Low MPG

[lucas.dargis]

Hey,

I'm an audi noob. I have a 99.5 A4 1.8T

I am getting under 20mpg on highway only driving. My exhaust is spewing out carbon.

I just changed the MAF and the air filter whichthat helped a little.

My check engine light is on so I pulled the codes. here is what i got:


4 Faults Found:
17604 - Oxygen (Lambda) Sensor Heater Circuit: B1 S1: Electrical Malfunction
P1196 - 35-00 - -
17913 - Throttle Idle Switch (F60): Does not Close or Open Circuit
P1505 - 35-00 - -
17536 - Fuel Trim: Bank 1 (Mult): System too Lean
P1128 - 35-00 - -
17544 - Fuel Trim: Bank 1 (Add): System too Lean
P1136 - 35-00 - -

There is no way the system is too lean. if anything it is basically flooding.
Any ideas?

[zrowcool]

that first code and possibly the second are your mpg issues. the primary 02 sensor is the one responsible for the fuel mixture and if your car is failing to idle consistently it might be running at a higher rpm than it should be. change that 02 sensor and clear/rescan

[lucas.dargis]

the car idles holds steady at about 900. Is that too high?

[Seerlah]

Maybe an exhaust leak? I had really bad mpg and I found out the PO had the exhaust chopped. Talk about an exhaust leak x15. I would have never known had I not installed an aftermarket exhaust system.

[lucas.dargis]

I will inspect the exhaust.

Which O2 sensor should I replace? As far as I can tell there are several of them.

[Seerlah]

Primary (one right after turbo on Cat). But when I replaced mine, I did all at the same time. MAF and both 02 sensors. And my advice to you- if it isn't broke, don't fix it. Check for exhaust leaks frst, get them welded shut. Then test out your mpg on full tank till empty (the cluster is not 100% accurate). Look in your manual. It tells how much gallons of fuel our cars carry. Set the trip computer till an empty tank and divide that by total gallons our car can hold (I am too lazy to go and look it up right now, but I will edit in a little while with number).

[Toyotatech]

An oxygen sensor tests the amount of oxygen being discharged from the motor, too lean means not enough oxygen content in the exhaust that's why if you look at freeze frame data it will usually show negitive fuel trims, the motor is removing fuel. You need the bank 1 sensor1 and to figure out the throotle idle switch issue. p1196, p1128 and p1136 are all the oxygen sensor.

[lucas.dargis]

I've been doing all my MPG calculations by hand and it has been scary low. last time it was 16mpg.

What is the best way to check for exhaust leaks?

I'm thinking jack up the car, turn it on and spray soapy water on the exhaust and see if any bubbles show up.

or do you think a leak would be very obvious?

[Toyotatech]

Yes it would be noticably louder. It's your o2 sensor just replace it. A crack in the exhaust would only affect air/fuel ratios if it was before the first o2 sensor.

[Toyotatech]

An oxygen sensor tests the amount of oxygen being discharged from the motor, too lean means not enough oxygen content in the exhaust that's why if you look at freeze frame data it will usually show negitive fuel trims, the ecu is removing fuel. You need the bank 1 sensor1 and to figure out the throotle idle switch issue. p1196, p1128 and p1136 are all the oxygen sensor.

Just to make sure you read it.

[lucas.dargis]

You need the bank 1 sensor1 and to figure out the throotle idle switch issue.

Can you clarify this please?

are you saying i need the back sensor and then i need to figure out the throttle issue?


I want to make sure i replace the correct sensor because the are over $100 each!

[Toyotatech]

You need to replace the one pre catylist (the one you can see when you open the hood right behind the turbo). The oxygen sensor after the catylist has nothing to do with air/fuel ratios.

[lukepl]

just curious have you or has anyone else fixed their low mpg problem. If so how did you do it?

[biketsai]

just curious have you or has anyone else fixed their low mpg problem. If so how did you do it?

Drive like a granny