Drastic drop in MPG M42 engine

[Jeremy Wilton]

I'm new to the site - my '91 M42 engine has gone from doing 38mpg (best) to 22mpg - haven't checked the plugs, but the exhaust looks black - obviously it's running rich. This hasn't happened gradually, the drop came quickly. Any ideas?

[brettski]

Have you checked your air filter?
Is it using oil or running hot?
Is it still running smoothly?

[Jeremy Wilton]

Haven't checked air filter - it's a large sponge version.
The idle is only slightly suspect. It starts fine - goes to 1100 drops to 900 goes to 1100, drops to 900.
Engine doesn't seem lumpy - but exhaust smells of petrol and white smoke for first few minutes. Looking on the web the coolant temperature sensor looks like the culprit - but my mate said that wouldn't make such a large drop in mpg (38 - 22)?
I reckon I'll change it and see.

[brettski]

There should be someone fairly experienced commenting on this soon, so maybe don't get too deep into the guts of your motor before some of the resident gurus comment. Good luck

[darkhorse737]

Originally Posted by Jeremy Wilton

Haven't checked air filter - it's a large sponge version.
The idle is only slightly suspect. It starts fine - goes to 1100 drops to 900 goes to 1100, drops to 900.
Engine doesn't seem lumpy - but exhaust smells of petrol and white smoke for first few minutes. Looking on the web the coolant temperature sensor looks like the culprit - but my mate said that wouldn't make such a large drop in mpg (38 - 22)?
I reckon I'll change it and see.

38mpg??? that is hybrid-class consumption and i don't think Bimmer of your year and model is capable of that. Mid 20's would be a more likely figure depending on your driving style and usage(city drive vis-a-vis freeway).

Idle should rest steady around the 750 are warm engine. Your RPM variation tends to point in a dirty ICV. and maybe also a dirty MAF... boith of which u can clean out with a good spraying of carb or brake cleaner.

Black sparkplug condition and smoke blowing... suggests u need a tune-up that includes a compression check on the engine cylinders.

TIP: O2 sensors contribute largely to a vehicles economy. BMW suggests replacement every 50,000miles

[witeshark]

Maybe his 38 MPG is based on the Imperial gallon.

[Jeremy Wilton]

Yup - maybe a bit optimistic and yup - the imperial gallon (I'm in Oxfordshire UK) but still whatever - it dropped from 32 - 22mpg almost overnight. I have changed the temp sensor - no change - checked vacuum pipes - no holes I can see - sprayed WD40 on them - no change. Will try to clean out the MAF and the idle control valve see if that makes a difference. Also I've been told that the fuel pressure regulator might be knackered. He says you should have 45psi at the fuel rail - if not, pressure goes up and squirts in more petrol - leads to rich mix. All I know is the plugs ar BLACK - still runs not too bad however.

[Jeremy Wilton]

Forgot to mention - it's an M42 in an e21 chassis - no oxygen sensor on the pipes.

[denyo77]

the38 gallons (would be some 30 US gallons) should be ok, as I'm currently getting about 27 US gallons with the auto LSD and 5-speed.
The swap paid off in more than one way

[MercBimerman]

Fuel filter, fuel pump?