RobsWS6

I have a problem that has been bugging me for years. I just can't get rid of this off throttle or very low throttle bucking. From 1600 on down, it drives me crazy. Feels like the driveshaft is going to come out.

I've tried timing, up or down, rich and lean, changed DFCO settings some, still there. Could it be too much airflow? If so, how to change it?

I have a 95 computer so its OBDI. Major mods are HOT cam, SLP dual cat shorties, and cat back.

Any help would be greatly appreciated.

TurboSS

try dropping your min pusle width threshold
and adding a healthy ammount to the Low PW offset table

RobsWS6

Never played with injector stuff since I am running stock injectors. Where and how much do I need to change?

TurboSS

if you made the motor very efficient at low throttle the min pulse with may be too lean at light light throttle

you could be usinge way less PW to move the car down the road then GM ever thought possible

add a healthy ammount to the LOW PW injt offset table!

Do you have a wideband in the car?

TAD

RobsWS6

I do have a wideband but not in this car. Do I add to the entire table? How much?

Thanks

ACE1252

I have been fighting this too, but I think I am well on my way to having it fixed thanks to Greg Banish's DVD and books on GM tuning. He is an OEM calibrator...so he knows his stuff.

To start with, Greg states that GM uses a blended model to calculate air flow. this means that even in MAF mode, the VE tables are playing a role in the fueling(more so at low rpm). The 400 to 2000rpm range of my VE table was very erratic when plotted it in excel, especially when compared to stock.

To tune it "properly", you will need a load bearing dyno, logging software, and a wideband.

His DVD is based on the LS1, but the principles apply to the LT1. Unfortunately, the logging software functions that make it pretty easy to do on an LS1 are just not there(to my knowledge) on an LT1(features within HP tuners and EFI Live).

Anyway it's a great DVD although it is not cheap(still not too bad given the quality of the training).

First, you need to know that I have not done extensive testing of my new tune yet. I DO NOT KNOW what the fuel trims look like yet, because my new long tube coating has taken out one of my pre cat O2 sensors....so use this advice at your own risk.

I had bad bucking off idle to around 1400rpms at light load. Most if it was on light tip in.....if you let the clutch out at idle and let the car pull itself at idle(no throttle) it would pull smooth....then if you just lightly tipped into the throttle.....major bucking.....I would get it in a parking lot or cruising around a gas station looking for an empty fill station.

This is what I did and the results are very encouraging.

First the injectors....(since yours are stock, don't screw with this part)

I feel I have calculated out the correct injector battery offsets for the SVO 30 redtops(fat ones, not skinny ones), but the low pulse adder still has me scratching my head. I calculated the battery offsets based on Ford's injector data sheet, but the low pulse adder values are handled differently by Ford and GM. In any event, getting these values correct are important to getting the correct fuel mass delivered to the engine. Since you are still on the stock ones, you are in excellent shape in knowing that the PCM injector data is correct(no need to question GM's data on this issue).

Next the MAF cal....

When you change the intake system to a cold air system, you are changing the calibration of the MAF sensor. Greg states that this can shift the MAF cal as much as 15 percent. On the DVD, he cal's a trailblazer with a cold air system and it's cal shifted around 12 percent on the average.

So, on my Z...I have increase the MAF curve(cal) by 10 percent. Just multiply that entire table by 1.10(if you needed an increase of 5 percent....it would be 1.05...if you needed a decrease of 5 percent it would be 0.95...just be sure you are working with stock values when scaling this table). Now again, since I don't have a wideband yet...this is just educated guessing based on an example from his DVD. So the shift of this value could actually be less or more....but one thing seems clear....this bucking appears to be a lean issue. So the fact that doing the above is telling the PCM that more air is coming in the system(due the cold air system)...does make sense.

VE table is up next.....

It's important to realize that GM has already modeled the airflow of a stock headed LT1 and stock cammed LT1(stock VE table). When you just change cams, it will need to be scale up or down at a certain rpms based on the cam selection. The LT4 hotcam is close to the CC503, so we are in the same ballpark.

In the VE table tuning example on his DVD, he is tuning an LS6 vette with a 228/230 range cam. So that is very close to what we have.....based on that I have made an educated guess as to what needs to be done to my VE table.

With these type cams and the overlap involved with them, the stock VE table is going to need to be reduced in the 400-2600 range....left the same in the 2800-3200 range, and increased on up.

Starting from the stock VE values, I stepped my 400-2600 rpm VE values 15 percent, 10 percent, 5 percent(I'll post up the exact percentage shift for each rpm range later...but the closer to 2800...the less the scaling). IIRC, I didn't adjust the values above that(from what PCMforless had), because I feel that area was in better shape than the lower part of the table....and low rpm is where I'm having the bucking.

I am happy to report, with my tweaking to the tune, that the bucking seems to have disappeared.

I still need to do some more testing, but I will post up my VE table, MAF tables, and the injector offset table(for those with SVO 30 fat redtops). I'm not going to post them just yet, because I want to see what my fuel trims do......once they seem safe I'll outline exactly what I done to the VE tables....and post them up in an excel spreadsheet...with chart's comparing it from what I had, to what I modified it.

Please don't feel I'm trying to withhold anything, because I don't want to post something that could throw the fuel trims into left field and/or destroy an engine. If I have screwed up, I'd rather blow mine up than someone else's.

So in a nutshell, I think you need to adjust your MAF table(if you have some type of cold air intake..you don't mention any...but I bet you do...) and the VE table. If you have the stock air intake system, the only thing that will need to be touched is the VE table. I am making the assumption that you are working with a mail order tune of some type...is that the case? If you are working with the stock tune, the spark advance tables will need to be tweaked as well.

ACE1252

If he has stock injectors, he does not need to touch ANYTHING related to them. Not trying to be rude, but the above is very bad advice.

RobsWS6

Thanks for the info. I have made a lot of VE corrections when in SD and from what I remember, the fuel trims were very close. I then went on to the MAF and did the same thing. I've tried to tune this car like I did with my GTO that drives perfect with the cam that's in that car. The WS6 drives pretty well except for this no throttle/very light throttle bucking.

As far as CAI, just a K&N and Fernco coupler. The rest is stock WS6. Once the weather gets better, I guess I'll start fresh again.

Thanks

TurboSS

based on what experience

ACE1252

The injector tables in the PCM are used to define an injectors electromechanical characteristics. They only need to be edited when changing to injectors different than stock. When you put in a different brand injector, it will react differently based on the flow pressure it was designed for along with it's own unique electromechanical properties. These are things that have to be defined in the PCM.

That is where data sheets like these come into play....

http://www.fordracingparts.com/parts...tKeyField=9043

See the calibration summaries near the bottom.

When installing a new injector, you have to use those sheets to input the proper injector data into the PCM injector tables.

It does not make sense to change the stock injector tables when no physical change was made to the stock injectors themselves. Doing so gives the computer bad data on what amount of fuel it "thinks" it is delivering as to what is really being delivered.

ACE1252

Here is some good reading....

"http://www.calibratedsuccess.com/Assets/Documentation/Fuel Injector Article.pdf"

TurboSS

wow I'm wrong you are correct

I'll stop here posting now.
Your E-kowledge is too strong for me.



Good luck with that light throttle bucking.

ACE1252

Well, all I can say is that I've put the engineering information right in front of you. If you would rather point and laugh than try to understand how the PCM actually works......more power to you.

Before viewing Greg's DVD I tried spark adjustments, VE table adjustments, screwing around with the injector tables....nothing cured it. After learning from Greg and applying it to my tune....on my first try, the light throttle bucking has disappeared. All this is just working on educated guesses too. So I still need to get to a load dyno and do some steady state testing with a wide band. This has nothing to do with "e-experience"....I've actually programmed my LT1 Z28 with JET DST software.

Part of my problem was searching high and low, on the net, listening to people who didn't have an ounce of engineering data to back up the adjustments they were making. That is not the case with Banish. He backs up what he is telling you with the engineering information and calculations. He also does this stuff for a living(as in working for car manufacturers....). Take it for what it's worth.

Lord knows I'm not God's gift to PCM tuning, but I'm no moron.....and I've got the engineering degree to back it up.

TurboSS

I'm only posting here to help out Rob

I have zero desire to take the time to review my resume with you or argue my point.





So I will help Rob via PM at his request inspite of your GB regurgatation fest.

ACE1252

I stand by my instructor and his methodologies....who has done a much better job of explaining himself than you. To me, you have simply offered up no good reason for screwing up an already calibrated fuel delivery system.