does your fried know how to drive a stick car good?? in case your car is manual, and the person driving it does not have a experience on it, it is very easy for them to downshift form 5 gear to 2 thinking they went to 4.
and if this happens, well then he overrevvv the engine, and since this engines have no piston, valve clearance, chances are he bent some of them,, this explains misfiring, poor engine performance, etc..
to begin i would do a compression test.. see if they are consistent and how it susteins that pressure
then if nothing seems strange,, try filling up the tank with good gas.
if you have a performance chip, and he used cheap low octane gas, the engine will suck and misfire, because of the gas.. performance chips advance timing, so high octane gas is essential for good driveability..
hope this helps.
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