Have P0304, replaced wires & went away. Came back 2 days later P0300 , P0304. Also piston slap started this winter w/ 143k. So after warmup, now since codes, it sounds like a its hammering but i believe its detonation. Man these 2.0's are loud. so i pull plug #4, its fine but noticed every wire on every injector is cracked between connector and old fused [brittle] taping of harness. Car runs good, but with poor gas mileage. So this should be the cause of codes, correct? What can be done for the wire problem? Is it time too junk car? Oh the car spent 10 years basking in the heat of the Arizona desert. And yes " IT'S A DRY HEAT "

 

Lots of unknowns.

Did the detonation problem come about after starting & running it after the 10 year heat soak in the desert? Did you just pour gas in & start it up, or did you clean the tank out & put new fuel in with a good fuel system cleaner, like Techron Concentrate Plus at the 1oz/gal treat rate, so to begin a tidy up of the fuel delivery system & combustion chambers?

Have you measured fuel delivery rate over time at the fuel rail, so you know the volume is there, such that the fuel injectors aren't lacking for fuel pressure & volume & giving a wimpy squirt/lean mixture?

Are the spark plugs the correct design, heat range & torqued in to spec?
Is the cooling system & coolant in good condition?
Is the air filter, air box, inlet air tubing clear & clean, no pack rat, or other varmint nests in there restricting air flow?
Has the PCV valve been replaced & it's rubber plumbing still in good tight fitting condition/no air leaks?

If all that is ok & you suspect combustion chamber deposits might be causing detonation, maybe consider trying running the engine to warm it up a little, remove the spark plugs, add a table spoon or two of Marvel Oil in each cylinder, screw the plugs in a little to keep debris out & let it sit at least over night to soak in good. This can also free up sticky compression, or oil control rings, to help them make a better seal.
Then remove the plugs, crank the engine to remove any excess Marvel Oil, so you don't hydro-lock the engine & bend something up. Then reinstall the pugs, torque to spec, start it up, warm the engine & go for a spirited highway drive with rpm run up to say 35-3600 for a mile or two. Do daily rpm run-ups during the treated tank & run most of the treated tank out & see how it goes. If things are really fouled up with deposits, perform another Techron treatment, with the daily rpm run ups.
Then change the oil & filter to remove any loosened carbon deposits or residual Marvel Oil, or Techron that might have made their way to the crank case & diluted the oil.
Use a Top Tier fuel to keep the fuel delivery system & engine intake side tidy & let us know how the Spring Cleaning goes!!!!

 

Its been running fine till this past winter in TN. Piston slap, which did get less after 1 min. Out of the blue P0304 appeared, changed wires cel light went off then came back on a day or two later. Checked Plug 4and it looked fine. Checked compression and found cyl 1=160, 2=160, 3=85, and 4=90. So i guess motors on its way out. Still driving it and it runs good, except that knocking under load. Though it seems when you can find the right spot with gas, it quiets down a lot. The knock only started after codes showed up, but I guess the compression tells the story. Right?

 

With the noise & compression numbers on 3 & 4, sounds like you might have a head gasket problem between the two cylinders. Maybe try a cylinder leak down test on 3 & 4.
Remove the radiator cap with the engine at cold idle & look/smell for exhaust bubbles in the coolant.
Look for chocolate milkshake color deposits on the engine oil dipstick.
Has the engine temp spiked or run hotter than usual lately?

 

When checking for bubbles under radiator cap, how fast do they happen, how big and i guess they would appear on the high side spot of the fluid level or do they appear random areas? Had paid mechanic to do compression check , he suggested too unload it. Didn't mention possible head gasket leak, said probably a stuck valve. If I touch # 4 injector connector the cel flashes, touch it again cel stops flashing.

 

Good find & feedback on the fuel injector connector response discovery. Disconnect it & inspect for wiring, pin, socket damage, corrosion, bent pins, spread sockets, cut or loose wiring crimp at the socket, or damage to the injector electrical connector side. Make sure the electrical connector is properly connected/snap locked into position,

On the radiator exhaust bubbles question, what you might see would depend on how bad the head gasket might be leaking into the coolant. If the engine isn't overheating, the leak might be small, so not many or large bubbles. In that case a sniff test for exhaust fumes with the radiator cap removed might be telling.

Get the fuel injector electrical connector finding put right first & let us know how it goes.

 

How to remove injector connector? Can't figure out and don't wanna break them, seeing that the wires entering connectors are cracked and /or exposed.

 

I don't rightly know, as I've never had to remove mine. Maybe a member following this thread that knows will answer up. EDIT: Might be helpful to know what year vehicle & engine we're talking about.

 

Was able to get connectors off without breaking them. Also carefully cut back some harness tape without breaking exposed, oxidized wires and put some vinyl tape around the bare spots. measured resistance of injectors , all the same. Still have the miss and hammer sound under load. From reading posts, is it possible a small breach in head gasket though i ain't seen no bubbles yet nor milky oil, would cause computer to make adjustments which would throw a P0300/ P0304 ?