ok heres the story my sisters magnum with the v8 hemi is giving code p0300-misfire on multiple cylinders the car will not stay on starts then dies. if i keep my foot on the gas it will stay on but run ruff if i let go it dies.please help

I already changed the spark plugs but same issue

 

What did she put in the fuel tank .. Diesel? Any smoke out the tail pipe?

 

no she put gas in it. it didnt happen till a few days after she filled the tank. and if I hold my foot on the gas there is smoke from the tail pipe

 

ok Update removed driver valve cover and found a broken spring here are some pics



Its the bottom left spring in this picture



Now the question is how hard is it to change? I've changed them before on my honda but this looks different any advice will be great

 

There easy enough to change. A good enough air supply to keep the valve up and block the oil returns at the bottom of the head so you don't lose a keeper.