+/- 16 mpg...why?

[rjnakata]

My '01 Legacy GT wagon is running great, but I'm getting +/- 16 mpg.  Any ideas? Help!

Regular gas

Newly cleaned K&N air filter

New Fuel filter

Newly gapped valves

New plugs/wires

Newish tires

Regular oil changes / coolant flush

New timing

Sometimes I get a CEL for CAT deficiency...

 

[doublechaz]

Possibilities...

 

Near failing front O2 sensor.

Exhaust leak at or ahead of front O2 sensor.

The new timing isn't correct.

Car full of extra weight, roof rack, and the loose nut behind the gas pedal.

 

I've been able to get that low with reasons 2 and 4.

[rjnakata]

@Doublechaz  Thanks!

Hmm, just replaced the front O2, can't hear any putt-putt exhaust leaks, no extra weight except for me...

Would the car run smoothly if mis-timed?

[doublechaz]

I fixed a friend's slipped timing belt a while back.  Just one tooth off.  Ran smooth, just off on power.  We never check mpg before and after as the car had other problems.

On my own car I jumped timing and it turned out to be three teeth off.  It would run fine at some throttle loads, but miss and buck terribly at others.  Idled pretty ok if you are used to V engines, but it was not as smooth as normal.  And it got like 8 mpg.  Testing showed it was actually firing two injectors twice for each cycle instead of once.

It would be good to get some live engine data to see if everything looks normal or not.

[rjnakata]

@doublechaz thank you.

At all loads the car seems to have power, maybe even more than before, BUT is has a new "growling" sound when loaded.  No missfires, or hesitations.

I forgot to mention pretty much all city driving, but still 16mpg?

Finally what is meant by "live engine data"?  Is that a dyno test?

[doublechaz]

I installed the binary pedals mod in mine, so it is always full send and pretty much all city.  I can get down to about 22 driving like that, so I can't see how you could get that low.  I used to get 29.5 to 30.5 highway, but that was back when speed limits were lower and I didn't have 150,000 miles of WOT on it.  Last I knew I could get 26 to 27 highway, but that is going 80 mph.

 

Live engine data means you get an electronic tool and plug it in to the OBDII port and it shows exactly what the computer sees and is doing in real time.  I have an elm327 OBDII to blutooth and the torque android app.  It will show all kinds of diagnostic data, but you have to be fairly advanced on your theory of operation for engines to know what it all means.  Still, the elm is about $10, and there is a free version of the app, (and a version for about $6) so it isn't a big barrier to starting to gather data and learn.  Working on cars through the internet isn't easy, but we can help.

[silverton]

Do you allow your K&N to dry completely before oiling? I think K&N's are more trouble than they're worth, if you gotta do it it's ideal to have two that way the other one is for sure dry and ready to be oiled, and then there is the possibility of over oiling and mucking up the airflow sensor..  They don't filter any better than a $5 filter you place twice a year.

the growling is likely engine noise that would escape through the exhaust, but it coming out the intake, due to a plugged catalytic converter. If you have a stock exhaust system and are getting a P0420 code, you need a cat.

[rjnakata]

@doublechaz@silvertonthank you.

Agreed 16 mpg is for cars much more exotic than mine!  I'll see about the phone supported OBDII - pretty cool.

I did dry the K&N but may have oiled too much(?)  Anyhow I've replaced the paper filter and we shall see.

@silverton: psychic - I did just start getting the P0402 code and roughly the same time as the growl. I just thought it could have been a number of other things I've been fooling with. I cleared the code and I used Rislone to see if I could clean the cat...the code returned after a few months.

I'm supposing the plugged CAT could cause a drastic decrease in mpg...correct?

 

 

 

[doublechaz]

Does the engine make a good deal more power at low rpm than at high rpm?  Plugged exhaust shows more and more as rpm goes up since it can't get rid of the increased air flow pumping through the engine.

[silverton]

A plugged cat could indeed cause a drastic decrease in mpg.

[rjnakata]

@doublechaz There may be some extra power at lower rpm.  Higher rpm seems ok, but I'll keep track of it some more.

@silverton Ok, a new California (CARB) cat is like $850.  I'll start a new thread on best options...

[doublechaz]

Your "felt power" should roughly follow the torque curve in this image, although my cat was somewhat damaged at the time of this run, so really my high end if off in this run.  If you would draw the graph for yours with the top end lower, I would check exhaust pressure.  I put an extra O2 bung in front of my front cat next to the normal one and made a pressure gauge adapter from an O2 bung block off plug.  You could briefly do the same by adapting a guage into your normal O2 hole.  The car won't run right without that front O2, but you can at least see if you have 1 psi or less pressure for normal, or more than 1 for broken.  For comparison, when I riched out real bad and plugged my cat I had up to 7 psi at 5500 rpm.

 

Dyno