Computer Diagnostics and Tuning Technical discussion on diagnostics and programming of the F-body computers

Dreaded P0300 code

Old 03-07-2015, 01:26 AM
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Dreaded P0300 code

My wife's seldom driven (18,500 miles now) 2000 model Z28 convertible with the LS1 has been throwing the P0300 code. Have checked, changed all the plugs, cleaned the throttle body, not consuming any fluids, took new coil and changed it to each cylinder trying to determine if a coil was bad all to no avail. Car runs fine until almost exactly 3 minute mark, then random missing, and white smoke. Not blue/grey, not black-----very light white. Double checked coolant and it was at exactly the same place as normal. Checked brake fluid correct level, checked oil and no mixing of oil/coolant apparent in either radiator or oil. Clear the code and it returns in about the same 3 minute. Before the 3 minute the car does not miss, no smoke and will go all the way to WOT if accelerated. This car is an export model and I cannot find any 02 sensors on the exhaust but do see two plugs. Had this car since new and have never done any work on it except normal servicing due to time and not mileage. Stumped on this one. All advice humbly accepted with thanks.
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Old 03-07-2015, 12:34 PM
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Re: Dreaded P0300 code

3 minutes is roughly the programmed "timer" for the PCM to switch to closed loop, assuming the coolant is warmed up and the O2 sensors are warmed up enough to be working. Could be pointing to problems with the O2 sensor(s) or some other component of the A/F control system. On the other hand, if you can't find O2 sensors, but can find unused connectors, and given the "export" issue, that theory may go in the trash.

To which country was it "exported"? That will determine which diagrams in the shop manual are applicable to the vehicle.

Find a scanner with "GM enhanced parameters". It will have the ability to pull up the misfire count by individual cylinder, which may point you to the location of the misfire, if it is not truly "random".

White smoke is typically associated with coolant leaks/entering the combustion chamber. Is it extremely cold where you are located? If so, is this possibly just the normal condensing of the byproduct water vapor produced in the burning of gasoline, which also frequently collects in the exhaust system of vehicle that are not operated for long drives, sufficient to warm up the exhaust.

When you changed the plugs, did the all look "normal"? Was there any evidence of a plug scoured by a head gasket leak?
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Old 03-07-2015, 12:37 PM
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Re: Dreaded P0300 code

Looked for a location in your sign-up info, but you did not provide that. Checked the IP address and it indicates you are posting from UAE/Dubai. You really hurt your chances of getting accurate responses by not including info like this. Only a moderator can check the IP address, no one else can.
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