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Don't know if they are legal or not, but I do know that they will cause your valve stems to leak. The extra weight of the cap will cause the valve stem to be pushed to one side when the tire rotates at highway speeds. This will put undue stress on the valve stem and eventually cause it to start leaking. Plus, someone will probably steal them anyway. Been there and done that myself. Don't install them.
egr valve bad.
the seat on the exhaust valve is dirty so the valve does not seal it.... not sure how to clean it
I had one do that and it was caused by a bad intake valve.
There are several things that can cause this effect. Ignition timing out of set or worn ignition parts being a large factor. Improperly set or worn valve train being another.
Depending on the vehicle, it most likely needs adjustment.
try cleaning the air idle control valve
Valve timing off, timing chain jump.
If its a 4 stroke it sounds like a burnt valve or head gasket, if a 2 stroke a bad pack,coil or keyway can cause this,...
timing problem or crossed plug wire or even possibly a burnt valve.
If any car backfires it is usually due to retarded timing, running lean or a problem in the valve train.
Follow the black hose from the top of the valve cover, over the top of the engine, then towards the drivers side to the front of the engine. Is is inline with the hose. Pretty easy as far as PCV valves go. See question about losing oil at highway speeds.