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2000 Impala LS 3.8L V6 P0300 Multiple Random Misfired has me Baffled

I have a 2000 Impala with a 3800 V6 series 2 with 165,000 miles. The only code I had was a P0300 Multiple random misfire that occured off and on at first, Worse in the city Better on Highway. It would start running on half the cylinders and bogged down, with the check engine light flashing, after getting onto the highway it would straighten itself out at first. I thought i had water in the gas so i ran some heat through it. straightened out for a day then happened again so I thought maybe a should run some premium gas in it, The car ran like total crap until the premium was gone, so i thought maybe it’s running too rich, I put 87octane in it and it ran great for a day or two, then it gradually got worse to where it would not straighten up on the highway, tried to die at stoplights and i had to drive it with 2 feet to get it home and park it. I’ve replaced the battery, spark plugs, wires, ICM checked good, coil packs are good, replaced fuel Pressure regulator, looked for arcing on the spark plug heat shields and it will not start. It turns over but doesn’t try to ignite and when I get the slightest rumble of ignition, it backfires up through the manifold and I see a puff of gas or light smoke by the exhaust manifold.

The car had a vacuum leak around the intake manifold cover gasket and a leak on the fuel rail. I pulled the fuel rail off my impala, snugged up the rest of the intake bolts then disassembled the fuel rail, flushed it out and cleaned the injectors the best i could, then reassembled it. before i re-installed the fuel rail I hooked it up to the fuel lines outside of the engine and tried to start the vehicle. all injectors had a nice spray pattern. So i installed the rail and it still didn’t start but it had more combustion than before although it’s still backfiring like before. Checked the fuel pressure by turning the key on and the fuel pressure went to zero very fast. I blew the rail out with air so to make sure i didn’t damage the regulator I replaced it with the old one, ran the test again with same results, so i reinstalled the new regulator. I performed the fuel pressure test a few more times while moving the fuel hoses and when i moved the back supply line the vacuum loss stopped then dropped to zero when i let off of it. So I replaced the fuel line connector and O-rings on the injectors. It no longer has a vacuum leak.

I cleaned the EGR Valve out and had no luck there, then I removed the Idle Air Control Valve-pushed the pintle in, re-ionstalled it and WooHoo! it started! Although it still ran like crap with the same random misfire I was still happy to hear it start.
I pulled the plugs on it to do a compression test and all 6 cylinders have between 180-190psi so the engine is in good condition mechanically. I installed new plugs before getting the car restarted and when i removed them for the test they were covered in thick black soot after running for roughly 15 minutes max.

I have (2) 2000 Chevy Impala’s so I’ve swpped sensors from my wifes car to see if it would fix it. I’ve tested the following MAP, MAF, IAC, coil packs and ECM. None of these made the car change. I believe it is a timing chain because the car has never had a new timing chain and is way overdue, but I haven’t had a code for a crankshaft position sensor. Can the timing chain be bad without throwing a code? It’s not out of time enough to change the valve timing because I had a good compression test but its not firing right and still has the multiple random misfire. I have not checked or received a trouble code for o2 sensors, crankshaft position sensor or cam sensor. Something has the timing retarded and is dumping extra fuel into the engine because my new plugs looked like they were in the engine for a year. If anyone knows the answer to this it would be greatly appreciated. I’ve had the car down for 6 months and need to get it going again. I’m not taking it to a shop until I’ve exhausted all possibilities that I can test on my own. Please help if you’ve seen this before. Thanks
Also, could it be the PCM Box that is located in the air breather that’s giving bad information?

I think you are going to have to pull the intake manifold off and replace all the gaskets, not just snug down the bolts. This engine has history with intake manifold gaskets and occasionally with a cracked intake manifold itself.

Right, I’ll have to replace the gaskets but for now there is no vacuum leak. I have stopped trying to start it because it backfires up through the intake hard enough to blow a vacuum line off and I don’t want it to crack the intake. I need to find out why it is running out of time and running so rich.

A random misfire code can often point to a faulty crankshaft position sensor.
By stating the new plugs looked like they had been in for a year do this mean they were sooty black on the tips?

(The compression is fine and it’s very doubtful the timing chain is the cause of this problem. Knowing what, if any, codes are present would help greatly.)

The plugs have thick soot around the ceramic, I can wipe it off and the plug would be like new again. The plugs have no deteriation, just soot from running rich. I’ve only had the very vague P0300 Multiple random misfire and after fixing the vacuum leaks i had a MAP Sensor High input value which I think popped up because of the better vacuum. I cleared it and it hasn’t returned. I also swapped the MAP Sensor from my wife’s impala that does run good and it had no effect. I’m starting to think that it could be the crank Position sensor but I’ve had no code for it. I’ve heard of them being bad without tripping a code.