Injector Opening Time Question


Boost Junkie
Feb 20, 2002
I have a few issues that I thought may be related to injector opening time. These were brought up by a friend who has been helping me with some tuning aspects.

I, like many others, have a difficult time getting the car to start on initial cold start. It will catch, then die a few times before wanting to actually run. Also, my VE table values in the idle area are higher than what most people use. Right now programmed at 59 or 60 on the VE table at idle.

My question/comment was that I was told that since I am running 50# high impedence injectors that my injector opening time should be more around 1.5ms instead of the 1.0ms that was in the original FAST program.

Does this sound plausible as a cure to both problems? And in what way would that effect the overall VE table. Right now the car does run very well, especially at WOT, so I am somewhat leary about making changes. My only real issues are start up and initial idle stability.

Comments, theories and any help greatly appreciated :)
The cold start problem is one thing, but don't be concerned about the VE values at an idle. Depending on the injectors you run, engine characterisitics, etc., the numbers could be anything. I've tuned motors that used numbers in the 20s at idle, and I've seen them as high as 80. Bottom line, there's no such thing as "right" and "wrong" numbers... just give it what it wants.

The startup issue is simply one that requires time tweaking the cranking fuel, after start fuel, and starting IAC position. Sometimes, setting the target idle speed a little higher when it's cold helps a bunch too.
the 4 VE cells around my idle RPM/MAP are around 70.

This gets me a 13.5-14 a/f at idle once engine is fully warmed up. I kept thinking I was doing something wrong with VE numbers that go above 100 in my base table. Craig is right it doesn't mater just put in the value that makes it run best.

Craig, what should I check if my idle speed surges for about 30 sec after a cold start? After that it settles down nicely.
Add after start fuel to cure the idle surge after startup. Surging comes from being too lean, and the added after start fuel will fix that. After start fuel decays out after time, so about the time it would have been done surging, the after start fuel will be gone and life will once again be good.

I realized that I never answered the real question in the first post. The injector opening time is a value that will be added to the pulsewidth that the ECU calculates to account for the delay from the time the signal is applied to the injector to the time that fuel actually sprays out the end of it. If you raise this value by .5 mS, you will add .5 mS worth of fuel to the entire map, essentially. Raising this value will probably mean you'll have to lower your VE numbers. The impact this would have on VE numbers will vary - big difference at idle, and less at WOT. If you felt better having smaller numbers in your VE table in the idle region, this would be a way to achieve that. I don't think that the injector opening time has any impact on cranking fuel though, so I don't believe it would impact starting.
Thanks for the replies, that's exactly what I wanted to know. :)

I wasn't too concerned with the VE #s from a running standpoint since the
car does run very well once it's warm and on the road.

For now I won't mess with the injector opening time, however I will mess
with the cranking and after start fuel even more than I have. Also I may
need to re-evaluate my IAC at start up conditions as well. I too experience
the surge at idle, moreso when the engine is cold. Idle is set at about 900.
The engine will idle there perfectly, but if I goose the throttle, it will fall down
to 400-500rpms and then back up a bit and level out.

Thanks again
Originally posted by ZBass28
The engine will idle there perfectly, but if I goose the throttle, it will fall down
to 400-500rpms and then back up a bit and level out.

Thanks again

I had that problem and fixed it by adding to the VE at the affected cells. Once you go overboard/too rich it'll pop out the exhaust.
A couple of thoughts based on the above...

For setting the cranking IAC position, figure out where your IAC is at idle, and use a number around there. If the IAC is too far closed, the motor will cough before it figures out that the idle speed is too low and it has to open up the IAC. If the IAC is too far open during cranking, the engine will rev way up just after it's started, until the ECU realizes that it has to lower the idle speed a bit. Just a rough guideline, you may have to tweak it a bit, but I found this worked very well for me.

Also, if your VE numbers seem off, then make sure you don't have an exhaust leak! This will allow air into the system, fooling the oxygen sensor into thinking that you're lean and that you need to fatten up the VE numbers. And vacuum leaks don't help anything either, but that's not quite the problem you describe.

-Bob Cunningham