Airflow meter - voltages - problem?

[Tobs]

I've been having a few probs with my chim lately (SEE <LINK>,">www.pistonheads.co.uk/gassing/topic.asp?t=243112&f=8&h=0</LINK>, when it has warmed up erratic idle, and miss-firing etc. Plus are very black - not sure if this is the problem or symptom.

Are the voltages given in the bible for the airflow meter output correct? As I’m getting higher voltages than those stated:

About 1 volt with the ignition on, but the engine not running. (Bible says should be .25-.5v
1.5-2v for the engine idling (or idling as well as it can anyway! Bible says 0.5v to 1.5v
2v-3v for 3000rpm ish. bible says 1.1v to 3v.

The car is more or less un-drivable at the moment when war missing and won't idle. However if I unplug the airflow meter it seems to run and idle OK - maybe running rich though. I assume its going onto a default 'limp home' map.

Do you think I’ve found the problem as the high voltages suggest? Or does unplugging the Airflow meter cause the ecu to go into a default map that also ignores other sensors like the lambdas?

[leebus]

Hi, i had the same problem with my chim 4.0 about 5 years a go! Unplugging the airflow puts the ecu in limp home mode! Mine was running like a dog intermitently, it seemed to over fuel on one bank (black smoke), i changed the lambda sensors, then airflow meter, usual plugs, then leads, then cats, then tps switch, I checked voltage readings at the ecu via pin data! And it still did it!
So then i came to conclusion it was wiring loom so i changed it for a new one! (that was fun)
Then 4 weeks later it still had the same problem!
I was gutted but not easily beat, I got it in the air and found a corroded earth lead to the gearbox (surely that wasnt the reason)
I didnt build my hopes up as i repaired the lead and bolted it back on!
IT FIXT IT!
I did another 10k in the car without a problem, eventually selling and buying a cerb!
Hope ive been of help

[griffter]

Your voltages are similar to mine (albeit on a 4.3 precat Griff). The voltages will vary depending on whether you measure at the meter or at the ecu, and presumably whether you measure under load or no load. Either way, I think the important thing is that the voltage varies with air drawn in - which yours does - and precise comparison of values between afms is not all that relevant, except perhaps for like tests on the same car. There are more tests on the RPi website, but these are different values again. The only solution for a duff afm is substitution, so perhaps try a substitute and see if your problem is resolved. Personally I think your voltages are OK - if only because the experts don't quite agree!

[Tobs]

Thanks for your replies. Last night I removed the AFM, and took it apart, by cutting the sealant and removing the black plastic cover, under which there is a metal plate (held on by 4 torx screws). After removing this I discovered a PCB inside which was wet! So I left it on top of our boiler overnight to dry out, and tested it again this morning.

Car is now running much better - although still not perfectly - but this could be due to me resetting the ECU and it having to relearn its settings, and also perhaps the plus could do with a clean. Seemed to get better the more I drove it (12mile drive into work!).

With the ignition on and the engine off I am getting a voltage of about 0.4v, with the engine idling 1.5v, and 2-3v at 2000-3000rpm. These seem to tie better with the values given in the book, although maybe still on the high side. I may also check the earths on the car when I am under it at some point in the future.

I assume the water in the AFM was evaporating and re-condensing on some critical parts inside when it warmed up (this is the only time I seemed to have the problem).

Now all I need to do is to find out how the water got into the AFM (think it maybe through the connector) and re-seal it up.