UK Car Repair Questions? Ask a Mechanic for Answers ASAP
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Yes, it could be from a vacuum leak. ok, you need to have someone crank engine and put in gear and while idling, you spray each vacuum hose, intake air tube, pcv valve, and base of intake with carb cleaner one at a time. If idle changes when one is sprayed, that is your culprit and you will need to replace it
Pcv valve is atop of valve cover. Air intake tube is the tube that runs from the air cleaner to the throttle body. The base of the intake is where the intake manifold bolts to the engine. And then any of the small black or grey vacuum hoses running in the engine compartment.
If it is idling smoothly, then likely not an air leak anywhere. The hiss may be from the exhaust due to a very small pinhole leak, or the secondary air system.
The current misfire may be from the oil soaked ignition leads, so we need to replace them and then see where we are on it
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