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Thread: 95 trans am lt1 diagnosing poor running/ running lean

  1. #1
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    95 trans am lt1 diagnosing poor running/ running lean

    Hi guys, ive been using this forum a bit as i started to build this car. though this is my first post, i want to thank everyone here for all the help.

    the car
    95 trans am with a 350 bored 40 over it has the stock heads and a comp 276 cam. something like 224/232. cam springs that are good for like 550 lift everything else is fairly stock, but new assembled by me so it should be correct :P . just installed the engine last week.

    The problem
    at "cruising " throttle , about 20-35% tps, or 0-35 % tps really. i have a huge bog, like im pulling a trailor. like your exhaust is clogged or intake is clogged.the a/f gauge reads lean at this point. as i push the accelerator it hangs in this "bogged state" until you push the pedal down somewhere past 35% tps at which point the a/f gauge jumps to rich and the car accelerates decently. The air fuel meter is a narrowband and attatched to the drivers bank factory 02 sensor which seems to be working properly. next the engine lacks power and does not want to pull at all above 4000 rpm.the a/f reads rich at this point. Now lastly. the scary part i realized tonight, that helps me diagnose the problem, but also lets me know to take action imediatly. I drove and tested for hours today. got to teh gas station only to realize my gas tank filler neck was actually hot to the touch. and exxtreme pressure buildup. although i dont have any sort of evap hooked up yet. this was clearly excessive. so this made me test something i had forgot. the exxhaust temps. the passenger bank was at 700 pretty even on all cylinders. the drivers bank was at 350 on all cylinders. of course the rest of the engine doesnt seem to be quit that hot by the coolant temps and external laser temperature readings. now for the last interesting part. and probably the problem. the passenger bank 02 is stuck at about 500 or so. although i have an understanding that i gathered somehow that if i unplug the maf the car wouldnt use the 02s to adjust fuel trim and would basically run off of the ve tables.ive been chasing this problem for days now and have removed the optispark to check, checked the plugs burning white on the right bank of course but clean on both none the less), checked the fuel pressure (its 35-40 depending on acceleration) , used a pressure gauge in the 02 hole to check exhaust pressure ( highest was about 3 psi at 4000 rpm, scared to rev much higher because of the way it bogs at that point.), i checked the resistance on the injectors ( all about 13) also tried tuning it myself ( to the tune of one fried pcm the first attempt) second attempt was a mail order tune from garrettuning on ebay. not real impressed with it, but im learnign tuning myself , have succesfully uploaded about 10 different tunes today and have a good understanding of how a car works and timing and whats too much etc. and im confident my problem is not the tune.. im sure ive tested other things but i cant remember. all the time of course not replacing the 02 i know is bad. anyway i have the 02 in my hand, waiting for my car to cool down and some of the evaporating gas fumes to float away and we will see what happens. although i have seen obd1 02 sensors do wild things, i find it hard to imagine its doing all this.


    so to anyone who made it throguh that, thank you, advise or criticism is appreciated. also i will give any more info that is needed. im using tuner pro rt to log and can figure out how to upload a log if i need to. thanks in advance.

    ps: im sure there are errors, im sorry, ive proof read it like four times, its late. thanks for the help!! i appreciate it so much haha
    Last edited by zoomo; 10-10-2014 at 07:11 PM.

  2. #2
    Fuel Injected! steveo's Avatar
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    lets keep it simple, send me your bin and a datalog and let us have a look at what you're working with. dont even look at that narrowband o2 gauge. it's useless.

    got to teh gas station only to realize my gas tank filler neck was actually hot to the touch. and exxtreme pressure buildup. although i dont have any sort of evap hooked up yet.
    the evap isnt hooked up? ... what about the external vent that runs down near your diff? if it's there, it'll be fine. these f-body tanks and evap systems are designed to build up a couple pounds of pressure. nothing should make the filler neck hot, really. that could be a lack of heat shielding between the exhaust and the tank which should be resolved immediately. the fuel in the tank can get a bit warm from recirculating heat soak from the rails, though.

  3. #3
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    ok sorry to be a noob, but how do i go about sending you my bin and data log? im using tuner pro rt and cats by the way. also the tank heat was a lot a lot like probably over 100 degrees at neck. all the shielding is in place, and the exhaust may or may not be stock i cant tell but its not obnoxiously close. i did not measure the exhaust temp under the car. anyway. like yous aid keep it simple. ha
    Last edited by zoomo; 10-10-2014 at 07:29 PM.

  4. #4
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    where you live may be your tank heat. some road temps will get 120-140 this time of year.

  5. #5
    Fuel Injected! steveo's Avatar
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    you said you tried to tune it yourself? post the bin file you're running right now to the forum

    then do a datalog with datamster or tunerpro or whatever and post that too

  6. #6
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    posted the bin and log in above post sorry, was being a noob. figured out how to do it

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  8. #8
    Fuel Injected! steveo's Avatar
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    your right o2 is barely working, it's stuck at reference voltage. is that where you plugged that narrowband o2 gauge into? didja splice the o2 wire or something? the o2 itself could be faulty. that needs to be addressed before going any further.

  9. #9
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    narrowband is on the working 02 sensor sorry for the confusion. just replaced with a new bosch 02. going for a drive. will report back in 15 minutes or so. also did remove the battery cable while i changed 02 to hopefully reset any screwed up trims from it being bad.

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  11. #11
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    so even with the maf unplugged, the 02 sensor still effects the fuel trim? i was under the assumption that with the maf unplugged it would run off of the ve table. but then again taht may not be correct ha. anyway new 02 installed. drove a little seems to do a lot better. much more driveable now, i think i may have still heard it pop a couple times under acceleration. and also still the power seems low and drops off around 4000 rpm and higher. though i know the timing definetly seems high at around 40 at 4000 rpm although it doesnt seem to be retarding the timing? before the 02 sensor change, the car was nearly undrivable it would bog and miss badly anytime you tried to hold steady throttle. anyway heres some more logs

  12. #12
    Fuel Injected! steveo's Avatar
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    that's not quite how it works.

    with the maf unplugged you're correct that it uses VE tables... but closed loop is an additional fueling calculation on top of either one of those.

    i wrote this a while ago might clear it up for you http://fbodytech.com/tuning.html#modes

    will look at your logs soon

  13. #13
    Fuel Injected! steveo's Avatar
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    in the cruising log, it doesn't even seem to make it into closed loop for some reason, but the highway one seems not too bad. your timing advance needs a bit of love (mostly due to the different cam) but it's a good start.

    you should lay off the 100% throttle until it's tuned.

    breaking up at high rpm might be due to the fact that someone leaned out your PE tables quite a bit. try this bin instead. i tweaked your timing table a bit too to get it closer, and a few other small things, but it still needs work.

  14. #14
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    Thanks so much steveo. the cruising was actually right as i left the house. i should have clarified that, and i went to highway within about 7 minutes of driving so it may have just been getting warmed up i read you link and its very informative, though i will need to read it a few more times ha. going out to install your bin shortly. thanks again for the help and ill post the results

  15. #15
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    well i went for a good long drive today. it does a lot better. seems to bog under load/cruising situations. almost feels like it starts to miss fire. is it possible im missing something here? is it possible im actually getting a missfire from a failing injector or some sort of wierd ignition misfire that we cant see in the logs well? (dont get me wrong i have checked for both these conditions very well and the injectors resistance test good, visual check and spray looks good, ignition wires are new although cheap and optispark is new from chandlermotorsports? ebay shit i know, but ive removed and inspected it and seems to be working well.)

    Anyways thanks for all the help so far steve. I did not realize that i had actually found your sites in all my searching and i was already using your ee file. im very interested in learning and understanding tuning this way. i work on cars as my "career" and have a very good understanding of tuning a car, and tune a carb and a distributor. I also do a lot of laptop-based scanning/diagnostic/programming on customer cars, but everything i do is back to factory. so this is very new to me, but im starting to get a decent understanding of it i feel. and im very thankful i came across this forum, and you to hopefully help me get a better understanding. thanks again to everyone. ok so heres some log files, let me know what you think. once again thanks for everything.




    EDIT: back again, looking at the way its bogging and popping occasionally at low rpm cruise/acceleration. wondering about the timing being too high in those situations so i was just playing with the timing curve, like i said im used to turning a distributor so its very linear, let me know if im completely out of line. thanks again

    EDIT2: check the second linear timing edit, more what im thinking. also adjusted corrcl. will let you know how it works. please feel free to give me advice im doing lots of searching ha
    Last edited by zoomo; 10-11-2014 at 07:17 PM.

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