Chrysler Neon 1995-1999 Repair Guide

Evaporative Emission Controls



The evaporation control system prevents the emission of fuel tank vapors into the atmosphere. When fuel evaporates in the fuel tank, the vapors pass through vent hoses or tubes to a charcoal filled evaporative canister. This canister holds the vapors temporarily. The Powertrain Control Module (PCM) allows intake manifold vacuum to draw vapors into the combustion chambers during certain operating conditions. The evaporation control system is made up of the following components:

Rollover Valve

All vehicles are equipped with a rollover valve. This valve is a safely feature which prevents fuel flow through the fuel tank vent valve hoses, should the vehicle roll over in an accident. All vehicles pass a 360° rollover.

EVAP Canister

See Figure 1

All vehicles used a sealed, maintenance free evaporative (EVAP) canister. Fuel tank pressure vents into the canister. The canister temporarily holds the fuel vapors until intake manifold vacuum draws them into the combustion chamber. The PCM purges the canister through the duty cycle EVAP purge solenoid. The canister is purged at intervals and engine conditions predetermined by the PCM.

The canister mounts to a bracket located behind the front passenger's side fascia. The vacuum and vapor tubes connect to the top of the canister.

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Fig. Fig. 1: The evaporative canister is mounted to a bracket found behind the front fascia on the passenger's side of the vehicle

Duty Cycle Evaporative Purge Solenoid Valve

See Figure 2

The duty cycle EVAP purge solenoid regulates the rate of vapor flow from the EVAP canister to the throttle body. The PCM operates the solenoid. During the cold start warm up period and the hot start time delay, the PCM does not energize the solenoid. When de-energized, no vapors are purged.

When purging, the PCM energizes and de-energizes the solenoid about 5-10 times per second, depending upon operation conditions. The PCM varies the vapor flow rate by changing the solenoid pulse width. Pulse width is the amount of time the solenoid energizes.

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Fig. Fig. 2: The duty cycle evaporative purge solenoid valve is located near the front engine mount

The solenoid is attached to a bracket that is secured to the front engine mount. The solenoid will not operate properly unless it is installed with the electrical connector at the top.

Pressure-Vacuum Filler Cap

See Figure 3

A pressure vacuum relief cap is used to seal the fuel tank. Tightening the cap on the fuel filler tube creates a seal between them. The relief valves in the cap are a safety feature which prevent possible excessive pressure or vacuum in the fuel tank. Excessive fuel tank pressure could be caused by a malfunction in the system to damage to the vent lines.

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