M50 fuel economy HELP please

Post Thu Nov 08, 2012 9:24 pm

Ive converted my e30 to run the M50 and i love it but its daft on fuel for my commuting etc!

the car pops and burbles on the down shift and smells of fuel.
The idle hunts but not realy erraticly.

It has a new 4 wire lambda sensor but the plug on the car side of the loom was missing (just 4 wires)

the question is, my lambda sensor has two whites, a grey and a black. My loom has a yellow, black, red and a blue and green....what connects to what? and what on an M50 makes the fuel consumption so shit. thanks.

Post Thu Nov 08, 2012 9:52 pm

Running without a lambda puts the ECU into limp home mode, which overfuels the mixture to keep the temps down. Biggest problem (apart from ridiculous fuel consumption) is you get bore wash, which will prematurely wear the bores/rings leading to low compression and eventually shagged engine.

Is the engine vanos or non vanos? E34 or E36 loom? I can have a look at a loom tomorrow for the wire colours for you

Post Thu Nov 08, 2012 10:48 pm

shes a m50b25 non vanos mate. its just flown through an MOT emmisions test mind but im still unsure! ive read that if the exhausts leaking badly near the downpipes etc then it will throw the lambda sensor readings out...this true?? thanks

Post Thu Nov 08, 2012 11:22 pm

This is true. A decent chip may also yield good improvements.

Post Thu Nov 08, 2012 11:32 pm

i think my ECU is already chipped speedtouch! how can I tell? the idles high at the momment, it hunts sometimes but not always...blah its doing my head in.

the plug on the e36 loom was destroyed too (lambda sensor)

so ive had to join the wires together...so ive a grey, black and two whites from the o2 sensor...then a black, yellow, red and then green i think on the car side of the loom...which goes to which? :/ aha

Post Thu Nov 08, 2012 11:40 pm

makman wrote: ive read that if the exhausts leaking badly near the downpipes etc then it will throw the lambda sensor readings out...this true?? thanks

It's true.

I've read that depending on if your car has a cat or not a bad exhaust can affect the fuel consumption by quite a bit. Your lambda sensor will sense too much oxygen relative to the ambient and use the feedback loop to pump more fuel into the system to get it to where it should be sitting, even though initially it is fueling fine, if that makes sense?

I was getting around 22 MPG out of a 318i auto, and I though I'd get my exhaust changed yesterday....
.......what looked like a very good exhaust had a 3 inch hole on the back box where the bracket held the box in place, and the mid box was completely shot, sounded like a rattling money box.

I'm hoping to see a slightly better mpg, albeit having a VERY council upswept DTM scorpion system on the car

Post Thu Nov 08, 2012 11:47 pm

mmm thats the next port of call then...my manifolds are pants to be fair haha theyve needed rebuilding and a custom system for a while now! so loud haha. il gum them up for the time been

council DTMs FTW! haha

Post Fri Nov 09, 2012 1:50 pm

makman wrote:i think my ECU is already chipped speedtouch! how can I tell? the idles high at the momment, it hunts sometimes but not always...blah its doing my head in.

Remove the ECU lid and look at the chip. An original BMW one will have a silver label on with a part number, e.g., 1267348357, whereas an aftermarket chip will either have a different label or possibly none at all.