Hot wire AFM

Gents I have a 3.9efi RRC running an entirely standard non cat hot wire system. Can anyone tell if there is a special procedure for resetting the relationship between the AFM and the ECU after I have run the engine with the AFM disconected.

As long as the CO trim is right and the air flow signal scales properly there is nothing special to do - why do you want or need to "reset" the AFM? Disconnecting it and firing the engine doesn't change the air flow meter in any way although it would cause a fault in the ECU and the EFI warning light would come on. Also, the ECU would go in to limp mode making the engine run extremely rich. You would need to disconnect the ECU to clear the fault.

Yeah just unplug the main ecu plug momentarily. Just did this at the weekend.

Make sure MAF is set to 1-1.5v on CO trim screw whilst engine is ticking over

Annoyingly my ecu warning lamp very rarely comes on. Didnt even come on when MAF was disconnected

It does work though because sometimes it flashes on bulb check very quickly when I switch her on...

I have reset the AFM to 1.5 volts but the gas analyser suggests the engine is running very lean not rich. Luckily it only starts on petrol then switches to LPG. I did, by accident, drive it with the AFM diconnected so I will disconnect the ECU as suggested and see what happens. Cheers Nick

countrymansouth said:

I have reset the AFM to 1.5 volts but the gas analyser suggests the engine is running very lean not rich. Luckily it only starts on petrol then switches to LPG. I did, by accident, drive it with the AFM diconnected so I will disconnect the ECU as suggested and see what happens. Cheers Nick

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What readings are you getting from a gas analyser and what makes you think it is running lean?

When I had it MOTed, the examiner commented that it was ok on LPG but very lean on petrol. Unfortunatley I didn,t keep a note of the petrol reading. Although only starting on petrol would like to have it set correctly so need to find someone with a gas analyser on Sussex/Surrey border. Cheers Nick

How do know it is very lean on petrol? With your CO trim set at 1.5v it is unlikely to be very lean. The other thing to bear in mind is that as far as I am aware there are some holes in the standard fuel mapping which does cause lean spots - these are intentional to improve fuel economy and also to meet certain emission standards at the time the system was introduced.

If your concern about running lean is based on a single measurement of exhaust gasses I would respectfully suggest you need to get more data. The lamda reading is th eone that typically indicates rich or lean but it is of little use on a non-cat system, if I really try hard I can get a lamda reading of between 0.97 - 1.0 but the air flow meter is turned all the way down I mean CO trim 0v. Currently, I have good CO and HC with acceptable lamda at a CO trim of about 1.2v but remember also that any change to exhaust system or free flow air filter will affect how the system responds, mine has a stainless exhaust with 4 branch headers and I also have a K&N air filter fed through a snorkel.

As you said, you need to get it on an exhaust gas analyser for a bit of time to tune it up, also remember that the ignition timing will have been altered to run on gas, this will affect the way your engine burns petrol and hence the emissions.

Smart guy is this Kev!!

Thanks for the advise. The ignition is set to 10 degrees BTDC for the gas use 97 ron petrol. I did as suggested and unplugged the ECU. After reconnecting car started instantly whereas before took a few churns to start, also hot restart was instant where it had needed a good cranking before. Will plug in a gas analyser and have a look. Again thanks for the help. Nick