P0102, what have I missed?

[RickC10NY]

1998 GMC Savanna 4.3L automatic, 161,000 miles

 

I had a problem with the ignition module a couple of months ago which caused a engine backfire and afterwards I had a DTC P0102 for the MAF sensor. and a code for the left ( bank 1) upstream 02sensor.

 

Assuming the backfire damaged the MAF sensor, I replaced the MAF sensor with a rebuilt from Napa and the 02 sensor . The O2 sensor code cleared but not the code for the MAF sensor. I have checked all vacuum hoses on the vehicle and replaced PCV valve. I removed carefully inspected the intake plenum hose and adapter. I Checked ENG1 fuse, Checking voltages on the MAF sensor at idle, I have 13.5 volts on the pink wire, ground on the striped wire and .75volts on the yellow wire.

 

Other than the DTC the van runs fine

 

Though not likely related to the above problem, I found a lot of carbon build up in the throttle body causing a high idle. I removed the throttle body and upper intake, I cleaned and replaced with new gaskets I also replaced the fuel pressure diaphragm and idle air solenoid. I cleaned and tested the EGR valve. I also replaced the bank #2 O2 sensor and the after converter O2 sensor.

 

 

Napa has ordered another MAF sensor just in case the rebuilt one is bad.

 

Is there anything that comes to mind that I may have overlooked?

and are there any tests that a home mechanic can do to bench test a MAF sensor?

 

Thanks

Rick

[RickC10NY]

This afternoon I picked up another MAF sensor, no change. At someones' suggestion. I got an AllData subscription. Using thier diagnostic chart I went through the steps. I was instructed to check voltages at the MAF connecter with the ignition in the "on" position but not running. I had 12 volts on the pink as required My ground wire was good. But the yellow signal wire from the MAF to the VCM showed only .75 volts instead of the 5 volts specified.( I must have misread the meter in my prior post). I also check the voltage at the connecter to the VCM . So according to the diagnostic chart the next step is to replace the VCM.

 

So before I buy a VCM can anyone think of something else that might cause low voltage from the VCM?

[RickC10NY]

I took the van to a local GMC dealer who has a good rep. Their techs confirmed the low voltage was a problem and replaced the computer, They said the computer was also shorting out the MAF sensors which is why I had two bad replacements. NAPA took both MAF's back without a hassle and gave me yet another. so now everything is working correctly.