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P0300 Fault on 2003 Chevrolet S-10 4.3L Engine - Misfire Only at Operating Temperature

Model: 93 Chevy S-10 4.3 Fault Code: P0300 Posted: 2015-01-31 06:48

Hello everyone, I'm seeking urgent help with a recurring P0300 misfire code on my 2003 Chevrolet S-10 4.3L V6 engine. The issue began about a month ago. Previously, around six months prior, the truck exhibited severe misfiring and excessive distributor play, which led me to replace the distributor, cap, rotor, spark plugs, and ignition wires — after which the problem temporarily resolved. Now, the P0300 code has reappeared. I’ve noticed a very slight misfire at idle, but it’s significantly worsening and now makes driving unsafe. The engine runs smoothly when cold (below 195°F), but begins to misfire consistently once it reaches normal operating temperature. I have performed the following diagnostics: - Vacuum leak test: holds steady at 18 inches of mercury both hot and cold. - Fuel leak-down test: holds 59 psi after engine shutdown for 10 minutes — within acceptable range. - Replaced the engine temperature sensor (no improvement). A faint odor near the rear of the truck has been noted, resembling burnt rubber rather than rotten eggs. The vehicle has accumulated 208,000 miles and retains its original catalytic converter. I believe this is not a catastrophic failure since the misfire only occurs at operating temperature — but I’m now concerned it may be related to fuel delivery or exhaust system restrictions. What steps should I take next? Any insight into potential causes would be greatly appreciated.

Related fault codes
P0300
Comments (4)
Anonymous 2015-01-31 12:17

Check for a clogged or restricted catalytic converter or exhaust system — this is a common cause of P0300 misfires at operating temperature.

Anonymous 2015-01-31 18:31

Thanks Mohawk. I cleaned the MAF sensor today with no improvement. Cleared the P0300 code, but it returned. I sprayed the intake and throttle body with carburetor cleaner while the engine was hot and at 2000 RPM — no change. I took temperature readings across the catalytic converter: inlet to exhaust showed a 178°F drop. Pre-cat temp: 440°F, inlet to cat: 303°F, outlet from cat: 481°F, outlet from muffler: 140°F. Notably, when I ran the engine at 2500 RPM under load, I observed the exhaust muffler pulsing in and out — it appeared to be bulging as if blowing and sucking. This suggests a potential exhaust restriction or internal failure. I plan to remove the catalytic converter tomorrow to test this theory. What do you think?

Anonymous 2015-02-01 08:01

This sounds like your issue has been identified — a likely culprit is a failing or clogged catalytic converter causing backpressure and misfire at operating temperature.

Anonymous 2015-03-08 11:46

This is an update from my earlier post. The weather in Kentucky has been terrible, and due to my garage being full of my wife’s belongings, I’ve had limited time to work on the truck. I removed both the catalytic converter and pre-cat pipe — the misfire persists. I initially suspected that the (almost new) fuel pump might be overheating and failing under sustained load, so I installed a completely new fuel pump. However, the misfire has only gotten worse and at times causes the engine to stall. While the leak-down test was positive, it seems possible there’s an underlying fuel delivery issue. The symptoms are consistent with poor fuel atomization or pressure drop during warm-up. I’m now considering a fuel pressure test and possibly checking for injector issues. Any suggestions would be very helpful.