DBW to DBC conversion Idle problem

[agentruley]

So this has been a w-i-p for quite sometime now.
Vehicle; Factory 2005 suburban with 8.1L and 4l85e.
Goal: Convert engine to DBC for customer.
What we have done so far:
Grab stock 8.1L tune, Grab stock 4.8L 4l80e Van ECU. Paste the VE table, Injector Size and cylinder displacement (just to get it to run) from stock 8.1Ltune to the 4.8tune+ecu.
Install Drive by cable throttle body along with pins for ecu in harness.
Problem: Using this method the engine will start up great, then slowly rise in idle untill 1700-1900 rpms with fuel trims bouncing back and forth. With the IAC unplugged at engine startup, the timing goes down and the engine idles fine. With it unplugged (and its in its almost closed position), truck drives great no problem. Plug it back in and slowly the timing increases along with IAC position untill its wide open, sucking massive amount of air (very loud). Engine immediatly starts to run hot.

So Lets try that again, Lets take a KNOWN WORKING GOOD TUNE for a otherwise stock 5.3 engine and just increase displacement to 8.1L, change injector size and VE table and try to see if it atleast wont idle. So we grab an entire complete ecu thats already been running, change the following and see if it wont atleast idle and it does the EXACT same thing. However I can turn right around, plug it back into my truck and instantly idles fine. Both engines have the same cam, crank position signal sensors, same IAC part numbers, same timing coils. Yet I then try that same ecu and tune on this 8.1L engine and it just idles really high. So I think vacumm leak?

While its running, if you unplug the mass airflow sensor it goes down in idle for a while, and then slowly but surely goes back up to about 1100-1300rpms.

In HPtuners with the tools thing in the menu I can go to spark control, activate negitive spark control and then go to -22ish degrees and BOOM! it idles like its suppose to, but since its commanding -22 degrees of timing any throttle and it just dies.

What is going on here? We have done the whole chaning the options matrix in the system tab in hptuners to "0" in the ECT control, yet all the ecu does on two different tunes is just open the IAC all the way open. fuel trims are a tiny rich while it idles there at 1300rpm, ltft is around anywhere from 3-10%/

Another problem: I havent used the vcm scanner in quite a hot minute, and I cannot figure out how to read things such as commanded timing, commanded afr, afr corrected, injector duty and so on but I do know its possible because once again, USE to be able to turn around and plug the computer into my trucks ecu and the config would auto load everything, now when I try with ANY ECU it says either option not supported or its blank. But I can grab the cheapest cantool in the world, bring up realtime data and there is everything I could ever want, so clearly the ecus DO support it it just for some odd reason only shows me LTFT and STFT along with engine rpm and thats it.

Anyone help us out? TRying to think outloud as perhaps someone can chime in with some similar experience.

What we are currently doing: Taking the original DBW throttle body and cable, reincorporating it into the harness and going to flash the DBC 4.8 ECU with DBW OS and see if we cant get the truck to right in anyway. Once again, if you take your hand and cover up the IAC valve on the throttle body (while everything is running in speed density) the engine idles PERFECT, along with load/idle adjustment such as going from park to drive. it all works so well with it closed.

[j_verrett5.0h]

did you reuse the iac from the dbw throttle by any chance? if so that may be your issue as the dbc ones are a little different from what ive learned

[85K5Jake]

What year ECMs are you trying to use? Earlier Ecms have the correct IAC driver chip in them already for the cable throttle.

[1970ss8.1]

Did you ever figure this out. I?m having a similar issue with a 102 on an 8.1?