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An engine might sound like a diesel at operating temperature if the engine was a diesel or if there was an issue with the manifold. Sometimes gases escaping the manifold cover could cause the engine to sound different.
It could either have a gasoline engine or a diesel engine
Excessive white smoke could mean a bad egr cooler which means it is burning coolant or it could be unburned fuel. You need to check and see what it smells like. Egr coolers are only on newer vehicles.
if the engine is knocking you could have bad support bushings for the mouter mounts or your engine could be warped do to excessive heat wear
you will have a bomb. your engine could explode from the increased compression of a diesel engine
No , a diesel engine doesn't have sparkplugs It could have glow plugs
Not really. You could start over and redesign an engine to be able to run on diesel but it would be completely different from the original.
Flex fuel does not mean you can use diesel. It means the engine can run on pure gasoline or on E85 gasoline. If you put diesel in this vehicle it simply will not run. Diesel can only be used in a diesel engine.
YesYes
You could damage the Engine because Gasoline is potent than Diesel
Realistically it wouldn't be feasible to convert a diesel engine to gas. Diesel motors require much greater compression - enough that it could cause an explosion with a gas-configured motor. Plus, diesel doesn't use ignition or spark plugs, and the fuel pump/fuel ignition systems are completely incompatible.
you do not say if your engine is gas or diesel. A gas engine has many sensors, any one of which could cause a problem, not to mention a borken catalitic converter, which could also have many causes. I have not found any reasons for a diesel to experience this problem. I think, that to correctly answer this question, you need to give a more detailed question.