RomRaider

I know we've defined (for most ROMs, I think) the fuel pump duty cycle values that the ECU selects from - by default they're 0, 33, 66, 100%, and some people have found it helpful to raise the 33 and 66 values to 100%. But what about the code that selects which of those values to use? I've found the code in my ROM (05 LGT/OBXT) but haven't really figured it out.

There are a pair of tables whose inputs are voltage and manifold pressure, and whose cell values apparently get compared against fuel flow to determine when to switch between 33/66/100% fuel pump duty cycle. It's sort of complicated because the function that does the table lookups actually just sets a couple of bits in a byte somewhere, and there's another function that selects the FP DC depending on the bits, and both functions are kind of convoluted.

The tables have only four cells - the voltages are 8 and 12 and the manifold pressures are -9 and +9. And the values in each table are all the same. One is 13 in all four cells, the other is 39. It's as if the ECU software developers thought it would be useful to vary the fuel pump duty cycle with voltage and MRP, but the OEM tuners decided not to bother with that.

I think the values in those tables are fuel flow thresholds but I'm not sure of the units. Fuel flow is calculated with a formulate that looks like IPW * 8.8 * RPM * 0.000000012. Given the tiny multiplier at the end, I'm guessing it translates to liters per hour, or something like that. The injector scaling doesn't appear in that formula, which probably explains why people with big injectors have found it helpful to raise the 33 & 66 values to 100. But it would probably be even better to either to modify the cells in those 2x2 tables, or modify that tiny multiplier, or both.

Has anyone dug into this stuff yet?

This was some time back...

viewtopic.php?f=40&t=8655

Bravo, thanks. I'm glad I asked, because you're way ahead of me.

NSFW, is IPW in this case before dead time is added?

Found the tables in the '06 ECU visually (see other thread). My injectors flow ~100% more than stock, so thus would I just half these values to have the same effective flow rate cutover points?

Latency not included in the value that is used for the lookups in these tables, it gets added later.

If you've added a bigger fuel pump, then that would also affect how much these pulse-width thresholds should change, but I don't really know how to do the math on that. In my car, I went from 550-ish stock injectors with the stock pump, to ID 1000s and an Aeromotive 340 pump, and I haven't noticed the issues some people have reported with large injectors. And I'm still running the default 33% and 66% fuel pump duty settings. I think I just lucked out, with the bigger pump compensating for the bigger injectors. Or maybe I need to watch for AFR issues more closely, but I think I would have noticed.

If there's an easily repeatable scenario where you can cause the AFR to go lean due to insufficient fuel pressure, you could try reducing the thresholds 10% at a time until the problem goes away. And then another 10 or 15% just to be sure. I'm just guessing here, but I think that approach would work.

Ok, thanks.

Runing the math by hand the scaling comes our extreamly close (within 4%) to deka-liters per hour, so at least on these earyer cars your hunch looks plausable. That would make sence though, as the engineers could just look at a flow chart from the pumps and see what point they'd need to switch. Too bad aftermarket pump manufactures dont provide a flow data sheet at various voltages as it'd make setting this easy.

Yeah, that would beat the aftermarket's standard guess-and-check approach. Thanks for looking into the math.

Did you see anything regarding the switch from 100% FPDC at startup to using these tables?