Fuel Trim 108??

Ok, After days of driving 40 miles one way to work trying to locate my engine stumble.. I have came to this question.. when cruising, (On cruise control) the car runs at 108 fuel trim, when part throttle is made to make up or a few miles an hour loss.. I get an engine stunble, and they jump to 160 fuel trim.. I know this is my problem, the mas is reading 37 gpm.. When you step on the gas hard.. They (being f-t) Jump to 128 or so.. But what I dont understand is this miss.. its driving me nuts..At any part of throttle if you barely mash the gash the car hesitates and when it comes back to life it jumps to 160 fuel trim an bang here comes the horsies.. I assume the car is running lean at cruise, being it is at 108.. But how do you correct for such when under wot, the car runs rich? Why is the iac at 50-60 when i get the miss.. The iac is working in the 50's while cruising..Shouldnt this be at zero? Even at wot its doing something..I have lt1 edit can anyone help? Also what is a safe knock count.. I have logs and can send them via email..please help me get his corrected.. Let me know if I need to post more info to help.. Thanks..
I Have a d-1 at 8 psi, dual intercooled, a 540 lif cam, 8.8 compression,shorty headers, 30# injectors,msd ignition... 3 in.. exhaust..

Post some data for us to look at.

Ben

I can foward the e-mail.. Will have to see what I can do for a post./. Thanks for your help.. Whats kso confuseing to me is the blms are at 108 when at cruise, but as soon as you move the throttle it goes to 128-135..
I have noticed that when the computer is in learn mode the blms are abouve or right at 128, when learn is disabled, it is 108.. Coincdence.. ?
Michael

Send me a log on excel and I will see what I can figure out...not that I have all the knowledge.

I will post what I find so others can also give input.

Ben

You've got it the wrong way I'm afraid...

Your rich at cruise (108) and your terribly lean at WOT (160)

The 108 is bottom of the line as far as the computer being able to take out fuel... And 160 is top of the hill in regards to dumping in more fuel cause your lean..

this was a question I was wondering however...

If you are running really rich, as the 108 indicates, it means the computer is pulling fuel... So if it pulls x amount trying to get the blm to 128, that should make your 108 change upwards correct?

As in, of course the computer will see the changes in BLM's as its making adjustments?

so its just the BLM really reads (example) 60! and the computer pulls as much as it can, down to 108 (20 points) so that only brings it up to 80, which is still no where near the ideal 128, so it still shows the lowest value present...

Vs. if it was say 105, the computer pulls fuel to bring it to 128, bringing your BLM's to 108 to pull the fuel, it sees that it worked, bringing the blm's back up...

OR does it just stay constant and the adjustments are seen in the Short term int.'s? So if your Short terms are low, meaning rich, it'll lower your blm's to bring your short terms up?

Or if your lean, high short term int's. It'll raise your blm's to add more fuel, lowering your short term Int's?

sorry if these doesn't make sense ;)

but if someone could explain the releation of lowering blm's or raiseing and the effect it has in the numbers, it would be great :)

Thanks guys!

Jeff

well uhhh.. I thought 108 was lean?

Ok well I have figured a lil more into things.. Car runs perfect even at idle.. As long as its in open loop.. As soon as it goes to closed loop the idle goes to sheat, and the fuel trims go to 108.. Most of the time car shuts off? Any ideas?

Easy way to look at it:


128 is perfect.

If you are below 128, the car is running rich and the computer is leaning it out. At 108 it cannot lean it out anymore (unless you program it to go below 108 to lean the engine out) and will be richer than it should be to achieve what the O2's want (450mV).

Think opposite for above 128.

But again, this is valid only in closed loop/non-PE mode.

Now is that simple or what :rolleyes:

Ben

Well thanks.. But that means Iam totally backwards.. How do I adjust for this in lt1 edit?

you need to get in contact with bryan herter at www.pcmforless.com he knows a thing or two about this sorta problem.

i dont have any experience tuning a S/C car but i can imagine it would be a nightmare at best.

i think the reason your fuel trims jump to 160 when you hesitate is because of a lack of fuel. one time i almost ran outta gas about a block from the station. i was stopped at a light and when it went green, i gave her gas as usual. well the car hesitated so bad it almost died! the car would move just fine with part throttle but when i floored it, the it would hesitate and try to stall on me. i instantly started flipping thru my scanmaster looking for anything out of the ordinary and what i found was that all my BLMs were pegged on 160. well after practially coasting into the station, i filled her up with 15.1 gallons of gas in a 15.5 gallon tank! and after a got about a mile away, my BLMs had all returned to normal and i never got the hesitation again. i am guessing you sorta have the same problem. you are simply starving it of fuel because of the S/C causing the PCM to dump in more fuel.

I had the same problem on my ati supercharged camaro. The discharge tube was to close to the maf sensor and I was getting erratic reading because of that. When you are at part throttle, the air flow coming back by your discharge tube is so great it came back thruw your MAF. I just changer my MAF after the intercooler and Bingo, perfect 128 BLM. Hope this help !

Well thanks, I have heard of that warning.. But when you say after the intercooler, you mean on the blow through side correct?
My problm is, If I dont do it right at the intake, i would only be seeing half the air (Car is dual intercooled) So each intercooler collects together right at the throttle body.. Also, while running a data logger, I have noticed at idle the maf is only reading 22-28 gp/s.. At wot in pe, most I have seen it is 68-80 gp/s.. this dosent seem right to me, does it to you guys?

Someone please correct me if I'm wrong but I just started studying this PCM stuff:

The computer always wants the INT's (short-term memory) to be at 128. It will adjust the BLM's so that the INT's will be 128, so the computer can get the O2 voltage at 450 mv (14.7 a/f ratio.) As long as the BLM's are within their range (108 to 160 as I believe was stated before, I don't know the actual numbers,) and the a/f ratio is correct, it doens't matter what the BLM's are. They don't have to be at 128, as long as the engine is getting the correct amount of air and fuel and the BLM's havent exceeded their limits, which in your case, it has. The car is running too rich and the computer can't cut back enough fuel. Maybe your injectors are too big?

Well your close on you figures.. But they all matter.. Short and long tem..
I either have a bad maf, or something tuned wrong, I could actually go with alot bigger injectors and throw the fmu away.. But I am trying to get the engine running correctly across the board, I am running rich at idle and lean at wot (pe).. Biggest problem is I have no ses light.. And when the car enables learn mode (While cruising) the numbers are perfect.. Just my luck., hehe