96 5.7 Vortec P0102 Troubleshooting

[Bighorns53]

1996 5.7 Vortec Stock

Troubleshooting my p0102 that only comes on after it hits first closed loop.

Ive read 1000 pages and videos…

5v at sensor yellow, 12v on pink, ground (black/white) has 40ohms of resistance to the battery terminals. Fuse engine A1 (20a) good 0ohms.

My question to a knowlegable vortec guy on here would be as states… is it possible a bad o2 sensor would disrupt this circuit at some point? Obviously when entering closed loop. The code doesnt come on right away. Tried a new MAF, ran better, but code eventually came back.

Ps ive been chasing this random misfire thats loves bucking and jerking the truck at 2k RPM when coasting on the interstate or highways. Does just fine under engine load but trying to coast is extremely aggravating. I did pull my maf sensor harness and the truck did not spit or sputter one bit anymore with it disconnected (except startup).

P0102
P0300

Think i had an EGR pintle code at one point also an evap. Been tinkering around on it too much for some of those codes to come back but p0102 and p0300 is always the first after a nice long drive.

Bad ECM maybe?

Thanks for any help

 

[east302]

Here is the GM diagnostic from a 98, maybe it will help. It may be close enough for 96. Manuals for most years can be downloaded here:

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

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[1997]

do you have a scanner or code reader?
you need to have a scanner that can enlighten you on which cylinder(s) are causing the PO300.
how old are you O2 sensors?

 

[Bighorns53]

Thanks for the replies. Ill check out those troubleshooting steps.

Unfortunately i only have a code reader at the moment. Any recommendations on a bluetooth adapter and iphone app setup? Or a cost effective scanner a rookie such as myself would benefit from?

The truck has 250k on it. Purchased it at 150k. Last tune up around 170k. No o2 replacement though. Figured id give the maf unplug a shot, and now im intrigued that it stopped it. Especially having the P0102 along with the symptoms, i went straight to maf issues, probably a poor assumption on my part.

 

[Jeepwalker]

Here's a couple things I've experienced which may or may not be of any help to you. General things I've seen on OBDII EFI vehicles:

1) I've had MAF sensors which were low end of the 'good' range which technically 'should' have worked fine, but caused driveability issues. After replacing with a quality Bosch unit, the voltage ended up being a little higher into the 'good' range which made all the difference. So, depending the condition of the sensor you purchased and the voltage is within the 'good' range ...it could be an issue if your new one happens to be just a little too low. Seen it happen. And when you say unplugging your MAF eliminated the problem, that's what made me think of that.

2) Quadruple-check the sealing on your air tube going from the MAF to your throttle assembly. And any area along that air-path. I've seen it occur more than once where sometimes ...on some vehicles, a mechanic in a hurry put on the air snorkel too quick, and it (the rubber part) becomes inadvertantly folded under at the point of the throttle assembly beneath the clamp. Sometimes it's very difficult to see when this occurs. But it can allow 'some' fresh air to intrude which is essentially air not going by the MAF. It can throw off what the computer 'thinks' the MAF should be 'seeing'. There could also be a crack or small hole in the tube between the MAF and throttle, or a bad PCV valve/hose/connector, etc. These don't cost anything to inspect other than a little time. But they can spoof other problems.

3) Double-check all the PCV hoses and assembly for hidden cracked fittings, etc. Sometimes they can leak and spoof certain codes you wouldn't expect.

Yeah, it probably wouldn't hurt to replace the upstream O2 sensor.

 

[Erik the Awful]

Unfortunately i only have a code reader at the moment. Any recommendations on a bluetooth adapter and iphone app setup? Or a cost effective scanner a rookie such as myself would benefit from?

If you're working solely on GM products, the ideal situation is to get a used professional scan tool with all the GM related software. Expect to pay a couple of hundred dollars.

If you're cheap like me and work on a lot of different stuff, get a $23 BAFX code reader and download the Torque Lite app. The BAFX works on most vehicles.
www.amazon.com/BAFX-Products-Wireless-Bluetooth-Diagnostic/dp/B005NLQAHS

 

[Bighorns53]

Really awesome help thanks to the both of you. I guess im getting way ahead of myself. Im having a hard time visualizing how the PCV/crank case air would be tied into the vacuum end of things. Idk how else one would get around it though, just venting into the engine compartment? My drivers side valve cover is completely soaked in oil. Im sure that grommet for pcv is not anywhere near sealed. Im confused on how it wouldnt be pulling in “unread air” past the maf all the time through the valve cover. Is it supposed to be that sealed it creates a negative pressure situation under the cover? Anyways… ill get working on a better scanner asap and have a look at my vacuum situation.

I have a 1000$ in ignition/tune up parts including a delphi MPFI. just waiting to go in this thing but im hesitant to waste them if there is something internally wrong with the engine. Whatever that may be… poor compression, floating valves (ive heard somewhere on here about that, 2k rpm spitting up). It backfires and pops like crazy when it starts acting up, but unplug the maf and all is well so im feeling hopeful the engine is in great shape. 250k is alot of miles though. Still on the fence about using my parts on this current engine.

 

[Jeepwalker]

Mine's got 265k mi....

 

[Jeepwalker]

The GM troubleshooting manual might be worth a good read. The first thing it says at the top of the charts (not just GM but other mfgrs are the same) is to begin by inspecting all the basic things, before digging into the electronic controls. The reason is, often basic thing are the problem. It's easy to get led down the road off the bat, thinking it's this sensor or that. It could be, but I've watched guys who were real GM "pro's" get thrown off by assuming it was something out of the chute. When it didn't work out, they took a step back and started looking over the basics. So even experienced guys can be thrown off.

Perhaps it's worth a step back ...perhaps start with a compression check, check base timing, put a vacuum gauge on it, write all the findings down. Youtube has some good videos on how to do a vacuum test. Quadruple-check your engine and body grounds, fuel pressure, dist cap, wires, and all that. Are the injectors clean? What do the spark plugs look like? (oily, worn, good?), Did the timing chain jump a tooth? Sounds like your engine could have a lot of oil blow-by?? What's that look like? How much? Replace that PCV grommet. OReilly's I'm sure has them. The more of the basic things you can cross off the list the better.

 

[Bighorns53]

Reviving this for an end to this thread if anybody finds themselves in my position.

It ended up being my distributor was off time throwing random misfires, no cam/crank correlation codes.

Very weird considering unplugging the MAF made the random misfire basically disappear. Not sure how that worked out.