So it starts typical... all of the sudden my fuel mileage has just tanked. from about 19-20 mpg to 14 mpg. I'm trying to figure out why.

How is this not a typical question? I've already checked out pretty much everything:

- No visible leaks on any fuel lines/components
- TPS is set up perfectly
- Pressure sensor is plugged in
- MAF plugged in and working
- All grounds are good
- Less than 10k on plugs/wires/fuel filter/02 sensor/water temp sensor
- less than 30xxx kms on motor

Recent work that i've done in the engine bay is limited strictly to painting my intake piping and then the engine bay. (and yes, i taped everything, removed everything from the fenders, covered the motor, and removed the MAF and set it aside). So i'm not too sure how any of that could've affected mileage.

The car is an N/A S5. My driving style hasn't changed. It's not really beating on my car, but at the same time not being too easy on it either. The car doesn't seem to have any kind of power difference. Any ideas guys?

 

Any way to check AFR's? Flooding problems? Your injectors could be leaking just a bit. Not necessarily enough to flood the engine out or cause starting issues, but enough to hurt MPG. Thats not the only possibility but just a guess. You could be running super rich also and there are a number of reasons for that. You may also want to check your O2 sensor as that *may* have gone out on you even though it has less than 10k on it.

Also (and this is kind of a long shot) but check your wheels for free spin. If your calipers locked up (even just a little bit) that would also hurt mpg even though you may not feel the dragging effect.

Just a few guesses of mine. Good luck.

 

OK guys, it's gotten worse in the last couple months. I put a new 02 sensor in it, pulled the plugs and cleaned them, played around with the tps, and nothing is helping.

When I pulled the plugs, the leading plugs had a fair amount of carbon build up (running rich), and the trailing plugs were both half carbon and half brown colored. What would that mean? Normal?

All wheels spin freely and great. No visible fuel leaks blah blah blah...

Just a thought....could it be the ECM? Another thing: how much does the TPS affect fuel mileage? B/C that's what I suspect the most. The car was running great, so I checked the tps with the test connector by the air filter and a multimeter. I had to turn the tps WAY in to get it to the correct setting, and the car ran like crap. Bucked everywhere, parking lots sucked, and so did going off and on the gas in any gear. So I turned it back out, but I'm not sure exactly where it was before. Could this be a bad TPS? And how much would that affect mileage?

Another possibility would be injectors. But I have ZERO starting problems or running issues (except crappy fuel mileage).

Last but not least, I checked timing as well, and it's dead on.

Thanks,

Dirk

 

one more thing.... maybe change the ECT?

 

All I an say is it is a sports car and around 14 mpg is correct for city driving. Especially if you redline it every so often. Personally when i bought my car I knew these cars weren't an economy car. I don't even bother checking my mpg because i know that it isn't great because the way i drive. If your looking for to get 20mpg city then you might want to get a different car. IMHO

http://www.fueleconomy.gov/feg/findacar.htm

 

Test the ECT as you suggest, or just replace it.

If the injectors have never been cleaned before, have it done.

 

put it this way. I was getting 19ish mpg city. Then it dropped to 14. That's when I started this thread. Now it's at about 12. I should be able to get at least 16-17 out of it

 

Wow, now your in the turbo mpg region. I would try cleaning injectors, if they haven't been cleaned before.

 

Are your tires properly inflated too?

 

35 psi

i'll change the ect and check the injectors. thanks guys

 

Pure gas or ethanol?

 

usually pure gas. the odd time i'll fill up with 10% ethanol. I don't notice any difference between the two

 

OK guys, I finally got around to checking all this crap-

Tested the temp sensor, was out of spec slightly, so I replaced it.

Tested the MAF sensor, within spec.

Tested the TPS, also slightly out of spec, but managed to adjust it pretty close, it has no flat spots, and isn't bad enough to cause fuel mileage this bad.

I pulled up the pass. side carpet to get at the ecu. As expected, the TPS readings were slightly out. Now this is where the fun begins. I checked the signal to the ECU from the water temp sensor (ECU pin 2E), and the voltage went down as the car warmed up! The way the sensor works is resistance gets lower as temperature gets higher, so shouldn't the voltage be going up as well?????

I also got a funny reading from the ECU Intake Air Thermosensor (ECU pin 2L). Spec at idle at 176 degrees is 1V-2V. But my voltage here is 5V! Where exactly is this sensor on the engine?

This is where it gets the worst. I checked the reference voltage signal from the ECU to the sensors (ECU pin 2I). Sepcification is 4.5V - 5.5V. But my ECU is giving the sensors a reference voltage of 7.5V!

So I figure I need a new N351 ECU? Any insight on that water temp sensor reading going down as the car warms up? To me it seems that the lower the voltage is that the ECU sees, the colder it thinks the engine is, and the more fuel it dumps in there!

I want this fixed before I sell it.

Thanks in advance,


Dirk

 

anyone have any insight on this?