are negative numbers rich or lean on the ltrims?


i put my bladed maf back on and sitting here idling, i get -5 to -8 on the ltrims where as my buddy with his 98 ported maf with the same cam gets +20 just sitting idleing.

 

Neg. ltrims mean you are rich. Your idle ltrims are fine.

 

Ditto,

Your Lterms are fine. In fact, it is better to have your LTerms slightly negative than it is to be positive. I wouldn't mess with anything.

Just make sure that your LTerms are around the same when you are driving around.

 

i am going to try and drive it around some this week and see what its doing under a load.


i guess thats why my car smells like gas when its idleing and my buddys doenst.


thanks guys

 

ok someone correct me if im wrong here but its quite a few posts in here about this..

LTRIMS dont really mean you are rich or lean, they are just what tells the PCM to add fuel at WOT or not. Correct?

Yes if his LTRIMS are +20 at idle, then its lean there, but that doesnt mean a rats a** at WOT. All that matters is what they are right BEFORE it goes WOT. Correct? So if they are +20 right there then the PCM is gonna dump fuel because it thinks its lean. Correct? Now if they are zero then the PCM isnt gonna add any fuel, which could, note could be bad because you may be leaning out on top end and the PCM isnt dumping fuel.

I just know on atap, yes i know ATAP isnt 100friggin precent correct. But when my LTRIMS are +6 right before WOT then my 02's read .92-.94(rich). Now if they are zero, my 02's read .86-.87(leaner). Anyone have a answer for this?

maybe im wrong, so if i am could someone let me know <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

i'd just rather have a little positive LTRIMS so the PCM dumps fuel instead of not dumping some fuel to richen the car up a bit <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

 

You do understand that with your -5% to -8% LTFTs you are running at about 14.7:1 AFR, where as your buddy needs LTFTs at +20% to bring him back to 14.7:1 AFR.

Initially you WERE running a bit rich as the others said, but the PCM did some learning and now is taking out a little fuel to keep the engine happy at 14.7 AFR.

Now it is time to check what the LTFT value is at WOT. I expect that you will see 0% for WOT LTFT, thats what you want.

 

98TAauto,

LTFTs are an indication of running rich or lean at part throotle. A car having all -8% LTFT values and a car having all +20% LTFT values are both NOW (provided the LTFTs have stabilized) at about 14.7:1 AFR,,, In the case of the -8% car, it WAS running rich due to some change to the engine and now the PCM has learned this and adjusted the LTFTs to -8% so that the car is now at 14.7 AFR. In the case of the +20% car, it WAS running lean (this could be due to installing nylon ported ends onto the factory MAF) but the PCM has learned about a lean condition (via the O2 sensors) and is now adding more fuel so that the car is running now at 14.7 AFR at part throotle.

You are right about that the only really important LTFT value is the one taken over to WOT operation. There are many fuel cells, each with their own LTFT value. I believe that the LTFT value taken over to WOT is the value from the last fuel cell while the car was at part throotle.
This could be fuel cell 15 but I am not sure.

I know that when my car is at 100% TPS, I am always in fuel cell 22. I have been at high RPM and at 90% TPS, but I was in fuel cell 15. So I think that the LTFT value at PE enrichment is the one from fuel cell 15, but its possible that it comes from fuel cell 14 or 13. It depends on how you got to WOT mode.

About the LTFT value at WOT or PE mode: If the LTFT value is positive (this is from the last fuel cell before WOT), then the PCM will carry this over into PE and add that amount of extra fuel up and above what the PE vs RPM table calls for. IF the LTFT value is negative, the PCM sets the negative LTFT value to 0% at WOT. Therefore, no fuel is taken out at WOT operation. This is done to protect the engine from running too lean at WOT.

What I said above fits perfectly with what you described in your paragraph about ATAP. With +6% LTFT at WOT, the PCM is adding in more fuel above the other PE tables resulting in a richer (but not necessary rich condition) at WOT,, so you see 920 to 940mV from the O2 sensors. When the LTFT value at WOT is negative, the PCM will set it to 0% and do nothing concerning PE mode outside of the normal PE tables,, so you see a leaner condition at WOT with O2s at 860 to 870mV.

Concerning O2 sensors,,, for my car, when I read 910 to 920mV from the O2 sensors, the wide band O2 says that I'm at 13.8:1 AFR (and I am on the verge of getting KR). When my O2s read 960 to 970mV, the wideband says that I'm at 13.1:1 AFR. So our factory O2 sensors can be very misleading.

I hope this helps.

 

ok if his #'s good what is happening when you have #'s like this -3.XX and -4.XX what do you do then make adjustment's with the maft. or leave it alone?

 

Well, if all of the LTFT values are around -3.xx% and -4.xx%,, AND the LTFT value at WOT is 0.00%,, then I would do nothing,, he is nearly spot on perfect for part throotle tuning. However, this is just the first part of a complete tune.

Next, its time to get a wide band check of the AFR at WOT. Work with the PE vs RPM table to achieve whatever AFR you want at WOT, like maybe 12.8:1...

Next, its time for ignition timing,, on and on,,,

 

well, i think i have mine figured out. i went in a adjusted the injector flow rate to compensate for my 2 fuel pumps and forcing 64 psi of fuel pressure thru the stock injectors and now my car seems to like that alot better.


it still smells like gas while its sitting there idling, but that may just be something i have to deal with.

 

My understanding is that you want to be zero or negative on the LTFT's to avoid dumping fuel in at WOT, which adds yet ANOTHER factor when it comes to trying to tune.

If you are neg or zero and your O2's are low enough that you think the car is too lean (assist this info with a dyno), then you adjust the PE/RPM table to compensate. You don't need the added variable of positive LTFT's making it so diffucult to tune.

Now, theoretically, if it was ALWAYS the same +LTFT EVERY time you go WOT, you could still get a consistent tune. I think TeamZR1 has shared this experience with us. But, it has been my limited experience so far that if the LTFT's are quite high in the positive direction, there is often quite a bit of variance in where the LTFT's will be when you go WOT. Also, monitoring and attempting to get LTFT's zero'd seems to me to be a good practice, because you can easily see if various problems that may occur have popped up... like vacuum leaks, etc.

If I could, with other than stock injectors, I'd adjust the Inj Flow Rate table to zero/negate LTFT before I tune on the dyno and use Autotap at the same time. This way, you get a feel for how close mV values for your O2's equate to the A/F ratio reported by the dyno. Then you can have a better sense of tuning when you get back on the street. This is where I also share TeamZR1's conviction that real-world tuning is VERY improtant. Also, in my experience, the car will lean out on the street, compared to the dyno.

My2Cents. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />