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post #1 of 17 (permalink) Old 04-08-2017, 10:50 AM Thread Starter
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Trans Am running rough

Hello all,

So, I was driving my 94' stock Trans Am down the street and was gunning it a bit but not much. Then it slipped out of gear so I let off and put my foot back down on the gas and reengaged just fine. I then stopped at a light and then all of the sudden it starts running rough...almost feels like a cylinder isn't firing or something. Has been doing it for the last 24 hours. 94/95 program show no DTC whatsoever( the connector is loose and caused the DTC seen here) Only popped up for 1 second. Ran the program for a minute without that DTC reappearing. Here is a snapshot of the program



Anything look odd to anyone? Need to know if it is ok to drive or needs immediate repair on something. Thanks!

Last edited by sc1984; 04-08-2017 at 10:55 AM.
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post #2 of 17 (permalink) Old 04-09-2017, 07:31 AM
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Because of the reflected light of pic, I can't tell if your in open or closed loop operation. But long term fuel trims show pcm adding fuel for lean running. Could be a vacuum or exhaust leak causing this.

Your voltage that pcm is seeing is about a volt too low. Should be 13.5 volts or maybe a little more.

Put a fuel pressure gauge on fuel rail. Check for around 43 psi @ key on prime. Start engine and look for mid 30's psi at idle. Has the fuel filter been changed on your car?

How about the plugs, wires, opti cap & rotor? These need to be changed from time to time as they either wear or break down with age.

Check for spark leakage by misting plug wires with some water. Do this at night and run engine to look for leakage. Have car up on jack stands as sometimes you can't see the leakage from up top.
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post #3 of 17 (permalink) Old 04-09-2017, 08:20 AM Thread Starter
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Thank you for your response. So, I replaced the Opti about 1 year ago, though it was a cheap knock off since I couldn't afford $400 for an AC/Delco. The plugs and wires were replaced about 4 months ago with NGK Iridium with 8mm wires. I am checking today now that I have time to see if any are loose or rubbing somewhere. Ill see if I can check fuel pressure, though the pump was replaced about 4 years ago and have driven maybe 500 miles since. I replaced the fuel filter about a month ago. I know this is a stupid question but where are the vacuum lines? I know pretty much every other componenet of the engine bay but that. Thanks!
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post #4 of 17 (permalink) Old 04-09-2017, 08:46 AM Thread Starter
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Also, according to Oreilly I have a bad regulator on my alternator. So, I believe that is where the low voltage is coming from. That will be replaced shortly
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post #5 of 17 (permalink) Old 04-09-2017, 09:51 AM
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Minot sure f you car but mine dose not like Iridium plugs so j jest use cheap copper plugs(Autolight 104 but I think the stock style plugs are 106s) as for the vacume lines there are a few on both sides of intake about mid way back. I think there is 2 on the passenger side and 3 on drivers side. There is also 2 on TB. Jest fallow them and or make sure thy are not off or missing with no cap.
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post #6 of 17 (permalink) Old 04-09-2017, 08:53 PM Thread Starter
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Ill use the plugs as a last resort. Its a pain in the ass to get to the #8 spark plug and the NGK were recommended. I believe on this forum.
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post #7 of 17 (permalink) Old 04-09-2017, 09:13 PM Thread Starter
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To be clear it does this both in neatral and while driving. It is sluggish doesn't operate the same as it did 48 hours ago. Found 1 spark plug that had a little bit of rub from behind serpentine belt but that was the only unusual thing. Now, with the water test am I supposed to spray a mist and see if there is any electrical charge outside of the boot?
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post #8 of 17 (permalink) Old 04-09-2017, 10:12 PM
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With car running at night you can spray some water with a spary bottle so you can tell where you may be loosing spark. It nakes for I nice light show if there is something wrong. I have found must of the time it is close to the headers or belt is where I find bad cracking or rubbed holes in wires
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post #9 of 17 (permalink) Old 04-10-2017, 08:22 PM Thread Starter
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Ok...potentially huge problem. I just replaced my alternator with a new one expecting it to jump from low 12's to high 13's. It started at 13.4 and then I turned on headlights, ac, and radio and drops to 11. Turn off all accessories and hangs at 12.1 ugh. I'm wondering if a mouse went to town on my electrical or something, but everything else works fine. I'm curious if low voltage is causing all my problems. If so, any way to test for voltage to ensure the correct voltage is going to spark plugs?
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post #10 of 17 (permalink) Old 04-10-2017, 10:17 PM Thread Starter
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To give you guys a slightly better idea here is a video that you can kind of hear it struggling almost like its about to die. It is in gear with the e-brake on in the video so it is running a little rougher than it would in neutral
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post #11 of 17 (permalink) Old 04-11-2017, 06:43 AM
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They make pc based automotive oscilloscopes that will show the waveform and voltage of your spark on all 8 cylinders at one time. You can find them on Ebay, Amazon, etc.
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post #12 of 17 (permalink) Old 04-11-2017, 08:19 AM Thread Starter
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OK, thank you. I will try the mist test, grounded screwdriver test, and check out that program tonight. Now, in the meantime I have checked all battery connections in my car and they are all solid. Is there anything that would cause such bad voltage? I have 4 gauge from ground to negative and 4 gauge from alternator to battery +. So, I should have decent voltage at least in mid 13's if not 14's considering there are no accessories running
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post #13 of 17 (permalink) Old 04-13-2017, 11:50 PM Thread Starter
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So, I did the mist test and it appears #8 is giving me problems. I see a very very small arcing between wire and a metal ring right next it. Should I replace the wire, spark plug, or both? Keep in mind both are new. Also, the mist test at the spark plug did not reveal anything. It is just arcing where the wire touches metal about a foot from the boot. I'm assuming that isn't normal? I also read somewhere if the voltage is low this can sometimes happen. I just replaced my alternator, however, my batter/alternator are reading 14.2 while my radio reads 12.1 volts. Not sure what voltage spark plugs are receiving because of this

Last edited by sc1984; 04-14-2017 at 08:11 AM. Reason: adding more info
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post #14 of 17 (permalink) Old 04-14-2017, 10:02 AM
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Spark plugs should get 30,000 to 35,000 volts in short bursts--that's what the ignition coil does with your 13.5V primary. Because it's short bursts you need a scope to measure.
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post #15 of 17 (permalink) Old 04-15-2017, 09:43 PM Thread Starter
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OK. So I got under the car and it appears the #8 spark plug wasn't quite seated correctly and was a little detached. I guess it was in enough to work and wiggled a little loose. So, after 20 minutes, a cut up arm, and crawling on top of my engine I managed to reseat it and seems to be running a lot better now. Personally, it still doesn't purr like it used to, but have no idea what the issue is exactly since the coil, spark plugs, wires, opti, and alternator have been replaced. Running out of ideas. Wish someone had a trans am near me to compare and see if there is an issue or im just crazy.
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