1999 chevy 350 runs rough at idle. #4cyl misfire. light gray build up on spark plug. compression good 180.

P0304 is a diagnostic trouble code (DTC) for a misfire on cylinder 4. This could be caused by a variety of issues, including a faulty spark plug or oxygen sensor. However, a rough idle and a misfire could also be caused by other issues such as a vacuum leak, low fuel pressure, or a malfunctioning fuel injector. It's best to have a professional diagnose the specific cause of the issue.

Have the engine computer scanned for trouble codes. Misfires are from loss of fuel, spark, or compression. Check the fuel pressure, check that all spark plugs are getting spark, and if all good so far, test engine compression. Low compression in a cylinder will cause misfires and loss of power.

It could also be a vacuum leak- a loose or cracked vacuum hose or an intake gasket leak.

Low compression does not require head removal. Do a compression test on a cylinder, then squirt some oil in the spark plug hole. Do the compression test on that cylinder again. If the compression goes up then the rings are worn. P0303 is a misfire in cylinder 3. This lets unburnt gas into the catalytic convertor. The cat can overheat, melt and be ruined. If the check engine light is blinking don't drive the car. A misfire is either no gas, no spark or low compression. The quivering light can be from idle or failing alternator. Put the probs of a multimeter on the battery posts. Set the meter to volts dc. Start the car the voltage should be between 13 and 14.6 volts. Anything more or less points to an alternator problem.

You have an interference motor. If you had to replace the heads due to the valves possibly touching the motor, you have to consider rechecking the compression after you replaced the heads. I had the same motor that had an intake leak. I found frozen rings on the number 1 piston as well as a bent pushrod. Ultimately, the camshaft had broken due to coolant getting to the bearings. If the spark is good, the fuel is good, then the only other problem you'd have would be compression.

Make sure plugs are gapped correctly and coil pack is good . If can't feel it missing could be a false code generated by computer . Hope this helps

is your car mostley electercal get a new alternater

check plugs -plug gap-vacuum leak-fuel mix-fuel pressure-timing-pcv valve-egr valve-air filter-oil sludge build up-problem in heads[valves adjustment-burnt-weak springs if only changed wires do a complete tune-up and do a compression check

Perform a valve clearance inspection and adjust if necessary. Tight or sticking valves will cause a misfire or rough running concern. Note: Perform valve adjustment when the engine is stone cold. 4cyl: INT 0.26mm EXH: 0.30mm (V6 engine are hydraulic and no adjust necessary.) Also check for a sticking or stuck open EGR valve. For diagnostic purposes only: Remove the EGR valve. Place a business card over the EGR passage and bolt the EGR valve back on. This plugs the EGR ports and if the misfire goes away the EGR valve is partially stuck open and is defective.
Check for low fuel pressure. Check for low compression. Check cam timing. Check for a weak ignition coil. Also don't over look the possibility of worn or broken engine mounts which cause excessive engine movement and engine vibration. -hope this helps

have an 02 misfiring on no7 cyl was catalytic converter

Spark plugs/wires might need to be replaced. Check your spark plug gaps and check your spark plug module harness connection on both sides of the engine. The check engine blinking indicates short/misfiring. If there is short in the harness, you can check it by moving around the wires close to connector one by one while the engine is running. You might need to remove and ckean the Throttle Body.