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1996 Neon P0300 OBD2 Code: After Replacing Plugs, Coils, Sensors Still Dies on Highway

Model: 96 neon Fault Code: P0300 Posted: 2010-11-21 11:31

I own a 1996 Neon with a 2.0L SOHC engine and automatic transmission. It has been experiencing intermittent stalling for nearly three months. I've replaced multiple components including spark plugs, ignition wires, temperature sensor, MAP sensor, TPS, fuel pump, and fuel filter. I've also cleaned and tested the injectors and tried three different ignition coils. All repairs resulted in temporary improvements—each time it runs well for a short period before stalling. The most recent incidents occurred after driving about 10 miles on the highway; the engine would suddenly shut off, as if fuel were cut off. It would either restart with rough performance (like a tractor) or fail to start at all. However, once the engine cools down, it typically restarts successfully. I recently purchased an OBD2 code reader and confirmed the presence of P0300—indicating a random misfire. I performed live data readings under two conditions: idle in park and driving in gear. At idle (park): - Fuel System 1: CL (no fault) - Load Calculation: 5.0 - ECT F: 174°F - STFT B1: -10.1 - LTFT B1: +14.8 - MAP: 9.1 psi - Engine RPM: 957 - Speed: 0 mph - Spark Advance: 14° - IAT: 75°F - TPS: 14.9% - O2S B1 S1 Voltage: 0.865V - STFT B1 S1 Min: 9.3 - O2S B1 S1 Voltage: 0.135V - STFT B1 S2: NA In drive: - Fuel System: CL - Load Calculation: 7.0 - ECT F: 176°F - STFT B1: -3.9 - LTFT B1: +0.7 - MAP: 12.1 psi - Engine RPM: 979 - Speed: 1.0 mph - Spark Advance: 16° - IAT F: 75°F - TPS: 14.9% - O2S B1 S1 Voltage: -2.3 - STFT B1 S2: 0.135 - STFT B1 S2: NA My next steps include clearing the codes and taking the car for a drive with live data monitoring to see if it stalls again, as well as checking fuel pressure during stalling events. Possible causes under consideration: - Faulty fuel pump relay (despite testing multiple units) - Damaged crankshaft or camshaft position sensor I'm seeking advice on how to proceed—any insights into misfire diagnostics for this model would be greatly appreciated.

Related fault codes
P0300
Comments (6)
Anonymous 2010-11-21 13:09

Just an update—I took the car out for a drive. After about one mile, it began idling too fast and wouldn't drop to normal speed. The scanner showed TPS at 76%. I swapped back to my original TPS and restarted. When manually turning the throttle shaft with a screwdriver, the idle stabilized and the scanner read 4.6% TPS—matching the expected value. When I reconnected the original TPS (the one on the throttle body), the car failed to start and triggered code P2195 (TPS voltage too high). After reconnecting my old TPS, it started immediately. This strongly suggests that the new TPS was improperly installed—likely damaging the internal disk where the throttle shaft connects. I believe this is now causing the stalling and misfire symptoms. I’ll verify this in the morning with a new TPS.

Anonymous 2010-11-21 18:17

The engine appears to be running rich at idle based on fuel trim values. Could an EGR valve be stuck open? Is camshaft timing correct? Did you adjust the throttle body's idle screw? That should never be necessary—any adjustment indicates a deeper issue.

Anonymous 2010-11-21 18:25

The engine temperature reading is clearly incorrect—it shows -174°F, which is impossible. The ECU interprets this as an error and demands excessive fuel injection to compensate. Also, the TPS sensor does not have manual adjustment—only a physical bolt-in design. If you unplugged and manually rotated it with a screwdriver, that’s not how it works. And adjusting the idle screw in the throttle body is not recommended—it can cause serious drivability issues. Did you replace the correct sensor?

Anonymous 2010-11-22 01:22

Thanks for clarifying—I should have explained better. The -174 was a typo; it's actually 174°F, not minus degrees. Regarding the TPS adjustment: I unplugged the new TPS from the throttle body and connected my original one temporarily to allow manual rotation of the throttle shaft using a screwdriver. This allowed me to observe that when the TPS reading was at 4.6%, idle stabilized and all parameters returned to normal. When I reconnected the factory-installed TPS, the car failed to start with code P2195 (TPS voltage too high), which matches the symptoms of stalling. This confirms that the new TPS is likely defective—possibly cracked internally due to improper installation. I heard a cracking sound when tightening the screws, and now I suspect the internal disk is damaged. I’ve ordered a replacement TPS and will test it soon.

Anonymous 2010-11-22 05:50

Where did you purchase your new TPS? It might have been defective—possibly an incorrect part or a low-quality offshore model that doesn’t meet OEM specifications.

Anonymous 2010-11-24 14:04

I bought it from a standard auto parts store. It's likely the same as most other brands available locally. I believe I damaged the internal disk during installation—probably because I didn't align the throttle shaft properly and over-tightened the screws. That’s my best guess. I’ll confirm this in the next few days when I install the new TPS.