1999 Suburban P0300 Nightmare - Please Help?

jstrong71 · August 24, 2014

I broke the distributor gears on my Burb and replaced the Distributor at 289,000 miles. Ran smooth and better than when I bought it used with 250,000 miles. I parked it on a Thursday and went to drive it the following Tuesday and it was chugging and misfiring like crazy. I started looking into what could be causing this since it hadn't thrown a code yet. I read that there are intake issues and so I looked for vacuum leaks. The following is what has been performed thus far with no change in the engine.

Checked plugs and wires which I replaced last year and they were all good still with no heavy deposits and coil was getting spark to each wire.

Checked fuel pressure when off - 160psi and bled down to 155psi and held.

Replaced Fuel Filter.

Checked fuel pressure when running - 150psi at idle and jumps to 160psi at over 1000rpm

All vacuum lines in tact and in good shape.

Replaced MAF and cold air sensor.

Inspected EGR and it looked okay but gave it a good cleaning anyway.

Replaced Throttle Positioning Sensor.

Inspected and cleaned IAC.

Then the fun began...Changed upper and lower intake gaskets and upgraded from CSFI to MPFI.

Fired right up then began to run shitty again right away. I checked and checked again to make sure that I had TDC and the Distributor was in the correct orientation on #1. Thought that this has to be a timing issue and threw a timing light on just to see, it appeared to be severely retarded. I took it to a shop and had them hook to a scan tool and it was -27 degrees. They tried adjusting then checked to make sure that it was TDC on #1 and verified. They had no idea but suggested that I start with a compression check, which I did. Values as follows:

CYLINDER DRY WET 1 190 220 2 182 235 3 190 240 4 180 225 5 182 242 6 180 225 7 180 220 8 180 225

I'm completely stumped. I'm going to unplug EGR and see if that makes a difference. If no difference then gonna drop the exhaust and see if maybe I have a CAT that's backed up but wouldn't that throw a code? What the hell at this point, 6 bolts ain't nothing and worth a try. I have a new EGR and a new Crankshaft Positioning Sensor to try if I don't get anywhere with the EGR and CATS.

ANY AND ALL IDEAS PLEASE REPLY. THANKS!

jstrong71 · August 25, 2014

Cleaned all Spark Plugs with wire brush and re-installed. Replaced Crankshaft Sensor and EGR valve. Still misfiring.The old ones looked like original equipment. Switched out Ignition Module and Coil with known good working newer ones and tested, sitll no change so I swapped back original equipment. I decided against dropping the exhaust as it seems that there isn't a restriction and plenty of exhaust flow when engine is rev'd above 3000 to 6000 RPM. Going to tear the front timing cover off tomorrow and see how the chain and gears look. Outside of that the only thing that I can think of is trying a Crankshaft re-learn at the auto shop again with the scan tool. The first time the engine was so erratic that he couldn't do it. The misfire seems to be consistent now.