Hi,
Question for the group: On my 1989 Formula 350 TPI with key on engine off my MAF shows 5 gms with my scan tool (OTC 4000e) and my laptop using TunerPro RT.
Is this normal? I search the internet, my factory service manual & on here and cannot find anything stating on what it should read with engine off.

My car at idle (engine at 170 deg f) was showing MAF of 10-12 gms at idle (675 rpm) and running rich. MAF was 10.31, BLM was at 108 and INT at 81 so ecm was trying to lean it out.
So I started checking my MAF circuit and found I had 15 ohms resistance on the black wire (MAF pin B to ecm pin A11). So checking I found some hack probed through
the wire insulation at the connector on all the MAF wires. I ended up installing a new connector with terminals and 12" wires and retested and now at an idle of 675 my MAF is 5 to 6 gms. So my repair fixed the rich idle condition.

But with key on engine off MAF still shows 5 gms! So does the ecm automatically have the MAF 5 gms for start up?

 

Well diagnosed.

 

On the Pontiac Firebird Formula 350 TPI (1989), seeing around 5 g/s with key on engine off is actually pretty normal for that system. Most GM MAF setups of that era don’t truly read “zero” KOEO — they often sit in a small baseline range due to sensor calibration and how the ECM interprets airflow voltage at rest.

Your idle numbers before the repair (10–12 g/s with rich BLM/INT correction) definitely point to a signal issue rather than true airflow. The fact that you had resistance in the MAF ground/return circuit explains it perfectly — that kind of added resistance will skew the frequency/voltage signal and make the ECM think more air is entering than actually is.

After fixing the connector and dropping to 5–6 g/s at idle, plus more normal fuel trims, that’s exactly what you’d expect from a healthy reading on these systems.

 

Thanks for the answer. I did not have another TPI car to compare the readings and could not find anything for koeo readings in my oem service manual.