Injector Duty Cycle/High Speed Miss/PE table q's

[Z06_BluByU]

I've developed a high speed miss after more than a year of driving with my 383 program.. I datalogged the ride in to work today and notice where the miss is occuring i'm seeing injector duty cycles in the 95%-120% range.. The fuel trim cell is in cell 15 and the 02 sensors are in the 915-950mv range.. Specs: 1993 383 LT1, Auto, stock everything except no cats and a diffrent cam (see sig for cam specs) and higher compression pistons 10.8:1 02 sensors are the non-heated stock variety.. Questions.. 1. I'm using the stock 1993 LT1 injectors.. what size are they? 2. Since I'm running the stock fuel pressure regulator and fuel pump should i just switch to bigger injectors or try getting an AFPR and trying to squeeze a little more out of the stockers.. since this problem just started to get worse i'm thinking a year of doing this to the injectors has probably tired them out.. but maybe i'm wrong.. 3. if i go bigger injector what size and manufacturer is recommended? 4. since VEmaster doesnt seem to do a good job of adjusting WOT fuel map.. where do I got to adjust it? Fuel map or PE map? Also, how should i go about changing it? 5. What should my target 02mv be at fuel trim cell 15 (WOT?). thanks for all your help..

[tjwong]

Originally Posted by 383_BluByU

I've developed a high speed miss after more than a year of driving with my 383 program.. I datalogged the ride in to work today and notice where the miss is occuring i'm seeing injector duty cycles in the 95%-120% range..

The fuel trim cell is in cell 15 and the 02 sensors are in the 915-950mv range..


Specs:
1993 383 LT1, Auto, stock everything except no cats and a diffrent cam (see sig for cam specs) and higher compression pistons 10.8:1
02 sensors are the non-heated stock variety..

Questions..

1. I'm using the stock 1993 LT1 injectors.. what size are they?
2. Since I'm running the stock fuel pressure regulator and fuel pump should i just switch to bigger injectors or try getting an AFPR and trying to squeeze a little more out of the stockers.. since this problem just started to get worse i'm thinking a year of doing this to the injectors has probably tired them out.. but maybe i'm wrong..
3. if i go bigger injector what size and manufacturer is recommended?
4. since VEmaster doesnt seem to do a good job of adjusting WOT fuel map.. where do I got to adjust it? Fuel map or PE map? Also, how should i go about changing it?
5. What should my target 02mv be at fuel trim cell 15 (WOT?).


thanks for all your help..

Your 93 should have 22lb injectors in it. The later 94 and up cars if I remember right made the jump to 24's. For a stock engine with just a bump in a cam the injectors you have should be adequate. If you want to go with larger injectors, a set of Ford Motorsports 24s, OE Multec 24s or someones spare LT4 injectors that are rated at 28lbs are more than adequate for your engine as it is now.

Have you checked the fuel pressure at WOT? That can be a huge issue when doing WOT tuning if the pressure falls off due to a plugged fuel filter or a faulty pump, that is the first place I would check. Seeing how this problem just sorta cropped up. Also while an indication from your O2 sensors at 900+ mV does indicate a rich condition, you have really no idea how rich it truly is. I can be just at 13.9:1 or it could be 12:1. Thats why they are called narrow band sensors, they are accurate at stoich but go one side or the other then, they are not very accurate.

I use VE Master to tune just in the idle to 2500 RPM regions. The reason is that with a larger cam the VE of the engine is degraded in the lower RPMs but is enhanced in the upper RPMs. I use it to enhance low speed drivability. Tune your WOT tunes using the PE maps, and double check to see where the ECM will go into PE mode. There is a table called TPS vs RPM for PE. If you don't have access to a wide band sensor, I would shoot for 910 to 920 mV for O2 sensor voltage. Your best bet is to get a wide band system and datalog with it, that will really tell you where you are. But the first thing I would do is to verify the fuel pressure at WOT. Make sure that it does not fall off at that time.

[Z06_BluByU]

Tom,

Thanks for the reply.. The motor has more than just a bigger bump stick.. I stroked it to 383 cid... would you still consider my injector size adequate?

Also, its seems odd to be running rich if the fuel pump is crapping out... or is that the reason for the huge spike in Inj DC? are you suggesting the computer might be tring to compensate for a weak FP (or low fuel pressure) by opening up the injectors longer? what should FP be at WOT?

[Z06_BluByU]

also what is ideal for A/F ratio at WOT?

[96GS#007]

Originally Posted by 383_BluByU

also what is ideal for A/F ratio at WOT?

Usually somewhere between 12.6 and 13.0 for best hp.

[tjwong]

Originally Posted by 96GS#007

Usually somewhere between 12.6 and 13.0 for best hp.

I agree thats what I shoot for as well. Your WOT fuel pressure should be at between 43.5 and 45 PSI. The PCM won't know if there is a weak pump or if its going lean because it ignores the o2 feedback. its really hard to say whats going on with the DC, for one reason is because the DC is a calculated number within the PCM, and depending on how the scan tool sees it, the number may not an accurate representation. I still feel that the injectors you have are adequate.

As for the high speed miss does it only occur at high RPMs? Is there a way you can get it on a dyno and simulate that problem with a ignition scope attached. That is what I do when I run into similar situations. I have to isolate the problem such as any ignition related miss caused by plugs, wires or other ignition problems. Then go from there, the ignition scope will generally show any problems and isolate it to a source when its done on a dyno. A shop down the street fought a problem like yours for 2 weeks then came down to my shop. It took me longer to strap the car down on the dyno than it did to find the problem with the scope. In his case it was a carbon tracked coil tower. But that is just an example of what can cause something like that.