LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

Only getting Positive fuel trims

Old 06-29-2019, 10:03 PM
  #1  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Only getting Positive fuel trims

So starting a few days ago, my car decided it wanted to stop subtracting fuel. The only "trims" I get are positive. I have a wideband and it seems to be adding fuel at the correct times. However, when my wideband goes lean (12-13 afr) my trims just stay at 0. .9xx volts on both o2 sensors at that time as well. I've done a few logs and the average mv of my o2s are around .5 and they go high when I get on it and go to 0 when i'm at 0%tps. So they seem to be working properly.

I just can't figure out why the negative trims aren't working? Anyone heard of this? I've tried different tunes, PCM's, and I still can't get the negative trims to come back.

If someone's heard of this let me know. It has me baffled. I cleaned my MAF again and it didn't even look dirty in the first place. Can't locate any vacuum leaks either.

I have no codes, and the car starts/runs just fine. Just obviously running pig rich at times because of the lack of trims.

I've measured fuel pressure with no vacuum and am getting about 47PSI so I have my injector flow rating at 25.89 in the tune. With vacuum it's about 41psi. So that all seems normal as well....

Last edited by slikrider20; 06-29-2019 at 10:09 PM.
slikrider20 is offline  
Old 06-29-2019, 11:23 PM
  #2  
Super Moderator
 
Injuneer's Avatar
 
Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 67,303
Re: Only getting Positive fuel trims

What positive % are the fuel trims in the various cells?

Are you talking about part throttle/moderate loads, where the PCM is in closed loop, fuel trim learning enabled, and targeting 14.7:1? Or are you talking about heavy throttle/high load operation, where the PCM goes into power enrichment (PE) mode, and targets a richer A/F ratio (typically around 11.7:1 in a stock tune, generally in the range of 12.8:1 to 13.2:1 in a custom normally aspirated tune).?

At closed throttle, are you decelerating (as opposed to sitting in the driveway)? The PCM cuts the injectors to prevent dumping raw fuel into the exhaust. The O2 sensors will read close to 0 volts. Is it operating in Cell 17?

Just to get the terminology correct, an O2 sensor reading of 0.500 means 0.500 volts. That is also 500 millivolts. If the fuel trims are stabilized and you are not in PE mode, the O2 sensors will average close to 0.450 volts, the bias voltage supplied by the PCM.

I know that doesn’t answer your basic question, but I need the clarification requested above.
Injuneer is offline  
Old 06-30-2019, 04:39 AM
  #3  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Re: Only getting Positive fuel trims

I'm just talking about light throttle and light loads. If I just barely get on the throttle let's say in 3rd gear and 2500 rpms, my wideband will go down to as low as 11.1 and the PCM will do nothing to compensate. If I get on it harder (not pe mode) it will get closer to 14.7. I just don't understand why it's only adding and not subtracting fuel. And yes when I'm decelerating it goes to cell 17 and my o2 volts are right or close to 0.
slikrider20 is offline  
Old 06-30-2019, 05:45 AM
  #4  
Super Moderator
 
JakeRobb's Avatar
 
Join Date: Jul 2005
Location: Okemos, MI
Posts: 9,315
Re: Only getting Positive fuel trims

Where is the wideband mounted relative to the pre-cat narrowband O2s the PCM is monitoring? And do you know what voltages those are reporting when the wideband says youíre running rich?
JakeRobb is offline  
Old 06-30-2019, 08:53 AM
  #5  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Re: Only getting Positive fuel trims

Wideband is mounted in my y pipe on the driver side about 8 inches behind the narrowbands.

And yes, when the wideband is rich the narrowbands are always .9xx, and drop to 0 when I'm in DFCO
slikrider20 is offline  
Old 06-30-2019, 09:04 AM
  #6  
Super Moderator
 
JakeRobb's Avatar
 
Join Date: Jul 2005
Location: Okemos, MI
Posts: 9,315
Re: Only getting Positive fuel trims

That rules out anything I had in mind. You left a few of Fred's questions unanswered, though.
JakeRobb is offline  
Old 06-30-2019, 09:06 AM
  #7  
Super Moderator
 
Injuneer's Avatar
 
Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 67,303
Re: Only getting Positive fuel trims

Positive fuel trims are the PCM's response to a lean condition reported by the O2 sensors. Is it happening the same on both banks? Knowing if the condition is the same on both banks determines what we have to look at - things that affect both banks, or things that can affect only one bank. If the %’s are the same in all cells, as opposed to increasing or decreasing with load, it can also suggest where to look.

Is this related to the same problem you posted a month ago, and I was trying to help you with? You never followed up on that thread. Two of us asked you questions, including the location of the wide band, and you never followed up.

https://www.camaroz28.com/forums/com...o-lean-887487/
Injuneer is offline  
Old 06-30-2019, 09:12 AM
  #8  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Re: Only getting Positive fuel trims

Let me post a log today with my wideband recording as well and maybe that'll help rule anything out.
slikrider20 is offline  
Old 06-30-2019, 09:15 AM
  #9  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Re: Only getting Positive fuel trims

Originally Posted by Injuneer View Post
Positive fuel trims are the PCM's response to a lean condition reported by the O2 sensors. Is it happening the same on both banks? Knowing if the condition is the same on both banks determines what we have to look at - things that affect both banks, or things that can affect only one bank. If the %ís are the same in all cells, as opposed to increasing or decreasing with load, it can also suggest where to look.

Is this related to the same problem you posted a month ago, and I was trying to help you with? You never followed up on that thread. Two of us asked you questions, including the location of the wide band, and you never followed up.

https://www.camaroz28.com/forums/com...o-lean-887487/
I ended up finding out that my wideband o2 sensor was bad. I replaced that and it's been reading accurately since. I've had so many different random fueling problems with this car that all or none of these issues could be related.
slikrider20 is offline  
Old 06-30-2019, 09:33 AM
  #10  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Re: Only getting Positive fuel trims

I was just looking at some old logs and it looks like my car never actually enters cell 17. It's almost always in cell 3 when decelerating and a few times in 2 when decelerating. I'm not sure if that has anything to do with this.

I have had issues in the past with fuel trims just not showing up at all, neither positive or negative. Reflashing the tune or tweaking the tune sometimes seems to have an effect on this, but I can't really pinpoint one thing or another. This is the first time i've ever seen it only show up with positive trims though. Solomon has been tuning the car for me and he's said there have been a few customers of his where the trims just don't show up as well and hasn't been able to figure it out either.
slikrider20 is offline  
Old 06-30-2019, 10:52 AM
  #11  
Super Moderator
 
Injuneer's Avatar
 
Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 67,303
Re: Only getting Positive fuel trims

Let Moe Bailey tune it. He just did a tune for a buddy of mine with mod's similar to yours, and my buddy couldn't believe the improvement. Moe has a good reputation on ltxtech.com, and tunes Tony Shepherd’s LT1. With the response from Solomon, disappearing fuel trims, previous problems with your wideband, etc., it's going to be very hard, if not impossible, to make any progress here.

I did review a previous log for you and the fuel trims seemed to be responding normally. I can’t go back and look at it because my PC crashed and Western Digital is still trying to resolve an issue with getting a “restore” out of my backup hard drive.

MoeHorsepower Tuning
San Antonio, TX
254-644-2656

[email protected]

Last edited by Injuneer; 06-30-2019 at 10:55 AM.
Injuneer is offline  
Old 06-30-2019, 01:21 PM
  #12  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Re: Only getting Positive fuel trims

I'll give him a shot. And you're right, with these compouding problems my ability to figure it out is 0. I was just hoping someone had heard of only getting positive trims.

At least if I have him tune it I can rule out any tuning issues. Thanks!
slikrider20 is offline  
Old 06-30-2019, 02:04 PM
  #13  
Registered User
 
ACE1252's Avatar
 
Join Date: Mar 1999
Location: Kernersville, NC
Posts: 1,971
Re: Only getting Positive fuel trims

What software are you using to scan and log it?
ACE1252 is offline  
Old 06-30-2019, 03:22 PM
  #14  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Re: Only getting Positive fuel trims

Ecm pro scan with enhanced Parameters, jet to tune
slikrider20 is offline  
Old 07-02-2019, 03:00 AM
  #15  
Registered User
Thread Starter
 
Join Date: Mar 2008
Posts: 66
Re: Only getting Positive fuel trims

So I went out again tonight and decided to see if my trims were still acting up. I hadn't started the car in about 2 days. As soon as I fired it up, it went into closed loop, and BAM! fuel trims worked great again, and wasn't running so damn rich. I also noticed that the car pulls SO MUCH harder and feels so much smoother when these trims are working. It seriously feels like 50+ more horsepower.

Well, just as soon as I get excited, I turn off the car to adjust my headlights and get back in, refire it and the fuel trims are DEAD again. Consequently, the car seems to be running differently, and my huge loss of power is back again. It's almost like it's in some sort of safe mode when this happens. I tried restarting the car several times after this and the trims stayed dead. I don't get what is going on. It's like I have to wait 2 days to drive the car and not restart it just to get it to run right.

Here are a couple logs. These are both the same tune but a different day. Nothing else changed other than the day. As you can clearly see, in the first one (ran a few days ago), the trims don't show up hardly at all and if they do they are only positive. In the 2nd one (ran tonight), the trims show up just fine. Keep in mind that even tonight after this 2nd log, it went back to it's old self with dead trims and crap performance.




Attached Files
File Type: csv
Solomon #7 no trims.csv (245.7 KB, 8 views)
File Type: csv
Solomon #7 trims.csv (299.7 KB, 7 views)
slikrider20 is offline  

Thread Tools
Search this Thread
Quick Reply: Only getting Positive fuel trims


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.