Bad gas mileage (Page 2/2)
theogre JUN 04, 01:34 PM

quote
Originally posted by Gall757:
Now that you have an SES light that works, check for codes to fix the mileage problem.

Big issue is MPG problems often don't set ECM Codes.

You can replace a temp or O2 sensor but still have iffy wiring will still have problems and often Not getting a code.
You test a Temp sensor w/ Ω meter and see right value(s) but have same problems and Not getting a code. (See my Cave, Sensors Quick Ref)
Wiring maybe good but can have iffy ECM causing bogus data and often Not getting a code.
You need an ECM scan tool to see if ECM sees close to the temps as Ω number in FSM, cave, etc.

Even then O2 values on scan mean nothing because how ECM uses O2 data that change in milliseconds. IOW Too fast "data" for even of ECM to stream info for scan tools. Only helps if ECM get bogus O2 and can't get or stay in Close Loop.
"Bogus" O2 means: (Assuming they work.)
Sensor too cold to work. Like just starting the car and just too cold but can cause by other things.
Sensor w/ bad wires. "1 wire" sensor is BS... All of them has a "ground" GM often use Tan wire to engine block.
ECM have problems.
Exhaust cracks leaking air upstream of O2 causing problems.
So O2 "volts" etc mean little or nothing but Close Loop = Yes/True is important on ECM scanners, more so w/ engine at normal operating temps. (More engine will go Close Loop as soon as O2 is hot regardless of engine temp. That happens in < 1 to 5 minutes on most OE exhaust setups.)

Again...
Top post saying X miles on a "tank" means little when doesn't say actual amount of gas used.
86 OM spec is 10.3 gal but owners can't use that for even new Fiero.
Should never allow a car to run out of gas to start w/ because that will kill the pump.
So most see empty on dash gauge and use ~ 9.x gal at best to fill up.

Then add above problems and many only get ~ 7 to 8 gallons per fill from "empty."
That's assume the Fuel Gauge doesn't have problems but many Fiero and other old cars have gauge lying for several reasons on top of bad tank sender.

[This message has been edited by theogre (edited 06-04-2020).]

Gall757 JUN 04, 02:11 PM

quote
Originally posted by theogre:

Big issue is MPG problems often don't set ECM Codes.




I thought we might get lucky.....

CTS and O2 sensor are often the issue, and cheaper to replace than finding a scanner.
M.aaron80 JUN 04, 02:18 PM
Will a bad egr Solenoid cause terrible gas mileage?
ltlfrari JUN 04, 02:54 PM
Vacuum leak?

------------------
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Dave

www.ltlfrari.com

theogre JUN 04, 03:04 PM

quote
Originally posted by M.aaron80:
Will a bad egr Solenoid cause terrible gas mileage?

Might but often will cause other problems And often set a code when switch in it doesn't work right etc. or Switch in it might work but solenoid doesn't.

Bad EGR or pipe can air causing idle problems and/or air leak into exhaust causing O2 errors.

You still haven't answer actual gallons use = "only getting about 100 miles on a tank of gas."
because 6g per 100m to 8g/100m isn't = 9.5g/100m
100 / 7 = 14.3 MPG vs 100 / 9.5 = 10.5 MPG

14.3 can be nothing wrong w/ the car, everything wrong about how and where you drive plus trans gearing, tire size and design, and more.
10.5 can be any/all of that and maybe something wrong w/ engine.
Many old V6 and V8 cars only get 20+ MPG driving on the highway for a long trip because keeps trans in highest gear all the time and stays Close Loop too.
Even many L4 only get best MPG doing same. Going to work etc locally cuts MPG big time for many cars including most brand new cars.
My Duke w/ auto only get 23-25 MPG most times but long highway trips gets 27-30 MPG

Tire design matters a lot more then many think. High MPG vs "Performance" tires can make a huge differences. Michelin and few other make tires to get better MPG but How much depend on same things...
M.aaron80 JUN 04, 03:32 PM
I put about 9 gal in and get 100 miles. Get tube good, no vacuum leak and idle is good
M.aaron80 JUN 04, 04:32 PM
I forgot to mention the only code I’m getting is 32. Also I have not replaced map or tps. They are like the only 2 I have not replaced
cmechmann JUN 04, 07:07 PM
You really need to find someone that has a scanner or Aldl program and adapter. You need to see if the readings coming in match the conditions.
If the readings are not out of range, The ECM is not going to fail the circuit. "Example", small crack in vacuum line to map sensor. The map sensor will respond OK, but not see high enough vacuum. The ECM would assume that there is some kind of heavy load and set conditions for it. Another example. Flakey ECM grounds causing the internal voltage regulators for the ECT and ATC to read cooler than what they really are. Another example. Vacuum hose to pressure regulator open or pressure regulator does not lower pressure when vacuum is back up. Any of these can cause bad mpg and still not be out of range enough to set a code.
I'm running a 3800na with pre ODBII PCM with a 4T60e transmission. I get 16-20 mpg around town. As much as 30-32 highway steady driving. However, if any of those condition happen, the mpg drops out.