Reviving a defective flow meter

The flow meter is fundamental to the fuel injection system. It provides a measure of air flowing into the engine. It has most effect at idle or light throttle. It doesn’t affect full throttle operation very much.

The mechanical air flow meter doesn’t last for ever. If you get erratic idle mixture, or can’t adjust the idle mixture at all, the flow meter may need some attention. Other symptoms can include rough running after a period of constant speed.

As far as I can tell, the flow meter circuit is like this (top view):

Connect a voltmeter between pin 4 and ground. With the engine idling, you should see a nice smooth reading. Gently open the throttle – the voltmeter should rise accordingly. If it’s diving about, the flow meter needs attention.

If the meter is reading erratically, proceed as follows:

NB: The flow meter is a delicate, expensive device. The procedure here should be seen as a last ditch attempt to save an otherwise knackered meter. DO NOT open up the flow meter unless you *really* need to…

1. Facing the engine, the flow meter is on the left hand side, between the air cleaner and the throttle valve.
2. Release the hose clip at the flow meter outlet, unclip the air cleaner cover, and detach the electrical connector.
3. Lift away the air-cleaner cover complete with the flow meter.
4. The flow meter is held on to the air cleaner cover with 4 bolts. Remove them, and carefully separate the meter from the air cleaner cover. Take care not to damage the rubber gaskets.
5. There is a plastic cover on the meter which is held in place with silicone sealant. Carefully peel away as much of the sealant as you can. Then, gently prise the lid off. You need to be patient, otherwise the cover will crack.
6. Aha! All is revealed… You can see that the flow meter is just like the volume control in a radio – there’s a wiper which moves along a resistive track. The erratic reading is caused by wear and dust on the track making a bad contact.
7. Carefully clean the track using a cotton bud soaked with WD-40. Deflect the meter flap to move the wiper out of the way.
8. Also note that you can adjust the “basic” mixture setting here: there is a spring hook on the toothed wheel, which can be moved around the wheel. I left this well alone.
9. I carried out some resistance checks on the meter before refitting it. The results were as follows:
   

   Pins	Resistance (ohms)	Adjustment
   2-4	50 – 950	LOW – HIGH
   3-4	410 – 580	LOW – HIGH
   2-5	0 – 390	LEAN – RICH
   3-5	390 – 0	LEAN – RICH

10. Now replace the cover and fix it in place with silicone sealant. Note that you really ought to use the special acid-free Bosch sealant for this. Normal DIY sealant can/will corrode the delicate mechanism.
11. You should now have a “revived” flow meter. Put the mixture adjuster to mid-way (it’s a 15-turn adjuster), and refit the meter, not forgetting to re-attach the electrical connector.
12. Warm up the engine, and adjust the idle CO. Either set it by ear, or do it properly with a CO meter: Clockwise is rich, anticlockwise is lean. I set it to 1.0% with a meter (I don’t know what the correct setting is). Alternatively, unscrew the adjuster slowly until the engine begins to hunt, then screw it in a turn or so. If the idle is unstable, especially with a hot engine and the headlights on, then it’s probably a little rich – try backing out the adjuster a half turn.Tip: Access to the idle mixture screw is awkward. Make it easier by fixing a small (10mm ø) control knob to the adjuster. Much better!