2004 Northstar Caddy Deville 90K miles
Scotty, Have my neighbors' 2004 Northstar and have a pesky P0300 random Misfire. We've done a ton ( a lot by weight) of diagnostics (electrical/fuel/mechanical) and yes DID throw some parts here and there--the obvious ones--and am still getting the code and the MIL light remains on at idle and even with no load, revving to 2500rpm for 5-minutes. However, engine started smoking from below as it began leaking oil (never saw this before). Also, driving the vehicle (under load), with even moderate acceleration will make MIL flash. If I drive super conservatively (slowly) the light stays on and doesn't flash until I hit 19mph--every time. Drop to 18, light stops flashing. Brake torque: Flashing MIL.
Do a compression test and a dye test on the cooling system to confirm your bad head gasket theory.
First, what is the fuel pressure supposed to be? Sounds like you should at least run some injector cleaner through the system and if that doesn't, help, replace the injectors with new. They are 18 years old now.
Thank you. As there are no bubbles in the pressurized expansion tank and antifreeze combustion contamination test evidence was negative (Bought test), I moved on to fuel pressure.
Checked fuel pressure: Key on, low at 36psi. Within 5-minutes dropped to 32. Started, at idle 38. Increased idle, no real change but pressure did blip a bit with throttle blip. Key off, 30psi. Changed filter--tried to blow through and it was clogged. Put on new filter. Reestablished fuel pressure and started. Pressure psi remains the same. Car still running rough. Will rerun codes. Do you think that if a filter is that clogged that other elements of the fuel system are compromised as well? Perhaps the injectors, as my Blue Driver Live Data did show 4 injectors at "fault." Also pointed to MAF, which I had cleaned. A cheap code reader had shown Open Loop faults on both banks.
Oil leak turned out to be valve cover and it is not leaking into spark plug tube.
it would overwork the fuel pump, which it sounds like needs replacing.
I have seen a wide range of pressure specs--so I don't know what it is supposed to be, but key on 30 seems low. I filled tank and added Techron and let it idle for an hour.
Will go injector route but they too can be expensive although I saw a set for 100, just hope there not junk. BTW, all flash with noid, but I think a couple are stuck open.