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Q: When vehilcle gets to 60mph or more it sputters / (small backfires) in the exhaust along with increased valve clatter, heating up.

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The vehicle begins to sputter when you reach highway speeds of 60mph or more. The sputtering is actually small backfires in the exhaust. There's no stalling or bucking, but there is a noticeable lack of power. There is valve clatter especially when going uphill. It will also run 3/4 up on the heat gauge. It starts fine, and runs reasonably well at lower speeds, except the gas mileage is really bad. There's also an antifreeze smell after you shut if off. So far I had the thermostat housing and sensor cylinder head temp. sensor replaced along with the ignition coil assembly.

My car has 100000 miles.
My car has a manual transmission.

It sounds like the engine is getting starved for fuel above 60 mph. I would first try and test for any codes in computer and then if no codes then check fuel pressure to see if it is normal. If okay then check the MAF to see if it is measuring the correct barometric readings for the altitude you are at and replace it if it does not register the correct readings causing lean condition without codes.

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