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1990 GMC Safari 4.3L V6 Misfires Off Idle When Cold - P0300 Diagnosis and Fixes

Model: 00 GMC Safari Fault Code: P0300 Posted: 2013-09-14 15:12

I own a 1990 GMC Safari SLE with a 4.3L V6 engine, 84,000 miles, RWD configuration. The vehicle misfires when the engine is cold and just started. It starts smoothly and idles steadily without any hesitation or skipping as long as the throttle is not pressed. However, once I attempt to increase engine RPM, it begins to skip and stumble—this occurs whether in neutral or drive. The issue resolves completely after the engine reaches normal operating temperature (around 200°F), at which point it runs smoothly for the rest of the day unless allowed to cool down significantly. A check engine light has triggered code P0300, indicating random misfires. I've already inspected: spark plug wires, distributor cap and rotor, distributor backlash, engine coolant temperature sensor (ECT), fuel pressure, and throttle position sensor. All components appear functional. Could this be a lean mixture issue? Is there a known cold-start misfire problem specific to the 1990 GMC Safari with the 4.3L V6? Any insights or common fixes for this behavior? Thank you, Leo

Related fault codes

P0300

Comments (9)

Anonymous 2013-09-14 15:30

Start with fundamental checks: > Perform a fuel pressure test and verify fuel pressure drop under load (post PSI). > Inspect secondary ignition components—check for arcing in the coil. > The 1990 model year had known distributor issues; remove the distributor cap and look for excessive carbon buildup or corrosion. > Conduct a vacuum system check. Clean the throttle body using a dedicated throttle body cleaner. > Use an OBD2 scanner to review fuel trims and freeze frame data—this can reveal key clues about misfire conditions.

Anonymous 2013-09-15 15:12

For vehicles like this, cold-start misfires are typically caused by low fuel pressure or insufficient fuel volume. In many cases, the issue stems from a fuel pressure regulator or injectors allowing pressure to bleed off—especially when the engine is cold. Wells Ignition offers excellent video diagnostics on common GM truck issues involving fuel injectors and distributors.

Anonymous 2013-09-15 16:02

Thanks for the input, Kev2 and Jeff. I tested fuel pressure under the exact conditions described—between 57 and 62 PSI depending on throttle position. Once the engine reaches operating temperature (around 200°F), which takes about 10 minutes, the pressure remains stable and consistent. The vehicle runs perfectly after that.

Anonymous 2013-09-16 08:57

Did you test the ECT sensor? Can your OBD scanner read engine coolant temperature? If so, check both cold and warm readings to ensure accuracy.

Anonymous 2013-09-16 12:00

Thanks, Mohawkmtrs. I tested the ECT sensor by removing it and measuring resistance at room temperature. Then I submerged it in hot water at various temperatures and observed its response—results were within expected range. After some troubleshooting, I purchased an ElmScan OBD2 scanner. The tool shows accurate readings: 70°F when cold and 200°F when warm. The data matches perfectly with the expected curve in OBDwiz software.

Anonymous 2013-09-16 12:18

With your OBD scanner, check the freeze frame data and fuel trims—this will help identify whether the misfire is due to lean conditions or sensor-related issues.

Anonymous 2013-09-17 00:04

Thanks, Kev2. The freeze frame data was not stored because the P0300 code cleared itself (I'm unsure how). Fuel trims remain between zero and one during operation. I’ll need to drive the vehicle under cold-start conditions again to trigger the code and capture the freeze frame data. Will update once available.

Anonymous 2013-09-17 15:44

Does my van have heated oxygen sensors? If so, does it operate in open-loop mode until the O2 sensors reach operating temperature (typically around 600°F), then switch to closed-loop control using sensor feedback? Could this open-loop operation be contributing to the misfire? How can I verify that the heater circuits are functioning properly?

Anonymous 2013-09-17 18:47

If your oxygen sensor has four wires, it is heated. Two-wire sensors are typically unheated. If you removed the ECT sensor, be cautious—reinstallation should be done carefully to avoid introducing new issues. Try unplugging the upstream O2 sensor before starting and observe how the vehicle runs. Does it behave the same or get worse? When revving in park, does fuel pressure drop from 62 PSI to 57 PSI? If yes, this suggests a fuel flow issue—possibly due to a failing fuel pressure regulator or leaking injectors. I’ve seen many trucks pass fuel pressure tests at idle but show drops under load during real-world driving. This could indicate a fuel pump that heats up and loses pressure. Also, blown intake gaskets can cause vacuum leaks inside the engine—especially when metal components expand and seal during warm-up. While more common in Ford trucks, GM engines are also susceptible to this. Fuel trims on a misfire-triggered code will show what’s happening: positive values indicate lean conditions (possible fuel delivery or intake leak), negative values may point to faulty O2 sensors, injectors, or pressure regulators. Note: On 75% of vehicles with four-wire O2 sensors, the two wires of identical color are part of the heater circuit. If either shows high resistance or an open circuit, they’re likely defective. This is a simple but effective diagnostic test—better than nothing if no other tools are available.