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2000 Safari 4.3L Random Misfire Code P0300: Diagnosis & Solutions for Intermittent Engine Performance Issues

Model: 2000 Safari Fault Code: P0300 Posted: 2015-06-04 09:17

My 2000 Ford Safari with a 4.3L V6 engine has been running intermittently since about six months ago, after a full engine rebuild by a certified and well-established shop. The vehicle currently shows only Diagnostic Trouble Code P0300 (Random Misfire) on the OBD-II scanner. The engine originally had 320,000 miles and was rebuilt with no cost-cutting measures — including an upgraded injector spider. Despite this, the issue has persisted for several weeks. The misfire only occurs under closed-system conditions (e.g., idle or light load), and it's not consistent — sometimes the check engine light doesn't come on at all for days. The vehicle starts reliably every time but begins to exhibit performance issues when the system is closed. Fuel pressure remains stable (58–62 psi) even with a loose doghouse setup used to monitor fuel delivery. After the misfire occurs, fuel trim values spike dramatically — dropping from near-zero (+/-10) to as low as -50 to -6-0 on one bank. This indicates the engine is attempting to compensate for an unexplained lean condition. Once a misfire event happens, it affects the entire drive cycle until I park overnight. The next morning, after a cold start, everything returns to normal — no symptoms during warm-up or operation. I recently cleaned the Mass Airflow (MAF) sensor using MAF cleaner without touching any components directly. However, this morning the vehicle began belching smoke and raw fuel from the exhaust — a clear sign of serious fuel delivery or combustion issues. On my scan tool, I also see the following reading: - Test $07 ID:0D Max:9b Meas:f9e4 Min:---- STS:HI MOD:$10 I'm unsure how to interpret this value — it appears related to EGR system testing but isn't clearly documented in my scanner's manual. Before considering a return to the rebuilder (which I suspect won’t cover any new issues), what troubleshooting steps can I take? The fuel economy has also dropped from 20 MPG to around 15 MPG post-rebuild — something I didn’t expect, especially given the precision of the upgraded metering system. The shop stated they weren't aware of this change. I'm looking for actionable diagnostics that don't require professional tools or a full rebuild.

Related fault codes

P0300

Comments (5)

Anonymous 2015-06-04 09:36

A fuel trim reading of -50 to -60 indicates a rich condition — the engine is receiving too much fuel. Check for potential issues such as a stuck poppet valve, grounded injector wiring, or faulty spark plugs (look for black, wet, or fouled tips). Use only OEM-quality spark plugs like AC Delco. Also monitor ECT (Engine Coolant Temperature) and MAP (Manifold Absolute Pressure) sensors during the misfire event. Since this is intermittent, gently shake the injector harness or connector while observing fuel trim values on your scanner to identify if a loose connection triggers the issue.

Anonymous 2015-06-04 09:49

The $07 test value likely corresponds to EGR (Exhaust Gas Recirculation) system diagnostics. If the EGR valve is stuck open, it can cause lean conditions and erratic fuel trim behavior — especially during idle or closed-loop operation. This could be a contributing factor in your P0300 misfire. I recommend checking for an active EGR code or verifying valve position with a scan tool.

Anonymous 2015-06-05 17:03

Thanks for the reply, Kev2! Before the recent failure, I recorded scanner data and video footage to document the issue. Everything else on the scan was normal — only fuel trim values were abnormal. All rebuilt components were OEM-grade. I tried gently tapping the injector harness while monitoring the scanner, but no immediate hiccup occurred. Now that symptoms have become more consistent, I'm considering taking it back to the rebuilder for further inspection. I'll update with results once available.

Anonymous 2015-06-06 08:16

Check the EGR valve operation — if it's stuck open, it can cause negative fuel trim readings and misfires under load or idle conditions. A sticking EGR may also trigger a MAP code in some cases. This is a common root cause for intermittent P0300 on 4.3L V6 engines like the 2000 Safari.

Anonymous 2015-06-16 09:40

I had a defective injector spider replaced — after that, the engine runs well. I monitored it closely and compared readings to pre-rebuild data. Surprisingly, only the fuel trim values changed significantly. I expected more noticeable improvements in performance or efficiency. Since I'm using a DIY scanner with limited capabilities, I suspect the issue may be deeper than just injector spider failure — possibly related to EGR or sensor calibration.