running lean on cold start

[mustangtw]

I am having a problem with my car running lean/rough when you first start it in the morning after its been sitting over night. I have a wideband on bank 2, which is reading 18 to 19:1 at idle for about two minutes after starting the car. Then it will drop right back down to 14.7 to 15:1 as if nothing ever happened. I hooked up the predator to see if I could find out what was going on and SHRTFT2 was at 25% when this was happening. And O2S22 was at 0 value. As the problem cleared up after about 2-3min SHRTFT2 came back down to around 0 and O2S22 started reading voltage. Is this a problem with my oxygen sensors on bank 2 or something else? This is very wierd and only happens for about 2 min on a cold start after sitting overnight. Will run fine the rest of the day. Thanks.

 

[spartansnake]

ttt same thing for me, my tuner said hes never seen it before and thought it was very wierd. ONly on cold starts or even initial start after its sat for a little while. Rpms also fluctuate during this lean period but after a couple of minutes everything is fine.

 

[Goodlife]

your in open loop. Once o2s start reading is because you have reached minimum ect for the pcm to go into closed loop.

 

[mustangtw]

your in open loop. Once o2s start reading is because you have reached minimum ect for the pcm to go into closed loop.

So are you saying its not the oxygen sensors? Do you know what it could be? Thanks.

 

[mustangtw]

I think I am going to go ahead and replace bank 2's primary oxygen sensor to see if that fixes my issue. I hooked it up to the predator again to datalog and I noticed within 30 secs of starting the car it was in closed loop. Before I started the car I looked at all the specs in the data list and I saw that all the oxygen sensor readings were at 0volts except for Bank 2 sensor 1 which was stuck on 1.6V for some reason. After starting the car it ran good for about 30sec in open loop, then when it switched to closed loop it started running very lean 19:1. I noticed 1 of the primary oxygen sensors (Bank 1) was switching from .2 to .7V rapidly while the other one (bank 2) was stuck, slowly falling down from 1.6v. It stayed above .8v for about 2min. Then it started switching normally as it should and air/fuel ration fell back into spec. The short term correction for bank 2 was 25%, which I believe means it is leaning out by 25% to correct for the false rich condition the oxygen sensor was giving. So I am fairly sure that I have a problem with my oxygen sensor, I am going to go buy one and replace it and will post results if that was it or not.

 

[04CobraMatt]

Well its the opposite, in OL the o2 sensors are ignored. I would start by looking at the cold start fuel table and check your STFT's between 150-300 maf counts.

 

[Got Torque]

Likely in your tune, since it is cold start it is not taking the O2 readings into account.

 

[mustangtw]

Well here is an update. I bought and installed a new oxygen sensor yesterday. I just went this morning to start it up and it seemed to have fixed the problem, the car is no longer running lean/rough at idle. When everyone talks open and closed loop, I have found on my car that there is only a 15 second time frame once you start the car before it switches to closed loop. I timed it on my diablosport. So, this was an issue in closed loop not open loop. If anyone else is having a similar issue with their car, I would advise them to check the output of their oxygen sensor voltages on a datalogger or scan tool. Also check the short term fuel trims to see if one is way out of wack. Thanks again.

 

[Goodlife]

Do a hard reset sometime also.
Yes that it was I was saying open loop o2s are ignored until certain engine temps are reached and other parameters...then the pcm/ecm goes to closed loop and takes all readings into account for fuel table adj. etc.