I finally got to drive my 5.3 swapped C10 to work yesterday (around 50 miles round trip). The truck ran fine, good oil pressure, 200* going down the road, idled okay coming to a stop but it had NO power, and the Brakes felt very spongy. I checked the manifold vacuum at the nipple behind the TB and it read about 12" of vaccum, which i feel is very low for a mild engine.

The engine is mildly built- 243 heads milled .030, stock 5.3 short block, NBSS intake/fuel rails/injectors, 92mm TB, BTR Stage II truck cam, long tube headers, mail order tune by Frost.

So I used the diagnostic tools that i have to gather some info (WiFi OBD2 / IOS data interface and old school vacuum gauge)
-between 5-9 degree ignition advance at #1 cylinder
-absolute throttle position at idle .49%
-intake manifold absolute pressure 8.1 inHg (when subtracted from the barometric pressure at the time, 29.1 inHg, I come up with 21" of vac?????)
-vacc via vacc gauage (12")
- total % TPS to get it to idle was 12.5%
-in drive its idleing low (450rpm)
-boost -8.1
- MAF 2.5 lb/min

Im thinking that possibly a timing issue (maybe a tooth off) but its really easy to start, like a stock truck.
Pushrod length? used stock pushrods

 

Sounds like the timing is severely retarded.
Too much throttle, low power are both symptoms...

 

I've heard good things about Frost but you have a fairly modified engine in a non-stock application. Working with a mail order tune there are only so many things anyone can do. I view mail order tunes as a "get it running so I can drive and troubleshoot then get it right" not the be all end all.

• You have 243 heads on a 5.3 that have been milled 0.30"
• non-stock cam (hopefully with appropriate springs)
• stock length pushrods - did you actually check the pushrod length or just toss in stock length pushrods?
• Gen IV style truck intake
• 92mm throttle body
• long tube headers

I'm not a cam guy and I don't know the specs on yours (nor do I care as it isn't relevant). Having changed the cam AND milling the heads sounds like a recipe for problems if the pushrod length hasn't been verified. My first check would be the pushrods and make sure they are right for your combo.
Next I think a dyno tune or at a minimum street tune could iron out some of your issues if and only if the mechanical bits are good.

 

WHOA. I might have mis-diagnosed... valves are being held open a weee bit..

 

I did check the length on the pushrods with a pushrod checker but my math/method may be off. I will def not drive it until I check them again this weekend. Also could I check this with a compression tester to see low compression or leaking compression on say cylinder number 1

 

Hate to be obvious but did you check for vacuum leaks?

 

I have checked the intake mating surfaces and any place there is a vacuum port. I am leaning towards the pushrods being too long and holding the valves open more so than the timing being off bc the truck is very easy to start, just bump the key and its on and running. my experience with Gen I small blocks when the timing is off you will get alot of cranking and back firing before it starts, if it starts. Im going to compression test the cylinders this weekend and then pull the valve covers and check push rod length with my checker.

 

Pop N Wood, that is a good thing to check but the only reason I wasn't thinking vacuum leak is the OP said the idle was in the 450 range.
Typically a vacuum leak on a fuel injected engine will do the opposite of an engine with a carburetor. The fuel injected engine will increase in RPM as the PCM adds fuel to compensate for what it sees as a lean condition.

Hopefully it's something simple.