Thread: PE Not locking in at Zero?

Wow. First time I've ever seen this on my car. This is the first time I've really taken a good scan of some WOT runs since my Plog, plugs, wires, new O2 sensor, and a few other minor changes. My LTFT is locking in at -6.25 in PE. . And it's putting my initial O2s in the low 800s.

Comments, suggestions, help?

You'll need to go towards the last 1/3 of the scan to see the WOT runs. The rest of the scan... seems my PTKR is nearly gone.

08092008_185007 from sabrewings -- Scan Depot dot net

Edit: On further thought, I wonder if my front cylinders are leaning out from the Plog. Would make sense... is it standard practice to skew the front cylinders some to make up for this?

The only reason I mentioned skewing is that it was mentioned on the Plog page. None of mine are skewed right now, but it made sense that the front cylinders are being exhausted so much better they might want more fuel.

Okay, we can forget about the skewing for now. What about the LTFT and my O2s? It says -6.25, which if I understand correctly means the PCM is pulling fuel in PE for some reason? But my O2s are really low. Until recently I was seeing much higher O2s.

I think I need to add some to the MAF up top, but I haven't even touched the MAF yet and don't really know where to go with it.

Can Eddie's table modifier thingy make the proper MAF changes? Or do I?

Higher 800s, from what I'm told. Like 880-890.

Originally Posted by TDCRacing

A negative number would mean the pcm is adding that amount of fuel to that particular cell. A positive number is the opposite. Kind of backwards if you ask me lol.

But I thought negative fuel trims meant it was pulling fuel. Okay, now I'm totally confused.

So, what would be my first step to get the LTFT back to zero? VE tune? MAF tune? Adjust the base PE? Kinda at a loss.

Right off the bat I noticed that your LTFT during normal driving is negative 9-10 most of the time. This MUST be zero or as close to zero as possible at idle and light throttle. The pcm references back to your light and part throttle ltft for PE mode and that is why you are seeing -6 in pe mode. Go back over the tune and get your fuel trims as close to zero as you can and that will fix your problem. Make sure you reset the fuel trims every time after you tune so you start at zero and the pcm will learn and display the correct difference the trims are off. A Negative fuel trim number means that it is pulling fuel out because it is running too rich, and a Positive number means the pcm is adding fuel because it is too lean. Fuel trims will only compensate with NOT in pe mode as once in pe mode the car runs off of maf input and fixed tables, not the 02 sensor. Durning normal driving the pcm wants 14.7 afr at all times and will constantly make fuel trim adjustments to see that. But what happens when the car runs too rich at idle or part throttle is the pcm needs to pull fuel out to achieve the 14.7 afr it is looking for, roughly 450mv on the stock 02 sensor, and wants to see the voltage go up and down from this number so it knows it is in the correct spot, this is referred to as cross-counts. If the pcm has to pull fuel out which gives you a negative fuel trim then this is carried over into your pe mode and shows up watching a scan and will pull fuel out when in pe mode also, so again it is very important that all of your ltft numbers are as close to zero as possible. LTFT reading negative or positive also throws off the DIC fuel economy as it is a calculated number. You are probably showing a few more mpg than the car actually gets because you have a negative fuel trim.

So, VE tune it is. I already tuned it once by VE but that was before I started having issues and I've been using a mostly stock file since.

Reptile, my PE is still stock.

And, big thanks to Trannyman. Very well composed response and very helpful. I suspected as much that it used the normal LTFTs to devise a PE LTFT, but I wasn't sure and no one ever actually spelled it out as such.

Would it be better to tune at night when its cooler? Or wait until the day?

Day time tuning is probably going to get you closer but once you get it dialed in it really shouldnt have much of any variations. I do a lot of late night tuning and dont have any trouble but sometimes for WOT fuel and timing adjustments you have to stay a bit extra conservative as the air temps have dropped off quite a bit compared to the extra heat during the day. One thing that will also help you out and can drive you nutz.... Unplug the evap solenoid connector under the hood. It will throw a code but its not a big deal and you can temporarily turn the code off while tuning. What happens when its warm out and you do not unplug the evap solenoid is that you THINK you finally have your trims where you want them and then pull up to a red light and then it hits, BOOM -15 fuel trims!! You get agrevated and it doesnt make sense how a good tune all of the sudden shows really screw fuel trims. When it is warmer outside, especially hot days sitting in traffic, the vapor levels in the tank build up much more so than when it is cooler outside and the pcm monitors this gas tank pressure/vacuum with a sensor and uses this data to cycle the evap purge solenoid. This is one reason sometimes you may notice in traffic the car wants to hiccup a bit coming to a stop or starts to idle slightly rough out of the blue. The fuel trims will go negative because once the evap solenoid opens up there is extra fuel entering the system and making the car run too rich, so the fuel trims drop to temporarily compensate until the solenoid is off again. I hope this helps you out, good luck with the tune!

Yep, when in question you can pop the hood and you will here a clicking sound. That is the evap solenoid pulsing to vent the gas tank vapors into the engine. Unplug it and the car will smooth out and things will go back to normal. It would be nice if there was a table for turn on time for this so it would not do this sitting at idle. With a stock cam its hardly noticable but with a larger cam it seems to show up more.

That's awesome! I've heard a ton of theories, but no one really had anything concrete on why the trims randomly dive bomb. But, that makes sense. I've seen them suddenly drop (on my Aeroforce) while pulling in a parking lot and it bugged the hell out of me that maybe an injector was sticking.

I also heard (and felt when I put my finger on it) the evap clicking away today. I always assumed it was the injectors. Thanks for the advice! I'll hopefully get to go tune it this weekend after I swap in my ZZP throttle body and I'll update you guys then. Good responses so far.