Over-fuelling When Cold

[StuBeeDoo]

My M20B27 seems to be over-fuelling when it's cold.

It's got a brand new blue sensor, the injector loom is known to be good (it was fine on the B20).
The AFM is the original one on the car, so I'm guessing that the CO screw needs adjusting - could I be right?
It seemed OK when I drove it home on petrol, once it was warm. It's perfect on LPG, but obviously it has to be started on petrol so I want it right.
TIA
Stuart.

[e30bmlover]

i would get the co checked mate... dosent cost very much and worth the peace of mind.

[pikanibbles]

Can anyone tell me the part number for the blue sensor?

[tomson]

The AFM is the original one on the car

Do you mean the AFM is the B20 item? B20 & B25 AFM's are different so the B25 might cure it ? ?

[daimlerman]

The AFM is the original one on the car

Do you mean the AFM is the B20 item? B20 & B25 AFM's are different so the B25 might cure it ? ?

This was one of the reasons I opted for the Miller MAF,did not fancy a s/hand AFM and the Miller unit was little more than a new AFM and got rid of the restriction in the air flow. Belive that LPG does not use the AFM,so it may be worth living with the poor running when cold,suppose it depends on just how bad it is and how much running you do under 'cold' conditions.How are you finding the 2.7,BTW?

[StuBeeDoo]

Belive that LPG does not use the AFM,so it may be worth living with the poor running when cold,suppose it depends on just how bad it is and how much running you do under 'cold' conditions.

You're right Malcolm, mine holds the AFM wide open when running on LPG - some don't, though. I had wondered about just living with it. Unless it's really frosty I don't have to do more than about 400mtrs before it will run on gas. I'll keep a look out for a 2.5 AFM but, like you, I don't want to buy just any old tat.

How are you finding the 2.7,BTW?

I'm well impressed with it. Done nearly 400 miles now. I've decided to stick with BMW's 1990 version of running-in theory - 1200 miles at no more than 4k rpm and no full throttle, then change the oil and gradually start opening it up from there. If it was good enough then.............
We went out to Gordale Scar yesterday - about 175 mile round trip. Wor Lass commented that the engine had a deeper tone and that I didn't seem to be changing gear so much. Of course, I knew this, but for her to actually notice as a passenger, it must be very obvious.

[StuBeeDoo]

Can anyone tell me the part number for the blue sensor?

13 62 1 709 966

[daimlerman]

I'm afraid I was very anal running mine in...2000rpm for the first 250 miles(remember it well,spent most of it drumming my fingers on the wheel rim) then gradual increase to 2500 over the next 500 miles and so on until I was confident it was bedded in.Changed the oil at 500 miles,the service indicator had three greens(did not re-set with the new motor) so I ran till they vanished and changed it again and then back to the usual routine.It may drop it's oil level a little between changes,but rarely enough to need to add oil.As you have already discovered,the added torque is what it's all about.Be interestec to hear a result when it goes on the rollers

[StuBeeDoo]

I'm afraid I was very anal running mine in...2000rpm for the first 250 miles.........

I did 1st 100 at <3k, then gradually moved up to 3.5k, which is where I'm at now, but it has been to 4k a couple of times in the last couple of days. Haven't been above 3k in 4th or 5th yet though.
I'll be sticking at 4k until 1200mile oil change though.

Only 800 miles to go.......

[Brianmoooore]

Try connecting a resistor in parallel with the blue temperature sensor. Start experimenting with something like 2200 ohms.

[StuBeeDoo]

Try connecting a resistor in parallel with the blue temperature sensor. Start experimenting with something like 2200 ohms.

Sorry Brian, far too complicated for me. I haven't got a clue with that sort of thing. My electrical capabilities only go as far as connecting 2 wires the same colour together with crimp-on connectors!

[johnlouis]

the blue temp sensor is a NTC type thermistor, thus the hotter the engine, the lower(?) the resistance. Putting a resistor in parallel with the sensor will bias it, the ECU will think the engine is hot when it is not (or hotter anyway) (and very hot when it is hot). I am not sure of the curve for the blue sensor normally, but you can work out the bias fairly easily from it. for each point (temperature integral) you would simply multiply the resistance of the termistor with the value 2200 and divide by the addition of the two.

While this all sounds very conviluted it will deduce weither your ECU is varying the fuel mixture correctly. You could always wire a potentiometer up instead of the blue sensor and vary it manually to bench check it, not sure what effect that would have on the mixture though...

[StuBeeDoo]

the blue temp sensor is a NTC type thermistor, thus the hotter the engine, the lower(?) the resistance. Putting a resistor in parallel with the sensor will bias it, the ECU will think the engine is hot when it is not (or hotter anyway) (and very hot when it is hot). I am not sure of the curve for the blue sensor normally, but you can work out the bias fairly easily from it. for each point (temperature integral) you would simply multiply the resistance of the termistor with the value 2200 and divide by the addition of the two.

While this all sounds very conviluted it will deduce weither your ECU is varying the fuel mixture correctly. You could always wire a potentiometer up instead of the blue sensor and vary it manually to bench check it, not sure what effect that would have on the mixture though...

Thanks for your time and trouble.
However, for all I understand of your explanation it may as well be Mandarin Chinese.
My plan is to fit a 325i AFM and get it correctly set-up for idle CO.

[Jon_Bmw]

You say its rich, do you mean rich on load or just rich at idle? If its more than just at idle, adjusting the CO screw is only going to change idle enrichment. I am pretty sure it is meaningless the rest of the time. Adjusting spring tension in the air flow meter is a crude but 'sometimes' effective/inexpensive way of making a small amount of adjustment that would affect the whole rpm range. You need it to be plugged into a wide band sensor to make any adjustments really.

Fit the 325 AFM, see where it stands then. Out of interest, I swap mine to run on LPG straight away, whatever the temperature. It never seems to struggle to swap over, I literally let it start on petrol, blip it to 1500rpm, as the solinoids flick(you can hear them) I blip the throttle once more ands its A-OK! It might be slightly warmer daaaarrrn saaarf I suppose.

[johnlouis]

out of interest does anyone know what resistance the blue sensor should be at hot temp and cold temp as I am not sure if mine is doing its thing, though I know the value does change...

[StuBeeDoo]

You say its rich, do you mean rich on load or just rich at idle?

Rich at idle

I swap mine to run on LPG straight away, whatever the temperature.

Mine needs 400mtr (or 1min standing) before it will run on LPG first thing in the morning. After that it will start on gas all day as long as it hasn't been parked-up for more than about 5 hours. Was the same on the B20. A couple of times in the summer it started on gas after muggy nights.

[Brianmoooore]

Around 300 ohms hot, and 4000 ohms cold.

[Brianmoooore]

Mine needs 400mtr (or 1min standing) before it will run on LPG first thing in the morning. After that it will start on gas all day as long as it hasn't been parked-up for more than about 5 hours. Was the same on the B20. A couple of times in the summer it started on gas after muggy nights.

What priming time do you have the gas set at? I'm assuming there's no temperature sensing element in your LPG set up.

[StuBeeDoo]

What priming time do you have the gas set at? I'm assuming there's no temperature sensing element in your LPG set up.

IIRC, I set it at 4sec. I've got the temp. sensor in the vapouriser, but it's not functional when the system is set on Alternate Fuel Start. I switch it manually.
When I had it set to switch automatically, I had it set at 15deg & 1500rpm on deceleration.
With the B20 engine, it often took 3 attempts to start on lpg. Since I've had the B27 I've never had more than two attempts and a couple of times it's fired-up first go.

[pikanibbles]

Can anyone tell me the part number for the blue sensor?

13 62 1 709 966

Thanks Stu

[Brianmoooore]

IIRC, I set it at 4sec.

Wind it up a bit!