Timing Issues

[BigRed76]

First things first here's some basic info on my swap:
'71 C10, 2000 5.3 & 4L60E using factory PCM, factory engine harness modified by me.
PCM setup by Brendan @ LT1swap to run as a standalone PCM.
Only engine upgrades were cold air intake, shorty headers, and dual exhaust with x-pipe.
Also replaced intake gaskets and knock sensors when the motor was out since those are common issues.
Spark plugs and wires are new.
I've had this swap running for a little over 2 months now.

I was previously having an issue with the MAF under reporting, but I moved the MAF farther away from the throttle body and that seems to have cured that problem.

My problem now is I've got what feels and sounds like a misfire...but it only occurs at steady cruising speeds, on decel, and in park It will do it at rpm's above about 1100. I've been watching scan tool data trying to figure this out and I've narrowed it down to my timing. The timing advance is very aggressive, meaning the timing advance goes well into the 40 & 50 degree range, and the misfire will come in when timing hits about 35-37 degrees. And while this is occurring the knock retard is just non existent, so this timing just goes through the roof. At warm idle (600 rpm) the timing sits at about 12-15 degrees. Short term fuel trims are right around 0, and long term fuel trims are right at 4 & 5%.

I haven't really seen much on the net about this type of issue, but from what I have seen stock LS engines don't like more than 30 degrees of timing advance. Are there some other things on the scan tool I need to be looking at or is this a potential tuning issue with the PCM?

[BR3W CITY]

I think stock trucks are around 17-19 at WOT, and fbodies around 25*, so ya you have a LOT of timing. Cruising would be like 33 on the cars. How good of a scan tool (can you monitor live data etc)?

A couple things can happen;
Trucks use "adders" to get total spark, which is a little different than the fbodies and vettes, in the sense that its not "straight" timing like twisting a distributor cap. There are things like Burst Knock, AFR spark correction, and even stuff like torque management can affect the number. SO, its hard to say what the tuner did.

Are you getting any codes popping up, or any other symptoms?

[BigRed76]

I've been using 2 different scan tools, the Tech 2 and the Snap-on verus pro. There are no codes coming up at all, and no other symptoms. It makes good power like it should, and I've been averaging 18 mpg. I'm not 100% sure on what the tuner did to the PCM other than the obvious things needed to make the PCM run standalone.

[BR3W CITY]

I feel like with that much timing you'd be libel to have at least one other symptom.
If you try to replicate the miss, CAN you get it to throw a code? like by getting to the sweet spot, and feeling out the miss. I'm wondering if its a miss, a knock, or something else entirely like an injector not pulsing intermittently.

[BigRed76]

I've tried to get it to set a code by running it in that sweet spot where it misses, but it won't happen. And I have noticed that occasionally the idle is a little rough. I guess I could scope to injectors and see how they look. They are the stock injectors and I never did anything to clean them.

[BigRed76]

So just an update on this...I moved my MAF again, even farther away from the TB because it was still under reporting at idle. That was a week and a half ago so it seems like that issue is cured now. As far as the timing, it only gets up into the 50 degree range in park idling between about 1100 and 2000, and it will misfire and pop out of the exhaust like crazy. I drove the truck while monitoring the timing advance and when the truck begins to misfire and pop on decel the timing drops way down to around 0 because the PCM is in fuel cut. During all driving conditions the timing advance never reaches more than 35 degrees. So this was beginning to make me think that the popping and missing a hear on decel is just a result of the louder than stock exhaust...but idk.

At my shop the other day we had a 2000 suburban w/ a 5.3 come in. So out of curiosity I put the Tech2 on it to watch the timing advance, it did the exact same thing as mine...50-60 degrees advanced idling between 1100-2000, and while driving the timing never exceeds 35 degrees, but absolutely no misfiring felt or popping heard from the exhaust