I have a 2004 Blazer 4.3L 4x4 with 88,000 miles on it. I occasonally get a P0101 MAF code when going hard on the throttle uphill with cruise control on. It doesn't happen very often because I try to take it out of cruise to prevent this. I am also getting less than desireable gas mileage (12 city/ 16 hiway). I have had a complete tune-up with plugs and wires, dist. cap, fuel and air filters, new o2 sensors, and seafoamed it twice. I rarely go over 2 grand on the tach even when accelerating. I would like to get the mileage up to where my 2002 blazer was (16/20), but just can't seem to get there.

Just a question I have about the MAF sensors role in fuel delivery. Is it only a factor during open loop? Or does it still help determine fuel ratio during closed loop operation. It is my understanding that the o2 sensors are the only thing that adjusts fuel in closed loop. Can someone help me understand this so I know what I should try next? Thanks

 

In open loop operation the PCM uses data from TPS, MAF, MAP, ECT and engine RPM to determine fuel mixture, oxygen sensor data is ignored. In closed loop operation, data from all sensors is used to determine fuel mixture.

Have you checked fuel pressure and leakdown? Fuel pressure is not monitored by the PCM and if it's low, it can cause a false MAF DTC to be set, (usually under load as you described).

 

I will give it a pressure check. Thanks for the tip and I will report what I find.

 

Key ON, engine OFF, fuel pump running: Fuel pressure must be 60psi to 66psi and must remain above 55psi for 10 minutes after the pump shuts off.

 

Results of my fuel pressure test are:

Key on, motor off = 52 psi
Bleeds down to 42 after 10 minutes

Key on, motor on = rapidly oscillates between 52-60
Bleeds down to 30 after 10 minutes

Again with key on, motor off = 52 psi
Bleeds down to 22 after 10 minutes

Looks like the pump is bad. Do you think the regulator could be bad too?

Thanks

 

Could be the pump or the regulator, or both! The results of your fuel pressure and leakdown test only indicate that there is a problem with the fuel delivery system, it doesn't tell what the problem is. To determine what the problem is, you can fabricate your own tool for ~$40 or you'll need to modify the fuel pressure tester to check maximum pump output pressure to determine if the pump builds sufficient pressure, and holds it.

 

Yeah.. hold off till you do the second part of the FP test: closed pressure/leakdown at the filter input line. While youre at it change filter then do the test at intake again.
https://blazerforum.com/forum/tech-a...agnosis-23038/

 

Ok. Thanks guys. I will let you know what I find out in the end.

 

I replaced the fuel filter and knew immediately what the problem was, when I took the old one out. A little fuel sprayed out when I undid the quick connect outlet and a lot of pressure was still there when I undid the inlet fitting. Had to be a plugged filter. I built the pressure test gauge and adapter, then tested the pressure after the new filter. 100+ psi at that point. Put it all back together and had 62 psi at the test port and maintained over 55 for 10 minutes. All it was was a dirty filter. I changed it less than 10k ago, so must have gotten some dirty gas. Now I wonder whats left? I will surely monitor the pressure and change the filter again in another 10k just for insurance. I can't believe that's all it was.
I am glad you guys kept me on track and stopped me from making more bad assumtions.

 

That's why we're here Let us know how it goes with the dirty gas.
Proof that low fuel pressure can and does, set a MAF sensor fault. Saved some $ there
Thanks for posting back with the fix.