Pontiac G8 Forum banner
1 - 16 of 16 Posts

· Registered
Joined
·
129 Posts
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
 

· Registered
Joined
·
129 Posts
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!
 

· Registered
Joined
·
129 Posts
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
 

· Registered
Joined
·
129 Posts
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 -
 

· Registered
Joined
·
129 Posts
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!
 

· Registered
Joined
·
129 Posts
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!
 

· Registered
Joined
·
129 Posts
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!
 

· Registered
Joined
·
129 Posts
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
 

· Registered
Joined
·
129 Posts
Discussion Starter · #21 ·
Thanks R1toG8 - the check engine light has not come back on (P1400 code), but the initial cold start is still very rough with a choppy idle, sputtering and shaking for about 10 seconds before the idle smooths out at 1,000 rpm. Honestly, not sure what to check next so this is why I thought perhaps the injectors (or some of them) were leaking fuel when the ambient temps are below 50 degrees.
 

· Registered
Joined
·
129 Posts
Discussion Starter · #23 ·
Update Hey there guys - made a quick 10 sec video - but can't get it to copy into my post... So, here is the still pic, and wondering if this may be the issue with the cold start issue (P1400 fault code). Could someone please let me know if the STFT B1S2 value that is unchanging (it is static at 99%) could possibly be the issue? All other values fluctuate while the engine is running, but this value never deviates. I don't know what to do if this is the issue, so any assistance here would be most helpful. Many thanks!
Gadget Communication Device Font Multimedia Electronic device
 

· Registered
Joined
·
129 Posts
Discussion Starter · #24 ·
Additionally, since my first post I have cleaned both the MAP and TB, checked all intake bolts and all clamps (all were tight), and as the G8 has 130K on the odo went ahead and replaced the injectors as well. Will be doing the carb cleaner spray test tomorrow if the weather holds as the next item on my list. Thanks in advance for any additional suggestions!
 

· Registered
Joined
·
129 Posts
Discussion Starter · #31 ·
Morning Gentlemen - both great points and appreciate the input. I'm sure the O2's will be a joy to remove from a car with 130,000 miles on the odo.... :oops: Guess I'll apply PB Blaster the night before and hope the threads remain intact upon removal. Thanks again, guys!
 

· Registered
Joined
·
129 Posts
Discussion Starter · #33 ·
Hey there Thawk - Thanks for the info and I'll try this tomorrow. As I'm new to the process of diagnosing trouble codes, are saying that because the rough idle occurs so quickly that the computer is going from open loop to a closed loop mode before the upstream O2 sensor can cause an issue? Thanks so much for the continued items for me to check!
 

· Registered
Joined
·
129 Posts
Discussion Starter · #35 ·
With the scanner plugged in, watch the O2s B1 S1 and O2s B2 S1 they both should fluctuate pretty quickly at close to the same values. The slow to respond one is bad. Hold throttle st s steady RPM when observing
Well, had great weather today so was able to spend more time with the scan tool in the G8. Compared B1S1 to B2S1 at a constant 1,000 RPM. The fluctuation in values looked pretty similar to me as best I could tell. I used the "live data overlay" to compare the two, and ranges were as follows:
B1S1 - .125 to .835
B2S1 - .075 to .850

Lastly, with 130,000 miles I imagine it wouldn't hurt to replace the O2 sensors, but based upon my limited understanding it doesn't appear that there is anything glaringly wrong. Do you see anything out of whack with these numbers?

Thanks guys -
 
1 - 16 of 16 Posts
Top