G8 GT 2008 - cylinder 5 and 8 not firing - Pontiac G8 Forum: G8 Forums - G8Board.com
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post #1 of 12 (permalink) Old 12-20-2017, 11:15 AM Thread Starter
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G8 GT 2008 - cylinder 5 and 8 not firing

Has anyone seen this issue with the car? I have taken to 2 different shops and they can not figure out what is causing the issue. Any help/suggestion would be appreciated.
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post #2 of 12 (permalink) Old 12-20-2017, 11:26 AM
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They seriously can't figure it out? That's quite odd.

Though, maybe you need to find a shop that has a smoke machine and have them look for vacuum leaks. I gotta think they'd have checked plugs, wires, injectors, and rocker arms for movement, air filter for clogging, compression, etc. So that would mean something is wrong with the mixture.

If they didn't check any of those things, then you need to go to a better shop..... I'm reluctant to chip in further...

'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
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post #3 of 12 (permalink) Old 12-20-2017, 11:36 AM Thread Starter
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They have checked all those things and more and still nothing. The car had Livernois tune on it and they also put it original GM tune to see if that solves the issue. But it did not.. Now the next option is to replace the cam, lifters etc. and see if that is the issue. It is still a guessing game. A very expensive guessing game.

BTW - your car looks badass!
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post #4 of 12 (permalink) Old 12-20-2017, 12:09 PM
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Did they do a compression test?

PBM G8 GXP M6-Self flex fuel tune,OBX LTs, Kooks A/B, Monster clutch, 3.45 gear,LSA S/C, cam, fuel system, stock block and stock heads, 12 PSI.
Best e.t 10.41/ best mph 138.7 Half Mile: 168.3 mph
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1991 GMC Syclone- Self tune/catback [email protected], 4,200ft D.A
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post #5 of 12 (permalink) Old 12-20-2017, 12:20 PM Thread Starter
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yes - it came out fine. I think it was 90 when cold and 170 when hot.
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post #6 of 12 (permalink) Old 12-20-2017, 12:28 PM
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Maybe switch the coil packs and see if the misfires go from 5 to 6 and 8 to 7 respectively? If not, they can switch the plugs directly over in the same fashion and see if it changes then...?

Is the mixture rich or lean according to the O2s?

'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
Power: 400HP/392TQ


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post #7 of 12 (permalink) Old 12-20-2017, 12:36 PM Thread Starter
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they did switch around the coils and the plugs. Still the same issue. It is running lean and the error code which shows up is P0174 which is also odd as it means that the driver side is running lean. It should be both sides as 5 and 8 are opposite side.
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post #8 of 12 (permalink) Old 12-20-2017, 01:25 PM
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Not necessarily. It just means that Bank 1 is running lean enough to trigger a trouble code and an check engine indicator.

I promise you the other one is lean, it just isn't AS lean yet to trigger a code. The Fuel Trims have to get up to 25% for the code to be logged and the MIL to be commanded on by the ECU. The other one isn't there (yet).

The smoke machine suggestion is to rule out vacuum leaks or to find one that does exist. If they don't have one, find someone who does.

Compression is fine. I bet they checked fuel pressure and it is fine and thus the grounds are likely fine (but worth checking). The injectors may be partially clogged. Did they also swap injectors? I am guessing that is a yes.

They seem to have ruled out a lot, but if they haven't gone at it with a smoke machine or other viable method for finding the most pesky vacuum leaks, then they haven't ruled out everything.

Did they also check the brake booster and it's check valve?

'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
Power: 400HP/392TQ


"If you want to criticize someone else's work, be sure that your own is in order first."

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post #9 of 12 (permalink) Old 12-20-2017, 01:46 PM Thread Starter
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I'll ask them if they did the smoke test. They did swap out the injectors but it did not resolve the issue.

I will also ask them if they checked the brake booster. What's the correlation of brake booster with cylinders? Is it for the vac leak?
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post #10 of 12 (permalink) Old 12-20-2017, 01:57 PM
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The booster uses vacuum to operate, and the check valve meters that vacuum.

When pressure in the manifold is higher than in the booster, then the valve stays shut. When it is lower in the manifold, the check valve opens to equalize. If the booster leaks it'll continually leak down and therefore will continue to use vacuum as the check valve opens to try to equalize the vacuum, but it can't.

If the check valve fails, then the booster can never achieve consistent vacuum and the booster consumes vacuum somewhat erratically. Brake performance would be impacted in both conditions.

I consider it to be a pressure system because even though the system is at vacuum, that is only as compared to atmospheric pressure. The system is still under absolute pressure.

'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
Power: 400HP/392TQ


"If you want to criticize someone else's work, be sure that your own is in order first."

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post #11 of 12 (permalink) Old 12-21-2017, 12:36 AM
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I went through a similar issue this spring. After getting car out of winter hibernation, I experienced a misfire Cylinder 1, rough driving, Codes, after 3hours and $200.00 at the dealer it ended up being a bad spark plug.
One week and 300 miles later, I start having cylinder 5 misfire, Checked plug and that too was bad. Changed all plugs and no issues since. Strangest part is car has 22,000 miles.
I use Stabil when stored and Chevron fuel injector cleaner in the first full tank of gas in the spring.
This fall I stored with Stabil and ethanol free gas thinking plugs were fuel fouled somehow. Good luck

2009.1 SRM Sport & Premium no roof, V6 tails, Complete GXP exhaust,Cats, midpipe and mufflers,Painted & modded lower grills, VG Sharkfin, Homelink visor, CTS-V 6-piston Brembo's with 2-piece rotors,Rear Brembo's with 2015 SS rotors,CTS-V wheels. Currently 22,200 miles .
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post #12 of 12 (permalink) Old 12-21-2017, 09:18 AM
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Check the wiring harness that is near the belt tensioner. I had one of my customers come in with a bad misfire, and it ended up being that his wiring harness got damaged due to a broken motor mount and the excessive movement.

PBM G8 GXP M6-Self flex fuel tune,OBX LTs, Kooks A/B, Monster clutch, 3.45 gear,LSA S/C, cam, fuel system, stock block and stock heads, 12 PSI.
Best e.t 10.41/ best mph 138.7 Half Mile: 168.3 mph
2007 TBSS-Self tune, bolt ons [email protected], 1,560 D.A
1991 GMC Syclone- Self tune/catback [email protected], 4,200ft D.A
https://www.youtube.com/channel/UC0o...xsBDzFImcyBfeA
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