Is there a way to control when injectors fire?

[dimented24x7]

Im wondering if GM ever included a way to control when the injectors fire relative to TDC? I know in some of the old OBD 1.5 PCMs, there was a way to do it, but I have yet to find it in my 98 vortec PCM.

The reason I need to offset when they fire is that the cam overlap is literally just sucking the injector fuel pulses out of the cylinders at low RPMs. Basically as soon as the manifold vacuum decreases, the apparent AFRs just plummet to < 30:1 and the engine has a horrible hesitation. From what I can tell from teh mass airflow, the engine fueling should still be good, but the engine just goes dead lean at low RPMs and high throttle. Once it get above the RPM band where it does it, the AFRs return to whats commanded. I tried it in speed density as well, and its even worse.

It would be nice to delay each injector firing until the exhaust valve has seated so I dont loose all my fuel out into the exhaust. Id rather do this than just use the VE table to drown it in fuel until the problem goes away.

[slow67]

Do a search for "injection boundary"

[blownbluez06]

Are you sure you're actually lean? I understand the WB is showing that, but high overlap cams can introduce extra oxygen into the exhaust even when your AFR's are actually legit inside the cylinders.

[dimented24x7]

Im pretty sure it is lean. You are right in that the reported AFR isn't trustworthy at low speeds, but the engine also acts lean. It stumbles and bucks in the area where the AFRs appear to plummet. It could certainly be something else, but if I can find the injector offsets, it may be worth while trying.

[dimented24x7]

I took a look around in the code, and from what I can tell, the 98/99 black box PCMs dont support the option to control when the injectors fire (anybody know for sure?). The only code I could find that references the sampling of the CKP time/period is the CKP relearn and P0300 misfire check.

But... thats irrelavent as the problem isnt the cam overlap at all. I finally got to drive the car for a long trip and the problem goes away as it warms up. It looks like its actually due to fuel settling out in the cool intake runners/ports at low intake velocities. Basically when the engine is cold, the fueling will go dead lean on tip in, and then pig rich when the throttle closes and the fuel flashes back to a vapor.

[slow67]

Take a look at Transient fueling in that case.

[dimented24x7]

Actually, the blackbox PCMs dont have any true transient fueling routines for MAP enrichment. Well, they sort of do with the transient airmass matrix, but theres no temperature compensation with that.

I ended up just adding a patch to provide MAP accel enrich/decel enlean. I had a routine that I wrote for my old MAF TBI setup that I used and re-wrote for the blackbox. Not as sophisticated as what the LS1 PCMs have, but it seems to be effective in providing crisp throttle response when the engine is cold.

Part of the problem is the intake I have. The edelbrock victor-E has the injectors pointing straight down at the bottom of the intake manifold floor instead of the intake valves, which gives the fuel lots of area to settle out on when the intake is cold.

[orange ss]

ditch the Vortec PCM and swap to an LS based 411 PCM

[dimented24x7]

Well, the blackbox is actually working pretty well. In MAF only fueling w/ SD on sudden transients, the AFRs track the desired within +/- 2% in open loop, which is about as good as it gets.

Money issues aside, Id also have the issue of not knowing how much HP tuners actually includes in their definition for the '411. Tuning with these PCMs isnt really tuning as much as disabling. The GM fueling logic is extremely conservative and overzealous. The use of heavy filtering in constructing the cylinder airmass as well as limiting when the injector drivers can sample really causes a lot of lag. It might work ok with an e-throttle or the little, restrictive vortec TB with its baffle, but not a performance TB and intake. I couldnt even touch the throttle without the engine lean popping. Basically I disabled all of the logic and just output the cylinder airmass from the unfiltered MAF flow directly to all the injector drivers. Works much, much better then the original logic. By nature, the MAF pulse accumulation read-in stratagy already does most of the filtering for you, so more isn't really necessary in a performance setup.

[dimented24x7]

Actually, I think I may have found it. Purely by accident, of course. Can never find these things when you actually look for them.

There are two tables that appear to control the relative offset of the injector firings with respect to when the CKP pulses are recieved. One table for engine on, one for engine off or cranking, and a static value used when fast injector response is desired (WOT/high RPM). Basically the time the injector fires will be (offset in table/4096) x cylinder seperation angle. The pulses come 45 degrees ahead of each cylinder reaching TDC. Playing around with a function generator, increasing the offset appears to move the inejctor firings as expected, with a value of 2048 being right at TDC.

Will know how well it actually works when I get a chance to drive the car again, but having the pulses come in after the exhaust valve closes does seem to cure the stumbling when I crack the throttle at idle.

[dimented24x7]

I finally got some time to play around with the car some more. I was wrong about how they fired. When I compared what I found with the function generator to the software code, it was obvious that the computer doesnt fire relative to 1 CKP pulse, but rather in terms of multiple pulses. IOW, the injectors dont fire when the valve is open. Rather the firings are delayed by a specified angle and fire somewhere inbetween each successive intake cycle. The factory set delay ranges from 150 degrees to around 550 degrees or so relative to the CKP pulses. This lets teh fuel sit in the runner and evaporate prior to being taken into the cylinder.

I changed it to actually fire when the valve is open, but it replaces the old problems with new ones. Obviously there isn't any time for the fuel to vaporize, so the mixture is stratified in the cylinder. The exhaust does sound much cleaner, and it doesnt hesitate on sudden low RPM tip-in, but the idle quality is worse and it has a tendancy to misfire when cold/at low RPMs. With the fuel allowed to dwell in the runner, the idle and P/T drivability is much better.

Oh well, guess it is what it is

[gsegse]

Actually, I think I may have found it. Purely by accident, of course. Can never find these things when you actually look for them.

There are two tables that appear to control the relative offset of the injector firings with respect to when the CKP pulses are recieved. .

which tables please ?

[dimented24x7]

What are you using to tune? HP Tuners encrypts their bins (for obvious reasons), so you cant hand edit the tables with a hex editor. I dont know if theyre defined in HP Tuners, but they should be referred to as "injector firing offset" or something similar if they are.