Help me understand what happens in PE

[BlackGS]

Just as the title says - can anyone explain the sequence of events and feedback that occurs once we enter PE mode?

I have been testing by adjusting up and down commanded AFR in PE. I have allowed plenty of time to learn. Whatever I set commanded to does nothing to change actual AFR. So what's the deal? Is it only MAF that is commanding AFR?

Also, does the narrowband signal come into play at all in PE? I would guess that it is being ignored, but I want to make sure.

Thanks for any info.

[Eddie-98GTP]

The O2 sensor is pretty much ignored in PE. The PCM uses ECT, IAT, TPS, MAP, RPM, and MAF to calculate fuel in PE.

I too have noticed very little change in adjusting my commanded AFR. The best way (in my opinion) to adjust fuel in PE is to use the MAF table. You would adjust cells from about 7500 up. Normally, you don't see MAF values this high unless you're in PE.

Just be sure your long term fuel trim locks at 0. The PCM uses this to correct fuel, and in PE this value is not updated. For example, if you enter PE with a LTFT of -5, the PCM will pull 5% the entire time you are in PE.

[ChrisV 02 - Z28]

I have always heard that if your trims are negative before WOT then during PE fuel will not be removed from the calcs, only added too if your trims are positve.

As my trims are all negative at part throttle and my WOT trim are zeroed out...

[Perforator]

Do you have any scans we can look at?

[Perforator]

I have always heard that if your trims are negative before WOT then during PE fuel will not be removed from the calcs, only added too if your trims are positve.

As my trims are all negative at part throttle and my WOT trim are zeroed out...

Not on the V6 3800. The trims are carried into PE.

[BlackGS]

Thanks for the info so far. The problem that I have is, what good is the PE Add vs. RPM table? If commanded is ignored and MAF takes control, there is no reason to adjust commanded over time. There must be something we are missing here. I find it hard to believe that table was put there to be ignored. Any thoughts?

[Russ K]

I always zero the Add vs. Rpm table, then set the Base AFR to 11.5, and rescale the Maf so the actual AFR matches the commanded AFR.

Russ Kemp

[Perforator]

When I had my rpm time table setup to richen over time, the a/f ratio would change with the commanded.

[JFDugal]

Commanded AFR is not ignored. Do you have a wideband O2 sensor? If so, tune the upper region of your MAF table so that commanded AFR ~ wideband AFR. Then you can adjust the base PE and PE adder tables for optimal fueling.

[BlackGS]

Commanded AFR is not ignored. Do you have a wideband O2 sensor? If so, tune the upper region of your MAF table so that commanded AFR ~ wideband AFR. Then you can adjust the base PE and PE adder tables for optimal fueling.

Yes, I have a wideband. I've tried to do exactly what you are recommending - set MAF to commanded, and then adjust commanded in PE base and PE Add. The problem is that the AFR does not change to meet the new commanded setting. I tried a series of tests where I kept lowering commanded in PE. I stepped it down and ran WOT repeatedly. Each time the new commanded was recognized, but the actual never changed. At one point, I had commanded as low as 10.5, but actual would come in 11.5-11.8. These tests are what prompted me to start this thread. I was hoping to understand if commanded worked in conjunction with MAF, or commanded was just secondary should MAF fail or something.

[Russ K]

Read this thread for more info.

http://www.hptuners.com/forum/showth...oop+maf+tuning

Russ Kemp

[Perforator]

The actual is never going to match the commanded 100% unless you are really good, lol. Was the actual lower than before you lowered the commanded? Do you have any scans we could look at?

[BlackGS]

The actual is never going to match the commanded 100% unless you are really good, lol. Was the actual lower than before you lowered the commanded? Do you have any scans we could look at?

I'll try to post up some scans when I can. I wouldn't expect a 100% match, but I would hope it would be closer than it is, and would at least trend along with it.

The actual has only once or twice ever been lower than commanded. One of those times was just recently when I had +7.0 trims in PE. The actual went down to 10.5 and commanded was set at 10.8 (I was testing). I started out with commanded at something like 11.8, and my actual was right around there. I recently moved commanded back to 12.4 base and adjusted it down to about 11.5 with Add vs. RPM. I have to get a couple more scans but the two I ran didn't show any change in actual.

[JFDugal]

The actual has only once or twice ever been lower than commanded. One of those times was just recently when I had +7.0 trims in PE.

That is to be expected with the +7 LTFTs in PE (I've seen that happen im my logs too). I would advise against adjusting the base PE higher as a fix to this though. The next time you reset your fuel trims and go WOT, you may find you're running leaner that you want.

Stepping back a bit, how are your fuel trims for the rest of the dirving cycle? Pretty close to zero?

Definitely post up a .hpl file so we can take a look.

[BlackGS]

That is to be expected with the +7 LTFTs in PE (I've seen that happen im my logs too). I would advise against adjusting the base PE higher as a fix to this though. The next time you reset your fuel trims and go WOT, you may find you're running leaner that you want.

Stepping back a bit, how are your fuel trims for the rest of the dirving cycle? Pretty close to zero?

Definitely post up a .hpl file so we can take a look.

Trims have been right on zero or very slightly negative in cruise and WOT for a long time. I just recently had a strange jump in WOT trims which I described in the other thread. I have a scan in that thread to look at also.

[BlackGS]

Here are some scans to check out.

[Perforator]

That's strange. Your IPW's are higher in 1 & 2 than 3, but your wideband shows a lower air fuel ratio in 3. You know something you may want to try, until you figure this out, is to zero your rpm time table so you run a flat a/f ratio in pe. Can you post the bins that go with these scans?

[BlackGS]

This bin was used for the 3rd scan IIRC. PE Add vs. RPM was tweaked for the other two, and I also probably played with timing a little bit, but otherwise I think they should be the same.

MAF settings did not change for these three scans.

[Perforator]

hmmm, no maf changes. Might be time to go back to stock and start over... I've done that two or three times

[BlackGS]

hmmm, no maf changes. Might be time to go back to stock and start over... I've done that two or three times

I think the tune itself is pretty good, but I would just like to understand why it is not consistent in PE with the commanded value. If I could understand what is driving the amount of fuel each time, I would feel better about knowing that I am running in a good range and just keep tweaking for the weather. I'm going to get to a dyno soon to check power with a few different tunes, and it would be also nice to get a consistent AFR on the pulls.