recently i bought a 1999 camaro 3.8L. it started but rough idle and found out milky motor oil. so, i removed the engien and disassembled it. i found out two problems, one was lower intake manifold gaskets were bad that's why water went into the crankcase. another one was water went into number 6 and 3 cylinders which might be caursed by bad head gaskets. besides these, all other engine conponnents were still in good condition. so, i put everything back with new bearings, gaskets, and seals, and make sure i was doing right, torque every bolts in specs. now, i start the engine, and the engine runs quiet but still has little rough idle. the engine light came on steady with p0300 random misfire. i had it test drive and feel ok when accelerated. just little rough idle and idle speed stay in 650 RPM. there are new plugs and wires. the coils and ignition module has been tested in good. all injector resistance are in 12 ohms which is good. the fuel pressure is 40 psi(should be ok), i checked no vaccum leaks for sure, EGR valve ok.
i have driven the car over 50 miles, there is no additional engine code, just P0300. That's all i know about this car. what else could causes the problem, Bad MAF sensor or Camshaft sensor...? i have no idea and don't want to spend too much money to test.

anyone knows what's going on my car, i need your helps thank you

 

did you have the heads machined?

 

Random misfire means just that. Either an ignition problem or a lean condition. A bad MAF could cause a lean condition. You can run with the MAF disconnected. It will set a MAF code, wich you can temporarily disregard, and force the PCM to run open loop. If engine now runs better, it indicates that one of the sensors is bad, and not necessarily the MAF. Sensors age and the output drifts. The PCM has no way to know that, so check the sesnor outputs with a scanner and see if they make sense,

This is from Alldata:
DIAGNOSTIC AIDS
The scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as DTC status Failed Since Code Clear, the misfire history counters (Misfire Hist # 1-# 6) will still contain a value that represents the level of misfire detected on each cylinder.

A misfire DTC may set if components that affect the crankshaft position (CKP) sensor have recently been replaced, and the CKP system variation learn procedure has not been performed. If the diagnostic table does not identify a condition, perform the CKP System Variation Learn Procedure. The crankshaft position system variation learn procedure should be performed if any of the following conditions are true:

• The PCM has been replaced.
• DTC P1336 is set.
• The engine has been replaced.
• The crankshaft has been replaced.
• The crankshaft harmonic balancer has been replaced.
• The crankshaft position (CKP) sensor has been replaced. See: Crankshaft Position Sensor\Service and Repair\CKP System Variation Learn Procedure

The scan tool displayed misfire counter values (Misfire Hist. # 1-# 6) can be useful in determining whether the misfire affects a single cylinder, a cylinder pair (cylinders that share an ignition coil (1/4, 2/5, 3/6), or is random. If the largest amount of activity is isolated to a cylinder pair, inspect for the following conditions:

• Secondary ignition wires-Check the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance. The wires should measure under 600 ohms per/ft.
• Damaged or malfunctioning ignition coil-Check for cracks, for carbon tracking, or for other damage. Also check coil secondary resistance. Secondary resistance should be between 5,000-8,000 ohms (5-8K ohms ).

Switch the ignition coils and retest. If the misfire follows the coil, replace the ignition coil.

IMPORTANT: If the level of misfire was sufficient to cause possible catalyst damage, and if the MIL was flashing, ensure that the DTC P0420 test is completed and passed after verifying the misfire repair.

If the misfire is random, check for the following conditions:

• System grounds-Ensure that all connections are clean and properly tightened.
• The mass air flow (MAF) sensor-A MAF sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition. Try operating the vehicle within the Failure Records conditions, with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the mass air flow sensor. Refer to Mass Air Flow (MAF) Sensor Replacement.
• A damaged accessory drive belt or driven accessory-A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC.
• The air induction system-Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, for an incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, the EGR valve, and the intake manifold mounting surfaces.
• Fuel pressure-Perform a fuel system pressure test. A malfunctioning fuel pump, a plugged filter, or a malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Diagnosis. See: Fuel Delivery and Air Induction\Testing and Inspection\Component Tests and General Diagnostics\Fuel System Diagnosis
• Fuel injectors-Refer to Fuel Injector Coil Test. See: Component Tests and General Diagnostics\Fuel Injector Coil Test
• Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire, refer to Alcohol/Contaminants-in-Fuel Diagnosis (Without Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (With Special Tool). See: Component Tests and General Diagnostics\Fuel Quality Diagnosis\Without Special Tool See: Component Tests and General Diagnostics\Fuel Quality Diagnosis\With Special Tool
• The exhaust gas recirculation (EGR) system-Check for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.
• Extended idle-Excessive Open Loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors (HO2S). The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing the OTC P0300 to set, review the Freeze Frame/Failure Records for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures below 80°C (176°F ), the condition described above is very likely the cause of the DTC to set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 g/s .

MORE:

CKP System Variation Learn Procedure The crankshaft position (CKP) system variation compensating values are stored in the PCM non-volatile memory after a learn procedure has been performed. If the actual crankshaft position system variation is not within the crankshaft position system variation compensating values stored in the PCM, DTC P0300 may set, refer to Diagnostic Aids for DTC P0300.

The crankshaft position (CKP) system variation learn procedure should be performed if any of the following conditions are true:

• DTC P1336 is set.
• The PCM has been replaced.
• The engine has been replaced.
• The crankshaft has been replaced.
• The crankshaft harmonic balancer has been replaced.
• The crankshaft position (CKP) sensor has been replaced.

IMPORTANT:

• Set the vehicle parking brake and block the drive wheels when performing the crankshaft position (CKP) system variation learn procedure in order to prevent personal injury. Release the throttle when the engine reaches the SECOND fuel cut off. Leaving the throttle open during the fuel cut-off will allow the engine to decel at an even rate. Once the learn procedure is completed, the PCM will return the engine control to the operator and the engine will respond to the throttle position.
• The scan tool crankshaft position system variation learn function will be inhibited if engine coolant temperature is less than 70°C (158°F ). Allow the engine to warm to at least 70°C (158°F ) before attempting the crankshaft position system variation learn procedure.

The scan tool crankshaft position system variation learn function will be inhibited if any powertrain DTCs other than DTC P1336 are set before or during the crankshaft position (CKP) system variation learn procedure. Diagnose and repair any DTCs if set.

The crankshaft position system variation learn function will be inhibited if the PCM detects a malfunction involving the camshaft position signal circuit, the 3X reference circuit, or the 18X reference circuit.

• If the scan tool indicates a problem with the cam signal, refer to DTC P0341.
• If the scan tool indicates a problem with the 3X crank signal, refer to DTC P1374.
• If the scan tool indicates a problem with the 18X crank signal, refer to DTC P0336.

1. Set the parking brake.
2. Block the driver wheels.
3. Ensure that the hood is closed.
4. Start the engine and allow engine coolant temperature (ECT) to reach at least 70°C (158°F ).
5. Turn OFF the ignition.
6. Select and enable the crankshaft position (CKP) system variation learn procedure with the scan tool.
7. Start the vehicle.
8. Apply and hold the service brake pedal firmly.
9. Ensure that the transaxle is in Park.
10. Increase accelerator pedal position until CKP system variation learn fuel cut-off is reached. CKP system variation learn fuel cutoff is reached at 5,150 RPM . Release the accelerator pedal when the second fuel cutoff is reached.
11. The crankshaft position system variation compensating values are learned when RPM decreases back to idle. If the procedure terminates, refer to Important above for instructions.
12. Observe DTC status for DTC P1336.
13. If the scan tool indicates that DTC P1336 ran and passed, the crankshaft position (CKP) system variation learn procedure is complete. If the scan tool indicates DTC P1336 failed or did not run, check for other DTCs. If no DTCs other than P1336 are set, repeat the crankshaft position (CKP) system variation learn procedure as necessary

 

i found out why my car has P0300 random misfire bacause the four cylinder compression are kind of low. #1- 98psi, #2-200psi, #3- 125psi, .#4-100psi, #5-175psi, #6- 120psi. and also did the wet test and got almost same compression as dry test, which means the piston rings are good. maybe the problem is about the valve train parts. but i not sure which are bad. i am for sure the valves, rocker arms, push rods, and springs were good condition. only things i am not sure are the lifters. the bad lifters will cause the low compression with no ticking noise. i know the bad lifters will make kind of noise when engines run. how can i check the lifters are good or not?

 

Just a pssibility, the valves could be mal-adjusted and not closing fully, or a head gasket could be blown. Adjacent cylinders 1&3, and 4&6 have low compression, which indicates the possibility that the gaskets between those cylinders may be blown.

 

Yep, gotta agree. You should have taken your heads into a machine shop and had them gone over, and had the surfaces shaved if needed. The heads could be warped, which if they are would be the cause of the leaky head gaskets. Also, you do not want to reuse the old head bolts. It sounds like one (or both) of these things weren't done.

 

i sent the heads to the machine shop to check, and they found out the four intake valve seats were bad that cuased losing pressure. i think that's the problem.
thank you all