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Q: Rough idle, surging, and hesitation - 1998 Chevrolet Blazer

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Idles rough, worse under load and surges at times. Rpm varies at constant speeds (occasionally), hesitates when trying to pass a vehicle or speed up. doesn't shift from 3rd gear to overdrive sometimes when hard accelerating, until I release the accelerator. Cruise control works intermittently. Going up on an incline, losing lots of power. No DTC except for p0446 emissions circuit. Replaced tps, iacv, cleaned throttle body, new plugs, wires, distributor, rotor, and coil.

My car has 165000 miles.
My car has an automatic transmission.

A: This may be related to the evaporative purg...

This may be related to the evaporative purge solenoid. The EVAP system prevents fuel vapors from the fuel tank from escaping into the atmosphere. The EVAP system collects and temporarily stores the fuel vapors in the charcoal canister. The charcoal canister is filled with activated carbon pellets that can absorb the fuel vapors. When the engine is running, the fuel vapors are purged from the canister and burned in the engine.

The vent control valve (solenoid) controls the flow of outside air in and out of the charcoal canister. The vent valve is controlled by the engine's computer (PCM). Normally, the vent valve is open. It closes when the engine computer tests the EVAP system for leaks. If a leak in the EVAP system is detected, the Check Engine Light will illuminate on the dash and the trouble code related to the problem will be stored in the engine computer.

If you have an air leak in the EVAP system, then the MAP sensor will not be able to determine the air density, and the MAF sensor will not be able to determine the volume of air entering the engine correctly. This will lead to an over-fueling situation or an under-fueling situation depending on the size of the air leak and engine operating speeds and load. I would recommend having an expert mechanic, such as one from YourMechanic, come to your location to determine why you are having a rough idle and a loss of power.

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