose before throwing parts at an issue) - haven't noticed crazy changes so I assume the old one was in fair condition.

Does it just clean the tips? I guess that the tank additive cleans the injectors from the inside?

Yep, just the tips.... you need in-tank additive or those kits that attach to the fuel rail to clean the inside. The additional stated benefit of that cleaner-through-tube is that it also cleans the combustion chamber better than an in-tank additive would do. I actually DID see an MPG increase the first time I used this kit.

 

[steve_oToy]

It would affect the entire tank. Vacuum in the manifold is highest at - you guessed it - idle, and lowest at - you guessed that one too - wide open throttle. So the vacuum will "suck" the diaphragm more at idle, opening the regulator and letting more fuel back into the tank, while closing gradually as throttle increases and vacuum decreases.

When the diaphragm gets old and brittle two things can happen: You get a diaphragm leak (sucking fuel into the vacuum line) or the diaphragm simply hardens up, and it's not flexible enough to open properly at high vacuum. The former (leak) seems to be very common on old American cars. The latter seems to happen more to Toyotas (I'm generalizing here...). You could be in that situation which could explain low MPG.

Would the latter (from above) also produce a higher idle speed? Best I can recall, that's something I've never found necessary to adjust on this car. The idle on it has always been just under 900 RPM.

Also, wouldn't that also affect the mileage more uniformly from start to finish than what I've noted (good mileage up to a point & then plummeting)?

Get Denso brand or NTK (owned by NGK).

Will do.

Short of that I like the 3M 3-in-one kit that has tank additive, injector cleaner and throttle body cleaner. The additive isn't the strongest but the injector cleaner (applied through a tiny tube through the throttle body) is very decent.

Thank you for the recommendations. I'll give those a try.

BTW- it just dawned on me that cumulatively I've spent the better part of a week just pumping gas into this car.:wink:

 

[94RollaDad]

Would the latter (from above) also produce a higher idle speed? Best I can recall, that's something I've never found necessary to adjust on this car. The idle on it has always been just under 900 RPM.

Also, wouldn't that also affect the mileage more uniformly from start to finish than what I've noted (good mileage up to a point & then plummeting)?

I wonder if you actually have two things going on and that everyone who chimed in here has a bit of the answer....

1. I absolutely do think you have an MPG issue.... and you should try some of the recommendations I've laid out.
2. I also think your gauge reading probably has changed from what it used to read, to the point others here have made.

Here's my theory: The combination of 1 and 2 above makes you believe that MPG is good the first half of the tank and bad the second half. When in reality it's "always bad" just the gauge tricks you to believe the car changes behavior.

Take Apple555's recommendation and do a few proper MPG calculations using an app (or just do it on a piece of paper). That will tell us if my theory is correct or if your car truly changes behavior halfway through a tank...

 

[jamesburton-ii]

I wonder if you actually have two things going on and that everyone who chimed in here has a bit of the answer....

1. I absolutely do think you have an MPG issue.... and you should try some of the recommendations I've laid out.
2. I also think your gauge reading probably has changed from what it used to read, to the point others here have made.

Here's my theory: The combination of 1 and 2 above makes you believe that MPG is good the first half of the tank and bad the second half. When in reality it's "always bad" just the gauge tricks you to believe the car changes behavior.

Good point 94. My 93 Corolla seems to delight in bringing up 2 or 3 issues at one time that would all appear to be related!