General 1990 low fuel trim stuck at 118%

[Nautica985]

My low fuel trim is stuck at 118%, mid and high are at 100%. I have the MAFT and i made the low/idle as rich as i can make it and the low trim still will not move. The only boost leak i can find is at the turbo housing but i have a GM MAS right before the throttle body so I assume any leaks before the MAS wouldn't effect my fuel trims or a/f ratio. I already tried reseating the ECU and its not the MAFT because the low trim was stuck at 118% with the stock MAS. It also knocks like a mofo

 

[Nautica985]

another thing that I noticed is when i reset the ECU, the low fuel trim goes to 118% within a couple of minutes of driving. The mid and high trim takes a good 15-20 min drive for them to adjust.

 

[spyderturbo007]

Is your O2 sensor functioning? If it is, it sounds like you have a tuning issue, as the ECU only uses O2 voltage to control fuel delivery at cruise/idle.

 

[Nautica985]

The O2 cycles but every once and a while it will just sit at .0v for a couple of seconds. Ill try another O2 sensor

 

[spyderturbo007]

Tho O2 sensor is most likely your problem. Provided it's fully warm, you should see a rhythmic cycling between ~ 0.2v and 0.8v.

As the O2 voltage approaches a value of 0.0V the ECU thinks you are running lean and adds fuel to compensate. This is how your FT's get maxed out with a dead O2.

 

[d1verDSM]

Wouldn't you be able too see you O2 cycling if you can read your fuel trim. You must be using mmcd or some sort of logger software? I'm sure you can see your o2 cycle?

 

[Nautica985]

They cycle but every once in a while it will stay at .02v until I give it some gas.

 

[spyderturbo007]

wouldnt you be able too see you o2 cycling of you can read your fuel trim ., you must be using mmcd ? or some sort of logger software , Im sure you can see your o2 cycle. ??

I have no freaking clue what you just said there??? Can you translate that to English for me?

Anyway, Nautica 985......

You said a couple of posts above that you were going to try a different sensor. Is this with the new O2 sensor or still with the old one.

 

[Nautica985]

The old one. I haven't found one to swap in yet

 

[d1verDSM]

I have no freaking clue what you just said there? Can you translate that to English for me?

Sorry there's days where my typing doesn't go too well, plus I'm a french canadian .. sorry

 

[blcknspo0ln]

^^ haha that would explain it


I think the answer you need is that if you're using a logger or some sort, the o2 voltage will be displayed. At idle or cruising (out of gear), your o2 voltage will oscillate. if it jumps from .30 - .8v or somewhere around there and goes up and down, your o2 may still be OK.

another two ways to tell is if you're cruising in gear and you let off the throttle, your o2 voltage should read close to 0v.

if you're at WOT, your o2 voltage should read .9v +

 

[mxpimp18]

Not trying to steal your thread but I'm having to same problem. My low fuel trim is stuck at 118.75. I have a new o2 which seems to be working correctly. I'm out of ideas.

 

[oldman]

Nautica985 & mxpimp18, what are your vacuum readings at idle?

 

[mxpimp18]

18/19. Just did a pressure test and not a single leak.

 

[lewroe59]

I'm having the same problem, its stuck! O2 trim goes up when I rev but it just sits at 100% and trim wont move after 30 mins.

 

[turbo2086]

Yeah I'm in the same boat with you guys, except mine is 139.8%

I actually posted in the newbie section about this. I did some diagnostics and going to update that thread, see if any other thoughts come to mind.

 

[Mits3gts]

I had the same problems with being lean at idle, try setting the base to a smaller injector (richer). The injector guide is WAY off and I have read a lot of people having problems setting up the MAFT. That's why I got an EPROM ECU and no more headaches, well no major ones anyways.

 

[dsmsucks]

Could be a ECU problem My fuel trims was stuck so I replaced the o2 sensor with a denso, nothing. I having steve repair my ECU.

Steve where are you?

 

[44pirate]

For 1g cars. The trims won't adjust until you reach operating temps above 184 degrees.

 

[TSIfreek]

BTW guys 90 fuel trims max out at 118 unlike the 91 and above.