5.3 LS Swap Problem

[Beaureed]

Hi all, I have been fighting a problem for quite some time now. i have a 2004 5.3 DBW set up on and engine run stand. I am setting this up to insure everything works as it should before I put it in my jeep. I have all the vats and rear o2's deleted, and I followed LT1swap.com to do the harness. My problem I am having is the motor fires up great, runs great until it goes into open loop and then has a rough idle. I am getting the P0300 code. The motor seems to rev up very well but when the RPMs come down it starts to stumble and surge. I thought it may be a screw up in the harness but i have built 2 harnesses now and it does the same thing with both harnesses. I have the o2's welded in about 4" past the manifolds. Here are the things i have tried.

2 different sets of o2s,
new plugs
new wires
new MAF, and Map sensor as well as TPS sensor
New exhaust and intake manifolds
cleaned injectors and Verified injectors with noid light.

Im running out of thoughts here, each cylinder has about 120PSI.

Could not having the VSS hooked up be giving me a problem. Is there a way to bypass it., since this motor is running on a run stand with no trans. It only acts up when its at idle, it wants to idle between 450 and 650. Thanks for any help

[pajeff02]

Upon entering open loop the engine is looking to the sensors for feedback to control fueling. Most likely culprit is a vacuum leak, but here are two easy things to check. If you have a scanner that you can view live sensor data take a look at the front O2's to ensure that the mV is rapidly cycling up and down for each bank. If it appears stagnant for either sensor then it is either bad (new parts do not always equate to good parts) or you have a wiring issue. Also, check you fuel trims - you should see both short and long term trims and these are what indicate how much fuel is being added or subtracted by the PCM. Ideally, these should all be fairly close to 0, but can range up or down by a bit without issue. Last thing, with the engine running and in open loop, unplug the MAF. This will force the PCM into speed density mode and often times the misfire will go away. This means that you either have an air leak at the throttle body, a defective MAF, or a mismatched MAF (there are multiple part numbers). Also, ensure that the MAF has not been messed with, ie. ported, de-screened or de-bladed, as this alters its calibration and will result in erroneous air reporting.

[pajeff02]

Update: Just saw that you are from Saginaw -- my parents are both from Bay City. Spent lots of time as a kid up in that area.