Simple question, or so I hope.

Assume one is looking at a data log of a car and the long term fuel trims are in the 108 range under various conditions. The datamaster help file says that this means the AFR is rich. I assume this means that the AFR is rich and as such it is giving the car less fuel than it would have expected based off of the other computer values?

What would I need to do to the VE tables for said conditions to bring it back into line? Basically if it's too rich does that mean that the VE value is too high, or too low?

As an interesting side note, this is on FAIRLY stock car. It has 1.6 rockers and long tubes. I've tuned it for WOT but I haven't touched part throttle because the driveability has been pretty good. I would have thought those mods would have made it too lean and it would have to add fuel? The only disclaimer I can give here though is that the car is not running heated O2's and the feedback it gets from them seems to be complete crap. I'll data log it again and see what it does after we get the new sensors installed. I just need to know what to do with it once I do finally get the O2's resolved, provided that doesn't correct the problem.

 

Nathan, have you tried VE Master. It worked really well on my 94.


http://home.comcast.net/~jgeorger/

 

Yeah I've tried it and it screwed up my tune so badly that I eventually went back to Ed Wright's tables and went from there. Sounds good in theory, but it didn't work so well for me. Thanks though!

 

Yeah Nathan, I think VE Master works alot better on the 94/95's. I've tuned 4 94/95's using VEM, with great results.

Sorry, I don't know too much about manually changing the VE tables. Maybe Tom Wong will chime in, he's my tuning guru.


-Mike

 

Well just for fun I ran VEMaster on one of my BIN files and it doesn't look like it F'd it up too terribly bad, I may give it a shot. I'm thinking that the last time I tried it I might have been fighting non heated o2 sensors as well.

 

When the engine is running rich the integrator will decrease below 128 to compensate for this condition. If the condition remains the BLM will drop below 128 to do a long term compensation. When this is done the INT will return to 128 and this indicates that the ECM has taken care of the rich condition.

Suppose the average BLM is 108 for all MAP and RPM. You want to reduce the injector duty cycle to decrease the amount of injected fuel, this is done by tweaking the VE tables (there are other methods).

108/128 = 0.84

Thus you need to multiply every cell in the VE tables by this factor. I'm using TunerPro and this programs allows me to very easily multiply entire table by some factor.

One more thing. My chip has the BLM min value set to 108. If BLM is this far off on a "stock" engine I would think that something is wrong. Perhaps a faulty injector, too high fuel pressure or wrong injector size in your chip.

Hope this helps!

 

Thanks for the info. I don't really think it's all that far off. The O2 sensors on this car are pretty unreliable. I think it'll be much better once he converts to heated O2's. Once that's done we'll look again and go from there.

 

It was my understanding that the heated 02's allowed closed loop sooner, other than that they operate the same. I would look at the injector constant, that is like making a global change to the overall fuel trim, then fine tune it with the tables.

 

Sorta. That's one benefit of heated O2's, but not the only one. You see with a non heated setup in a car with a very free flowing exhaust system the O2's can actually cool off and start sending crap data back to the computer. If you're lucky the computer recognizes this poor data and drops back to open loop. If it doesn't then the car runs really crappy.

The heated O2's might not make a huge difference, but they are not acting like they are supposed to, that's very obvious when you look at the data logs.

As for the injector constant, that's a quick way to tune, but not really the best way. It's like installing an adjustable fuel pressure regulator. They are global changes intended to fool the computer. The car still has stock injectors, so the computer is going to remain programmed to know that. When you start playing with the injector constant you mess up your existing WOT tune and you screw up all of the MPG readouts. Making small changes to the VE tables only where they are needed takes a lot more time but ends up being a much better tune. You want to program the car for what it has, not fool it and let it try to adjust the best it can.

 

Nathan,

I have also created a program similar to VE Master that I use on my vette. i have gotten the tune very close for cruising..

If you want to send me an Excel copy of your scan, the BIN that you scanned and I can run it through my program. I have not built an application for my program yet, but will run it with your info if interested.

vettxtc@comcast.net

 

Well it's not my car, and it seems to run OK. I'd be hard pressed to convince the owner to let me screw with it right now. I will keep you mind though if the problems persist after the heated O2's are installed.

 

Okay,

if you want to try it out just let me know

 

Hmm, never heard of O2 sensors cooling off as long as they are positioned correctly i.e. not too far down the exhaust path.

How did you tune for WOT?

As for the MPG readouts, mine showed over 30 MPG after I installed larger injectors but before I changed the injector constant. It was my understanding that this was because the ECM was reducing the injector pulse width because I was running rich all the time. After I got my part throttle BLM's back to ~128 my MPG readout was back to normal. All this was done by only changing the constant. I don't think I fooled the computer, I told it what I needed to to get my tune, which was in fact a higher constant value than my injectors were rated since they are SVO injectors which are rated at a lower FP than we run.

 

You're exactly right about the O2's, but that's exactly the problem. With a set of long tube headers you really have no choice but to position the sensors a little further downstream than optimal, thus they cool off.

With the injectors in that case, you're right you're not fooling anything. You suggested though that I play with the injector constant on a car with stock injectors, which would be fooling the computer. Of course, if you change injectors, change the constant accordingly. If you have stock injectors leave it stock.

As for tuning for WOT I tweaked the timing tables and the PE % change to A/F vs RPM and PE% change to A/F vs coolant temp tables until the A/F ratio was right on target on the wide band.

 

Well we installed the heated O2's and the car is much happier. I tweaked the computer a little bit and it manged to go closed loop after 96 seconds at idle. Previously it would never go closed loop at idle, it took several minutes of sustained highway driving to ever get it to go closed, and even then the sensors looked lazy. Now they look fantastic.

 

Nathan
Thanks for the update. Reading your post is begining to make me rethink heated O2's would be a wise move for my application. Question, with the exception of the open/close loop senario, what other symptoms were identified? Rough idle?
thanks
Hank

 

When my car was running standard O2 sensors I encountered what I could only describe as a bastard problem in cold weather that almost caused me to pull my hair out. Here's the scenario:

I would get in my car in the morning, heated garage. I would start it up and drive in to work NO problems whatsoever. Later in the day when I had to go to lunch or to go home it was a different story. The car would start immediately and drive fine for about a minute. After that it would start running VERY poorly. It would idle OK, but if you tried to give it any gas at all it would stumble and buck. If you gave it enough throttle it would blow past all the bucking and run great. Before I would get home it would be running perfectly again.

I eventually figured out that when I started the car up in the morning it was warm to start with AND my drive in to work was over a mile before I ever had to slow down. This constant speed and warmer initial temperature allowed the O2 to warm up and the car ran pretty good. When I would leave from work the car would have been sitting outside and the temp was very cold. I was also immediately placed into stop and go traffic. Basically the O2's never got hot enough to work properly, yet the car would try to go closed loop anyway. It would see that the O2 thought it was running lean and try to richen it up. The rich mixture would further cool off the O2 and it would still look lean so it would try to richen it up more, etc, etc, until it barely ran at all.

As soon as I put my heated O2's in the car ran like a dream. I guessed too small on the fuse size for the sensors and it blew, the car was up to it's old tricks. I checked the fuse and sure enough it was blown. I put a new fuse in it and the car ran perfectly again and has ever since.

 

If I'm not mistaken, not only are the O2 sensors heated, but the heater element is regulated to keep the O2 sensor's element temperature fairly constant. This gives good and accurate output under various engine operating conditions.

I base this on some Honda engines that use wide band O2 sensors. I was looking at adapting a Honda sensor to our race engine for data logging. These are heat regulated to give very accurate O2 measurements. Maybe our sensors are not as "advanced"? I don't know for sure.

 

Some Honda cars uses a cheap version of the NTK wide band sensor. The newer Bosch LSU4 sensors are quite accurate as well and are considerably less money and easier to find than the Honda sensors. There are already complete wide band monitoring and datalogging systems available. In these days its so much easier to use someone elses mouse trap

Here is one you can check out:

www.innovatemotorsports.com

Their system uses the bosch LSU4 sensors and works very well. There are others out there as well. This one has a lot of features and is expandable.