What causes PCM to send rich command in close-loop other than O2 sensor?

[mena24]

Here's my problem - PCM sees high O2 voltage (980 mV, but PCM still sending rich command!

I'm hoping hoping that this question is a generic concept for an OBD II vehicle. I know this is not the right forum for this vehicle I'm working on,.. but please bear with me. I needed some info regarding the fuel trim scheme that could shed tlight on this issue. Thanks in advance.

Here it is...

It's OBD II, 1996
Smog failure for very high CO (9.00%), high HC (amost two times the limit) under load
(15mph and 25mph).

Scan reading says O2 detecting rich exhaust (980 mV) but PCM keeps sending rich command.

LTFT goes up to 24.8 %

All reading goes back to normal at idle.


Here's some of the info @idle RPM using Solus scanner:

Closed loop
ECT = 198 F
MAP = 20
O2 switching fast from around 150mV to 900mV
STFT = 0 +/- 1
LTFT = 0

Now, here's some of the info @2500 RPM using Solus scanner:

Closed loop
ECT = 201 F
MAP = 22
O2 stays 980mV (O2 detecting very rich mixture) !!!!
STFT = .... (Don't remember, it keeps changing)
LTFT = 24.8 % ( from 0, it keeps going up till it riches 24.8 and stays there)

Is this PCM problem? Why is PCM ignoring a rich O2 reading by sending rich command under load?
What else can PCM look for to calculate fuel other than O2 sensor during close-loop??



Any input will greatly appreciated!!
Thank you!!

[FormulaLT1]

PCM's look to Map and Maf(on cars that have both) sensors as well as IAC sensor count to know how much to pulse the injectors. Do you have the ability to log a PCM run ?. If you can log your PCM commands so I can look at your block learn cells I am sure I can find the problem. Basically what is happening is that PCM's use short term and long term readings to send commands to things like the injecter and iac. So when it registers something that needs to be changed such as injector pulse or operning and closing the iac more. It registers a block learn cell. Build up enough of the short term ones and it makes a permanemt change. Also under WOT conditions all rules and cmmands are thrown out the window. So you can be running super rich and the PCM will continue to add fuel. Like I have said though. I would need to see a engine PCM log to try and see what was happening.

John

[mena24]

Thanks for the reply John. It's been a baffling case(to me atleast).
Could you please explain what PCM log is? Is it similar to Freeze frames? We have Snap-On Solus ar work, so if you're familiar with it perhaps you can tell me if I have the capability to log a PCM run. Solus is supposed to be an update to the regular Snap-On scanner.

Btw, this vehicle has MAP, but no MAF.

And you mentioned "Block Learn". I thought it's the same as Short-Term/Long Term fuel strategy? Isn't block learn equivalent to Long-term fuel trim? If so, then block learn is probably in the 30-ish on the scale of 0 to +128.

Regarding the WOT condition, I understand that if the vehicle is in one, then wouldn't it go back to "Open-loop" status? The scanner is showing it's still under "Close-loop" while it's trying to richen the already rich mixture. Or am I mistaken?

Thanks again.
Mena

[FormulaLT1]

A PCM log is when you hook the car upto a scanner or computer and monitor all the sensor feed as well as PCM adjustments its making as well as learned block cells it already has. So you can see why it either has taken away or has added fuel to get the perfect a/f ratio. A computer uses both long term learn cells & current O2 reading to know how much fuel to add to the mixture/pulse your injectors,. The current changes it made this run are short term learn cell. The pcm also looks at its history to see what changes its made in the past to create the perfect mixture. As for WOT conditions the computer does not go back into open loop but does not look at the O2 sensor either, what it does is takes a premapped fuel/ignition timing in your computer and looks at your long term learn cells and adds fuel/spark as it see's fit and then goes right back into adjusting for short term changes based on TPS sensor feedback. Sorry I can't explain this any easier but you really have to see a log to understand it. If you would like I can send you one of my logs to check out so you get a better idea of what I am talking about.

John

[mena24]

Oh I will greatly appreciate it if you can send me a sample of your logs! Please send it to [email protected].
Once again thank you!!

Mena

[FormulaLT1]

OK I sent it and your going to want to copy the info and paste it in a graph so it all lines up.

John

[mena24]

Originally Posted by FormulaLT1

OK I sent it and your going to want to copy the info and paste it in a graph so it all lines up.

John

Ok, just got back from work. I got your info and I must say it's pretty interesting! I have not seen data like it before which makes me wonder if Solus is capable of getting PCM log,.. but I sure will try to figure that out tomorrow.

I'll let you know if I can get it and If so, I'll show you what the vehicle got.

Thank you!!
Mena

[FormulaLT1]

Originally Posted by mena24

Ok, just got back from work. I got your info and I must say it's pretty interesting! I have not seen data like it before which makes me wonder if Solus is capable of getting PCM log,.. but I sure will try to figure that out tomorrow.

I'll let you know if I can get it and If so, I'll show you what the vehicle got.

Thank you!!
Mena

No problem Mena, I look forward to helping you figure this out.

John

[mena24]

Originally Posted by FormulaLT1

No problem Mena, I look forward to helping you figure this out.

John

Well, I came to work that morning and found the shop manager had released the vehicle back to the owner because we were just swamped. and the owner needed the vehicle fixed pretty soon. We gave the owner all the data we found so far and there's no charge. Of course the shop manager apologized and the owner was cool about it.

I wanted to really figure it out.... but I guess there'll be more opprtunities ahead.

Sorry I wasn't able to get back with you right away John, but I really appreciate you trying to help. You gave me some pretty good pointers earlier.

Thanks again!

[FormulaLT1]

No problem Mena. Hopefully next time we can figure it out.

John

[Chevyracincamaro]

did we just get a shop mechanic askin for help on the forums? thats bitchin...