Brand new member here!

My 1999 CL was not heating up, so I replaced the thermostat. Now it heats up fine, but it started running rough the next day and popped multiple codes (P0300, P0302, P0305, P1399). I do have 360,000 miles (hooray!) but I can't help believe that something I did caused the problem. Is it possible a poor connection on one of the wiring harness/ connectors could cause this problem? I had to disconnect 3 or 4 and I did spill a little coolant.

 

I checked my fuel pressure and it was at 48 psi, but did not drop when I reconnected my Pressure Reg vacuum hose. Took off the top of the intake manifold to get at the Press Reg, and noticed oil in the manifold. Took off the distributor cap and noticed oil in there too! But, I previously checked the compression and all cylinders were 170-185 psi and no leak down! Could it be as simple as a PCV Valve?

 

Oil in the intake manifold is fairly common and not a huge concern. Oil in the distributor cap probably need to replace a seal. 48 psi for fuel pressure is pretty high. you could have a bad fuel pressure regulator, that's probably what I would replace first. but still, you have misfire codes, so I would inspect spark plugs closely, and check your EGR valve and passages for clogging. If EGR is stuck open, it will cause engine to run rough or even stall a lot of times.