I wanted to give you guys some insight I discovered last night while logging some data. I recently had the cats cut off my truck & had posted a thread on the improved results at the track. The last time out @ the track I had carried along a tech 2 scanner to to record my passes. I would reset fuel trim data after each pass to ensure consistansy. I have noticed that doing this puts me back@ the same fuel cell block everytime. On the way home I was looking @ iat temps & never paid attention to ltft.

Last night I had a fairly long interstate drive incool weather so decided to look @ some data. What I found out was that my fuel trim values were WAY POSITIVE at prolonged hwy speeds 16-19% . When this lean condition occured I noticed my fuel cell count was really abnormal. When I pulled over & launched the truck i had NO LOW END TORQUE. Then I reset the ltfr values, launched, & all felt great.

So I adjusted the IFR table to them back to -2 to 0% . Doing this fixed the no torque problem that I believe was caused by the lean condition. My point being that some of the threads involving loss of torque with the cats off could possibly be caused by an excessive lean condition. I am guessing this is caused by the increased airflow across the O2 sensors.

If anyone else has looked @ this or has any other info let me know.

 

Have you looked for exhaust leaks? If fresh air hits the o2's, that will make the PCM think there is a bad lean condition, and looking at data, it looks like it as well.

It doesn't seem logical that fresh air can get in an exhaust leak, but trust me, it is.

Maybe the o2's are on the verge of going out too. How is the switching rate?

 

The cats should only be effecting the readings on the rear O2 sensors, and, to my understanding they only monitored emmisions and the PCM makes no adjustments based on their readings.

 

That was the first thing I checked, I had a problem when I first installed my headers with a variance between banks. I had an exhast leak that we found smoking it here in our service dept. when that happened I had two new ones (O2's ) covered under factory warranty. I watched the for there response rate & everything is fine. On the way to the dealership this morning everthing was still good.

 

yes, you are right. I have them turned off. My point was that if the airflow across the front O2 sensors was increased due to less restriction than the oxygen count could be be higher based on the fact that the oxygen count (saturation) would stay the same. The O2 then would see more unburnt oxygen (due to increased airflow) & show a substansial amount positive fuel trim values. The other thing that makes me believe this is that it only occurs @ faster hwy speeds & can be related to the increased maf flow.

I am not certain but I am basing this hypothesis based on what I have seen. If someone has an answer other than this I would love to hear it.

 

Where you driving confusiosly??????????????????????????

 

Front 02s: Fuel Trim
Rear 02s: Catalyst Efficiency

 

geting rid of the cats will effect the ltrims just like any mod that effects air flow. if the ltrims get very far off then yeah you should try and bring it inline. the ltrims being +16 means it WAS lean. the computer has compensated for it and it isn not lean anymore. WOT fueling should be handeled completely seperately once the ltrims are brought inline. by adjusting the IFR rate you effected the WOT fueling. your ltrims are now better but your cruze a/f ratio is the same as before since the pcm corects them for you.

if the ltrims have a positve average then they will be positive at WOT. if they are positive at WOT then fuel will be added on top of PE so the ltrims do effect WOT some but will only make things richer. i wonder if when you reset the computer maybe you were part way to the low octane timing and you got all your timing back when you reset it.

it has been my experience that fueling doesn't make a huge diference in power. na or boosted it seems like i can run 11.0 or 14.0 at WOT and i make good power. the timing is HUGE, 5deg of timing makes a big diference in power.

 

Dr phillips!

 

I checked timing & I was @ 34 degrees @ the track, I know it sounds like alot of timing but that is what made the best power onthe dyno.

I would bet I was getting detonation from my lean condition the other night, I did not even think to look, ther ltft had me blown away.

I never used the wide band again to check afr during power enrichment, do you think it has changed now that I have removed the cats? I figured that since I got the fuel cell back in the same block during part throttle closed loop driving the power enrichment multiplyer would not be affected since it goes to open loop & uses that table for enrichment. I kept the same as it was when I tuned it with the wideband. I hope it has not because I spent a ton of time changing my pe vs rpm table to where it would start out at 12.0 on the bottom end & would go to 13.0 on the big end. What worse I am going to the track after work & don't have a wideband right now. Do you think I should look @ the O2's during pe just to make sure they stay above 950 mv's ?