Not another problem :(

I'm so close to selling my caprice, but i'd hate too, i just love the car, when it doesn't have these days.

So, my caprice has obtained a new problem. It knocks ONCE when i start the car, and never does it again.

It only does it with the official PCM, and not my home-made "intelligent" PCM.

What can be causing this? The knock sensors and module are within spec.

(the only reason I'd want to use the official PCM, is incase mine breaks, i can have a fully working backup. my PCM has given me that "edge" i wanted, while the official PCM still has problems).

I even got a new PCM that is official to the car, same problem. My PCM is the only one that won't knock when I start the car.

By 'knock', do you mean a detonation-style ping when the engine fires? This is normal in MOST engines, even in my car. Even in my old '72 Impala of 20 years ago.

By 'knock', do you mean a detonation-style ping when the engine fires? This is normal in MOST engines, even in my car. Even in my old '72 Impala of 20 years ago.
Yes, it's always a single "knock", and it's that ping sound you hear on TV shows when someone runs out of gas somewhere...

I could have sworn mine never did that before. And it doesn't do it with my home-built PCM (only thing I can think of, is that my PCM doesn't use a knock module like real PCM's, mines all ADC based and uses audio filters via software)

I asked a mechanic about this about a year back because I always heard on newer (and some older) cars. He said it's because when you first start a car everything hasn't completely "booted up" including the knock sensor. That one ping is natural because when the engine first starts up, at that first second things are set to "default" and that includes a higher than normal ignition timing. The combination of that and the rich mixture in the cylinders to fire up the engine at that exact moment often causes a knock. Perfectly 100% normal. Nothing to worry about.

P.S. Trust me.... things will get better with this car for you. The first year I drove my Caprice I called my car's "Yeah of Hell." Things progressed in this order from when I first got it on the road: FALL: Doesn't pass inspection because an emission hose was detached, exhaust needed replacement, rear brake line goes, front brake line goes, other rear brake line goes, carburetor needing rebuild, ignition coil goes, washer fluid unit goes... WINTER TIME: Lower radiator hose freezes = no heat on sub-zero day, my door freezes OPEN and my passenger held it shut with a rope tied to the handle just to get to school, someone smashes the front fiberglass piece in the parking lot because they "couldn't see it", my battery dies, chip in my winshield spreads to a crack across the whole thing, gas line freezes killing the engine... SPRING: lower radiator hose springs a pinhole leak, rear brake cylinders needed replacement, radiator goes. That summer went completely problem free as did the following years up 'till this point, aside from very minor stuff. Since I gave that car the care it needed for that year, the rest up till now it's repayed me taking me everywhere reliably and problem free. To school, to work, to the beach, to the indian reservations, and even on a few camping and road trips, through the adarondac mountains.

Currently it's out of commission waiting for an engine transplant at my choice, but that ol' 305, even though it wasn't treated the best, could have kept on going. After an autopsy on the engine it turns out the only thing wrong with it was a few worn cam lobes.

Unless you're that truly overwhelmed, keep at it, and your car will repay you.

Well I guess that would explain how my PCM don't have this issue.

My PCM follows the "use last good timing" system. Meaning, the last time the car was running, it follows the timing. And while driving, it will occasionally change the timing for better performance if it can without knocking.