Ian.g

Have an 85, completely stock, except for straight pipes, no check engine light on, No visable fuel leak, no gasoline smell. Average 5 miles per gallon. At highway speed, the slightest touch of the gas pedal drops the instant gas mileage down to 6mpg just rolling with the clutch down it hovers around 12-15 mpg. In Manhattan traffic around 3-4 mpg. The car has a new, IAC, air meter, spark plugs, wires, fuel filter, fuel pump, Tps. The cluster isn't off its a rebuild that was just put in and the car went through a 1/4 tank in 10 mIles. Please any advise is welcome

Tom400CFI

Your data is junk. How are you acquiring these numbers?

5 mpg x 10 miles = 2 gallons burned...not a 1/4 tank as you say, which would be 5 gallons (more, going by the gauge).


I don't believe that it's possible for the car to consume gasoline at the rate you claim...and still run "fine". At that rate, it would be barely running and chucking black smoke out the exhaust.

I'd suggest that you have the display set to metric and you're reading km/liter, which would be way closer to accurate than the mpg data that you're claiming. You need to start be reexamining your data and how you acquired it.

gerardvg

Hi
Does it idle and run ok?
You should get around 400 miles on highway use, I would look at the fuel pressure does it drop straight away when engine is turned off? it should stay at 35 psi for a minute or more...
Check your sump ... any fuel smell in the oil? if the fuel pressure diaphragm tears it will go into the engine via the vacuum lines and fill the sump.
Check the timing, check throttle position sensor is at 0.54vdc with ignition on.
Also check the oxygen sensor... Running retarded ignition will need more pedal travel to maintain speed, also check your torque converter locks up
(going uphill at steady speed the rpm should no go up)

In Oz we drive at 100 /110 kmh on highways, with 3.07 rear gears that is 1800rpm at 100kmh (62mph)

My instant fuel consumption will hover at 7 to 9 liters per 100km
on flat highway, will see 40 or 50 L/100 km taking off from standing start then settles.
My average is usually around 10L/100km or 25 mpg,

Check your gas tank hoses are not leaking, the fuel has to be going somewhere?
Time to check a lot of things ... good luck

billschroeder5842

Don't rely on your gas gauge.

Fill your tank. Drive for about 50 miles then refill. Divide your total miles driven by the amount of gas you added. That will give you an idea on your MPG.

Sounds like your gas pump needs to be pulled and the sending unit rebuilt.

chevyman_ky

I have a 85 when dash says 150 or less miles to go on fuel it drops a lot more than the miles i drive will drop 30-50 miles in driving 20-30 doesn't do it when have more gas in it

L.V. Vette

since you didn't mention your car not running well, this would be the first place to start

Ian.g

Today, ran it till the reserve light came on, put 6.5 gallons in, drove it 38 miles till the reserve light came on again. thats 5.8mpg, the sender works and the gauge isnt off. no black smoke, no signs of a leak so far, O2 sensor maybe?

also maybe this is unrelated but went the gas cap is opened, it feels pressurized and hissing can be heard. something must be clogged.

and to tom, there's no need to be rude. Did that 10 year member status go to your head? Maybe in Utah you don't have traffic. Here in new york city there are 8 million people, you can sit in traffic for an hour idling, and when you do move you're going 10mph. its ridiculously low but completely possible. my data is a rough estimate, but you don't need scientific method to tell something is really off. 5 mpg is the average shown on the cluster, so yea 1/4 tank in 10 miles, heavy traffic.

Tom400CFI

I wasn't being rude. Direct? Sure. That is the fastest way to get to the fats...which help to diagnose. Do you want help? Or not? _Some people just like to post to "vent" and don't really want/need help. If you want help, we need to have accurate data...not made up bull ****. Can you agree with that? If not, and "made up data" is good enough, then it is going to be very difficult to diagnose an problem...or if there even IS a problem.

That test doesn't work. You need to do it they way that Billschroeder5842 recommended:
Fill your tank. Drive for about 50 miles then refill. Divide your total miles driven by the amount of gas you added
Why? The gas gauge is most any car is no where near accurate enough to asses consumption....on an '80's GM product, the gauge is even less accurate. On the C4 the gauge is totally inaccurate on the bottom end of the scale, and so is the reserve light. Your reserve light can come on when you have 5 gallons left...or when you have 1/2 gallon left, so trying to assess your consumption rate using that method...I'm sorry but it just won't work. Ya need to FILL the tank, and start from there; a known quantity. When you FILL the tank after driving, that's as close as you can get to accuracy.


Unrelated. Don't worry about that -it's normal.



I agree...and here is one example why facts matter. In your first post, you said:
At highway speed, the slightest touch of the gas pedal drops the instant gas mileage down to 6mpg just rolling with the clutch down it hovers around 12-15 mpg.So which are we talking about here? Highway speeds? The numbers you produced would be totally unacceptable -something would have to be wrong with the car...or your data. Or are we talking sitting idling in traffic, which depending on the traffic and the time spent idling, could be normal. So trying to assess mileage when sitting in traffic is pretty futile. A better diagnostic measure for that scenario would be gallons/hour.



You certainly DO need to be "scientific" to diagnose situations like this. Absolutely. Here again, we have an example of why facts matter:
The car's gas tank is 20 gallons. A 1/4 of 20 is 5 gallons. So to burn a "1/4 tank in 10 miles" means that you'd have burned 5 gallons in 10 miles...getting 2 mpg. You said you got 5 mpg, which is 150% difference. So you see what's going on there? Your numbers don't work. Something is wrong with what you're telling us and therefor, nobody can determine if there is actually anything wrong with your car. We can not help or diagnose your situation when the data that you provide can't possibly be real.


SO...back to what I originally said;
I don't believe that it's possible for the car to consume gasoline at the rate you claim...and still run "fine". At that rate, it would be barely running and chucking black smoke out the exhaust.
Based on what you've provided for information so far, I don't think that there is anything wrong with your car. I think there is something (several things) wrong with your data and the way that you're trying to assess it. I think that you need to take Billschroeder's advice, go fill the tank, highway drive, fill and hand calc the result. You may not like what I'm saying...but 25 years of working on and owning these things...I know what I'm telling you.

biggiefl

Fake News


I had a 71 Nova with a radical 355 in it. Thing would practically drip gas out the pipes it was so thirsty. It still got 8mpg but was double the hp you got. Something is goofy alright.

Ian.g

the data isn't made up. the tank was essentially empty. i put 6.5 gallons and drove it for 38 miles till it was empty, not essentially empty. if the fuel gauge was way off, I would know. no matter how much was already in the tank its clear there is a problem. so your telling me that if its getting 9 or 7.2 miles per gallon as opposed to 5.8 or 4 miles per gallon that completely changes the problem or the steps to repair it. That's ridiculous.
.
i said "at highway speed". 55mph, tap the gas and the instant gas mileage drops to about 6mpg

and i said it averaged 5mpg so for that to be an average sometimes it would have to be less than 5, like say 2. it seems pretty real to me.

i don't like your attitude so i'll pass on that help. there's tons of other people.

856SPEED

Need to fill the tank....drive it about 100 miles or so in traffic and highway combined. Fill it back up and see how much fuel it consumed. Divide the total miles driven by the gallons consumed. Don’t top it off either time on fill up. Otherwise you have NO idea.

FWIW; I average 12 to 16 MPG on my 85....but I run 3.73 gears and run 12’s in the quarter on street tires. And it runs great!

c4cruiser

To really test for a MPG figure, you need to fill the tank, not just put in a few gallons. With the tank full, drive the car for some distance such as 75 to 100 miles. Then fill the tank again. At that point, manually compute the gas mileage by dividing the miles driven by the amount of gas needed to fill the tank again. City driving with lots of traffic lights will result in lower MPG. Getting out on a freeway will increase the MPG. If everything is in good shape, you should see a good 25 MPG or so.

Doing the calculation this way takes the computer-calculated data out of the problem. The answer to that math step is the "real" MPG figure.

Something you didn't mention was the configuration of the car. Automatic trans or manual? Gear ratio? Taking the car out on a freeway, drive at 55-60 MPH; whatr's the RPM at that speed?

Does the fuel gauge appear to be showing an accurate level? If the fuel level sender is not working right, that may be why you are seeing incorrect readouts on the display. And it's also possible that even if the display had been rebuilt, the computer may not be working right in displaying values.

ddahlgren

You have to fill the tank until the pump stops it from filling drive 50 to 100 miles and fill it again until the pump stops it. Small samples are not your friend. Do highway first. Easiest way in LI is get on 495 drive to the east end lunch in Greenport drive back to the station and fill again. Forget the reserve light setting etc. you have no way to prove it trips on or off at the exact same place every time. The pump shut off is not perfect either but the distance traveled greater with no real traffic if you pick the right part of the day.

biggiefl

C'MON I used a smiley
Not only what he said above but many of the older systems would compute MPG via vacuum even if fuel injected. If you have a vac leak, not gonna read correctly. Newer cars use injector pulses to calculate mpg, much metter but still not as accurate as filling the tank and computing it manually.

383vett

I'd kill for 5 miles a gallon. I'm more like 8 passes for 5 gallons.

ddahlgren

I like my 91 the best when getting 3.5 miles per gallon LOL. It means my foot is in it burning up some pavement.

Bfenty

I'm going to pile on here and agree-you need to fill the tank until it clicks-don't top off at all-then drive 50 miles and fill up again (preferably at the same pump for best accuracy) and manually compute your mileage.

Here's the reason what you did doesn't work:
if your fuel sending unit isn't working (which I highly suspect it is malfunctioning) then your gas gauge isn't correct, so you can't believe it when it says the tank is 'empty'. It could be 3/4 full still! By manually calculating and relying on the gas station's cutoff to tell you when the tank is full, you're eliminating a bad sending unit from giving you bad data.

It would also be very helpful to know things like the RPMs you're running at what MPH and in what gear-things like that will tell us a lot about how your engine is running. For instance, if you're running 2k RPMs at 70mph, that's pretty typical in my experience for 4O. I don't believe you've mentioned how the car 'feels' to drive, either-for instance, does it hesitate? Feel like it has no power? Or does it try to break the rear wheels free at the slightest touch? Stuff like that could indicate a problem or modification that's causing the behavior you're seeing. I know you said it's stock, but HOW do you know? A car I thought was stock when I bought it turned out to have all sorts of mods done to it by someone else before me.

Just some thoughts. Please don't be combative, people here are trying to help.

Ian.g

The car is bone stock, except for straight pipes. i put the car together myself. the engine is from an identical 85 corvette, the rear axle is from an 86 4+3. its 4+3 manual. the gauge appears to work fine.the car shuts off at empty and the guage reads full when the gas pump cuts off, it doesn't jump around or anything. the car has the normal power you'd expect and doesn't hesitate. ill fill the take reset the info and drive it tonight.

Tom400CFI

If the gauge is inaccurate (and it is) HOW DO YOU KNOW? Ya don't.



HOW? Are you "sticking" your gas tank?



Are you here for help? Or to argue with people who know what they're talking about? I literally showed you with some basic math, how the numbers that you posted contradict each other (5mpg ≠ "1/4 tank in 10 miles")....which immediately throws up a red flag.



You know....at least 6 people (who know what they're talking about) have told you that you need to fill your tank/drive/fill/calc. Same thing I said, and they're saying it for the same reason; that your data is junk. You going to argue with them too? Best of luck! This should be interesting....



.

Ian.g

Don't have to "stick the tank". Pump at the station showed 6.5 gallons, I drove it tI'll the reserve light came on then I put the car on a lift and ran it until it shut off.

I doubt the gauge is off because the cluster is new and the sender is new. The car runs out of gas at empty and reads full when the gas pump cuts off.

What don't you understand? AVERAGE of 5mpg. You say 1/4 tank in 10 miles is about 2 mpg. So it got 2 miles per gallon and then later it got 8. Average it together. You've been doing this how long and you don't understand its not going to be exactly on the average all the time?

what do you care what red flags come up? It's none of your business. You don't like my description, then go play village elder someplace else. I didn't come to you to ask for your help, Get off my thread.

6 people came to me nicely and gave me the advice. Even though I don't think it's needed because it seems more geared towards verifing the problem which I already know is there, I'm going to do it anyhow. Just because they seem genuinely helpful.