Mazda Tech Tip: Curing Rough Idle During or Immediately After a Cold Start – UnderhoodService

Mazda Tech Tip: Curing Rough Idle During or Immediately After a Cold Start

Automotive fuel with high alcohol content may damage the fuel pressure regulator diaphragm. The damage to the diaphragm may allow fuel to enter the vacuum supply line for the fuel pressure regulator. The vacuum supply line uses the No. 3 intake runner as the vacuum source. The unmetered fuel entering the No. 3 cylinder on cold starts may cause a rich mixture misfire.

Some 2000-2002 Mazda 626 models with the FS 4-cylinder engine may experience a rough idle, hesitation, stumble and/or misfire during or immediately after a cold start. A DTC P0303 may also be stored in memory. In extreme cases, the engine may stop running and multiple misfire codes, like P0300-P0304, may be stored in memory.

Automotive fuel with high alcohol content may damage the fuel pressure regulator diaphragm. The damage to the diaphragm may allow fuel to enter the vacuum supply line for the fuel pressure regulator. The vacuum supply line uses the No. 3 intake runner as the vacuum source. The unmetered fuel entering the No. 3 cylinder on cold starts may cause a rich mixture misfire.

The diaphragm material has been modified, and a new fuel pressure regulator has been established as a service part (P/N FSY1-13-280). Customers with this concern should have their vehicle repaired using the following procedure.

Repair Procedure:
1. Verify the concern by inspecting the fuel pressure regulator vacuum supply hose for evidence of fuel entering the vacuum supply line.

• If no evidence of fuel or fuel smell is found, refer to the repair manual (section 01) for further diagnostics.
• If evidence of fuel or fuel smell is found, proceed to next step.

2. Replace the fuel pressure regulator with a new part according to the repair manual (section 01-14 Pressure Regulator or Removal/Installation).

3. Verify the repair by allowing the vehicle to fully warm up, then allow at least a six-hour cooldown (overnight, if possible) before restarting the engine.

For additional information, visit www.mitchell1.com.

Technical service bulletin courtesy of Mitchell 1.

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