Hello All, new here. First week with the CobaltSS. 2009. Checking over a few things and not sure if these are OK or not based on historical knowledge.

Not sure if this might be a source for a possible vacuum leak, intake rubber bellow just after the plastic elbow has a small area that looks collapsed. (I don't necessarily think I have a leak and the area in question does not look all the way through, but maybe cracked).

And the rubber section just off the turbo compressor outlet has a crack near the clamp, but it looks like it is just a crack in a surface cover and not the actual hose, as it does not look deep or through.

Thoughts on if these are problem spots for me (or on the cars in general)?

It seems to run pretty well overall (GMS1 and drop in K&N only) boosting 17psi quickly at under 4000rpm, and climbing to 19-20psi by 5500 rpm. It does seem a bit rough like a little consistent mis/bobble at idle when cold that mostly goes away when warm (but is still there a little). But part throttle and WOT driveability and power seem good from what I can tell.

I logged a few PID's on a drive (OBDlink MX with APP) and see that STFT seems to bounce around between -5 to 5. LTFT was at -10 and went to -9 during the driving. I know the LTFT is a little bit off so not sure what that might mean.

TIA for any help and insight on both the hose/bellow issues, and also the fuel trims.

 

For the LTFT and STFT I did some more logging. At WOT my STFT goes to 10-13 positive. LTFT is at -10 at the time, so total is that it is close to zero fuel trim at WOT. So it seems to be running well that way.

But at light throttle STFT stays -5 to +3 range, so total fuel trim is quite negative (-7 to -12). Anyone know what causes this?

 

What is ur ltft normally? If ur ltft is plus 10 or more at idle u have a boost leak somewhere, if it's -10 I could be exhaust leak. But then again it could also mean u just need to get tune adjusted. I had a weird rich trim issue that ended up being a leaky injector , went away when I changed Em.

 

I will keep an eye out for exhaust leaks, but it seems whisper quiet on the stock exhaust system. Car is all original and was very well cared for, so it seems this might not be it.

I have just had this car a week now. LTFT I just watched over 1 day since I got the OBDlink setup to watch it. It was -9 to -10 for LTFT. STFT is usually a bit negative, but goes from around zero at idle to -5 to +3 when driving out of boost at light throttle. When I go WOT STFT goes to +10 to +15. So at WOT the STFT pretty much cancels out the LTFT making total fuel trim about zero or a little positive. So it seems WOT tuning is pretty close to spot on.

Boost is a little lower than I expected with GSM1 from reading that I have done here, but it feels strong and pulls well and consistent. And in the cold weather the tune is probably going to see its target load values at a lower boost level because of the dense air. At about 30F it will hit 16-17psi right away at 3500rpm, then climb to 19-19.5psi by about 5000-5200 rpm and hold that until 6300. In warm weather I am guessing that will go up since the air will be less dense and it will need to use higher boost to hit the same load values.

But the total fuel trim while driving lightly was -10 to -13 (-10 on LTFT and -2 or -3 on STFT). So that is a bit odd. I did reset the ECU to put LTFT back to zero and pulled the line off the purge solenoid and plugged the solenoid. (I have read that the solenoid or evap purge valve can seem to cause problems sometimes). I plan to try the vacuum tank delete (or at least just bypass it and leave the tank in place) and see if that helps a little later also.

More driving will tell though before I do the vacuum tank thing. Since I did the evap purge valve plug and reset the ECU (so LTFT is now sitting at zero after I started it back up) I will drive a few days and see where the fuel trims go.

Thanks for the ideas.


edit: Are leaky injectors common on these engines? I could see a mild leaky injector not affecting WOT much (which is my case), but affecting idle and part throttle quite a bit (which is where my fuel trims seem to be off more).

 

+ or - 15 will set a code for LTFT.

-10 is pretty bad. GMS1 should be pretty close to 0 depending what gas you put in it.

I ran a K&N drop in and it was never off by much. For sure not 10 percent.

Either you have an air leak or your car is tuned (not GMS1) and the previous owner returned it to stock and left the tune on.

Your boost numbers do sound typical for GMS1 though.

 

Thanks for the reply. I feel pretty sure it is a GMS1 and not any other tune and that it has been completely stock (except KN drop in filter) as the former owner stated. He was original owner, mature driver, commuted the car and not raced it, the car was immaculate inside, outside, and under the hood, and he had every single service record for the life of the car. Always a chance at another tune, but just does not seem likely in this case.

Good to know boost seems correct. After resetting the ECU (therefore LTFT), after a few commutes it sits at about -7 LTFT. Short term is still leaning negative more than positive.

For air leaks, from my knowledge, that would add air (when driving in vacuum range, not boosting), therefore fuel trim would go positive to add fuel for the unmetered air. But I have negative fuel trims subtracting fuel (less air than the engine is expecting).

And when I boost heavy the STFT goes positive about the right amount to offset the LTFT, equaling about zero fuel trim. So WOT boost air/fuel is pretty good to the tune and what the ECU is expecting. It feels nice and smooth and strong with no signs of misfires. WOT throttle also has zero EGR.

It's that darn part throttle vacuum driving that's off. I have read that too much EGR could cause negative fuel trims. EGR is supposed to be active only at part throttle (not idle or WOT), from my general knowledge of it. And there is a slight bobble/misfire at idle, especially when it is cold. Not massive stumble, but noticeably not smooth. I have had a previous 2 liter turbo platform that had too much EGR and it did a similar thing with misfires and stumbles at idle or very low throttle.

Anyone know of likely issues with the EGR system on these engines? (too much signal to an EGR modulator if there is one, or the EGR valve gets dirty and starts to stick?). I could see that being my issue possibly if it does happen. Anyone know the best way to test the EGR system?

edit: Just found the LNF does not have EGR system because the variable valve timing eliminates the need for it.

 

MAF reads air through intake. Small air/boost air leak would cause more pressurized air to leave the system which means more fuel than air at the 02 sensor which causes negative trims.

With that said, a lopey idle sounds like an air leak or a bad tune as well.

If you watch the timing/ advance at idle, i bet you see it hunting a lot. Typically you see it at 0- 6 degrees. When the idle is not good it will jump over 10 a lot to compensate.

I would check the entire intake tract, then do a boost leak test. Then if you don't find one, i'd find a local tuner to read the tune on the car and see what it is.

I'm mature, civil engineer, daily commute my car and have all service records. My ECU is not stock LOL.

Luckily your car isn't far off so hopefully it's an easy fix.

 

Yes, I get the boost leak causing negative trims (when boosting). I have pretty much zero total fuel trim when boosting though. It is only when I am in vacuum that I have negative trims. So with a boost leak I would expect positive trims when in vacuum (extra air getting sucked in so more fuel is needed for it), and positive trims when boosting (loosing air which goes out because the tract is now pressurized, therefore need to reduce fuel for the lost air). I don't have either which is strange. Only the opposite of negative when in vacuum. Might mean a leak before the turbo (never pressurized piping), which is only small. But enough to effect idle and part throttle when air volume is low. When air volume is MUCH higher during WOT runs, it is small relatively so, so does not really affect fuel trims.

Good tip and thanks for the information on the ignition advance at idle. I can check that easily tonight on my way home.

Everything looks good visually on all the air piping, but I will go over it again and see if I can also check the clamps to make sure they are all tight.

edit: scratch all the EGR thoughts I had. I think I just read the LNF does not have EGR at all because the variable valve timing eliminates the need for it.

 

correct about EGR. Forget that.

There is a hose that connects to the intake. Has a plastic connector. Make sure you don't have a leak there.

 

I just ordered the evap purge solenoid just in case that is a problem.

I will check the plastic connector on the intake piping.

 

So I replaced the evap purge solenoid. No change.

But , as with so mang things always check the simple things first even if you "think" they should be fine. One of the first things listed if you have negative ltft's is a restriction in the intake tract, namely a dirty or restrictive filter. I have a k&n drop in that looked pretty clean visually. Must have been either old, or my guess is over oiled. Changed to a brand new stock ac delco filter and after just a few miles of driving the ltft was already back to only negative 3.1-3.9 range.

So lesson that even a clean looking k&n can be a restriction and always start with the simple stuff first.

Thanks again for the ideas on things to check.