'07 Cobalt Bad engine misfire

[J-Ri]

Originally Posted by manicmechanix

Yeah but again the LTFT applies to operating ranges and conditions that are similar to the one at closed loop. I think it's mostly telling you how much the fuel trim was leaned or enrichen to maintain lambda at the operating condition of closed loop. It will compesate some for power enrichment maybe or trim some through out the operating ranges, but it's still going to be slightly too lean outside those ranges. Ethanol has a stoichiometric of 9. If you do the math the the AFR will be off by .5 or so. This doesn't sound like much but at power enrichment/near WOT where it matters for engine overheating. That might not sound like much but if for some reason you are at 13.5 or so with straight gas, this ethanol can tip the ratio too lean.

In closed loop and PE the PCM is programmed to run a bit rich to provide some safety margin... if you snap the throttle and watch the upstream O2 sensors, they go .9xV (and generally stay there until the throttle is released) I have never had the chance to play with a wideband sensor (but I'm adding one to the Cavalier soon, want to be sure I'm OK before I really get into the boost ), so all I know is "rich", it may be 14.4:1 at WOT, I really don't know there.

I think you're backwards on the AF ratio, 13.5 is pretty rich. First number is air, less air=more gas=rich.

[manicmechanix]

Originally Posted by J-Ri

In closed loop and PE the PCM is programmed to run a bit rich to provide some safety margin... if you snap the throttle and watch the upstream O2 sensors, they go .9xV (and generally stay there until the throttle is released) I have never had the chance to play with a wideband sensor (but I'm adding one to the Cavalier soon, want to be sure I'm OK before I really get into the boost ), so all I know is "rich", it may be 14.4:1 at WOT, I really don't know there.

I think you're backwards on the AF ratio, 13.5 is pretty rich. First number is air, less air=more gas=rich.

The O2 sensors will probably max V at 14.2. That's the nature of narrow band O2S, they make a big jump when you get away from stoichiometric. I know the first number is the air and higher the ratio the leaner, lower the ratio richer. Power enrichment and WOT is usually tuned around 12.8-13.3. It's suppose to be richer at heavy load. If you are in the 14's or higher than about 13.5 not only are you losing power but that's too lean for high engine load and risking damaging the engine. Yeah, a wideband tool like the Innovate is an easy and affordable way to go and is almost a neccessity to get the tune right. I'd like to have a 5 or 6 gas analyzer too. That's what the manufacture engineers use.

[J-Ri]

Ah, ok, I thought you meant 13.5 was a little lean for cruising conditions. I gotcha now. I was told that 14.7 is OK until you get into boost. I didn't have to tune anything on my supercharger, it was done with a PCM recalibration that came with the kit from GMPP. Tuners could talk circles around me all day. I'm working to change that tho

[rodeo02]

Jezzus-H... you guys got me all paranoid with the E10 now. I gotta think it's mostly a non-issue though. I know in the Buffalo NY area, almost all of our stations have sold nothing but "up to 10% ethanol" for about 20yrs now. It's nothing new for us, nor have I ever had a fuel related concern. Maybe I'm lucky!

Joel

[manicmechanix]

Originally Posted by rodeo02

Jezzus-H... you guys got me all paranoid with the E10 now. I gotta think it's mostly a non-issue though. I know in the Buffalo NY area, almost all of our stations have sold nothing but "up to 10% ethanol" for about 20yrs now. It's nothing new for us, nor have I ever had a fuel related concern. Maybe I'm lucky!

Joel

It's not really a big deal, it's if your car is switching between regular gas and E10 stations AND you have a fuel deliver problem like a dirty injector it gives you less margin for error for leaning out. If your always getting E10 then like J-ri said your PCM LTFT will mostly compensate for it.

[rodeo02]

Gotcha. That makes sense. I've never had a fuel injector issue that I know of, but I'm wondering if adding a fuel system cleaner like Chevron Techron concentrate a few times a year would be a good thing. I used to use that and 'experimented' with a product called "Fuel Power" and used Lucas fuel system treatment years ago. Kinda gave up on'em. Never really saw a benefit for my usage.

Joel

[manicmechanix]

Originally Posted by rodeo02

Gotcha. That makes sense. I've never had a fuel injector issue that I know of, but I'm wondering if adding a fuel system cleaner like Chevron Techron concentrate a few times a year would be a good thing. I used to use that and 'experimented' with a product called "Fuel Power" and used Lucas fuel system treatment years ago. Kinda gave up on'em. Never really saw a benefit for my usage.

Joel

I think it's a good idea to add Techron fuel system cleaner a few times a year. When you're using the fuel cleaner the idea is not so much noticing a difference as keeping your engine running smooth and your injectors from clogging up and causing misfires. Fuel should have a minimum amount of cleaning additives, but some brands and locations do not have enough. Also so engines and injectors are more susceptibl;e to deposits than others. Maybe you use good gas in your area or you car has a good injector design, but I think it's a good idea to use Techron a few times a year. It won't hurt and it's cheap insurance. I've had to clean my injectors on the rail and replace clogged injectors before, so I use Shell gas now and add Techron occassionally.