I have a 1996 Toyota Corolla, with 184k miles on it . A few weeks ago, it started idling rough. It idles rough in drive , reverse, and park, but not in neutral. It is slow to accelerate but once it gets going it’s good until it gets to around 50mph then I start to hear a loud noise under the hood, like the sound of the bass from a car stereo. Also I smell fumes ( like something is burning) from the hood After continuous driving of it or when I turn the car off… The check engine light has been on ever since and it blinks when I step on the gas… and The code that came up was P0303.
I changed the spark plugs and fuel injectors a few days ago and the car ran good but now the problem is back with all the same symptoms and the same code is showing up.
I am lost for answers… I was thinking maybe the coil or a vaccum leak but irdk at this point
Thanks in advanced!!
That code means cylinder 3 misfire. Did that plug look different from the others? If you swap that coil with another does the trouble move to the other cylinder?
Hey , the plug had some oil I believe it is… on it and I’m not sure how it got there… so there is possibly oil leaking into the third cylinder some way…
I think it’s time for a compression test.
The engine needs a new valve cover gasket set.
https://www.rockauto.com/en/moreinfo.php?pk=10571324&cc=1275612&jsn=427&jsn=427
The round seals are for the spark plug tubes or wells in the valve cover.
When these leak, oil pools in the tube/well and creates a poor connection between the plug wire and plug, which causes a misfire.
Tester
Thank you! I will try this asap and let you know the results 
Single cylinder misfires are diagnosed by swapping components between cylinders to see if the misfire follows the swap or not. Spark plugs, spark plug wires, coils (if applicable), fuel injectors are the most common things to swap. So if the above ideas don’t pan out and absent any other clues that’s where to start. Internal engine problems can cause this too, such as a sticking/leaking valve, poor compression, etc. The common advice here is to not drive the car at all when the CEL is flashing as it can damage the cat.