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1998 Oldsmobile LSS 3800 Engine P0300 Misfire: Diagnosis & Solutions with P0442 Evap Leak

Model: 98 Olds LSS Fault Code: P0300 Posted: 2008-08-16 15:21

Hello everyone — this is my first post, and I'm hoping for some expert advice on a persistent issue I've been struggling with. My 1998 Oldsmobile LSS (3800 N/A engine, 280k km) has had a recurring P0300 misfire code that simply won't go away. I've already replaced the spark plugs and plug wires with new ones, properly gapped. Fuel pressure readings are within specifications: 52 psi when primed but not running, 42 psi while running, and it returns to 52 psi when the fuel regulator's vacuum line is disconnected — indicating a healthy system. I've also replaced both the fuel pump and filter, swapped out two ignition modules (one of which was faulty), and changed all coil packs. The check engine light has intermittently turned off on its own but returns within days. I recently used an OBD-II scanner to retrieve freeze frame data during a P0300 event: - Fuel SYS1: OL, drive conditions - Fuel SYS2: OL, drive conditions - CALC LOAD(%): 6.6 - COOLANT(F): 190°F - ST FTRM1(%): 3.1 - LT FTRM1(%): -10.9 - MAP (in.hg): 11.8 - ENGINE RPM: 1679 - VEH SPEED(MPH): 54 - MAF (lb/min): 1.515 - ABSLT TPS(%): 9.0 The scanner reports 'Misfire Monitor OK' but does not provide any diagnostic values. Additionally, I've been experiencing a recurring P0442 code — indicating a small leak in the evaporative emissions system. The gas cap is recent and tight, so this may be related to the charcoal canister or vacuum lines. Occasionally (very rarely), while driving, my 'check oil' light flashes for just a second, followed by noticeable engine stumbling. On rare occasions at idle, the engine stalls but immediately refires. Otherwise, the car runs smoothly and feels strong despite these codes. I'm wondering if this could be caused by a faulty oil level sensor — perhaps it's sending intermittent signals that cause the ECU to momentarily shut off fuel delivery, resulting in a misfire. I've already replaced the oil level sensor with no immediate improvement, but I haven't driven much yet (only about four trips to work). The P0300 has not returned during this period. The P0442 code still appears intermittently — once or twice recently — and previously often coincided with the P0300. I'm now planning to focus on resolving that issue if it persists, but I'd like to rule out other possibilities first. Key details: 1998 Oldsmobile LSS, 3800 N/A engine, 280k km. The charcoal canister is located under the hood beneath the airbox and is controlled by the PCM.

Related fault codes
P0300
Comments (2)
Anonymous 2008-09-02 18:35

Have you performed a compression test? A vacuum leak — especially in the intake manifold — could be causing the P0300. The 3800 engine is known for its plastic intake manifold, which can develop leaks over time. Also, consider having your vehicle smoke tested at a dealership to detect any hidden vacuum leaks. The P0442 code might point to an issue with the evaporative charcoal canister or its associated hoses.

Anonymous 2008-09-04 19:44

I haven't done a compression test yet, but my intake manifold was replaced about five years ago due to the known vulnerability of the 3800's plastic intake. I've already replaced the oil level sensor — hoping for a miracle fix. Since then, I’ve driven it only about four times (to work and back), and neither the P0300 nor the intermittent 'check oil' light with stalling has returned. The latter was very rare and brief, so I’m cautiously optimistic. However, the P0442 code still appears once or twice on its own. Before replacing the sensor, both codes would appear together. I’ll keep monitoring for a few more weeks — if the P0300 stays gone, I'll shift focus to resolving the P0442. As an aside, the charcoal canister on this model is located under the hood beneath the airbox and is managed by the PCM.