2004 Cadillac Deville 4.6 Northstar
90,000 miles
Recently had all coil overs all changed. Ran well for a few months, then it began misfiring.

I am assisting the car'* owner, as he has fallen ill. I have spent many hours and performed many diagnostic checks and initiated seemingly justified solutions, but the code still remains. Dash light on and has flashed but at idle it does not flash. Originally, I had an evap code as well as the random misfire, so I replaced the EGR and replaced a leaking gasket at the secondary air injection valve. Cleaned throttle body and MAF but did not test either. Although yesterday, I disconnected the MAF and car died. Emissions code went away, however random misfire remained. Got a coil over paddle tester and it was not conclusive, but I assume new CO'* are okay(?) I checked the plugs, they were beat up, so I replaced. Checked injectors with screwdriver, stethoscope, and noid light and all appear to be working on the electric side. Will pull all plugs and inspect to see if there are any wet with fuel or antifreeze(?) When I start it cold it runs very roughly and then calms down within 30-seconds, but an intermittent misfire can still be felt/heard. There is no discernable bad cat odor. Battery needs replacing but for now have a trickle charger and have used jumper cables. It starts but has low voltage. Alternator 14.4. Did a smoke test and found a leak at PVC connection at plenum. Fixed, and replaced PVC. Misfire code remained; no other apparent vacuum leaks (also sprayed carb cleaner).
Did not check fuel pressure but bought a new filter but have yet to replace. Considering purchasing a fuel pressure tester.
My cheapy Ancel OBD2 scanner does not show misfire by cylinder nor fuel trim but its live date indicates OL Fuel Fault on both banks. Have not checked O2 sensors. Considering a better scanner--but unsure on which one ($). Would an auto parts joint scan show a misfire count by cylinder and fuel trim?
An ex-Cadillac mechanic came by yesterday and phoned a current Caddy Master Tech. Not sure how but they believe the head gasket(*) are faulty. Today I will check oil (can you detect antifreeze by smelling the oil dipstick?) and antifreeze level and do you believe I should get an antifreeze color diagnostic tool?

ShitStar engines, I mean NorthStar engines, are KNOWN for head gaskets. Typically, if the cylinders are sucking coolant, you will get the P0300.

Oil, if the underside of the oil cap appears milky brown. You have coolant mixing with the oil Lets just hope its only going in the cylinders. Otherwise, rebuild is in order.

Thank you Mike. I will investigate asap. Incidentally, I agree with your assessment of the engine'* lack of prowess.

It'* a good engine when it'* good. I guess that'* true of many though. Northstars are definitely more delicate than most others though.

I agree on the head gaskets though . . . but they got a bit better about that in later years.

What codes are stored in the PCM right now?

What is the fuel pressure?

Thank you CC. The persistent code is P0300, random misfire. I will check oil momentarily, but do you believe in those color changing antifreeze testers?
I have not checked fuel pressure and am set to buy a gauge on Amazon.

I'd check fuel pressure first.

Just because they're prone to head gasket failures doesn't mean yours had one . . . unless there are other symptoms that I am not aware of.

A flashing check engine light indicates misfires. No use barking up the head gasket tree if the misfire tree is indicated.

Thanks again. I did check this that and the other thing in 10° weather. Pulled plugs and one was wet with perhaps antifreeze and didn't smell of fuel. Had checked all CO'* for spark and they all passed. Had a considerable amount of condensation below exhaust pipes as well as a moderate amount of white exhaust but due to weather both seemed inconclusive. I am not a registered antifreeze feel and taste expert but the exhaust fluid seemed like AF. Checked reservoir and it was low. Marked and filled
Checked oil and no milkshake apparent.
I added Techron and filled tank (further in the hole $$$). And as car idled, MIL is steady, car is rough, and once I accellerated light began to flash. Hmm. Remember, I still have open loop codes? O2'*?
So I pulled off CO'* pulled and checked plugs, only one wet. Hooked up a spark check and they seemed robust(?)
Ordered antifreeze contamination test. Will rent fuel pressure gauge tomorrow.
Since I lost my Blue Driver I ordered another one but I wish I had a full function vehicle-specific bi-directional scanner but I've gone goofy try to figure out which one, plus 300-1200? Again, no revenue on this hell-in-a-handbasket pro bono job.
And then I watched 3-hours of various vids. Confusion.
Then, I contacted an automotive instructor and his answer was quite indepth, however has preaching to the choir. He teaches ASE course so shouldn't he be the Gold Standard?
Brake torqued it ... flashing MIL.
Fuel pressure tomorrow.
So, beginning stage of a failing head gasket? Why else a wet plug?
P0300 remains.
Thank you

In 10 degrees, you'll see condensation from the exhaust pipe most of the time. Drips coming out happen if exhaust flow rate is low, like at idle.

It could be that the suspect cylinder has low compression due to a valve or ring issue. I recommend checking easier things first.

After fuel pressure, next I'd check for spark on the suspect plug.

I bought the Blue Driver and will rent fuel pressure gauge today. On suspected cylinder it appeared to be wet with antifreeze(?) Oil is not milky. I topped off antifreeze and marked to see if it is gettIng sucked out.
Ordered color test as well. Going further in debt but he is a good fella. Battery is sulfates so I am trying to resuscitate. Otherwise, another $125.
If head gasket(*) is/are compromised I can do it because dealer wanted $5K+!!!
My ill friend (car owner) came over so I could explain car and his hand was bleeding. He is on thinners so I redressed his wound to stem the bleeding.
Are you by chance into tractors/implements?
As far as easier things to repair, I just got a vibratory compactor as I help out with gravel road leveling/compacting down at my property. Too much to do, too little time.
https://y6s4i8f2.rocketcdn.me/wp-con...R1570-Main.jpg

If the cylinder is getting wet from coolant, this will not always cause the oil to get milky. It just depends on where the leak in the gasket is.

Dealer wants 5K because they drop the entire engine/trans/cradle assembly to do the work.(I guess its next to near impossible to do while its still in) There are newer style ARP headbolts that apparently correct this issue with these engines. You will need to Google this though.

if you really want to eliminate the ignition/fuel system for the P0300, drain all the coolant out while the engine is cold, start it up, and run the RPM high for about 10 seconds. Then shut it off. If the MIL isn't flashing, you are not misfiring anymore because the cylinders aren't sucking coolant.