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P0300


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Hey Everyone, I need some help. Iv been trying to solve this randm misfire for a while now. The only codes im getting are P0300. My engine has been running pretty rough and its performance is lacking. So far iv changed to plugs with oem Acdelco platinums and iv tried replacing the coil assemblies front and rear and still have no luck. Iv checked all vacuum connections best i could and found nothing. FPR was removed and inspected and i didnt see anything wrong with that either. Also, it might be worth mentioning that when i replaced my plugs in the Fall and when i replaced the coil assemblies this past weekend i noticed that oil had pooled in the spark plug wells on the front bank.. Im starting to suspect that maybe the coil boots and plugs on that bank have gone bad because of the oil.. Iv failed emissions testing because of this p0300. Please Advise.. Paul

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You cannot diagnose a bad FPR with a visual inspection. You either have to pull the vacuum hose at idle and watch for fuel leakage from the nipple, or apply vacuum with a hand held vacuum pump and be sure it holds.

A little oil seepage into the plug wells is somewhat normal, but should not be excessive.

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Thanks Ranger,

Do you think my fuel filter or pump could be my problem? Id like to troubleshoot my fuel system again. What should i look at, other than the above mentioned?

I wish i had more codes to work with

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I'd check the FPR and fuel pressure, but I would suspect ignition before fuel. Maybe one of the coil boots was carbon tracked. If so and it was not replaced, the misfire will return even after new plugs are installed and will likely carbon track the new plug as well.

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I have a feeling 1 or 2 of the boots might be the problem. The 2 or 3 closest to the oil filler cap on the front bank have oil on them and those are the only plug wells that have the oil going in them. I'll be on the phone with my local dealership in the morning to see if they have boots available.. In the mean time ill keep poking around and keep you informed as things progress... Thank You

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  • 3 weeks later...

Sorry about the late update. After replacing both coil banks, plugs and boots im about ready to rule out the ignition sysem. Iv double checked the FPR by pulling the vacuum hose and looking for fuel or even a smell. No problems found there. Iv double checked all vacuum connections and although i cant find any leaks, I swear i can hear a faint HISSING noise coming from the area around the TB and EGR valve area. I cant pinpoint the noise, Iv also tried the propane torch leak test in that area and around the manifold, no findings. My misfire is consistant at idle and it might be worth mentioning that it seems the higher the rpm the less it seems to miss. For example, she ran like a champ when i did some WOT's on Easter Sunday...I havnt ran the carbon out in a while because of the missing. Its amazing how diffent the drive feels afterwards. But anyhow, to date iv replaced: Coil banks (module included), plugs and boots. All A/C Delco parts.

Where else should i start troubleshooting? This missfire seems more like a puzzle than anything else.

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The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensors and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring variations in the crankshaft rotation speed for each cylinder, the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause 3-way catalytic converter damage. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for catalytic converter damage are present.

Conditions for Running the DTC

• DTCs P0101, P0102, P0103, P0106, P0107, P0117, P0118, P0121, P0122, P0123, P0125, P0218, P0335, P0336, P0340, P0341, P0385, P0386, P0502, P0503, P0705, P0713, P1114, P1115, P1121, P1122, P1336, or P1372 are not set.

• The engine speed is between 500-5,850 RPM.

• The system voltage is between 9-18 volts.

• The engine coolant temperature (ECT) indicates an engine temperature between -7 to +120°C (19-248°F).

If the start-up ECT is below -7°C (19°F), then the test is delayed until the ECT is more than 21°C (70°F).

• The throttle angle is steady.

• The A/C compressor clutch state is steady.

• The exhaust gas recirculation (EGR) flow test is inactive.

• The fuel level is above 10 percent

• The deceleration fuel cut-off (DFCO) and the torque management are not active.

Conditions for Setting the DTC

Five out of 16 consecutive 200-revolution blocks containing 22 or more misfires is considered a non-catalyst-damaging misfire.

OR

Any 200-revolution block containing an excessive amount of misfires (above a predetermined threshold) is considered a catalyst-damaging misfire.

Action Taken When the DTC Sets

• The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

• The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

• The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.

• A current DTC, Last Test Failed, clears when the diagnostic runs and passes.

• A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

• Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

Check for the following conditions:

• Incorrect CKP system variation data stored in the PCM--Run the Crankshaft Position System Variation Learn using the scan tool.

• The system grounds--Ensure that all connections are clean and properly tightened.

• 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. Refer to Symptoms - Engine Mechanical in Engine Mechanical 4.6L.

• The 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 .

• The fuel injectors--Refer to Fuel Injector Solenoid Coil Test

• Contaminated fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• An extended idle--Excessive Open Loop operation caused by extended idling or by short trip driving may leave deposits on the heated oxygen sensors. The deposits cause oxygen sensors to respond slowly to the exhaust oxygen content, affecting the fuel control and causing a misfire to be indicated at idle. This condition is not permanent. In order to determine if this condition is causing the DTC P0300 to be set, review the Freeze Frame/Failure Records data for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC P0300 to set. If the DTC P0300 occurs during idle or during 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 P0300 being set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows (MAFs) above 15 g/s.

• Running the vehicle low on fuel--This DTC may set if the vehicle misfires while running out of fuel. If no misfire is currently present, but the History Counters indicate that a random misfire was once present, ask if the vehicle has been run out of fuel. If this is the case, no further repair may be necessary.

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

If the condition is intermittent, refer to Intermittent Conditions .

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Here is something to try that works wonders and may solve your issue. Get a can of Mass Air sensor cleaner, and take the sensor off and spray down the elements very nicely and remove the carbon biuld-up. If there is one spec of carbon or debris on the elements the car will run horrible, No power and crap mileage without giving codes. I would like to see if this is your issue. I have replaced mine and It was nite and day difference But i clean all my mass airs on all my cars every 6 months. To much dust and dirt in NY. Good luck!

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Here is something to try that works wonders and may solve your issue. Get a can of Mass Air sensor cleaner, and take the sensor off and spray down the elements very nicely and remove the carbon biuld-up. If there is one spec of carbon or debris on the elements the car will run horrible, No power and crap mileage without giving codes. I would like to see if this is your issue. I have replaced mine and It was nite and day difference But i clean all my mass airs on all my cars every 6 months. To much dust and dirt in NY. Good luck!

This is interesting, I have never cleaned mine. I don't even think I have seen Mass Air Sensor cleaner.. hmm, thanks for the heads up

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Something to look at..personally doubt it's the PO300 issue..but worth a look.

Very common problem and well known problem on the 3800.

Very fast fix.

Cheap air filters, dust or fibers can collect on the MAF sensor wire.

Enough to throw off the PCM calibration maybe 10%.

Oil soaked sport filters can destroy a MAF also with oil contamination..GM TSB..and I have seen them..

We clean them with a Qtip.

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I gave the MAF cleaner a shot. $6 at autozone so i figured I'd give it a shot. No changes. Now, I want to get rid of the oil in the plug well problem. Someone suggested in a recent thread that the pooled up oil could be robbing the plug of its spark. Does anyone know the GM part number for the cam cover gasket or whatever it is that i need to correct this oil leaking into my plug wells. And if anyone has the torque specs for the bolts i could use that too. It's my front cam cover (gray). Thanks!! ---Paul

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Well, that's quite a bit, but consider that it took 118K to do it. Cam covers are a little difficult to remove as I recall so you might just consider cleaning out the plug wells every year or two.

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I gave the MAF cleaner a shot. $6 at autozone so i figured I'd give it a shot. No changes. Now, I want to get rid of the oil in the plug well problem. Someone suggested in a recent thread that the pooled up oil could be robbing the plug of its spark. Does anyone know the GM part number for the cam cover gasket or whatever it is that i need to correct this oil leaking into my plug wells. And if anyone has the torque specs for the bolts i could use that too. It's my front cam cover (gray). Thanks!! ---Paul

YES, but once you clean the plug port OUT of oil, and dry off the plug and wire, the mis should go away

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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