[Gmecm] HELP: MAF Burn-Off problem

[Mark Tonazzi]

I got MAF error codes when my ECM was crapping out.  It would run fine until
warmed up then intermittently it would throw a MAF  voltage high or voltage
low code and go into limp home mode.  If you can get your hands on another
known good ECM may want to try that.

-----Original Message-----
From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf
Of Rick McLeod
Sent: Friday, September 29, 2006 2:57 AM
To: gmecm at diy-efi.org
Subject: [Gmecm] HELP: MAF Burn-Off problem

IIRC: running $32b on '165 in '86 L98 vette, it's setting the 'obligatory'
code 36 MAF sensor burn-off voltage missing. I've been down the road of all
the classic 'fixes' such as replacing the relay, cleaning the contacts,
etc., and now tearing into wiring to try to find the problem.
 
Part of the problem is that this signal only occurs after a warm engine
shutdown, and IIRC the engine has to have been running for some period of
time prior to trigger this diagnostic/service circuit into action at engine
shutdown. Makes for difficult diagnostics at best, but I've got to find the
headache.
 
Plan of action is to 'tap' the wires to the MAF for monitoring signals. Once
that is done, I'll be able to monitor the activity since the 'tap' is going
to be a 4-wire trailer light socket so I can plug a 4-wire 'extender' into
it and temporarily monitor it in the cabin until I get the problem fixed.
Then I'll weather seal the stub connector and tuck tidily away into the loom
in case I need it again (suspect I may, this is a chronic problem w/ this
version of MAF).
 
Now, my question is: What does the ECM/code look for at shutdown to
determine that the MAF burn-off cycle completed. IIRC the ECM has a line
that enables the burn-off relay a few seconds after shutdown (which is what
I'll be monitoring at the MAF diag dongle above) but since there is NO air
flow, does it look for some delta in the signal from the MAF (and, isn't the
BOSCH MAF a frequency based signal vs. the voltage based on the DELCO MAF
variant?). Once I confirm that the MAF is receiveing burn-off voltage
correctly at shutdown, then my next step is obviously to diag the signal at
the ECM, but what am I looking for, is it just the presense of signal due to
the MAF being energized post shutdown, or is it a change in the signal from
the ECM due to the burn-off causing a very hot heated wire, then cooling,
hence that should cause a delta in the MAF signal seen by the computer.
 
My hunch is that I'm actually fighting a poor current path, like maybe a
high resistance fusible link or a corroded ground connection to the MAF
which would cause it to work correctly but the high current demand by the
burn-off cycle causes excessive drop in the supply/return path and not
enough current passes to actually burn-off the element correctly.
 
At any rate, knowledge is key, I can get lots of data, but what is it going
to tell me? What is normal opperating condition, so I know what to 'fix' on
this pesky critter.
 
BTW, it works wonderfully from a drivability standpoint if I reset the codes
on the ECM and then drive, it warms up fine, and performance is normal but
after that the next restart will have a check engine light and the code will
be set for this, and performance sucks big time, since it is not using MAF
data I presume is using static or even worse I suspect LIMP mode?
 
Sorry for wordy post, but info is key to getting quality responses and this
group has that quality.
 
Cheers, thanks in advance
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