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Discussion Starter · #1 ·
Hello folks - hope you guys can provide some guidance on how to best diagnose the cold start/rough initial idle issue at hand. Here are the details:
  • When the temps drop below 50 degrees F, initial start produces a rough idle for approx 10-15 seconds
  • Engine coughs & sputters, and a smell associated with a rich mixture is present
  • After the idle smooths out, operation is normal with no ill effects
  • 2009 G8 GT w/ 130K on the odo, OEM tune, and bone stock engine w/ exception of Roto-Fab
  • Today the check engine light came on, and error code pulled is P1400.

Lastly, I did notice that two of my OEM exhaust manifold bolts are broken off below the flange (on each on PS & DS). As most of the P1400 threads on the Forum seem to be with G8's that have been modified, just wondering where to start the diagnosis on my stock G8.

Thanks for reading, and any suggestions please share your knowledge! Thanks much - Dave
 

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Out of curiosity, have you ever changed the plugs and wires out? If not, it would be a good thing to do. Even if it doesn't fix the issue, it wouldn't be a waste of time to do so at 130k.
 

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Discussion Starter · #3 ·
Hello T-Type - thanks for your quick reply and meant to include in my comments that I did change out both plugs (NGK) and wires (Taylor) just 12,000 miles ago. I also added the RotoFab at the same time. Thanks much!
 

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What kind of plugs? Iridium? If just regular plugs 12k is half its life. At least check the gap. I would look at injectors being dirty. They may not sealing 100% cold and leak causing the rich condition. Run them a few minutes and warmed they seal and the car idles better.
 
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Discussion Starter · #5 ·
Thanks for the assist, GT Drew. Yes, high end Iridium and I did check the gap on each plug prior to installation. Regarding your mention of the injectors, do I check by the plastic bag method to determine if leaking down? Or, would it be better to send them off to be cleaned and tested? Thanks much! Dave
 

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That is common with post maf vac leaks, as the P1400 sets also from low engine air flow mostly from vac leaks, it smothes out because it goes to closed loop and takes over lean/rich conditions. with eng warm watch 02 voltages while spraying carb cleaner around intake gaskets/hoses/etc and watch front 02s skyrocket as you find a possible leak first
 

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Discussion Starter · #7 ·
Appreciate the input, Thawk. I am going to borrow an Innova 5410 scan tool tomorrow and will use this plus your process to see what I can determine. Thanks much -
 

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Discussion Starter · #8 ·
Update I secured an Innova 5410 scan tool and confirmed the P1400 code (first check was via O'Reilly's). IAT & ECT indicated temps shown before cold start w/ ignition in the "on" position matched ambient temperature. Can someone advise as to what other values I should be looking for on the other critical items? I see what appears to be fuel trim measurements for each bank, along with associated % values - but not sure what is normal or acceptable range. Will pull the fuel rails tomorrow and try the plastic bag test, but just thought I would check with scan tool once more - just not sure what all to check...:oops: Thanks for any assistance, and just learning as I go! Happy Thanksgiving to all!
 

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OP check for leaks as suggested.

That is common with post maf vac leaks, as the P1400 sets also from low engine air flow mostly from vac leaks, it smothes out because it goes to closed loop and takes over lean/rich conditions. with eng warm watch 02 voltages while spraying carb cleaner around intake gaskets/hoses/etc and watch front 02s skyrocket as you find a possible leak first
 

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Discussion Starter · #10 ·
Yes, will be trying to locate any leaks tomorrow but just wasn't sure what O2 voltage I should expect to be seeing. All connections on the RotoFab are tight so will start with the manifold. Thanks for chiming in!
 

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You are looking for vac leak after the throttle body.
 

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With eng warm O2s should be cycling from 200-800 mv. look also at long term fuel trims on both banks should be close to zero while anything over/under 10% is a potential for vac leaks in the + plus 10% range, when you hit the leak with carb cleaner the values will skyrocket if a leak is there
 
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Discussion Starter · #14 ·
Thank you for the detailed reply, Thawk. Was able to take a look at the LTFT on both banks, and LTFTBank 1 was at a -10.2% value, while LTFTBank 2 read out at -7%. O2's were indeed cycling between .250 to .750 - so I believe that is in range, correct? Lastly, will start the spray test with carb cleaner and see if I can pin point any leaks. Thanks for the assistance!
 

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I recommend cleaning the throttle body, I had to do an idle relearn after doing so on my GT at 160k miles. Idle went from a normal 550 RPM to well over 1k RPM before the relearn. Your negative value LTFT's are indicating it's running rich in closed loop and pulling fuel. Cleaning the MAF couldn't hurt either, just be sure to use only MAF cleaner on it. My buddies Snap-on scanner shows desired idle RPM, and the actual idle RPM below that, should fluctuate above and below that RPM and occasionally hit the exact desired RPM. At full operating temperature my desired idle RPM is 544. A good injector cleaner with PEA could help with overall running, REGANE by Gumout is what I use, about $6 at Wal-Mart for 8oz. I believe.
 

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Just googled P1400 GM, 2 of the first 3 causes are what I mentioned, dirty throttle body #1 and dirty MAF #3. P1400 is only triggered while in open loop, the fact that your LTFT's are in the negative would lead me to believe its not an air leak causing your cold idle issues.
 

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...A good injector cleaner with PEA could help with overall running, REGANE by Gumout is what I use...
Awhile ago, Gumout discontinued the one-n-done mixture and began using the multi-system tune-up albeit prefer the former.
 
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Injector cleaning is item of the past as current, ok, last 15 year designs are self cleaning. Good friend in a dealership shares they do not see cleaning - injector failures only. I can share, on 2008 LS2, I pulled injectors at 168K miles, and benched them, all sprayed perfect.
 
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Discussion Starter · #19 ·
Thanks guys, and I was thinking that perhaps the injectors (one or more) may be leaking under pressure due to damaged or deteriorated O-rings - is this a possibility? Checked into getting the injectors cleaned and tested, and this appears to be about $175. Replacing with all new injectors is $400 - so not that much more to have all new injectors. Cleaned the TB and MAF yesterday and checked all manifold bolts (none were loose) as well as all lines and fittings (all appear to be in good shape). So tonight, I'll pull the fuel rails to lift out the injectors and then bag each to see if they leak any fuel overnight. Any other ideas please feel free to chime in! Thanks much - Dave
 

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The o-rings only seal at the fuel rail and manifold, the leak would be external of the engine if the fuel rail one was leaking, the manifold one could suck air. I would see if the code stays cleared or goes away before disturbing the fuel system.
 
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