chris604

Hi All. I have an issue which started recently. I hadn't driven it in about 5 months, but fired it up a few weeks ago, and it seemed to run fine. But, the engine just seemed to cut out every few seconds while driving normally, and under intense load, would just sputter and backfire. And, on top of that the gas mileage has dropped to half of what it was before, and it smells of gas after I get out of the car. I didn't see any leak, so it's possibly just dumping too much fuel in when it runs. Its got a catless downpipe, I installed a few years ago, but it still ran fine for a year and a bit, just recently started running bad. I guess it's not the fuel pump or fuel filter, as that would mean it wouldn't run at all, and it would just cut out and not accelerate that well, but it wouldn't dump gas in like it is now. Is it injectors or a fuel/air sensor or something along those lines? Any help is appreciated. Thanks!! OH, also, I did get a superchips tuner for it for 91 octane. And, it runs worse when the boost is turned up, so I tuned it back to stock, and it ran better, but still runs horribly. It was also giving misfire codes on all 4 cylinders P300, 301-304. Thanks.

steelmesh

First reply doesn't mean the best reply, here is what I got for ya: Reading this I was thinking about you should be getting misfire codes, and last sentence you confirmed. Being all four cylinders probably rules out a failed injector. If it is dumping fuel then yes that can cause a misfire, also ignition problems could too and that would also tell-tale with fuel smells.

Do a once over of the engine wiring and connectors on everything you can get at, and even run through your fuses to make sure we rule out all the easy stuff.

chris604

Cool. I'll try that. Could it be the MAF or something like that? Probably not ignition coils, as only one would die at a time most likely. What fuses would likely be the culprit?

C0balts5

Mine did the exact same thing. I am catless also and have full bolt ons and running 23 psi. It was so bad raw fuel would literally drip out the tailpipe when I shut it off. I chased it for months until my car finally popped a lean code for post fuel trim. Turned out my down stream 02 sensor went south but didn't throw any actual 02 codes. Replace my downstream 02 and it fixed it. These cars like to eat 02 sensors especially if your catless. Might want to check into that. I'm on my 3rd in 4 years

chris604

Ok. Thanks man. I took that sensor out a few weeks ago, and it threw the code for bad sensor #2, so I thought that wasn't the problem. I guess I could just swap a new one in and see if that was it.

chris604

That's the one near the turbo right? I guess there is only one left, since the downpipe doesn't have one any more.

C0balts5

Mine still has a downstream 02. They will mess up your air fuel ratio bad and cause multiple cylinder misfires and poor gas mileage. Ecu is still looking for some sort of reading. I have a spacer so I don't throw a cat code. For me it was the one further down, not the one at the manifold near the turbo

C0balts5

Mine still has a downstream 02. They will mess up your air fuel ratio bad and cause multiple cylinder misfires and poor gas mileage. Ecu is still looking for some sort of reading. I have a spacer so I don't throw a cat code. For me it was the one further down, not the one at the manifold near the turbo

chris604

You mean there are 3 sensors? I took out the one in the cat in the down pipe, and there's one near the turbo, which is a bit hard to get at. Where's the 3rd one? Oh, mine is a 2008 Cobalt SS Turbo, lnf.

C0balts5

It's just a 4 cyl. You have one upstream and one downstream. Only 2 sensors altogether. Do you have some sort of trick sensor plugged into the downstream o2 to trick the computer? Or is the plug for it just there with nothing attached to it? I still have both my sensors but I have a tube welded into the down pipe to pull the downstream o2 out of the stream so it tricks the computer and doesn't throw the p0420 cat efficiency code.

chris604

Nope, just pulled it out, and hid the cable. It ran fine, but it showed the code as you mentioned. But, it was running great until a few weeks ago. So, do you think it's worth putting in a new upstream O2 sensor?

C0balts5

Honestly I would replace both the sensors. The computer relies on the oxygen sensors to rich and lean the air fuel mixture. I can't remember exactly which one the computer uses for cold start operation, the computer relies on one of them to base the air fuel mixture off of and then once it warms up it reads both sensors and fine tunes in the mixture. Throw a new down stream one in first and erase all codes and see what happens. I have yet to have to replace my one by the turbo. The computer needs to see fuel trims to know to lengthen or shorten the fuel injection timing.

chris604

OK. I'll try it. I guess I would have to drill a hole in the down pipe and just stick it in there, I'm assuming. Or is that a good idea? And, ever since I got the catless down pipe, I smelled fumes inside the vehicle. Is that from the catless down pipe, or I just didn't install it right? I did it myself.

C0balts5

Sounds like exhaust leaks. You may want to take it to an exhaust shop send have then drill a hole and weld a nut on for the sensor to screw into or have them just straight up weld a spacer on for the sensor to thread into. Or buy a catless down pipe that has a hole for the downstream 02. Exhaust leaks will screw with the readings and will cause the computer the richen the mixture cause it will be reading oxygen. That's also why the exhaust sensors have a crush washer so they get a perfect seal.

chris604

Yeah, I think I'll just get a proper downpipe. The one I have on sucks anyways, and then it will probably seal properly when it's installed, unlike this one. It did have gaskets on either end of it, and I wrenched it super tight, so thought there would be no leaks, but that wasn't the case. And, the motor will run better anyways, with a proper downpipe and O2 connector on it.

C0balts5

Hopefully. Gotta start with that. Most likely that's your issue. Without the downstream 02 readings the fuel trims are all out of whack. It probably ran ok for a lil until the long term fuel trims got screwed up and now it went full rich

C0balts5

Hopefully. Gotta start with that. Most likely that's your issue. Without the downstream 02 readings the fuel trims are all out of whack. It probably ran ok for a lil until the long term fuel trims got screwed up and now it went full rich

chris604

Ok, I'm just hoping it's not something else like fuel pump or anything like that. I guess it's worth a shot to replace O2 sensors, etc. as they aren't too expensive.

chris604

Fixed the issue. It was the upstream O2 sensor. All the codes went away after installing the sensor. I needed the special tool for it, as it's got the long thich wire coming out of it, and it's right next to the turbo, with very limited elbow room.. Crappy tire loaned it for free, so no problem there. Part was about $100. all the codes disappeared except P0100 for the MAF, but that's because of the aftermarket intake I installed. I still get a few exhast fumes, when i turn on the Vent, but that's probably cause of the downpipe I installed. I'll get a better one with an O2 sensor for downstream.