P0303 check engine light when cold.

Update: after sitting overnight, the gauge still showed just above 0.7 right where I left it last night with ambient temp still showing 19degF. I pumped it up to 0.9 and now over an hour later it's still at 0.9 bar so I'll call this a win. I'd usually pull the spark plugs and pressurize each cylinder while the cooling system pressure tester is in place, to see if 100psi in the cylinder pushes any pressure into the cooling system...but it's darn cold in the garage and the door won't close with the truck in there so not going to happen.

No solid conclusion as to what screwed the O2 sensor but I'm finding info that certain chemicals can contaminate the sensor's reference element and cause the symptoms I saw.

If the new sensor fails I'll resume troubleshooting. The truck runs the best it has since I bought it at 134K miles a few years ago, no telling how long that Coolant Temp Sensor had been failed but now that it's replaced the engine is happy.

Thanks again everybody for your help!

 

Huge help and it's greatly appreciated. I can't imagine spending all day turning wrenches, then coming home and telling other people how to turn wrenches...but I'm really glad you did!

 

Update: misfire returned, long term fuel trims again negative 12% or lower at idle, took it to another dealer this morning. Cyl 3 has 80psi compression, cyl 4 has 110psi, 1 & 2 are fine at 185+.

They pulled the valve cover (remarkably clean under there for 199K miles, yay synthetic oil) and valve lash is well within spec across the board. The mech scoped the bore on 3 & 4 and saw no scratches or damage, thinks stuck compression rings are likely.

Orders are to put it back together at this point...I'm not ready to pop for an engine teardown. Will try aerosol top engine cleaner (GM or Dodge) soak via the spark plug hole this weekend to see if a stuck ring might loosen up.

 

Update: it goes into the garage today. Leakdown tester showed cylinder 3 and 4 could only hold 20 and 24psi, respectively. Big hissing noise at the exhaust manifold, so burned exhaust valves are the likely culprit.

Next troubleshoot will begin with a compression test...I threw over $300 worth of parts at this between the injectors, plugs, and coolant temp sensor. Apart from a rough idle and vibration going up hills, it runs well despite the valve sealing problems.

 

$300 isnt too bad, luckily once you have it figured out then you know everything else will be brand new

 

Update: both exhaust valves burned on cylinders 3 & 4. Head gasket was fine. Head is at the machine shop getting new seats as they're scored beyond re-grind, I popped for new valves as well. Should have it back Thurs or Fri.

 

Update: head is also cracked. Rebuilt head is on the way, old one goes back as a core and I'll stop by the shop on Monday to take a look at it.

 

I just picked up a 97 with a 3.4 and it's doing the same thing; when it's cold it misfires and runs poorly for the first couple seconds, then smooths out and runs fine. I also have the P0300, P0303 and P0304 codes popping up.

 

I guess I'll provide closure on my case...the rebuilt cylinder head fixed it. I inspected my factory head and it had small cracks radiating outward from the spark plug holes on cyls 3 & 4, plus cracks between the exhaust valves on 3. The exh valves on 3 & 4 were visibly burned.