Nexus9

For a lot of you "old dogs" on the forum, this will seem kind of basic, but my experience is that there are a lot more new folks trying to figure it all out; so if you are one of THEM (like I was), this is for you. The information in this post refers to using an HP Tuners MPVI on an EFI motor.

"Tuning" a car requires making changes to the computer software that controls the motor, and before we make changes, we need to know what we want (or expect), and compare it to what we actually have. There are many basic changes we can make like setting fan speeds and eliminating skip shifts; but most of the performance tuning revolves around Air Fuel Ratio (first) and Timing (after our AFR is accurate).

The challenge in finding out what the car is actually doing is that the computer keeps trying to "fix" any errors and bring things back to center, and even "learns" certain corrections and keeps them in place semi-permanently - so if the computer keeps adjusting the ECM's output, gathering accurate data is like hitting a moving target. We can fix that.

Let's work with AFR. Your car has Oxygen sensors mounted in the exaust that are constanly monitoring the results of combustion and translating that into AFR. The ECM receives that data in a constant stream. If the data from the O2 sensor doesn't match what the ECM is trying to produce, the ECM makes a correction, in the form of "Fuel Trim". This is called "Closed Loop Operation" - there is a loop between the ECM and the O2 sensor, constantly reading and correcting, reading and correcting.

"Short Term Fuel Trim" is an ongoing, very fast process. The ECM takes instant data from the O2 sensor and makes instant adjustments, adding fuel or subtracting it. Even on a well tuned car, you can watch the STFT going up and down as much as 5% constantly, making fine corrections. But if the STFT keeps going the same direction over a period of time, the ECM "learns" that the motor just needs more or less fuel consistently - and it feeds in (or takes away) a percentage of fuel on a steady basis, which then allows the STFT to operate over a smaller (and quicker) area. This is "Long Term Fuel Trim", and it will stay in place until the ECM learns something new and moves it again. LTFT can modify fuel input as much as 13% in some motors, and hold it there.

Wait a minute... if my ECM will do all this for me, why do I need to spend $600 on a tune...?

Well, two reasons. One is that the ECM is always, well, whatever the opposite of PROactive is. Counteractive...? In other words, it responds late. It's much better to have the AFR adjusted so that the mix is correct right up front - the motor will run better. The other (probably more important) thing is that there are times when your motor comes OUT of closed loop (like full throttle) and is running on a pre-determined mixture for max power. At this point, whatever LTFT is in the system "freezes" at that reading and adds to the open loop predetermined mix. If your LTFT is adding 10% fuel and you go 12.5 at WOT, you're going to be blowing a lot of black smoke and missing a lot of power. Best to keep the LTFT to within 5% up or down max.

Okay, so what does all of this mean to us when we're scanning? Well clearly, we need to disable all the ECM controls that are going to adjust our AFR, so we can see what the "actual" AFR is without any ECM support. To do that, we need to create a tune that we can install in the car that eliminates all those controls. Here's what I do:

Open VCM Editor, and open the most recent tune that you want to work with.

Select Engine/Fuel/Open and Closed Loop.

Click on "ECT vs Startup ECT" and set to the maximum setting, 306*.

Click on "O2 Readiness" - set to 493*.

Click on "Gas (Gear)" - set to 1.

Click on "IVT Gain" - set to 1.

Click on "AIR Gain" - set to 1.

Click on "AIR Airflow Gain" - set to 1.

That will pretty much keep the loops open. Now we need to be sure we don't get any enrichment from anywhere else.

Click on the "Power Enrich" tab. Under "PE Enable" (not Piston Protection) set the Enable RPM to 10,000.

You may want to click on "COT, Lean Cruise" and disable the CAT overtemp protection (which adds extra fuel), but if your CATs got that hot, it's probably better to ruin a scan than ruin your CATs. Your call.

Now go back up to the main tabs and click "Airflow". Click on "the "Dynamic" tab. Set "High RPM Disable" at 400 (below idle) and "Re-enable" at 350.

Save your tune with the word SCANNING (or something similar) in the file name. This file is for short-term data collecting, you want to be sure this program doesn't inadvertently get installed in a car you're driving around on a regular basis.

When you are ready to scan, be sure you save the existing program in your car, then install your SCANNING program that you just created. Now open up the VCM Scanning software, and connect up to your car. We're going to do one last thing to be sure we don't get skewed data. With the car on and hooked up, click on the "VCM Controls" in the toolbar (next to the white motor icon). A table will open up. Click on the "Fuel & Spark" tab. On the top left you will see "Closed Loop" - select "Off". Under that, "Fuel Trim Learn" - OFF as well. Then click on "Reset Fuel Trims" one time.

That's it! (Jeez, that's ENOUGH, right?) Now go do your scan, and take a look at your Config Table - you should see all ZEROs next to your STFT and LTFT PIDs, which tells you that you got it right.

DON'T FORGET TO RESET ALL YOUR SETTINGS AFTER YOU SCAN YOUR CAR! As a matter of fact, print this post for easy reference so you don't forget anything.

Have fun tuning!

Jon

MJEngineering

Very good writing, should help some new people for sure.

Mmicheletty

This is good, simple, easy to read and follow instructions that simples what some of us had to learn by reading page after page of instructional lingo. Good post.

Nexus9

Thanks you guys. I remember trying to learn on the forums and getting that same feeling - always getting fragments and trying to piece it all together. I hope this makes sense to some of the guys trying to learn, hopefully it will make things a little easier for a few folks before fading off into oblivion behind a hundred other posts.

Jon

BanditDave

iTrader: (11)

This is a great post...thanks a million. I am definitely a newb when it comes to Fuel Injection Tuning. That being said, when using this method, do you need a wide band installed or can you rely on the narrowbands to get this measurement?

Sorry if at any point in this post I said something stupid....hahaha.

Thanks,
Dave

Transamtx

I agree great job as always LS1 has awesome info

stangtrader

iTrader: (2)

Awesome write up. One thing I have learned in my misadventures in tuning a quick google search can usually find lots of info on specific tunning issues or methods,how ever you still need to use common sense cause there is either old methods that are outdated or incorrect.learning the software is definitely a challange but the reward is well worth it, and the vcm scanner is your friend and goes a long way

bunker

iTrader: (1)

Great write up, only thing missing is what to do next.

I assume you are saying to plot STFT and use those to make corrections to the MAF table since in this write up the MAF is still functioning.

Plot STFT against MAF %error and make corrections through % multiply?

Or are you implying to use a wideband and set it up for lambda error.

Sorry just at work trying to word it quickly, but awesome write up very simple for new ppl to undesrstand Just missing last part about what you'll be measuring to make corrections and thats it