Misfiring/ backfiring around 5000rpm
 

I just bought a 2009 cobalt ss/tc from a dealership with unknown mods besides the ones that I can see which is an after market charge pipe-straight black across the motor with no name on it, magnaflow exhaust, k&n drop in filter and interceptor gauges on the apillar. other than that idk what else has been done.

Anyways i wanted a louder blow off sound so i purchased the k&n sri. installed it no problem, started it up and took it for a drive. car sounded much better, no check engine lights no problems at all. So two weeks has passed now and all of the sudden when under a heavy load i can not go above 5000 rpm, the car just shuts down almost until i shift into third and get around 5000 again then same problem. so thinking it was the plugs i changed that, no luck same problem. so i drove it for it for a couple of more days finally got pissed off and took the sri off thinking that was the problem. put the stock one back on, no luck still cuts out.

The weird thing is that i have no check engine light, everyone else that seems to have this problem also has a check engine light. The only thing that is showing me the misfires is my interceptor gauges. So i called the dealership and they also have never heard anything like this.

What did you gap the new plugs to and did this start after you put the new BOV on?

I would imagine you need a tune if you got it from a dealership.

I baught my cobalt from a dealership and it was modded and it neeed a tune.

When the dealerships get these cars in, they reflahs the PCM to stock settings wiht no reguard for modifications done to the vehical. I have an 06 so I was running too rich throwing P0172 Not sure what the TCs do with mods and a stock tune

I never got a bov, it is just a black metal charge pipe. The gap on the new plugs were pre gapped to the recommend .35. i didnt want to mess with them since they were already set to the gap. This all started after i put the sri on. it would misfire once the car was started a couple of times (atleast thats what my gauges were saying), but never while i was driving. Now i cant even get on the gas with out a misfire around 5000, almost feels like a rev limiter.

That is just the thing no check engine lights at all! that is what is weird and for how bad it is misfiring it should light up the whole dash

I think it has to misfire mroe then so many times a minute to throw a code.

Instead of useing the misfire scan, go to the menu section of the interceptor and select misfires, and it should break down the misfires based on cylinder # Ignore 5 - 8 and watch 1-4 see if its all 4 of them or just 1 cylinder.

My first thought was spark blow out... Maybe your pulling timing becuase of Spark knock...

You are useing 91/93 Octane gasloine.. right?

Try a diffrent brand? I always use Shell V-Power

i am using 93 octane (thats what i thought maybe a bad gas since that seems like when it really started last fill up). and on my interceptor it is all the cylinders, since i have been having this problem i always put it on the misfire setting in the menu just to watch it.

What's your a/f ratio like and how much boost?

runs the normal 15 boost and i am not sure what my a/f ratio is because idk what those numbers mean i will have to check on that and get back to you

And back to having to misfire so many times before it throws a code, on my interceptor it has said as high as 67 misfires when under a heavy load

scan through your interceptor and watch your ltft and stft while driving normal and at wot.... if they are all sorts of whacky they will make the car misfire like crazy without throwing a code lots of times.

What can cause these to be all whacky?

noob question what does ltft and stft mean?

Long term fuel trim and short term fuel trim

http://www.obd-codes.com/faq/fuel-trims.php

Does the interceptor gauges show that ?

It's advertised that it does show LTFT and STFT, but since I don't have one someone else will have to speak up as to whether or not it really does.

I will have to check it out later

The interceptor shows LTFT and STFT. Select it in the setup so you can see it under the scan setting.

Okay thank you !

Specifically, the intake itself or an air leak post maf but before the turbo will do this. Even though the k&n sri has been shown to skew trims less than other intake systems, this is only true if the filter is messed with and is clocked in a certain direction.

Assuming there is no air or boost leak, (seeing as how you have an interceptor) the easiest way to attempt to remedy the trims without actually tuning is to watch your stft's at idle and see where they are. Idealy, you want the trims to be as close to as zero as possible, but seeing as how you aren't exactly tuning here, as long as you (or the OP in this case) get it within +/- 5% (meaning somewhere between -5% and +5%) this should be fine.

Before you start messing with the filter angles you should first disconnect the negative battery cable for 5 minutes or more to allow the ecm to reset itself. Now that you reset the ecm, go ahead and set your interceptor to view ltft and stft, and then start messing with the filter angle and clocking while the car is idleing until you get it within the -5% to +5% mentioned above. After you get close to where you want to be, tighten the clamp on the fliter back down and take the car for a drive. Keep monitoring both ltft and stft and total misfire if you are able watch them all at the same time. You will probably still see some random misses, but honestly, most lnf's tend to see the random miss and kr (knock retard), so as long as the misses aren't crazy like they were before, you should be in the clear more or less. I would personally still tune for the intake, but at least the car "should" function better than it did before once this is resolved.

Sorry this was so long, but considering the OP is saying that he is a "noob", I figured more details would be beneficial here. I hope this helps.. :)

In order to get the interceptor to read AFRs on a Coblat, you must have the Wideband sensor kit.

He has an ss/tc. The car comes with an onboard wideband and the pid is already in the ecm ready to be monitored by the interceptor or any scan tool.

Plus we weren't talking about monitoring afr's anyways.. ;)

Thanks for the help, ended up taking it to the dealership. $250 later they said that the plugs i put in were wrong thanks to autozone(ohm rating on them are wrong according to the gm tech). They also said that for some reason there were codes in the computer but it will not throw the check engine light (the check engine light will not illuminate). Also they said that they believe that there is a tune on my computer because it does not the factory tune on my car. So now i wonder what kind of tune has been done and if i were to get a trifecta would that erase the old tune? Also since i have the factory airbox back on the stft are at 0.0 to 1.5.

my stft ranges from-3.0 to +5.0 when i am driving. Then my ltft is -3.1 but I had reset it two days ago so it should be fairly acurate. Also that is with the stock air box back on. Is that normal fuel trims?