Unplug MAF=does not run

[mysunnshine]

I'm trying to do the initial SD tune on a 427ci 05 H2 Hummer and I'm running into a problem. When I disable the MAF in the software and unplug the MAF, it fires up but immediately dies and will not run. My assumption is that there is not enough fuel in the fuel map to support an unplugged MAF. Either adding or taking away fuel does not make a difference. It still will not continue to run even after making those changes. When it's running OL, you can unplug the MAF and it'll continue to run so the fuel map can't be that far off. Any ideas?

[BRD-PREY]

check the map sensor or map wiring, it just happened to me
ed

[mysunnshine]

The MAP sensor works because I have been logging it in closed loop fueling to determine that it needs some fueling changes.

[Taz]

Hello mysunnshine,

I take it your vehicle is using a 5 pin integrated MAF / IAT sensor. Are you completely unplugging the MAF - or just removing the yellow signal wire from the connector ? You need to report IAT to the PCM during the AutoVE process.

What AFR / Lambda / EQ does your WBO indicate when you start the vehicle, then unplug the MAF ? As you know, while using the AutoVE technique there is no type of STFT. Add at least 15% to the VE table before you begin. On unknown parts combinations I have initially added 15%, drove the vehicle about 100 yards, stopped - added another 15%. You may have to make a series of these short trips until you get into a safe "ballpark" for initial fueling.

Also, if your tune uses B4206 (O2 with open loop) or similar parameter - this needs to be set to disable.


Regards,
Taz

[mysunnshine]

The MAF I'm using has a separate IAT sensor. I do understand that there is no LT or STFT when in OL using the AutoVE. I hear ya on the adding 15%. I actually added 10% at a time up to 50% and did the same with taking the fuel away from the fuel map assuming that it may be that far off with the MAF unplugged. Going both ways on the fuel map did not change anything. Is there anything else in the tune that can not allow it to run SD and only MAF. I just hate to get ambushed on this one because it was tuned by somebody else before I got it.

[Taz]

What did your WBO indicate during all of those attempts at fueling changes ?

If the MAF fails mechanically / failed via tune / is disconnected - the PCM defaults to the VE table.

What DTCs are being set ?

I take it this is a DBW a setup - did you also adjust the ETC predicted airflow (to avoid limp mode while tuning) ?


Regards,
Taz

[mysunnshine]

The WBO2 indicated about 20% lean during OL datalogging. As I was adding fuel to the fuel map no changes were indicated by the WBO2 which made me take a closer look at the MAF. That's when I found out that when the MAF is unplugged, it would not run. I actually assumed that if the MAF was disabled in the software that it would not contribute at all to the fueling. It is a DBW set-up and I did not modify the predicted airflow but I have not had the limp mode activated yet.

[joecar]

The PCM fails the MAF (i.e. runs from VE) only if a MAF DTC is present... check for this.


i.e. without a MAF DTC, the MAF table is still being used.

[joecar]

... As I was adding fuel to the fuel map no changes were indicated by the WBO2
...Which table are you referring to...?

Are you saying that changes to B3605 resulted in no change in wideband lambda...?

Can you sanity test the wideband...?

[joecar]

Does it run in MAF only (set B0120 to disable VE)...? In this mode does wideband agree with B3605...?

[mysunnshine]

The PCM fails the MAF (i.e. runs from VE) only if a MAF DTC is present... check for this.


i.e. without a MAF DTC, the MAF table is still being used.

I'll assume you're talking about DTC enabler P0101-P0103 right? My current config is P0101 not reported but the other two are set at 2 trips.

[mysunnshine]

Which table are you referring to...?

Are you saying that changes to B3605 resulted in no change in wideband lambda...?

Can you sanity test the wideband...?


Sorry...I used the wrong terminology for the factory programming. I was adding and subtracting fuel in the VE map. If I change the B3605, it will respond accordingly.

[joecar]

I'll assume you're talking about DTC enabler P0101-P0103 right? My current config is P0101 not reported but the other two are set at 2 trips.Does P0102 or P0103 trigger immediately...?

The PCM will continue to use the MAF table until one of P0101, P0102, P0103 trigger.

Those DTC's set to 2-trips will not trigger immediately (requires the MAF fail for 2 trips)... set them to 1-trip, and check for DTC to trigger.

[mysunnshine]

I now see that the B0120 has been set to 400 which means that somebody had the programming set so it runs MAF exclusively correct?

[joecar]

I now see that the B0120 has been set to 400 which means that somebody had the programming set so it runs MAF exclusively correct?Yes correct, down to 400 rpm.

[mysunnshine]

Well there we go....So now I understand why it would fire up and then immediately die because it would go past the 400rpm threshold and not see anything to fuel the motor with if the MAF was unplugged.

So on a side subject...Isn't it unusual to tune a twin-turbo motor to run MAF only and not a blend of the VE and MAF together?

[joecar]

Yes, set B0120 to zero.

But, watchout, the PCM limits MAF to 512 g/s.

[mysunnshine]

But, watchout, the PCM limits MAF to 512 g/s.

Which is probably why the last motor broke I'd imagine. Is there any reason to run a MAF only tune?

[joecar]

Which is probably why the last motor broke I'd imagine. Is there any reason to run a MAF only tune?Any good reason to run MAF only...? No, not really.

With B0120 set to the stock value 4000 rpm, the PCM uses VE during transient conditions under 4000 rpm...
i.e. GM considered the MAF to be slower to respond to transients than VE which is computed from MAP;
the MAF would have a slower response to transients than VE/MAP for two reasons:
- implementation details/limitations,
- laws of physics: MAF (mass) cannot instantly change since mass has momentum; MAP (pressure, has no mass) can change as fast as the throttle blade.

Also, if the MAF sensor fails, the VE should be able to run the engine with no noticeable difference (except maybe throttle response is quicker)... the VE table must be correct for this to be possible.

[mysunnshine]

I'll see what I can do with this thing tomorrow. I really appreciate the help-

[joecar]

No worries :cheers: