Hi All,

2000 Blazer 4.3.

MAF reads 0.74 @ 650 and 2.0 @ 2000. These are without load, and so I am not 100% sure they mean much.

No codes.

Symptoms include lack of power and intermittent misfire (not enough to throw codes.) Funny shifting.

I unplug the MAF and drive it, and... no codes!

However, it does perform a LOT better. Lots of power, and of course full power shifts.

I did not expect a lack of codes with the MAF unplugged.

Any thoughts?

Thank you,

Josh

 

Engine at normal operating temperature, (200F), rpm at idle, (650 rpm), trans in park or neutral, all accessories turned off, MAF should indicate 4.6 g/s (grams of air per second). Your reading of .74 g/s is obviously low, or incorrect, or both. Clean the MAF sensor elements. If that doesn't fix it, the next thing to check for is air intrusive, (unmetered air being consumed by the engine). Possibly a leak after the MAF sensor. Could be the inlet tube between the MAF and the throttle body, the gasket where the tube attaches to the throttle body, or a vacuum leak downstream of the throttle blade. Fuel trims are probably positive, and oxygen sensors are probably indicating lean, post the readings of them. Also check fuel pressure and leakdown, and post those results too.

 

Try cleaning it. If the wires and such on the inside are dirty, they'll look brown or black. Hit it with some MAF cleaner (only MAF sensor cleaner) and see if it helps. The metal in the middle should look gray to silver. Had the same issue with my Alero. Different vehicle yes, but same exact problem.


If cleaning it doesn't work, it could be bad. MAF problems won't always set a code.

 

Hello,

I recently did the manifold gaskets and checked a lot of stuff during and after that time. The vacuum leaks due to the gasket were masking a lot of other problems.

Now, as for those readings, I expected what you are saying. However, the complete list of those I wrote down are these:

MAF

0.74 @ idle
2.00 @ 2000rpm

Absolute TPS

0.0 - 0.4 @ idle (gotta check this! No pedal movement.)
8.0 @ 2000rpm

Ignition Advance

20.0 @ idle
35.5 @ 2000rpm

Fuel Trim 1

-10.2 @ idle
- 2.3 @ 2000rpm

Fuel Trim 2

-12.5 @ idle
- 3.1 @ 2000rpm

(That last I was trying to get to go positive and simply could not.)

... and I totally forgot to record the O2 sensor readings. They were within normal operating range. All three are new, as well.

I need to get the multimeter out tomorrow and check the values at the MAF terminal.

I had cleaned the MAF during the intake gasket job, and made a gasket for the top of the throttle body as I never did like that one that's inside the hat, and there was a small intake leak there due to the erosion of that built-in one. It ran better after the cork gasket was installed.

I've been through the entire air cleaner assembly several times. I did induce a malfunction (but not a SES light or even pending code) by pulling the PCV vent off the air cleaner hat.

I did finally induce a SES light and MAF code by messing with the MAF plug enough. But, once I cleared it, I couldn't get it to come back.

The MAF sensor has over 150,000 miles on it, so I assume it's bad. The readings just don't make sense.

Regards,

Josh

 

Hello,

I plugged in the MAF and unplugged the intake air temp sensor.

It ran like a top.

Could the air temp sensor cause low readings on the MAF the way it's set up?

Regards,

Josh

 

The PCV inlet line, (the one you unplugged) is the filtered intake air supply for the PCV system. It is a calibrated vacuum leak created by the PCV valve. Unplugging the line will have no affect, other than the line could suck dirt into the engine It will not affect any sensor, it will not affect how the engine runs, and it will not set a DTC. It's one of those things that the PCM can't/doesn't check, just like camshaft retard, fuel pressure, and leakdown.

Which brings us to checking fuel pressure & leakdown at the service port, and at the fuel filter outlet. Then post the readings. Low fuel pressure and or low fuel volume can cause the MAF to report incorrect data to the PCM.


EDIT: unplugging the IAT sensor basically told the PCM that inlet air temp was -40F. The PCM increased injector pulse width which richened the air fuel mixture. Pretty much what I suspected in my earlier post regarding positive fuel trims and lean O2's. Just for grins, you might want to compare ECT, IAT, and ambient temperature on a stone cold engine, (KOEO before starting it). Definitely something stupid going on... question is what.

 

Simply need to watch the units you are working in. The numbers you report are right for lb/min. However, typically mass air flow is spoken about in g/sec.

Captain, doesn't the fact that Short Term Fuel Trim goes close to zero (much less than +/-10) at speed but is significantly low (negative) at idle indicate that O2 sensors and MAF and fuel pressure are OK - but something is causing it to go lean (from PCM calibration) at lower RPM such as a vacuum leak. A real question from me.... just trying to learn something today. (First sentence above was not a question - I'm sure about using the wrong units)

Hope this helps!

Les

 

Good thought on g/s versus lb/min. I'm so used to using g/s and Hz, because they are far more accurate, lb/min never crossed my mind


O2 & MAF data alone is not reliable enough to determine if fuel pressure is a problem. Best to check it at the service port and fuel filter. I have found, (more than a time or two) that P0171, P0174, & P0101 will all set simultaneously if fuel pressure/volume is low. I haven't been able to pinpoint what pressure or driving condition triggers it though.


Haven't put a finger on this one yet, still gathering data But, negative fuel trim means the PCM is decreasing injector pulse width to lean out the air fuel mixture. The mixture does lean out as rpm increases, which could mean dirty injector nozzles or the pintle is not sealing, (rather doubtful cuz both banks are so close to each other). Probably not a pressure regulator leak as that would affect only bank 2. Need to be very careful using O2 sensor data. The sensors measure oxygen content in the exhaust. Without combustion, or too much fuel being injected, oxygen content and sensor output voltage is high. If there is grossly too much fuel, and the heater can't keep up, the O2 sensor cools off, voltage output drops, and it indicates lean. Need to make sure the sensors aren't lying Gotta use common sense and your nose at the tail pipe Definitely something stupid going on, maybe even two problems.

 

OK successfully learned something new.... I was thinking that negative fuel trim was doing the opposite. Thanks!

Like you say, need to know how things work............

Les

 

Gents,

Know something? I got to looking at my maintenance logs on this Blazer because I'd thought I changed the wires two years ago, and turns out it was actually 2009. One wire fell in half while I was matching the new ones up.

Know what else? I thought I'd changed the fuel filter at the same time. However, I have no logs for this. It appears that I haven't changed the fuel filter since I got the truck in early 2009.

I figure it's about due. Could be causing the whole lean mess, I'm assuming.

Any special tools needed on the 2000 model year, or will needle nosed pliers suffice as they always have?

Josh