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Q: I have an issue with my Tahoe its idling rough the hand stays on 500 rpms but you can hear it in the engine

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I have a 2003 Chevy Tahoe with 5.3L flex fuel. It's idling rough but the rpm hand stays on 500 rpms but you can hear it in the engine it will idle fine for a minute but then it will idle weird for about 30 seconds then go back. When I put my hand at the end of the exhaust pipe it blows out barely. Also my vapor canister surge soleniod has a vaccum going to it and a plug I checked the power to it and it has power but when I pull the vaccum line off of the vapor canister purge soleniod it is not sucking in air through the line at all so what do I need to do? Also it says misfire on cylinder 5 but I changed the injector, changed the coil, changed the plug and wires also checked the wiring to cylinder 5 and all others they have 12 volts going to it the injectors and coils has 12 volts and the compression is good all the ground wires are good what do I do to fix this its drinking gas bad

My car has 240300 miles.
My car has an automatic transmission.

A: It would appear you have already attempted ...

It would appear you have already attempted to perform all of the normal repairs to fix this issue. Due to the high number of repairs made, you may want to start by retracing your steps and confirming that all the repairs are holding and functioning well. With that being said, one of the only aspects it appears you haven't considered would be air.

Are your oxygen sensor working correctly? Have you looked at things as simple as the air filter and mass airflow sensor? Any one of these components can be sending bad signals to your computer and causing issues. The misfire is the reason you are experiencing bad gas mileage. Its most likely the reason you are having rough idling as well.

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