gas consumption... plus a bunch of other little problems...

[hungrymonkey]

My wife ran my car out of gas today. Its the second time Its happened.
I have been filling up around every 130 miles just to be safe, I computed the gas mileage at about 17mpg. Which from what I read is on the low side of average. But today It ran out at 143 miles.

I am guessing the tank capacity is 16 gallons. Which should around 270 miles per tank. Where do I start with trying to figure this out? I really do not know much about what has, or has not been done to the engine. both maintenance or performance. It does not sound like the cam has been changed. it has the stock intake on.

Here is what I do know about the car.

89 GT Vert, 107,xxx miles.
alternator is charging at 11.8 volts at idle.
smells like it is running rich.
pings intermittantly under load.
The car will die if I down shift through first and come to a stop.

I pulled 2 codes using a "brain scanner" code reader that my buddy loaned me. It was for 34, and 91. I did a search and came up with EGR and fuel pump.

I have not been able to check the timing to see if its been advanced. It idles smoothly and starts on the first crank.



My first thought was that the fuel pumps gasket had failed, but then I realized the fuel pump was not attached to the motor

Where should I look first? I would assume the low output of the alt is causing the dying problem. It is hard to replicate, it happens about 1 out of ten times when coming to a stop.
My exhuast is not blowing smoke, But I can smell gas in it when I have the top down. It smells a lawn mower.


sorry if my post seems a bit out of order. I have both my babies and my wife hollering at me.

 

[hungrymonkey]

forgot to mention, the gas guage is always on full. That is why I have ran out of gas twice.

 

[Fox8950]

Pretty sure fox's only have a 12 gal tank, someone correct me if I'm wrong. The gas level sending units crap out pretty commonly lol I know I've changed a few .. as for the other problems, If it smells like its running rich and still pinging maybe a timing issue I'd double check it just to make sure nothing jumped a tooth, EGR problems may be making this worse as your timing gets advanced as the egr valve opens and exhaust gases are reintroduced back into the combustion chamber. also these cars have TERRIBLE stock alternators, take this opportunity to grab a 3g alternator (94-95 mustangs) and do the swap theres a ton of posts in the forum about doing it.

 

[hungrymonkey]

I have been reading the 3g posts, I will probably buy new, since the junkyards around here are very overpriced.

I put in 12.9 gallons earlier today, along with 2 gallons from my gas can.

I am starting to think the PO might have bumped up the timing, It will knock when I hammer the throttle.

I just found a thread where somone stated the code 91 is an O2 sensor. That would explain it running rich. I doubt it would have halfed my gas mileage though.

 

[lang williams]

o2 sensor could have a major impact on your mileage. I had one go bad in a new van on a trip an it cut my mileage by 7 mpg. i got 23 going there an 16 on the way home.

 

[Fox8950]

Looked around and saw 15.4 gals as the tank capacity, yeah a bad O2 can kill your mpg pretty badly, fuel delivery is pretty dependent on the readings from them unless you are WoT and they are cut out of the loop

 

[SuperStang83]

Tank capacity is around 15 gallons, but I am n0ot sure exactly what it is.

 

[speedytang]

15.4 gallons on all Fox engine and body combinations. Since you smell gas are you sure your not leaking fuel at the filter connections or that the fuel filter has not rusted out and leaking. Lift the car and start it and see if you have a leak first.

 

[jrichker]

The code 91 is not a fuel pump problem. It is the code that is pointing to your fuel consumption problem.

Code 34 Or 334 - EGR voltage above closed limit - Failed sensor, carbon between EGR pintle valve and seat holding the valve off its seat. Remove the EGR valve and clean it with carbon remover. Prior to re-installing see if you can blow air through the flange side of the EGR by mouth. If it leaks, there is carbon stuck on the pintle valve seat, replace the EGR valve ($85-$95).

If the blow by test passes, and you have replaced the sensor, then you have electrical ground problems. Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1.5 ohm. Next check the resistance between the black/white wire and the negative battery post. It should be less than 1.5 ohm.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.

Let’s put on our Inspector Gadget propeller head beanies and think about how this works:
The EGR sensor is a variable resistor with ground on one leg and Vref (5 volts) on the other. Its’ resistance ranges from 4000 to 5500 Ohms measured between Vref & ground, depending on the sensor. The center connection of the variable resistor is the slider that moves in response to the amount of vacuum applied. The slider has some minimum value of resistance greater than 100 ohms so that the computer always sees a voltage present at its’ input. If the value was 0 ohms, there would be no voltage output. Then the computer would not be able to distinguish between a properly functioning sensor and one that had a broken wire or bad connection. The EGR I have in hand reads 700 Ohms between the slider (EPV) and ground (SIG RTN) at rest with no vacuum applied. The EGR valve or sensor may cause the voltage to be above closed limits due to the manufacturing tolerances that cause the EGR sensor to rest at a higher position than it should.

The following sensors are connected to the white 10 pin connector (salt & pepper engine harness connectors)

This will affect idle quality by diluting the intake air charge. If it does not close beacuse it is leaking, it can cause the engine to ping under acceleration.

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor. Code 41 is a RH side
sensor, code 91 is the LH side sensor.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up the driver’s side, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the crossover tube and thoroughly clean the insides to that there is no carbon blocking the free flow of air to both heads.

 

[ninety15.0]

Yup i was going to mention something about code 91 as well. This is for an indicated lean condition. This is probably because your O2 sensor is stuck and not switching. Therefore you are dumping fuel to "compensate".

If this is just an O2 sensor problem and they tested bad then just replace the bad sensor...or both if you have never changed them. If this does not solve your problem you need to start looking for vac leaks.

Im guessing you may have a related problem with that code 34 for the EGR. If you are getting un-metered air into the engine due to a faulty EGR valve that could be causing your lean condition and your subsequent rich condition. Check all vacuum lines. good luck man