High fuel consumption

[TS1997]

Hello!

I have a 1999 523i E39 that I’ve noticed has really high fuel consumption. I’ve only noticed it through the needle on the gauge cluster and the actual mileage per tank so far (I haven’t checked anything yet). The fuel consumption needle shows 30l/100km even with only slight throttle and a 200km trip used about 45 liters of fuel. The 3l/10km average is on shorter trips while it averages 2.25l/10km on longer trips. I’ve also noticed that the car smokes a bit and has a fuel smell in the exhaust.

My guess is that an O2 sensor is broken but I’m just wondering if there’s something else I should check as well? How can I test if it’s an O2 sensor?

i have to keep using the car and also wonder which is best, keep driving like now or disconnecting the O2 sensor until it can be replaced?

Does anyone here know a good place to get high quality O2 sensors? I live in Sweden so I’m mainly looking for places in the EU. The prices are really high locally which is the reason for asking. Should I get Bosch sensors or something else? Are they model specific or should I just get a universal one?

Thanks in advance!

[scye39]

Any fuel leaks at the injectors? Replacing o-rings on them are pretty simple.

Vacuum leaks of course would be a plausible next check. All the vacuum tubes and intake boots around the manifold.

DISA valve is common failing point, easy to repair with all the available repair kits online for <$20.

[taz.]

Use an elm scanner and scanmaster elm to look at the behavior of the O2 sensors and the fuel trims. Also diagnose with inpa, ista or bmw scanner for codes.
An O2 sensor costs around 500 sek and you have 4, 2 pre catalist and 2 post catalist.
So do a thorough research before, there are related videos on YouTube.

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[pleiades]

TS, does your car warm up fast enough in that cold winter weather? If it's taking a long time to get up to operating temp, that will hurt fuel consumption, especially if you use the car mainly for short trips.

Also, sometimes the thermostat on these cars can fail, usually in "open" position, preventing the engine coolant from warming up and causing the engine computer to run the fuel mixture too rich.

[TS1997]

Any fuel leaks at the injectors? Replacing o-rings on them are pretty simple.

Vacuum leaks of course would be a plausible next check. All the vacuum tubes and intake boots around the manifold.

DISA valve is common failing point, easy to repair with all the available repair kits online for <$20.

I didn't see any fuel leaks when I checked. I'll check for vacuum leaks as well.


Use an elm scanner and scanmaster elm to look at the behavior of the O2 sensors and the fuel trims. Also diagnose with inpa, ista or bmw scanner for codes.
An O2 sensor costs around 500 sek and you have 4, 2 pre catalist and 2 post catalist.
So do a thorough research before, there are related videos on YouTube.

Sent from my SM-G960F using Tapatalk

I plan on scanning for codes with INPA but I have to find a cable for it first. Do you know a good one?


TS, does your car warm up fast enough in that cold winter weather? If it's taking a long time to get up to operating temp, that will hurt fuel consumption, especially if you use the car mainly for short trips.

Also, sometimes the thermostat on these cars can fail, usually in "open" position, preventing the engine coolant from warming up and causing the engine computer to run the fuel mixture too rich.

It warms up fine and stays at operating temperature after that. I'm going to do a complete cooling system overhaul after new years as my radiator is bowed so the thermostat will be replaced soon. One of the trips I made when I noticed an absurdly high fuel consumption was a 200km long trip which I did without stopping. This trip used about 45 liters of fuel and there were no spirited driving.

I think I saw a few drops of fuel in the exhaust one time as well which makes me believe that it's running rich rather than leaking.

[taz.]

On Ali Express or Ebay are dozens and they all work ok, just look at the reviews before choosing one. But get the scanmaster elm tool also it's not expensive( 10-20$) but it's worth the money.

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[TS1997]

I've finally fixed this issue today and it ended up being the exhaust side cam position sensor that had died resulting in that the car were running rich.