I have recently acquired a '99 FRC. After remedying a clutch issue, I think my car may have a fuel leak. I average 27 mpg (I have a highway commute) according to the average the car calculates, yet I only got 21 when I filled up today. I'm barely able to get 350 miles per tank, every time I have filled up. The car cranks and runs fine, yet everyday after work/ before work, the gas gauge is a decent bit lower than where I left it.

Is this a common problem? Perhaps the average the car calculates is not accurate? I reset it along with the trip odometer. Anyway, I searched and couldn't find anything.

Thanks for taking the time to read.

Blaine

 

...might want to replace your fuel filter.... sometimes the
little o-rings will cause a leak. ...easy job if you have a lift.

 

Well lets hope it's not too much more difficult with jack stands...

After more Googling, I ran across some people having issues where the entire tank needed replaced (at a hefty price). I just hope it isn't that.

 

First off don't go by the DIC, I don't think I get 350 to the tank, Most of my driving is town, I would think if you have a leak you would smell gas and see spots under the car.
I don't think your mileage is that far off.

 

I thought that too about the DIC, but my commute is 20 miles of perfectly flat, 60 mph driving all the way. My truck got 21 mpg on that commute, and I know this car should do better than that. And with all the other factors like the gas gauge dropping, I am suspicious.

I appreciate the reply though. Can anyone comment on their mileage/miles per tank so that I can have a reference?

Thanks for being so helpful folks!

 

You could go old school. Note your mileage, fill up until the pump clicks off, note gallons pumped, drive, then repeat at the same pump. Do that 3 times, it's not perfect but will give you a fair estimate.

 

That's what I did to get the 21 mpg average. I just cant see the DIC being THAT wrong. Plus it still doesn't make sense that the gas gauge drops after it sits a while.

As a side note, I just went out and crawled around it a bit. There is a very slight gas smell on the drivers side just behind the door.

 

Sounds like you need to google c5 fuel leak. There are lots of posts on that subject. My 03z has a mild cam and gets approximately 25mpg on flat terrain per DIC. I never smell fuel though.

 

Just so you know the DIC can be manipulated, I can get my to read 50 if I try.. lol

You might do a search on the cross over pipe.

 

Oh I know that is true, I can get 99.9 for a bit. LOL

I suppose from watching the "instantaneous" it looks to be about right for 27ish, plus, my experience from other GM cars is that the DIC is fairly accurate as long as it gets reset with each fill up. I really don't mind if the car only does get 21. However, premium's expensive. I'd rather use it doing donuts than it making the pavement wet.

 

I'm getting 18 mpg........tremendous considering my lead foot.

 

If you have a fuel leak bad enough to affect the MPG, I would think you would find a wet spot somewhere on the ground after sitting awhile. A couple of drips of fuel on the ground will leave a quarter sized spot. You might put something under your car to help identify spots. I have masonite under my car for that purpose. It is coated white so I can wipe it off once-in-awhile. You will get water spots from normal stuff like rain or AC condensor drainage. You could temporarily put cardboard down. Make sure it lays flat so it doesn't contact anything hot when you park over it. (Get a large appliance box from a store or neighbor.)

You say your gas gauge drops after sitting. Does it seem to drop with a full tank or partial? The reason I ask is the car has two tanks and as I understand it, when the car sits, the fuel will level out between the two which could then affect the initial gage reading. (someone correct me if I'm worng) Once the car starts and fuel pump is working, the left tank will become more full and the computer will read the two sending units and display the calculated fuel level.

Look at your fuel gage when you start it and then after driving a couple of minutes. The fuel you "lost" might come back after the car has been running and the computer reads the fuel senders again.

The left tank has the fuel pump and the right tank has a venturi pump. The right tank will be consumed first. The smell of fuel behind the driver's door doesn't suprise me as that is below the fuel filler door. You might notice a stronger smell of fuel after filling up if the pump overflows the tank even a little.

I'd start with tracking your MPG manually and putting something under the car. If you see a deterioration without cause, then you know you have something going on. The other thing is these cars are known to have issues with the sending units. You might try some Techron to see if your gages start reacting more to your expectations.

 

It does this with both a full tank and one that isn't full. And after the few minutes of driving, sometimes it doesn't return to where it was, maybe a needle-width lower, yet I never see any leaking gas anywhere which is puzzling. Perhaps it leaks when the gas is being transferred between the tanks while and the gauge is getting back to the correct reading?

I actually did a quick test today. The DIC and my calculated mileage from yesterday to today was off by .5 mpg, not bad off, it could be that the tank didn't fill as much as yesterday, I only had 40 miles on it.

I will continue to track my mileage, filling up at a quarter tank some times, empty others, half tank, etc. I think that will help me to see if the .5 mpg difference at 40 miles will extrapolate to 5 mpg at 350 miles.

Thanks again for all the replies, here's to hoping it's just the DIC not reading correctly.

 

Well, I believe I may have narrowed it down a bit more. I have found out that the DIC is accurate over short trips (60 miles or less). Today, I took a longer drive. At one point about 15 minutes into the drive (about 70 or 75 miles on the tank of gas), I watched my fuel gauge drop almost 1/4 tank. It took about 30 seconds. Unfortunately, I was on a bridge (literally just got on the sixth longest bridge in the states, the Jubilee Parkway), and unable to stop and look under the car.

I'm starting to think that the crossover between the left and right tanks is leaking profusely when the car pumps gas from the right tank to the left.

Has this happened to anyone else? Does this sound plausible? I know that the crossovers have leaking problems historically, but would it only leak when the car is transferring gas?

 

fuel gages aren't the highest quality in these cars. first there is the sulfur problem, second, they even when working right aren't as gradual as a honda for example. they are however better than the c4 so that's an improvement

if you have a fuel leak you should smell it or see it when the lines are under pressure

350 a tank all highway sounds very normal to me

 

I guess what puzzles me is that I actually DO get around 27 mpg, both by DIC readout and by hand, on short 60 miles or less trips. After I saw the gas gauge drop today, the DIC still reads 27 mpg (it wouldn't know if the tank was leaking) but my hand calculation comes out to less than 20 mpg.

EDIT: By short trips, I mean that I am taking the same daily commute, just filling up after one full day's commute.

 

at 20mpg and 350 miles a tank you would need 17.5 gallons to fill which is running on fumes.

are you really putting that much in? kind of dangerous to run that low. my average fill is 13-14 gallons and 250 miles per tank making 17-18mpg avg. lot of mine is in town though

 

I put in around 16 gallons. It has just worked out that way the last few fill ups. I'll leave in the morning with a quarter tank, get back home it will be on reserve. All in a 20 mile one way commute.

 

If you had a leak you would smell it. If your that worried about mpg you should trade it in for a prius.

 

I also think that if you had a leak of this magnitude you would definitely smell it.
I'm thinking you've got the fuel sending unit sulfur issue. Try some Techron and see what happens.