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1993-97 Gen II Small Block V8

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Old 04-12-2011, 10:40 PM   #1
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Default What are the symptoms of a bad MAF?

Hey guys, if you remember my old thread(rough idle = optispark), once in a while it would idle rough. Well it hasnt done it in a while, just has a slight hiccup when i stab the gas at idle. It only hiccups when its in closed loop as well. So it made me think that possible the o2's but when i hooked up datamaster, i was watching the o2 sensor's and they were fluctuating fine. Also when the car is in open loop, it idles perfect at about 1k rpm. When it goes to closed loop, it idles around 650 in park then 550 in drive. I could be wrong but that seems low to me. Then out of curiousity, i unplugged the maf while the car was running, it immediately died out. I plugged it back in, and went to start the car. It immediately started to have the huge bad idle like before. I tried to give it a little gas, and it back fired so loud, i set off a car alarm about 20 feet away. It sounded like a gun shot. I turned it off right away and havent started it since. Now, could a bad maf sensor be causing the bad idle(that seems to only happen once in a while) and low idle? Im still thinking it could be the opti, but before i throw parts at it, I would like to get some more opinions. Thanks in advance.
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Old 04-12-2011, 10:43 PM   #2
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Also, the reason i think the maf might be bad is because I disconnected the maf on my other car and it didnt die out. It stayed idling just fluctuating idle a little bit. Could it have died out because the idle was so low to begin with?
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Old 04-13-2011, 01:53 PM   #3
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bump....any ideas?
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Old 04-13-2011, 02:14 PM   #4
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In closed loop for a stock 94 auto, 650 in neutral/park & 550 in gear is normal.

Normal MAF readings should be between 6-10 gps (grams/second), ideally 8gps at 650 RPM, closed loop.

I'm also chasing some issues with a stock 94 auto right now, but with screwy O2 values that I don't trust. But no backfiring... that comes from gas lighting off at the cat because it didn't burn in the chamber. You may have a leaky injector. Also, you might want to look at valve adjustment- unless that was something you already covered in your other thread...
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Old 04-13-2011, 02:23 PM   #5
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ok so the idle rpm is fine. I will have to run it again with datamaster to check the gps on the maf. I do have a code for high/low res failure, which i am mostly certain is the opti. Im just trying to make certain that it is that before i make the plunge. Also, my security light is on, not sure if that has anything to do with it. It never stops me from starting the car, its just always on. Thanks for the help.
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Old 04-13-2011, 02:27 PM   #6
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if you've got opti codes, and no other ones present, that makes sense. The backfire fits in with that. Sorry, assumed you had already replaced a bad opti. Mano (1Badzee) posted up a guide for the opti swap I used a couple years ago to do my first opti. It's over on lt1engine.com. Do 2 or more and you can do it in your sleep in about an hour.
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Old 04-13-2011, 02:50 PM   #7
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Well one of them is for the opti. The codes were 42, 23, 48, 36. I know one of them was for the maf but thats because i started the car without it on.

23: Intake Air Temperature Sensor Circuit High Temperature Low
36: High Resolution Pulse Circuit
42: Ignition Control Grounded Circuit
48: Mass Air Flow Circuit
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Old 04-13-2011, 02:53 PM   #8
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If you unplug the maf sensor, it should throw a code, but engine will still start and run off of VE tables programmed in the tune. I've never unplugged it with the engine running, but it seems normal to me that the motor would kill and crank back up. If you have MAF failure I think you would have a consistent problem, not a sporadic one like you describe. When the pcm gives you the hi-res and low-res trouble codes, it's time for a new opti. That was the case for me on my 2nd opti. 1st opti didn't even throw the SES light on. But both of my failed opti's gave me backfire at one point.
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Old 04-13-2011, 02:56 PM   #9
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23: Intake Air Temperature Sensor Circuit High Temperature Low
36: High Resolution Pulse Circuit
42: Ignition Control Grounded Circuit
48: Mass Air Flow Circuit

Do the opti first. Space the ICM and coil bracket off the head per writeup at shbox.com and see if 42 comes back. Make sure the 2 grounds on the coil post are good while you're at it. You may need an IAT sensor. Thankfully those are cheap. Ignore the MAF code, just clear it.
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Old 04-13-2011, 02:59 PM   #10
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Yea its most likely the opti. Im still waiting to change that out. I was just trying to see if it could possible be anything else.
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Old 04-13-2011, 03:01 PM   #11
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Ok what two grounds on the icm stud? When you take the icm off from the head, there are two bolts that hold it on correct? Are you saying that there is suppose to be 2 ground wires coming from one of those bolts? I added one there so far going to a ground bolt on the framerail right underneath the air box.
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Old 04-13-2011, 03:18 PM   #12
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One is the copper braid to the driver's side frame rail. the other disappears inside the harness and loops over the inboard stud like the braided one does. On the outboard side in this pic:

Click the image to open in full size.
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Old 04-13-2011, 07:59 PM   #13
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Ok so I didnt have any grounds going to that stud then. I did add a braided strap to the one stud to the framerail. Im missing the other two. Can I replace them? or just add two more ground straps to the same spot on the framerail? Could those missing grounds be causing all my problems? I appreciate all the help bud.
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Old 04-13-2011, 11:55 PM   #14
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Carefully peel back the harness you see in the pic above and see if you can find that black ground and any part of the white pair.
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Old 04-14-2011, 01:49 PM   #15
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Quote:
Originally Posted by 95chevyz28 View Post
Well one of them is for the opti. The codes were 42, 23, 48, 36. I know one of them was for the maf but thats because i started the car without it on.

23: Intake Air Temperature Sensor Circuit High Temperature Low
36: High Resolution Pulse Circuit
42: Ignition Control Grounded Circuit
48: Mass Air Flow Circuit
Deathbed confession for the opti, however, clear your other codes before you go to work on the opti to isolate it.
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Old 04-15-2011, 08:04 AM   #16
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Ok some good news. I reset the codes. When I went to start it, it started up perfectly fine. It's only throwing one code now, 42. Well atleast the opti code went away....for now. I ran the car for about 20 mins. Everything seemed fine. No popping either. BUT...the picture you showed me of the harness that has the two ground wires going to the icm stud, is that the same harness that has the wires for the icm, and the maf sensor in? I took some pics that I will put on here in a little bit since I'm on my phone right now. I don't see any ground wires coming out of that harness...what exactly does that code mean and how do I fix it? Thanks.
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Old 04-15-2011, 01:44 PM   #17
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Here you go:

Click the image to open in full size.
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Old 04-15-2011, 01:47 PM   #18
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And the flowchart from the GM manual:

Click the image to open in full size.
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Old 04-15-2011, 07:46 PM   #19
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Ok so I went to take another look. I think I do see those ground wires. It looks like someone before me put them on a bolt that is under the icm. I will move them around tomorrow but I dont think that is going to make a big deal. If after I space it out as well, if it still doesnt go away, then I will start looking at that diagram. Any other ideas, let me know. Thanks for all your help...
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Old 04-15-2011, 07:46 PM
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