E46 320i M52TUB20 High fuel usage...

[Arata 28]

Hey guys it's me again.

I have another E46 which I'm getting it ready to sell soon, however it has somewhat very high fuel usage.

So far no fault codes/check engine light on dash or by scanner(INPA).

It also has slow acceleration when cold.

It idles fine whether cold or hot, shift gears smoothly (automatic ZF).

 

 

Things I've done since owning this car in the last 2 years...

New fuel filter as of yesterday (plus the fuel hoses around the filter ends)

New spark plugs.

New upper/lower intake boots.

New throttle body gasket, new idle control valve gasket, new intake manifold gasket, and all intake system/components are cleaned.

New throttle cable.

New vanos seals.

New valve cover gasket with grommets.

New CCV system/hoses.

New upper/lower coolant hoses, expansion tank, radiator, thermostat, water pump, coolant temp sensor, heater hoses, coolant level sensor.

New battery.

New transmission fluid and filter.

New air filter.

 

(I might have forgotten more stuff...)

 

I also have E46 M54B22 and that fuel consumption I would say it's average for the age of E46 M54, but this M52TUB20 uses almost twice the amount.

Now I've also done repairs/maintenance on M54 ones pretty much the same. Only things I've done more was replacing all 4 O2 sensors on M54 which did make little bit better (wasn't as bad as M52TU fuel consumption to start off with anyway)

 

I wonder if it's O2 sensors possibly too old/faullty, or the throttle body/position sensor issue, or simply throttle cable needs adjusting?

Thanks in advance guys.

[Karter16 323]

Could be worth checking MAF sensor. I had one go bad years ago, no codes, but played havoc with performance and fuel consumption.

[F10er 62]

Check fuel pressure regulator function. Sometimes the diaphram splits and leaks fuel into vacuum hose

[Eagle 1522]

Have you checked the live data in INPA?

[NZ00Z3 186]

Mixture problems are reflected in the fuel trims, so check the 4 fuel trims with your scanner.  If any are > 8% at hot idle then you likely have a vacuum leak.

Short term fuel trim = INPA integrator

Long term fuel trim = INPA Multipicative

[KwS 2373]

Define "high". They're a underpowered engine for the car, with an auto, so will tend to use more fuel than a bigger capacity engine in the same car as it's working harder. 

Is it a new issue or has it always been like this? 

[Arata 28]

Hey guys, thanks for the input.

I’ve checked my mass air flow sensor and it’s original BMW Siemens sensor and working fine.

as for fuel pressure regulator, it’s not leaking and reading around 50psi so it’s still working (I did have some suspect but oh well)

Then today, after work I was thinking maybe the O2 sensors are gone.

My M52TUB20 only has 2 O2 sensors (no CAT) and checked with RealOEM.com for parts diagram and does show that this engine only uses 2 O2 sensors. (My other M54B22 has cats and 4 O2 sensors by the way)

So I unplugged each sensors one at the time, checked for resistance value apparently between 2-3 Ohms is standard, then no resistance shown on BOTH sensors!!!

 

Pretty sure they are original sensors from 1999…

So at this stage I’ll replace them and see what happens hopefully that’s the only culprit.

I mean, using 15 litres worth of petrol in 10 mins non-stop driving is insane isn’t it?

Will keep you updated as soon as my O2 sensors arrive.

[KwS 2373]

42 minutes ago, Arata said:

I mean, using 15 litres worth of petrol in 10 mins non-stop driving is insane isn’t it?

This means absolutely nothing. You need to do full to empty to full, and calculate the average over the distance to get something useful in l/100km or km/l.

If it was using 15 litres in a couple of kilometers, you'd be fouling plugs, misfiring, chuffing black smoke or straight up just pouring petrol onto the road. Thats worse than my Rover which averages 23L/100km, and doesnt do any of the above.

[NZ00Z3 186]

12 hours ago, Arata said:

So I unplugged each sensors one at the time, checked for resistance value apparently between 2-3 Ohms is standard, then no resistance shown on BOTH sensors!!!

2-3 ohms is for the heater circuit.  The sensor circuit will be kilo-ohms when cold.

INPA can be used to check the O2 sensors.  There is the heating %, usually > 80% and  the lambda reading, which should switch between 0.1 to 0.8V at hot idle.

[Arata 28]

9 hours ago, NZ00Z3 said:

2-3 ohms is for the heater circuit.  The sensor circuit will be kilo-ohms when cold.

INPA can be used to check the O2 sensors.  There is the heating %, usually > 80% and  the lambda reading, which should switch between 0.1 to 0.8V at hot idle.

NZ00Z3,

Thank you very much for that info. Well I know that there's no resistance at all anyway on both sensors, I'll plug in INPA and will have a look at the readings.

HOPEFULLY, what's left in the fuel tank will be enough to monitor those readings...

Anything else I should be watching on while I'm looking at live data?

[Arata 28]

So I'm looking at values right now, heating voltage on both sensor at hot idle, is 4.25V-4.26V.

TV probe heating is 99.21%, on both sensors. Voltage is switching from 0.12V-0.78V.

Is this normal? Or the sensors on heating side is actually f**ked?

 

[NZ00Z3 186]

That sounds like normal operation.

The heater circuit needs to be operational to get the O2 sensor hot enough to work.  The voltage switching you are seeing is normal.

 

What are your fuel trims at hot idle?  See my earlier post on what to check for both banks.

[Arata 28]

Ok. Thanks! I’ll check the fuel trim and see what it says.