SCCAIT7

I need some help... Yesterday I went through 3/4 of a tank of gas in 100 miles.... This is my commuter car right now and it is killing me on gas!! I am not turning more than 5000RPM simply to conserve gas... Any ideas why it would be so bad??? Can't be the AC could it? YIKES! Its like 100 degrees!!

BinaryRotary

Fpr.

Aaron Cake

iTrader: (1)

First, read the mileage topics linked in the FAQ. 2nd, do a full and complete tuneup. 3rd, start looking for actual problems.

BinaryRotary

Doesnt sound like its been happening for long, I doubt its a 'tune up' problem.

Aaron Cake

iTrader: (1)

The TPS, ECT, plug wires, plugs, etc. can fail at any time and cause poor mileage. A tuneup is the first step in troubleshooting stuff like this. And if the car is like most here, it needs one anyway.

SpooledupRacing

oxygen sensor

Dave

Aaron Cake

iTrader: (1)

Oxygen sensor has virtually no effect on fuel economy, ironically enough.

ryderX

iTrader: (1)

yeah i went 500 miles with the wire for the o2 sensor disconected, and got the same gas milage when i reconected it............

Sideways7

iTrader: (7)

Did you actually fill up the tank, go 100 miles, then have to add 12 gallons to fill up the tank? If not, then you didn't use 3/4 of a tank. The stock gauge, at least in my experience, can be very innacurate. I have no doubt that you are getting bad milage, but I doubt its 8 or 9 mpg like you said (thats what 3/4 tank in 100 miles works out to).
Also, there's no need to take it past about 3k-3500 in daily driving, except maybe to get on the highway.

ericgrau

iTrader: (1)

+1 full tune-up then mpg FAQ, then ask again.

Also keep under 3800rpm during normal driving.

Easy mpg check. Fill gas to full. Set trip odometer. Drive. Fill gas to full (record this number). miles / gallons = mpg.

O2 sensor can still cost 1-2mpg, and a bad O2 sensor can be worse than no O2 sensor, I think. But mainly do the full tuneup, change or disconnect O2 sensor (?), change the TPS (?). Check for a fuel smell and/or leaky fuel pulsation dampener while you're at it (above the exhaust manifold, below some other junk), and make sure you have an OEM thermostat. These 2 things have nothing to do with mpg, but as long as you're doing a tune-up you might as well save your car from a possible engine fire or overheat.

scrip7

iTrader: (6)

If it were in my shop, I would use a lab scope to see what the injector duty cycle was reading under varying rpms. This would tell me if the ecu is indeed sending a signal to inject lots of fuel when it isn't needed possibly due to an incorrect input signal from the airflow meter or pressure sensor. If the duty cycle is within range, then I would suspect a leaky fuel pressure regulator (common problem with S4s) or perhaps a restricted / kinked fuel return line to the tank (rare). The oxygen sensor can affect fuel mileage, but not more than 1 or so mpg.

SCCAIT7

Changed the plugs and air filter tonight... The plugs were TOAST... WHITE/TAN tip......seems to be getting a good burn....however it would seems like it would not be burning all the food...electrode was almost non-existant... best case the damn thing was not burning all the fuel going in....

Waiting on plug wires, O2 and fuel filter...

I am doubting a TPS simply because the car runs well...h

Lab scope? Damn... you must have some money...

Mason Rx7

Take off the radiator cap... Slide 7 back... Slide Honda in... Replace radiator cap...

Problem solved!

Check your timing. I was getting 11mpg no matter how I drove. After I reset the timing I jumped up to 18/24.

Wht90Rx7Conv

One of the first things I would do is timing then a fuel pressure test. No money spent for good answers.

scrip7

iTrader: (6)

If the spark plug insulators are white, that indicates lean fuel mixtures....it should be a medium to dark tan color when the mixture is right...and if it were dumping too much fuel you'd see a black coating on the insulator....so I'm wondering if it really is using too much gas or if your gas gauge / sending unit is on the fritz. And no I'm not wealthy just yet....30 years of car repair for a living has let me collect lots of equipment instead lol.

Aaron Cake

iTrader: (1)

Everyone should have a scope. I have 2. Old test equipment is cheap on eBay.

As for timing, I wouldn't immediately assume timing since it doesn't move unless someone has messed with the CAS.

bigdaddyp

mine is horrible on gas too. It runs really rich, it is all the time popping through the exhaust where it is running so rich. It smokes black some and smells like gas really badly.

NZConvertible

If you're implying that not using the secondary injectors saves fuel, you're mostly wrong. The total amount of fuel injected depends almost entirely on engine load, not how many injectors are firing.

No it can't. 1-2mpg is a huge percentage change (6-11% if you normally get 18mpg), and the O2 sensor simply doesn't have that much effect on average fuel consumption. In most non-highway driving it's not even being used.

Having said that, it's worth replacing the O2 sensor anyway, since an original sensor would've stopped sending useful signals a long time ago and they're cheap to replace. You might get up to 5% better mileage if you do a decent amount of driving at cruise conditions.

In theory this might work, but unless you have an intimate knowledge of exactly what duty cycle you should see under what conditions, the numbers won't tell you anything. I have an injector duty cycle meter in my car and I can tell you the readings change vary rapidly and over a wide range even during normal driving. It would be very hard to pick out a trend that showed you where averaging 5-10% more fuel than normal.

Somthing nobody's mentioned is checking each of the sensors that determine fuelling as per the FSM and also checking their voltages at the ECU. Instructions for doing this are in the FSM. Check the AFM, MAP sensor and coolant thermosensor.