MAF sensor readings

[sierrap615]

hello, i've been working on a 2000 astro RWD 4.3 in the shop for the last few days, trying to track down a P0300 random missfire and a P0339 cranksensor intermittent. i think i haven the P0339 taken care of, caused by the last tech who worked on it(he replaced the CKP without doing the crank learn), but the P0300 is still there, most of the missfires are on cylinder#4(gotta love live data) i'm thinking plugs, about 6-8 months old, bosch double platinum, i only had a chance to pull #4 plug, with some funny wear on the inslator. tomarrow i'm going to put in a set of copper plugs, see if it helps then recommend Delco plugs. but the other item i see off in the datastream is the MAF. it reads 5.5-6.0 g/s in park at idle, most cars i work on (small 4 cylinders of course) i see a reading around 2.5-3.5 g/s. LTFT was about - 11-12% negitive, i think injector duty was 3.4-3.6ms. do these readings seam off to anyone elses? i'm i looking at a bad MAF?

much thanks - anything to get this van out of my bay!

[maxwedge]

Originally Posted by sierrap615

hello, i've been working on a 2000 astro RWD 4.3 in the shop for the last few days, trying to track down a P0300 random missfire and a P0339 cranksensor intermittent. i think i haven the P0339 taken care of, caused by the last tech who worked on it(he replaced the CKP without doing the crank learn), but the P0300 is still there, most of the missfires are on cylinder#4(gotta love live data) i'm thinking plugs, about 6-8 months old, bosch double platinum, i only had a chance to pull #4 plug, with some funny wear on the inslator. tomarrow i'm going to put in a set of copper plugs, see if it helps then recommend Delco plugs. but the other item i see off in the datastream is the MAF. it reads 5.5-6.0 g/s in park at idle, most cars i work on (small 4 cylinders of course) i see a reading around 2.5-3.5 g/s. LTFT was about - 11-12% negitive, i think injector duty was 3.4-3.6ms. do these readings seam off to anyone elses? i'm i looking at a bad MAF?

much thanks - anything to get this van out of my bay!

You are ok on the maf readings for that engine, that's about what I see on the 4.3's, the lt fuel trim maybe a result of the misfire. No Bosch plugs in GM car/truck!

[MT-2500]

Originally Posted by sierrap615

hello, i've been working on a 2000 astro RWD 4.3 in the shop for the last few days, trying to track down a P0300 random missfire and a P0339 cranksensor intermittent. i think i haven the P0339 taken care of, caused by the last tech who worked on it(he replaced the CKP without doing the crank learn), but the P0300 is still there, most of the missfires are on cylinder#4(gotta love live data) i'm thinking plugs, about 6-8 months old, bosch double platinum, i only had a chance to pull #4 plug, with some funny wear on the inslator. tomarrow i'm going to put in a set of copper plugs, see if it helps then recommend Delco plugs. but the other item i see off in the datastream is the MAF. it reads 5.5-6.0 g/s in park at idle, most cars i work on (small 4 cylinders of course) i see a reading around 2.5-3.5 g/s. LTFT was about - 11-12% negitive, i think injector duty was 3.4-3.6ms. do these readings seam off to anyone elses? i'm i looking at a bad MAF?

much thanks - anything to get this van out of my bay!

As maxwedge says no bosch for gm.
Just drop them in the trash can.
Also check the cam retard setting. If over -2 /+2 it will need to be set.
Here is some guide lines on MAF checking
And match your allitude to baro reading.

On a fully warmed up engine, look at Long Term Fuel Trim at idle, in Neutral, A/C off, (LONGFT1 and/or LONGFT2 PIDs). If it is more negative than -12%, the fuel system has learned lean corrections which may be due to the MAF sensor over-estimating air flow at idle. Note that both Banks 1 and 2 will exhibit negative corrections for 2-bank system. If only one bank of a 2-bank system has negative corrections, the MAP sensor is probably not contaminated..

3. On a fully warmed up engine, look at MAF voltage at idle, in
Neutral, A/C off (MAF V PID). If it's 30% greater than the nominal MAF V voltage listed in the Powertrain Control/Emissions Diagnosis (PC/ED) Diagnostic Value Reference Charts for your vehicle, or greater than 1.1 volts as a rough guide, the MAF sensor is
over-estimating air flow at idle.

4

If at least two of the previous three steps are true, proceed to disconnect the MAF sensor connector. This puts the vehicle in to Failure Mode and Effects Management (FMEM). In FMEM mode, airflow is inferred by using rpm and throttle
position instead of reading the MAF sensor. (in addition, the BARO value is reset to a base/unlearned value.) If the lean drive ability symptoms go away. Replace or clean the MAF sensor

BAROMETRIC PRESSURE REFERENCE CHART
Barometric Barometric BARO/MAP Altitude Above
Pressure Pressure PID Sea Level
(in. Hg.) (kPa) (Hz) (ft)
3.5 11.8 89.3
5 16.9 92.8
10 33.8 104.6
15 50.7 117.0 14,000
20 67.5 129.6 10,000
21 70.9 132.5 9,000
22 74.3 135.4 8,000
23 77.7 I 138.3 7,000
24 81.1 141.1 6,000
25 84.4 144.0 5,000
26 87.8 146.9 4,000
27 91.2 149.8 3,000
28 94.6 152.8 2,000
29 97.9 155.8 1,000
30 101.3 158.9 0 (sea level

[sierrap615]

much thanks for the info. here is the update for today -

swaped #4 plug for a copper gapped to spec (0.060), no change in the missfire counts(which BTW the missfire only happens off idle), swaped #2 and #4 wires, no change, removed doghouse so i could watch fuel pressure as i'm driving(between 61-64 psi, its not a very good gauge), while driving i can hear arcing, check cap/rotor, no problems, listened with vacuum line, sound like wire to coil is arcing but no visible arc and no appearent damage to wire, (wires were replaced 6 months ago with plugs) i wet the wire and still no visible arc, i touch the wire and it arcs to my finger. tech in the bay next to me also sees a arc at wire connection to coil, removed coil and there is a little rust peice i beleave from the old wires jammed in the coil, not allowing the wire to fully seat. ordering new wires and coil tomarrow, going to recommend replacing the plugs with coppers under warrenty to prevent another comeback and some fuel injectior cleaning while i have the doghouse off anyway. hopefully that will solve the problem, but with this van everytime i fix one problem another one shows up!when the missfire is gone i will recheck the MAF and LTFT, thanks again!

[Blue Bowtie]

Like the BigWedge and MT tried to advise, NO plugs other than dual platinum (brands really don't matter) with a wasted spark system. Anything else is just asking for trouble/uncompensated warranty work. Avoid any of the designer plugs, such as "SplitFire", "+4", "V-Groove", or any fixed-gap types. If you pull the standard copper cored plugs in about 10K miles you might already detect erosion on the side electrodes on half of them, and erosion of the center electrodes of the other half, with increasing gaps on all of them.

FWIW, "strange" MAF indications are very, very rarely the fault of the MAF itself, and more often from a connection or air plumbing problem.

[sierrap615]

i beleave the problem has been solved with a new coil and wires, would have never found it if i didn't drive with the doghousing off to check fuel presure. after the repairs were done i rechecked fuel trim which with the better burn is now in the realm of negitive 3%.

thanks for the help guys

[Blue Bowtie]

So, the old optical scanners picked up the corona and arcs during your test drive, eh? Sometimes the old technology works the best.

Advising your customer that the data shows they should be using up to 3% less fuel should take the bite out of the repair bill.