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1998 Mazda Millenia 2.5L P0300: Recurring Idle Misfire After Warm Restart - Fuel Sys 2 OL Fault Explained

Model: 1998 Mazda Millenia Fault Code: P0300 Posted: 2008-02-26 09:11

I own a 1998 Mazda Millenia with a 2.5L V6 engine and 47,000 miles on the odometer. The check engine light activates every few days, even after I clear the P0300 fault code (multiple cylinder misfire). This issue only occurs at idle speed following a warm engine that has sat for at least five minutes after being turned off. Freeze frame data shows: - Fuel System 1: CL, using H02S - Fuel System 2: OL (fault) - Calculated Load (%) : 27.8 - Coolant Temperature: 194°F - ST FTRM1 (%): -2.3 - LT FTRM1 (%): -7.8 - ST FTRM2 (%): 0.0 - LT FTRM2 (%): -6.2 - Engine RPM: 722 - Vehicle Speed: 0 I have already replaced the spark plugs, ignition wires, distributor cap, and rotor. The vehicle runs smoothly under all other conditions. I'm wondering if others have experienced this issue—what does a 'Fuel System 2 OL fault' specifically indicate? Any guidance on diagnosing without replacing parts that are likely functioning correctly would be greatly appreciated.

Related fault codes

P0300

Comments (4)

Anonymous 2008-02-26 10:08

Have you checked the EGR system recently? Many users report similar symptoms after a clogged EGR port or intake manifold, especially in older Mazda models.

Anonymous 2008-02-27 08:57

Thanks for the input. I've received conflicting advice—some suggest vacuum leaks, others point to faulty fuel injectors or a dirty MAF sensor. I did remove and clean my MAF, but it wasn't visibly dirty. I'm trying to avoid unnecessary part replacements. Can you clarify what an 'OL fault' on Fuel System 2 means in this context?

Anonymous 2008-02-27 11:46

Don't jump to conclusions about the O2 sensor—there's no active O2 code reported, and scan tool data may not be fully accurate. The O2 sensor is likely working correctly and just indicating a fuel mixture issue. First, check for intake gasket or vacuum leaks. Then verify fuel pressure. On many Mazda models, especially older ones, the EGR valve isn't always the problem—carbon buildup in the EGR port (often behind the throttle body) can cause idle misfires after warm restarts. Cleaning this port with carb cleaner often resolves the issue. This is a known TSB for Mazda Millenia and similar vehicles. Removing the throttle body to clean the intake port is straightforward. A blocked port restricts minimum airflow, causing poor idle performance when restarting a warm engine. Fuel pressure and vacuum integrity should be checked first—O2 sensors are typically unnecessary unless you can prove they're failing or not switching properly. Given the low mileage for this model year, it's likely the car has been driven infrequently, leading to carbon accumulation. Vacuum leaks are also common in Mazdas—check for them thoroughly or consider a professional diagnostic check before investing in O2 sensor replacement.

Anonymous 2008-03-26 18:37

Thanks again! I cleaned both the EGR valve and the intake port leading to the manifold, and the issue is now fully resolved. A simple spray of carb cleaner cleared the carbon buildup—no need for expensive repairs or replacements. This confirms that the root cause was likely a clogged EGR port.