Silly Dorito

Well, I've gone through the search feature, probably read every thread on gas mileage there was, and I'm still stumped as to why my car gets about 5Kms/1L (about 10-11mpg). I've tried almost everything, and that's why I'm posting this now, I'm out of ideas.

The car is a 1987 S4 GXL, with roughly 215,000Kms on the body, and apparently, 75,000kms on the new crate engine. So far I have replaced/done;

-New fuel filter (not in tank, under hood)
-New o2 sensor
-New plugs/wires
-New Pressure Sensor (old one made o2 sensor read rich 100% of the time)
-All fluids changed
-All belts changed
-Timing Checked (dead on)
-Grounded Pressure Sensor

That's it for now, my next step is getting the injectors cleaned and balanced, after that, if the problem isn't fixed, I'm all out of ideas. I've even checked to make sure the 5th/6th ports are working. I'm not sure, but they look stuck open, could that cause my bad mileage ?

Anyone help ? I need answers to why it's eating gas!!

Thanks for helping a n00b out.

w_hari

Hi.

Maybe one of your front brake pistons is stuck. When mine did, fuel consumption went up to +2L/100km. Drive about 40km and then check if one of your rims is noticeable hotter than the others!

OverDriven

If I were getting crappy milage, the very first 2 things I would check are the air filter and the cat. My bet is that your cat is clogged.

-Joe

91_fc

does your exhaust smell like your running rich?

VroomVroomVroom

I can think of several reasons for such poor performance.
First off, your engine must be in good mechanical condition, or all bets are off. (check the compression).
Your exhaust system, must not cause undue restriction. (check back-pressure of cat/muffler if you are not sure of this).
Assuming the above items are good, then you are injecting too much gas. You could be leaking from an injector (fuel system leak-down test to eliminate that: hot starts would be a real problem), or most likely the computer is not going into "closed loop" control where it adjusts the fuel/air mixture based on feedback from the O2 sensor. Open loop operation can be caused by a temperature sensor telling the computer that the engine is still cold, and demanding a rich mixture (check out the coolant and temp. sensors both cold and hot to see if thay are in range).
Then it gets tedious!
I'm afraid the Mazda Workshop Manual is little help with poor fuel economy, and for closed loop control, inexplicably covers the cats & muffler system.
Anyway, I hope this gives you some direction to work in.
Dave

Silly Dorito

Forgot to mention, I'm running with K&N intake, and Bonez high flow cat. Intake was just cleaned a few days ago, and cat seems pretty clean, but should probably be checked. My exhaust doesn't smell rich at the rear, but under the hood, the engine sometimes smells a little rich. There is a very small exhaust leak though at the manifold, which might explain the smell.

I'm going to try the suggestions so far and see what I come up with. Thanks for the replies, and keep them coming! Always good to have different views and different opinions, it's sometimes the most obvious things you look over, hehe.

Anyone else ?

hugues

I will give you the most obvious of all answers:

Check for gas leaks (under the intake or along the gas metal lines)

If you want to be super thorough, you can check the fuel pressure and the input voltages from the sensors that matter (like air/water temps). The FSM gives the voltage you are supposed to have. A lot of sensors influence the fuel injection duration, some more than others.

Hugues -