I was just wondering how do you guys tune the idle? I am seeing about 7-8gps on the MAF and I was wondiering how can I make the car leaner or the car burn all the fuel that is going in?

I have an XE 224/236 cam and I think this is the "problem" my blms are near 128 all the way but when stopped you can smell the raw fuel...

Is open loop the only way to correct this???\
Thanks

 

You cannot really run rich unless there is an exhaust leak telling pcm it is lean. If you are running with no can you can get perfume to add to your fuel to cover up smell.

 

My theory is that when you run a bigger cam there is always more overlap than stock... in that overlap, more fuel will go un burned to the exhaust and it will not be detected by the O2 Sensors since they detect CO2 not O2.

I wanted to eliminate this problem due to fuel consumption... LMK what you guys think

 

Im having the same problem...I dont have cats though so that could be part of the problem too.

 

If you have LT-1 editor or Tuner cat, then you may be able to tweak the MAF calibration at the 7 - 8 gms/sec ... move that frequency down to correspond to 5 - 6 gms/sec and that will lean things out.

Is yours a 383?? I ask because I'm about to install the same cam into a 9:1 comp 383 and was hoping it would run good enough on the stock 95 program.

thx

LWM

 

Yes its a 383...

Anyways.. if I lean it out all that will do is move up the blms higher than 128. It will help in open loop though, but it will just throw the blms off.. so what is the gain??? the computer will compensate and add the fuel again...

 

Im running the same cam in my 396 LT4, it smells fine. I have it at a stable 800 RPM idle speed and its tuned nearly perfect. Did you remover your CATS? If you did then that is the source of the smell. A cam like the 224/236 will have a thumpy idle, and no matter where you set it up with out the cats you are going to get a smell.

Even a stock engine without mods will stink at times without its cats. I removed the cat on a 454 suburban and it stunk of raw unburned fuel on decel and the same thing will happen on a LT1 car.

There is no sense to adjust the MAF tables or the injector constant unless your fuel trims are way off. The long term trims can be off as much as 20 points and the PCM will still be able to control it. Just look at the short term trims and see where they are, if they are near 128 (averaging) then the PCM is in control. No matter what you do to it, the PCM strategy will try to maintain its stoich condition by adjusting its long and short term trims to 128 so unless you adjust the injector constant way off or the MAF so far off so it can't get within its range of control the PCM will maintain its programmed setpoint automatically based on what it sees from its o2 feedback.

 

No... the O2 sensors can detect only O2. They can not detect CO, CO2, N2, unburned HC, etc. They detect oxygen.

As pointed out, the mixture should stay right at 14.7:1 in closed loop, IF the O2 sensors are accurate.

The sensors can be faulty, from old age, contamination with anti-freeze, silicon, lead, etc. They can be crudded up with carbon. They can also be contaminted from the outside, since the sensor is comparing the O2 content of the exhaust gas to the O2 content of "air". If you get oil on them, or other contaminents, or if you have an exhaust leak blowing on them, their accuracy could be off. They can also give bad data if they are not hot enough, or if they don't have a good ground, there are poor qualtiy extensions or splices in the line, or there is RFI interference from the high voltage spark plug wires.

The sensors can also provide bad feedback, if there is an exhaust leak before the sensor (O2 in the air), there is a misfire (unburned O2 from the cylinder), sensors located too close to cutout.

If the BLM's are at or near 128 at idle, ,and it still smells, AND you still have your cat, you need to check for all of the above.

 

As a general equation for a fuel

C2H2 + 5/2 O2 ----> 2CO2 + H2O

So how can they detect O2 since you do not know exactly how much O2 is comming in the engine. It seems to me that they do detect CO2 since it will be stechimetrically correct to do so... All the fuel will be burnt as the fuel will be the limiting reactant...

Another thing is:

If they detect O2 why is a missfire seen as a lean condition right??? Lean condition is low O2 sensor millivolts... If it read O2 it should read a high count since none of the O2 was used since there was no combustion, then all the oxygen is at the exhaust
If it were O2 a missfire should be a high count??? I am not saying you are wrong but that is my logic and I have always wanted to clear that up...


I dont have CATS

But its interesting to know... When I had the wideband installed on the dyno the A/F was at 14.5 or so, but dont remember, anyways I have brand new O2s that I can put on... And maybe they are a bit contaminated with some oil...

But ... I dont get it... if the car runs fine at WOT, that when the sensors are off it runs like CRAP, they should be OK.. but I will check.. anyways let me know what you think

 

O2 sensors are essentially little voltage generators. It is more of an electro-chemical reaction to what is in the combustion gas stream than just a chemical reaction as a result of combustion.

The sensors are constructed with a porous Zirconia bulb, the outside of the bulb is coated with platinum this bulb and platinum layer is porous. Inside the bulb are two electrodes and the inner chamber is what is refered to as the reference chamber where there is always a reference gas from the outside atmosphere.

In the older designs of early sensors there was an actual hole in the sensor body to allow for this atmospheric reference air to enter into the body of the sensor. Now days with better materials the reference gas travels into the inner chamber via the wire harness insulation. The air actually travels into the chamber via the insulation of the wires that connect to the inner electrodes. I guess in essence you can say that the sensor breathes through the wires. So that is why its very important to never use any kind of grease on the O2 sensor connector, if you plug off the reference gas to the sensor you will render it useless.

That very thing happened to me once when I contaminated the O2 sensor connector with antifreeze. It worked for about 10 or 15 miles until the sensor used up its reference gas and then the sensor told the PCM that I was lean, so it started to pour the fuel to one bank on my LT4. And I mean it was pouring the fuel to it because it was pegged at .115 mV.

The greater the difference between the reference chamber and in the exhaust side of the sensor the greater the voltage output of the sensor. So in a very rich running condition there would be less oxygen in the exhaust stream, compared to what is in the reference chamber which is outside air and is usually about 21% oxygen If I remember right and a very high output from the sensor.

Do this test with your scan tool connected, while in closed loop and idling, disconnect one of the injectors and watch what happens to the O2 reading, it will take a dive as the O2 level goes way up with one cylinder going so lean that it will no longer fire.