LS 66 Impala no start issue

[66_ImpalaSLW]

New to forum -
My 66 driven to tuning shop March 21; adjustments made to tuning with laptop, after which it no longer started. Almost 12 weeks later, 2 mechanics have said a short or wiring issue. ECM re-flashed more than once, throttle body replaced (clicks loudly with ignition on).
I'm doing my best to stay patient but extremely frustrating. Owned the car less than 3 months and it's spent about 12 days in my garage.
6.0 LS, GM harness and ECM from 08 Silverado, auto trans from same truck.
Any thoughts or suggestions welcome.

[Vimes]

First rule of troubleshooting - what has changed? You drove it to the tuning shop, and they touched the tune, and now it isn't working. Did they take a backup before making changes? If so, put the backup back in.

Sometimes during a tune, there is a risk of the reflash not taking and bricking the ECM. It's never happened to me, but every single time I've flashed one there's a HUGE warning flag that warns of this unless you check a box saying don't show the warning anymore. You may need to replace the ECM. A replacement ECM for a 2008 Silverado should be (off the top of my head) around 300 bucks.

In any case, the tuner should be the one figuring this out, since it broke while he was working on it. Hope this helps.

[66_ImpalaSLW]

Thanks. I agree 100% the first guy responsible but I got tired of waiting at the 5-week mark, had it towed and now second shop is going on 7. These are LS experts with decades of experience building and tuning and it's almost impossible the problem is tune or ECM-related since they both have tried re-flashed computers more than once. Resetting backup not an option since the original ECM refused to fully program again. Second shop currently chasing wiring, but short of ripping out the entire harness and starting over - which I can't afford anyway being 20-30 hours labor - there aren't options I'm aware of, nor is anyone I've spoken with. I have no choice but to continue being patient or choosing to sell the car at a huge loss. I refuse to accept it can't be fixed, just a question of how much time and money is reasonable.

[dhutton]

Who modified the harness for the standalone swap? Is it throwing any codes?

[66_ImpalaSLW]

Don’t have original build or code info. Big part of problem is unknown wiring scheme

[dhutton]

I would give serious thought to getting a new standalone harness from a reputable source.

This is the risk you take when you buy an unfinished project with unknown history. No one wants to work on someone else’s mess…

[Vimes]

OK, so other ECMs were already tried. Did any of the ECMs come online enough to set any codes? Seems strange that the original ECM would let you drive the car to the tuning shop but failed when the tuner loaded the new program though. Did he remove the ECM from the car? Some tuners have bench harnesses that allows them to reprogram the ECM from the comfort of their office. If so, I wonder if some of the pins weren't quite set when whoever pulled the harness apart to clean it up and one or more pins backed out when the harness was reinstalled. The little fingers that hold the wiring pins in place are pretty flimsy and it doesn't take much to damage them when working inside the plugs. This is usually what happens when a backyard mechanic uses a screwdriver instead of a proper pin removal tool to remove the pins. When damaged, the pins can push out of the plug when you try to plug it in, something I learned from using a screwdriver to remove the pins . It's easy enough to tell by looking through the plastic to see if any pins are backed out if you're looking for backed off pins, but you wouldn't be able to tell from a casual glance.

One other thought - battery voltage. If the battery is low, it plays merry hell with the ECM. Swapping in a known good battery is a quick and easy test for that.

In any case, if other ECMs were tried and you don't see any pins backed out, and the battery is not the cause, then I agree with dhutton - sometimes it's best to scrap what some unknown person did. An aftermarket standalone harness would be the next logical step.

[66_ImpalaSLW]

Thanks again for input everyone. Car only qualified as unfinished paint- very light body. Started instantly and rumbled smoothly every time until this mess. I had some needed brake work before tuning snafu but everything else great shape.
Vimes question - car actually started ONCE with programming done up thru injection but reverted to clicking throttle body (also replaced) after. Won't crank, don't know code answer, assume both guys went all thru any codes.
I'm no way qualified to replace harness and thousands labor so still stuck indefinitely