Hello. I’m having an issue with my 6 liter LS in my Silverado. It is an 2003 2500HD.

I have just rebuilt it with new pistons/rings/cam, as well as all new sensors for the entire engine.

I am using a TBSS intake with a Corvette style filter/regulator. I didn’t use the 90mm TB, but used my original with an adapter plate.

I’ve gotten a base tune for a local tuner. The vehicle mostly starts and runs.

However, I can’t seem to get past these very annoying issues.

1). It doesn’t start on the first key-on every 4-8 startups, but starts immediately on the second try.

Usually I get a backfire on the first try when it doesn’t start up. And, increasingly, I get to a point where it goes into “Reduced Engine Power” with no throttle control and will not turn off. I have been pulling the FP relay.

The dash lights also flash on and off when I try to turn it off, before I pull the relay.

These are the codes with the “reduced engine” message:

P0120, 220, 1516, 1518.

Generally, I can clear them after the vehicle has been shutdown. Most of the time, it allows the engine to restart properly, but not always.

I’ve got 50 miles on it since the rebuild. Good oil pressure, temperature where it should be, sounds good. But not dependable for longer trips.

Any ideas where I should start? Is this a tune issue? Being that I’ve never seen these before, I’m only guessing. I’m an old smallblock guy, not used to all the electronics.

Jeff

2003 2500HD Silverado, 6 liter, 4L80e, 4.10s, 4wd

 

Cam sensor is bad or disconnected.
It will run without it, but often only on the 2nd start.
Its only function is telling the ECM when #1 is on the firing stroke.

 

It is a new cam sensor. I guess it could be bad from the getgo.
jeff

 

It happens. Make sure the new one is real GM stuff.

 

Yup, "often backfiles and fails to start on first try but then starts on 2nd try" indicates a missing cam sensor.
Could also be a broken wire or mixup on the cam sensor harness.

 

UPDATE: We found a ground around the center back of the engine. I had found the one on the passenger side and though that was it after putting 5 grounds on. Larry's Electric in Ruma, IL found a ring connector floating and hidden back center of engine. Put it on the head and all issues except bump and restart fixed.
As per this forum, I did replace the cam sensor with a GM (had a new Walker on it at first) and that seems to have taken care of the problem.
Once again, thanks for all the help with this!
Jeff

 

Thank you for giving us an update and details of what it took to solve your problem.