86 XJ6 Mas Air Flow Meter Faulty
 

There air flow meter was working fine when the car arrived, the car has sat for about 1 year without start or drive and I have recently noticed the MAFM is faulty and as the car could not stay running idle nor drive. I changed the air flow meter and everything works fine, put back other old air flow meter the the sickness come.

What could cause this fault?

Is there a way to fix this faulty air flow meter or is there any company to send to for rebuild?

A little-known feature of the MAF are the contacts inside that supply the fuel pump relay. The contacts are held open when the flap is in the rest position, (i.e. engine not running). On engine start, the flap moves and allows these contacts to close to keep the fuel pump running. On engine crank, the contacts are bypassed electrically until the MAF flap moves. So the engine will start but if these contacts are in bad condition the engine won't keep running. These contacts are really tiny things, the wonder is the system works at all !! After a lot of faffing around in the mid-nineties with my Series 3, I finally found out about these contacts and the effect of poor contact. So I managed to get the top off the MAF, found these contacts and cleaned them thoroughly, incl with a bit of emery paper, carefully bent them a bit to give more pressure on the contact, and all was well ever after. You can check this is the problem by wiring up a 12v bulb with some long leads with clips at one end, then attach the lines to each side of the fuel pump in the boot. On engine start, the bulb should light-up and stay lit. If the bulb lights-up and goes out immediately, the MAF contacts may well be the problem.

if I understand you here, I can bend the arm inside there downward when i open the MAF to have well contact you mean?

You've got to be very careful ! First thing to do before you start fiddling with the MAF is to do the check with a light bulb like I did. Once set up, what you should see when you crank the engine is the bulb light up. If the contacts inside the MAF are OK, the bulb will stay alight after you stop cranking and the engine runs. If it goes out, this shows the contacts are bad. If the bulb doesnt light even when the engine is cranked, the fuel pump relay is suspect.

Hi Fraser,

Can the fuel pump relay be a suspect when another replaced MAF on same car start up and run with no single stress?

No, because if the fuel pump relay is suspect, a replacement MAF will make no difference. If the relay doesn't work, the car won't start. There is a quick and easy test for the relay. Put the car in Drive and try to start the car. The inhibitor on the shift will stop cranking, but for some peculiar reason, doesnt stop the fuel pump, so you'll hear the pump running.

The peculiar reason is that the pump circuit has two branches, starting and running.

For starting, voltage from the starter circuit triggers the fuel pump relay. For running, voltage from the AFM switch triggers the fuel pump relay.

So, when you turn the key to 'start' the fuel pump runs to fire the engine. As the engine starts the air flap in the AFM opens, actuating the little internal switch.

The red diode pack separates the two branches.

Selecting "D" (or anything except "P" or "N") prevents the starter from turning but the starter circuit is still triggering the fuel pump relay

Cheers
DD

Thanks, well understood, the car starts from both N & P shift position with the new AFM changed.
So now the deal is to work on the faulty AFM and fix it to perform its function