Wrong AFM, or faulty?
Discussion
TLDR: Bought new AFM, Rovergauge reports extremely high long-term trims, even up to 100%. Put old (but good-ish) AFM back, trims return to sensible values. Have I bought the wrong AFM, or am I justified in returning the AFM as faulty?
The full story:
In January my car had a new cam fitted by a well-respected TVR specialist just off the M40 in Oxfordshire. When I got it back I took it to Surrey Rolling Road and discovered it was running way too lean ( thread here). Rovergauge frequently (but intermittently) reported long-term fuel trims of +100%, so the ECU was trying very hard to enrich the mixture but it couldn’t do enough.
I took it back to the specialist, and after a long time I now have it back with a new fuel pressure regulator and 8 refurbished injectors. On a test run I found that the fuel trims were distinctly better, but still would go to +100% on occasion. So I wasn’t convinced the problem was solved.
I decided to try swapping the AFM by fitting the old one (probably factory original) that was on the car when I bought it, but I had changed it for a new genuine AFM as a precaution when I bought the car. I wish I’d done this swap to start with, because immediately the long-term fuel trims went to sensible, reasonably stable values.
So clearly the replacement AFM that I put on 10 years ago has now failed, presumably under-reading and causing the ECU to start too low in the fuel map. It tries hard to correct the error, but ends up at +100% with no further trimming possible and the mixture is still lean (16:1 or more, as seen at SRR).
I could just stick with the old original AFM, but I don’t feel comfortable relying on an ancient AFM, so I bought a new one from Racing Green. I fitted it today, and guess what? It gives very very high fuel trims, with one bank hitting +100% at one point.
I put the old original AFM back, and I now have sensible trims again. Around +20% or thereabouts.
So it looks like the new AFM is wrong - have I bought the wrong part? I thought it was one part across all engine variants (mine’s a 1999 450).
Or am I justified in sending it back as faulty?
The full story:
In January my car had a new cam fitted by a well-respected TVR specialist just off the M40 in Oxfordshire. When I got it back I took it to Surrey Rolling Road and discovered it was running way too lean ( thread here). Rovergauge frequently (but intermittently) reported long-term fuel trims of +100%, so the ECU was trying very hard to enrich the mixture but it couldn’t do enough.
I took it back to the specialist, and after a long time I now have it back with a new fuel pressure regulator and 8 refurbished injectors. On a test run I found that the fuel trims were distinctly better, but still would go to +100% on occasion. So I wasn’t convinced the problem was solved.
I decided to try swapping the AFM by fitting the old one (probably factory original) that was on the car when I bought it, but I had changed it for a new genuine AFM as a precaution when I bought the car. I wish I’d done this swap to start with, because immediately the long-term fuel trims went to sensible, reasonably stable values.
So clearly the replacement AFM that I put on 10 years ago has now failed, presumably under-reading and causing the ECU to start too low in the fuel map. It tries hard to correct the error, but ends up at +100% with no further trimming possible and the mixture is still lean (16:1 or more, as seen at SRR).
I could just stick with the old original AFM, but I don’t feel comfortable relying on an ancient AFM, so I bought a new one from Racing Green. I fitted it today, and guess what? It gives very very high fuel trims, with one bank hitting +100% at one point.
I put the old original AFM back, and I now have sensible trims again. Around +20% or thereabouts.
So it looks like the new AFM is wrong - have I bought the wrong part? I thought it was one part across all engine variants (mine’s a 1999 450).
Or am I justified in sending it back as faulty?
Edited by Dr Mike Oxgreen on Wednesday 4th May 16:49
That does indeed sound like the new AFM is faulty.
Over recent years I've studied these AFM's and have developed a simple statistical analysis that will diffinitively establish a good/bad AFM. In a nut shell a bad AFM has much greater variability in its output than a good AFM.
If you wish I can do an analysis on your AFM's for you, all I need is some RoverGauge log data from you (by email).
Run the engine up to temperature and then record 10 minutes of data with engine at idle. With these options in RoverGauge
DIRECT MAF,
ABSOLUTE THROTTLE,
SHORT TERM TRIM
Send me the logfile and I will analyse them and send back report.
Over recent years I've studied these AFM's and have developed a simple statistical analysis that will diffinitively establish a good/bad AFM. In a nut shell a bad AFM has much greater variability in its output than a good AFM.
If you wish I can do an analysis on your AFM's for you, all I need is some RoverGauge log data from you (by email).
Run the engine up to temperature and then record 10 minutes of data with engine at idle. With these options in RoverGauge
DIRECT MAF,
ABSOLUTE THROTTLE,
SHORT TERM TRIM
Send me the logfile and I will analyse them and send back report.
Edited by blaze_away on Wednesday 4th May 19:17
There are lots of copies out there, and I doubt you could get a genuine Lucas old stock / new stock. A simple test is to simply start RoverGauge and blip the starter (don't start) to get it to read the AFM standby voltage- it should read 6% with no airflow or about .3 volts . If its outside this range the whole scaling will be wrong. Blazes log file analysis is more accurate as it will pick up scatter at higher airflows so certainly worth doing if both AFMs give you 6%. The very early AFMs are Hitachi Units and called the Lucas 3AM and have fewer pins but the two are plug compatible wiring wise. Notably the Mark Adams Tornado chip is specific to the 3AM or 5AM AFM, but far as I can tell the two units are fully compatible, but I have to admit I've not flowed the two together to check if the outputs match 100%
Wow!
Once again I am blown away by the depth of knowledge and generosity of a PistonHeads member!
Many many thanks to blaze_away for analysing the log files of my newly-purchased AFM, and also the bad one that I recently removed and the original that I replaced 10 years ago.
The conclusion is that the recently removed bad one is indeed properly bad - under-reading and with a wide scatter of readings. It's no wonder that this sensor was causing the engine to run so lean that the ECU couldn't correct it with the fuel trim.
The old original is pretty decent, and for the time being I will definitely be going back to it.
The newly purchased one has a fairly tight distribution of readings, but is clearly under-reading overall. This explains why I see very high positive fuel trim with this sensor - so high that I'm not happy using it. And I did actually see +100% at one point.
So with this ammunition I will be sending the new AFM back to the supplier for a refund.
Thanks again to blaze_away!
Once again I am blown away by the depth of knowledge and generosity of a PistonHeads member!
Many many thanks to blaze_away for analysing the log files of my newly-purchased AFM, and also the bad one that I recently removed and the original that I replaced 10 years ago.
The conclusion is that the recently removed bad one is indeed properly bad - under-reading and with a wide scatter of readings. It's no wonder that this sensor was causing the engine to run so lean that the ECU couldn't correct it with the fuel trim.
The old original is pretty decent, and for the time being I will definitely be going back to it.
The newly purchased one has a fairly tight distribution of readings, but is clearly under-reading overall. This explains why I see very high positive fuel trim with this sensor - so high that I'm not happy using it. And I did actually see +100% at one point.
So with this ammunition I will be sending the new AFM back to the supplier for a refund.
Thanks again to blaze_away!
Very kind comments by all thank you.
These are the charts from data supplied and my good and bad AFM's
Chart 01 OP NEW AFM
Chart 02 OP OLD GOOD
Chart 03 OP OLD BAD
Chart 04 my ref good AFM (20AM)
Chart 05 my ref Bad AFM (5AM)
My evaluation is this.
"02 Old Good" is not bad, shows good tight MAF readings
"01 New" is definately reading low and does result in ECU under fueling.
"03 Old Bad" is more erratic and reads very low
"04 good 20AM" really tight very little variability reads slighly lower than 5AM's but exactly as you'd expect. Runs perfectly with good Wideband results and well balanced spark plug colour)
"05 Baf 5AM" very varoable and reading high caising over fueling (which it did)
Hope this helps others with somilar predicaments.
These are the charts from data supplied and my good and bad AFM's
Chart 01 OP NEW AFM
Chart 02 OP OLD GOOD
Chart 03 OP OLD BAD
Chart 04 my ref good AFM (20AM)
Chart 05 my ref Bad AFM (5AM)
My evaluation is this.
"02 Old Good" is not bad, shows good tight MAF readings
"01 New" is definately reading low and does result in ECU under fueling.
"03 Old Bad" is more erratic and reads very low
"04 good 20AM" really tight very little variability reads slighly lower than 5AM's but exactly as you'd expect. Runs perfectly with good Wideband results and well balanced spark plug colour)
"05 Baf 5AM" very varoable and reading high caising over fueling (which it did)
Hope this helps others with somilar predicaments.
Dr Mike Oxgreen said:
Wow!
Once again I am blown away by the depth of knowledge and generosity of a PistonHeads member!
Many many thanks to blaze_away for analysing the log files of my newly-purchased AFM, and also the bad one that I recently removed and the original that I replaced 10 years ago.
The conclusion is that the recently removed bad one is indeed properly bad - under-reading and with a wide scatter of readings. It's no wonder that this sensor was causing the engine to run so lean that the ECU couldn't correct it with the fuel trim.
The old original is pretty decent, and for the time being I will definitely be going back to it.
The newly purchased one has a fairly tight distribution of readings, but is clearly under-reading overall. This explains why I see very high positive fuel trim with this sensor - so high that I'm not happy using it. And I did actually see +100% at one point.
So with this ammunition I will be sending the new AFM back to the supplier for a refund.
Thanks again to blaze_away!
How was the new one badged? Can you say who the suppliers was in a round about sort of way? Just to help to know what to avoid in the future? I closely looked at some Bosch replica AFMs off Ebay and they were terrible, specifically the grid that goes across the air intake ,it looked like a portculluis and they where not temperature stable, the output would change as the engine bay got hotter. Not seen a replica 5AM to test yet.Once again I am blown away by the depth of knowledge and generosity of a PistonHeads member!
Many many thanks to blaze_away for analysing the log files of my newly-purchased AFM, and also the bad one that I recently removed and the original that I replaced 10 years ago.
The conclusion is that the recently removed bad one is indeed properly bad - under-reading and with a wide scatter of readings. It's no wonder that this sensor was causing the engine to run so lean that the ECU couldn't correct it with the fuel trim.
The old original is pretty decent, and for the time being I will definitely be going back to it.
The newly purchased one has a fairly tight distribution of readings, but is clearly under-reading overall. This explains why I see very high positive fuel trim with this sensor - so high that I'm not happy using it. And I did actually see +100% at one point.
So with this ammunition I will be sending the new AFM back to the supplier for a refund.
Thanks again to blaze_away!
Gassing Station | Chimaera | Top of Page | What's New | My Stuff