520 (M20) very poor economy

[Yaninnya]

Hi everybody,
Thats my first post on that forum.
The problem that I have with my 520 is a little bit strange. I was reading foult codes several times without sucsses but car is very badly overfueling all the time. The niddle on econometer usually is out of scale. The economy is around 10 mpg during very slow normal driving. I have checked HT leeds, rotor arm, distributor cap, replaced spark plugs and it is still the same.
Any sugestions?
Cheers,
Jan

[DaveVoorhis]

I assume you've verified that it actually is running rich, and it's not just a bogus reading on the meter. Does it have an oxygen sensor? If so, check that. Also check the AFM and coolant temperature sensor for proper behaviour. It might be perpetually running in cold enrichment mode.

[Yaninnya]

I assume you've verified that it actually is running rich, and it's not just a bogus reading on the meter.

Yes the real fuel consumption is dramaticly too high.

Does it have an oxygen sensor? If so, check that.

No, it haven't lambda sensor.

Also check the AFM and coolant temperature sensor for proper behaviour. It might be perpetually running in cold enrichment mode.

Do you know what impendance shoud be on that sensor when cold and hot?
Cheers,
Jan

[DaveVoorhis]

Do you know what impendance shoud be on that sensor when cold and hot?

It's the sensor on the thermostat housing with the blue connector.

According to the Bentley manual, at -10c it should be 7 to 11.6k ohms, at 20c it should be 2.1 to 2.9k, and at 80c it should be 0.27 to 0.40 k ohms. It's a continuous resistance sensor, so the actual value will depend on actual temperature.

If it isn't the coolant temp sensor, I'd suspect the AFM. The flap can get sticky, or the resistance track inside can get dirty or worn.

BTW, are you sure it's an '87 520i and an E34? E34 didn't start until the '88 model year.

[Qube]

The economy is around 10 mpg during very slow normal driving.

Okay, so what's the overall economy on a tank of gas, or is that what you're trying to say?

Assuming you're talking about US Gal, then it's about 23.5L/100KM... which is usually an entire tank of spirited driving for me...

[Yaninnya]

It's the sensor on the thermostat housing with the blue connector.
According to the Bentley manual, at -10c it should be 7 to 11.6k ohms, at 20c it should be 2.1 to 2.9k, and at 80c it should be 0.27 to 0.40 k ohms. It's a continuous resistance sensor, so the actual value will depend on actual temperature.
If it isn't the coolant temp sensor, I'd suspect the AFM. The flap can get sticky, or the resistance track inside can get dirty or worn.

Many thanks for that info, I will check it soon.

BTW, are you sure it's an '87 520i and an E34? E34 didn't start until the '88 model year.

You are absolutely right. My second BMW (318i) is '87, that's the reason of that mistake. My 520iA was first registred in August '88.

Okay, so what's the overall economy on a tank of gas, or is that what you're trying to say?Assuming you're talking about US Gal, then it's about 23.5L/100KM... which is usually an entire tank of spirited driving for me...

I'm taking about Imperial Gallons, I'm living in Jersey on Channel Islands (near England ). But because I,m polish it is easyer for me to state it in L/100 km. It is around 24-27 L/100 km. A lot to much. Even with auto it shoud be around 12 L/100 km.

[Qube]

Many thanks for that info, I will check it soon.

You are absolutely right. My second BMW (318i) is '87, that's the reason of that mistake. My 520iA was first registred in August '88.

I'm taking about Imperial Gallons, I'm living in Jersey on Channel Islands (near England ). But because I,m polish it is easyer for me to state it in L/100 km. It is around 24-27 L/100 km. A lot to much. Even with auto it shoud be around 12 L/100 km.

Hmmm... is 12 L/100KM optimistic? Of course I've got a 535 and can only pull down to 17/18 L/100KM at best. Of course, in the 1.8T Passat, I average 7.5L/100KM (yum!).

[BillionPa]

does the engine idle unstable?

can you smell gasoline from the exhaust during idle?

[Yaninnya]

Hmmm... is 12 L/100KM optimistic?

I don't think so. Here on Jersey is 40 mph speed limit and the roads are so twisty and narrow that it is not so easy to drive with higher revs than 1600 - 2000 rpm (it is auto tranny).

does the engine idle unstable?

Now (when it is not running for feew weeks) they are quite stable. But when problem began it was hard to keep it running on idle so unstable it was.

can you smell gasoline from the exhaust during idle?

Not too much and during driving there is not black smoke from exhaust even during harder acceleration.
Cheers,
Jan