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Q: High RPMs, but throwing the error code P0340.

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Went into get a hair cut, came out and my jeep started running at a high RPM and was throwing 3 error codes; P1391, P0171, and P0340. I bought a new Camshaft Position Sensor and installed it, I also did a visual inspection of all the wires, tubes, and springs related to the problems, and I rain 1/3 of a bottle of Seafoam through the engine. After that I cleared the codes and went for a drive to burn out the rest of the Seafoam. On the drive the P0340 came back up and the high RPMs returned. I have checked the wiring and, as far as I can tell, none of my vacuum lines are broken. I did not remove the lines, just visually inspected and ran my fingers over them to feel for abnormal imperfections. I am at a loss because the P0340 is suppose to get you a rough idle, not the high RPMs. So currently, with my foot off the clutch and gas, I can idle my car along in 3rd gear at 25 mph.

My car has 142000 miles.
My car has a manual transmission.

Hi there. These codes may or may not be related to one another. I would be most concerned with the high idle first. Most of the time, on this engine it is due to a bad vacuum line. It sounds like you have checked those though. If the vacuum lines are fine, then most of the time the leak comes from the intake manifold gasket.

It is common for this to fail and cause excessive air to get into the engine and cause this high idle. The engine needs to be checked further for vacuum leaks. I would then check the crank sensor signal. If the crank sensor is not reading right, then it can throw a camshaft code like what you have. If you are looking to have this fixed, consider YourMechanic, as a certified technician can come to your home or office to diagnose the high idling symptom.

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