P0304 Code (out of ideas)

[mike_245]

My 2001 SL SOHC (177,000 miles) has a P0304 setting at idle and idles rough. Normal driving I don't notice any issues. I have read many posts and tried nearly everything. Here is what has been done:

Front O2 changed (already had this code set from before)
Plugs changed from bosch to AC gapped to .40
Wires changed
Coil packs changed
Ignition module swapped from my brothers Saturn
PCV changed
Coolant temp sensor and connector changed (was cracked and connector corroded)
TPS swapped from spare 1.9L
IAC swapped from spare 1.9L
Compression checked wet #1 200, #2 180, #3 200, #4 180
Injectors removed and cleaned (checked via 9v battery on switch) (swapped #1 injector with #4 injector) ( 1 time I did get an P0301 after this, but now consistantly get only P0304)
EGR changed

Items noted:

Carb cleaner used to find leak, but no impact (noted that engine nearly stalled rather than increase when sprayed into throttle body - running too rich already). Terrible fuel smell and moisture from exhaust at idle.

Pull #4 plug wire and no impact to engine idle, also would match with P0304 and unclean burn on plug. Pulled all other plug wires and engine nearly stalled. Same applies for pulling injector wires...although I pulled #1 injector and didn't notice much difference either.

Can hear a mechanical clicking noise (4 or 5 clicks in succession) and then nothing for 8 or so more counts...can't seem to locate source using a screwdriver on engine, injectors, EGR, exhaust, valve cover, etc. Seems to come from area near firewall. No difference engine hot or cold.

I haven't pulled the valve cover to see if cam lobe is warn on #4 exhaust yet, but that seems like it would also affect normal driving.

Any ideas? I just picked up a low mileage 1.9 that I could swap, but I was hoping to get at least 200K with this one. I have never had any issues and it still runs strong as when it was new....except for the idle issue!

[rc1488]

P0304...usually its a p0301.

Anyway, you have a '01 SOHC with a rough idle. My guess is your intake manifold gasket needs to be replaced. Spray some brake cleaner near cylinder #4 and #1 around the intake manifold gasket for confirmation. If the idle changes, then you know what is wrong.

[BarnOwl]

rc1488 is right that the typical intake gasket leak on a third gen SOHC is on cylinder one. There's a fairly rare intake gasket leak that can leak coolant into cylinder 4 and cause a miss. However, I don't think that would stop cylinder 4 from banging at all.

Try swapping injectors with another cylinder to see if the miss moves.

[mike_245]

I tried that with carb cleaner and ether, but didn't notice any change. I will try with brake cleaner.

I also swapped 1 and 4 injectors with no change...well I think I did...I need to verify I didn't put them back in the same place. I was going to swap the new fuel rail/injectors from the new motor for final confirmation. I also was going to see if the tech at work could hook up a multi channel O-scope to check the PWM is the same for all cylinders. It sounds the same using a screwdriver to the injector, but that is subjective.

The coolant leak concept may make sense...I have notice some coolant loss, but didn't think too much with it. I didn't really want to change the intake gasket without confirmation.

Any possiblity that fuel pressure is too high? I don't have a pressure gauge, but was thinking about getting one.

The other idea I has was that the computer is not working correctly during idle (open loop operation).

I acts very much like a simple injector or ignition problem - but the plug is firing and the injector seems to be firing...I just don't know if it is at the right time or not.

Correction to my post...the compression numbers were dry not wet...I did the wet test on 4 and it only increased by 5 psi.

[BarnOwl]

You can also try swapping the coils and see if the miss moves.

[mike_245]

Both coils are brand new but on Saturday just for second check I took the coils, ignition module and wires from my brothers SL1 and still had same problems)

This thing has got me stumped.