B2 air flow sensor - how much resistance is acceptable?

David42U856 · 12-05-17, 10:35 AM

Bosch K-Jetronic air flow sensor resistance - I've not found much on the internet on this subject but I'm sure there will be members who know the answer. I've just re-built the fuel metering unit for my 1985 B2 Audi 90 quattro and before I assemble it I would like to check that the air flow sensor is serviceable. The height of the flap is fine and it's concentric with the cone but I'm not sure if the resistance is acceptable. When I move the flap up and down the resistance is consistent but the it doesn't return to the closed position under its own weight. Should it? Does the fuel pressure in the metering head apply sufficient force on the control plunger to return the lever to the closed position? I'd rather not put it back together if there is a problem. The various manuals I've looked at don't really help.

msh · 12-05-17, 03:27 PM

All the metering heads I've touched with no pressure in system moves freely and easily....

David42U856 · 12-05-17, 03:57 PM

So you believe that the air flow sensor plate should fall to the closed position under its own weight?

msh · 12-05-17, 04:10 PM

sinbin5t · 12-05-17, 08:13 PM

msh is absolutely correct, when you hold the bolt head holding the plate in place and lift the plate, then just release it, it should just drop down again, there should be no resistance in either direction without the fuel distributor being in place.

Hope you get sorted.

David42U856 · 22-05-17, 12:31 PM

Thanks both. I've removed the circlips/closing plates from the pivot and with the liberal application of WD40 and a lot of cycling it's now free and the sensor plate falls under its own weight. I've reinstalled the unit to the car and the metering head now works correctly. The metering head plunger was stuck open before I rebuilt it but now it behaves as it should, no fuel at the injectors when the air flow meter is closed and good spray patterns and even fuel volume at the injectors when the air flow sensor is manually operated with the fuel pump running. I still have the original problem though ...... the engine starts and runs for a few seconds before cutting out. It won't start again until it's been left to rest for a while. Almost everything in new .... fuel pump, fuel filters, fuel accumulator, spark plugs, hall sensor, distributor cap, rotor arm, tank drained and new fuel added. While it's running there is no misfire or lumpiness so the timing and firing order must be ok. Today I checked the air duct to make sure there is no restriction but it's fine. I'm running out of ideas ...... any help would be much appreciated.

msh · 22-05-17, 04:43 PM

Overfueling?

sinbin5t · 22-05-17, 06:26 PM

Pull the plugs out to see if they are sooty, as msh said over fueling could be the problem as i had with my 5T.

12-05-17, 04:10 PM	#4
msh 4 ring whore! Join Date: Jun 2008 Location: Rucava, Latvija Posts: 3,816	Yes.

12-05-17, 03:27 PM	#2
msh 4 ring whore! Join Date: Jun 2008 Location: Rucava, Latvija Posts: 3,816	All the metering heads I've touched with no pressure in system moves freely and easily....

12-05-17, 03:57 PM	#3
David42U856 Newbie! Classic Audi Club Member Join Date: Aug 2015 Posts: 17	So you believe that the air flow sensor plate should fall to the closed position under its own weight?

12-05-17, 08:13 PM	#5
sinbin5t 4 ring whore! Classic Audi Club Member Join Date: Jan 2010 Location: Where a spike points skywards Posts: 12,960	msh is absolutely correct, when you hold the bolt head holding the plate in place and lift the plate, then just release it, it should just drop down again, there should be no resistance in either direction without the fuel distributor being in place. Hope you get sorted.

22-05-17, 12:31 PM	#6
David42U856 Newbie! Classic Audi Club Member Join Date: Aug 2015 Posts: 17	Thanks both. I've removed the circlips/closing plates from the pivot and with the liberal application of WD40 and a lot of cycling it's now free and the sensor plate falls under its own weight. I've reinstalled the unit to the car and the metering head now works correctly. The metering head plunger was stuck open before I rebuilt it but now it behaves as it should, no fuel at the injectors when the air flow meter is closed and good spray patterns and even fuel volume at the injectors when the air flow sensor is manually operated with the fuel pump running. I still have the original problem though ...... the engine starts and runs for a few seconds before cutting out. It won't start again until it's been left to rest for a while. Almost everything in new .... fuel pump, fuel filters, fuel accumulator, spark plugs, hall sensor, distributor cap, rotor arm, tank drained and new fuel added. While it's running there is no misfire or lumpiness so the timing and firing order must be ok. Today I checked the air duct to make sure there is no restriction but it's fine. I'm running out of ideas ...... any help would be much appreciated.

22-05-17, 04:43 PM	#7
msh 4 ring whore! Join Date: Jun 2008 Location: Rucava, Latvija Posts: 3,816	Overfueling?

22-05-17, 06:26 PM	#8
sinbin5t 4 ring whore! Classic Audi Club Member Join Date: Jan 2010 Location: Where a spike points skywards Posts: 12,960	Pull the plugs out to see if they are sooty, as msh said over fueling could be the problem as i had with my 5T.