DTC P0102 MAF CODE

1997 GMC C1500
160,000 MILES • 5.7L • V8 • 2WD • AUTOMATIC
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PMACZIK
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I have couple of problems that I cannot figure out if they’re related. I’m working on my 97 GMC 1500 2 Wheel dirve 5.7L engine. I have a low idle problem below about 750 rpms. It will cycle down until it stalls. If I unplug the MAF sensor it will run smooth around 800 rpm and up. I have an OBII diagnostic scanner. I have no other codes.
1. MAF sensor P0120 code. With the engine off and the key on, unplugged from the sensor the plug has 11.9v at ignition pin to battery ground. I have .5v at the MAF sensor signal on the plug pin. Both are in spec value. I did a continuity test between the ground pin on plug and battery ground and it is good.
I’ve taken my whole air intake units off my other 97 & 98 GMC Yukon 5.7 engines that I drive with no problem, but come up with the same issues. So, I don’t believe it is the sensors. I replaced the Idle Air Controller on the carb and cleaned the port.
2. My scanner shows me that the #6 cylinder is missing. I clip my timing light on the wire and watch it flicker. It appears to me it is flashing at a steady rhythm. I pulled the wire off plug and grounded it. It was firing showing the same. I ended up replacing all the plugs wires, rotor, and cap. It still shows 6 as misfiring.
Apr 12, 2016 at 12:12 PM
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HMAC300
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misfires are really hard to diagnose best thing here is to have a local mechanic freeze frame the sensors and see if any are off. you can check for low fuel pressure and vacuum leaks which can cause this. also take a compression test. if 20% out of any other cylinder you have a problem.
Apr 12, 2016 at 3:22 PM
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PMACZIK
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My fuel pressure at the test port by manifold is in the expected rang. Specs call for 55 to 62 and it is running at 58-59 psi. I changed the fuel filter anyway. while the fuel filter was off I tested the pressure at the feed side of the filter. It was about 69-70 psi. I did check for vacuum leaks all around the exterior top engine with carb cleaner. No change in idle speed. As far as the misfire, how does scanner know that there is a misfire (~177 count) and as I stated before I can see it firing. I will check compression.
Apr 12, 2016 at 5:46 PM
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HMAC300
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fuel pressure on this engine should be 60-66 psi key on only. the 55-62 is for a 7.4L if itdoesn't meet minimum that may be your problem.
Apr 13, 2016 at 9:30 AM
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PMACZIK
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I am not sure where your info is coming from. I am sure the range is 55-62 for my GMC 1500 truck 5.7 L. here in MI 600 ft. above sea level. I have two other GMC as stated in the original post. I use the 98 daily. I just checked the pressure with key on and it is steady at 58 or 59 psi. My GM code flow chart spec. is 55-62 psi according to my VIN number. Thanks for you time anyway.
Apr 13, 2016 at 3:07 PM
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PMACZIK
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The pressure you are referring to is for a different fuel injector system. It is the central SFI my is the SFI.
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Apr 13, 2016 at 3:22 PM
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HMAC300
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didn't think they put sfi on these trucks thought it was only big blocks. Oh well I worked in engineering at Gm for 32 yrs which I worked in motor room and not many had sfi most were cpi do a freeze frame and check your sensors the maf may be low on voltage and that will cause a misfire.
Apr 13, 2016 at 3:33 PM
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PMACZIK
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I bought this from a fellow that retired from GM in 97. It was a retirement option to have a vehicle instead of money I guess. It had 68K+ miles. I noticed a knock around 120k mi. When I pulled the engine and broke it down I found that the front main bearing was almost lose and the next one was maybe about 30 ft. pounds. I ended up installing a new crank and bearings. While I had it out I tested injector the rest and ended up replaced one. This was about 50k mi ago. I don’t drive this once the snow flies. I just got it out of storage about a week ago. The thing is in almost mint condition.

Now the misfire on #6, yes I agree with you. At this point I feel like I am wiping my rear end with a hula hoop. That is where I started. As I stated in the first post. If I understood the flowchart for code P102, the electrical connector checks out ok. Again I took both air intake units on the 98 & 97 GMC Yukon with the same engines and installed them on this pickup. I come up with the same issues. Tomorrow I am pulling a compression check on all eight cylinders.
Apr 13, 2016 at 7:31 PM
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PMACZIK
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I checked my compression. I had one at 152 the other 7 was 160-165 on a cold engine. Specs call for nothing less than 20 psi lower than the rest and nothing less than 100 psi. I am a happy man there!
Apr 14, 2016 at 7:34 AM
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HMAC300
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actually it's 20% but your good
Apr 14, 2016 at 9:26 AM
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PMACZIK
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Yes, actually its 20% of the highest reading
Apr 14, 2016 at 10:36 AM