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P0300 code for 2004 Chevy Express 2500 4.8 V8

(2004 Chevrolet Express 2500)
in Oxford, MI on October 06, 2010
I get flashing P0300 flashing code when driving above 60+ mph. New plus and wires, No problems with start, idle or excelleration, vacum checks ok, yet when I use Actron scanner to clear code and leave attached while driving, I will get NO Check Engine Light or freeze frame data regardless of what speed I drive. If I stop and disconnect the scanner, and resume driving conditions to exceed 60+ miles per hour the CIL will flash and lock into a solid CIL as soon as soon as I slow down to below 60. The scanner reads "ok "for all MIL's it is capable of reading when attached while driving. Could engine ground be the possible culprit, but if so why only after 60+ mph. Is there a TSB regarding this ground issue. This issue has been ongoing for 3+ months whith no change in vehicle performance.
Displaying 2 answers & 18 comments
Popular Answer
on October 06, 2010
The P0300 is a MULTIPLE misfire code. It sets when there are more than one cylinder misfires, therefor more DTC should be registered i.e. P0302, P0306 for #2 and #6 cylinders. Do you see any other P03xx DTCs as well beside the P0300??
How about EGR codes?
Is this problem started after the new plugs and wires?

on October 06, 2010
The problem was pre existing prior to the plus and wires and it never will set to a registered cylinder. Its always the same P0300 only. No other DTC's or EGR codes. When i do reset and test drive, the numer of times registered by the reader is alwys listed as 2 times with one pending no matter how many times I start and stop and exceed 60+mph the scanner shows the same 1 code P0300and 1 code P0300 pending. on occassion after 6 + warm up cycles the CEL will clear itself, but run it back up to 60+ and it starts all over again.
on October 06, 2010
It might be an ECM ground issue. Check out the following TSB and let us know what happened. If you want to see graphics I need your e-mail.

Bulletin No.: 06-06-04-046

Date: September 12, 2006

Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430

1999-2007 Cadillac, Chevrolet, GMC Full-Size Pickup and/or Utility Trucks

with 4.8L, 5.3L, 5.7L, 6.0L or 6.2L VORTEC GEN III, GEN IV, V-8 Engine (VINs V, C, T, Z, B, 3, M, 0, J, R, U, N, Y, K, 8 - RPOs LR4, LY2, LM7, L59, L33, LC9, LH6, LMG, LY5, L31, LQ4, LQ9, L76, LY6, L92)

with Active Fuel Management(TM) and E85 Flex Fuel

If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition.

The wire terminal (G103) attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup and/or Utility Trucks. If the ECM ground terminal has been found to be corroded, then follow the service procedure outlined in this bulletin to correct the corrosion issue.
Remove either the nut or bolt securing the main engine wiring harness ground terminal (G103) to the right cylinder head. Refer to the above illustration to determine where the ground is located on the vehicle (1).

Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt.

Position the main engine wiring harness ground terminal and install the nut or bolt.


Tighten the retaining nut or bolt to 16 N.m (12 lb ft).

Apply some type of electrical moisture sealant to protect the harness terminal from further corrosion.
on October 06, 2010 I will let you know where ground is located when I find it and if it resolves my problem.
on October 06, 2010
OK, check your e-mail. It has a Chevy TSB subject. I hope you will receive the graphics.
Let me know.

on October 11, 2010
Cleaned ground terminal, reassembled and test drove , no change to P0300 code but for the first time I was able to get freeze frame data on the Actron scanner that I left attached while I did a 1-1/2 hr road trip. I am sending you the data as I don't know how to interpet.
Freeze frame data for Code P0300
Throttle Position 15.6%
Engine RPM 1769
Engine Speed 49mph
Load Value 10.5%
Air Flow Rate 23.32 Gr/Sec
Map Sensor 44 KPA
Coolant Temp. 194 Deg
Short term fuel trim 1 1.5%
Long term fuel trim 1 16.4%
ST FT 2 -4.6%
LT FT 2 17.1%
ST FT 3 83.5%
LT FT 3 95.3%
ST FT 4 -5.1%
lt FT 4 53.9%
Fuel System 1 Closed
Fuel System 2 Closed
on October 11, 2010
Do you happen to use K&N or similar air filter?
The ECM sees lean conditions.
Make sure there is no intake manifold leak, disconnected or leaking vacuum hoses. The best thing would be to check it with a smoke pump. The "not so safe" leak test is to spray carb cleaner to the intake runners and see if the idle changes. Don't do that when the engine is hot.
Make sure there is no exhaust leak.
There is a good MAF sensor cleaner made by CRC, most parts store sells it, clean the MAF sensor as directed, reset the adaptations and drive it again.
The best thing would be to see the live data instead of the freeze frame.
Do you have access to any other scanner to read codes?

on October 11, 2010
No K&N filter installed, new Fram when i changed plugs and wires, will clean MAF sensor and check vacuum again. I do not have access to another type of scanner at present. Do you do a smoke test wile engine is off ? I have heard of thois before but never tried it.
on October 11, 2010
Yes, the engine has to be off. The smoke pump will blow a thick smoke into the manifold through a disconnected vacuum hose. The intake hose and tail pipe has to be plugged.
If there is a leak you will be able to see the smoke, so you can pinpoint the location.
I'd do the test with cold and warm engine. Sometimes you can't find leak on a cold engine, but it starts to leak when warm.
Could you tell me what kind of spark plugs did you install?

on October 11, 2010
AC Delco 41-110
on October 11, 2010
OK, that should be the correct one.
on October 12, 2010
Cleaned MAF sensor but got same result at 59 mph. Simalar freeze frame results except ST FT #3 was 50.% and LT FT was - 78.4% Does large varience in #3 point to any specific area to look at more closely?
on October 12, 2010
Thanks for the update.
I was thinking about the whole situation and I found this pretty weird, not a "normal or usual" problem.
I did some more diggin' and came upon this. It seems to explain the P0300 random misfire and the sudden reach condition. I wonder if the 59 Mph would trigger this.
It would be great if a GM specialist would chime in I'm mainly an euro tech- however I'd like to get to the end of this.
Please read:

Accessories - DTC's P0300, P1380, P1381

Bulletin No.: 02-06-05-004b

Date: February 14, 2006

Misfire DTCs P0300, P1380, P1381 and Catalytic Converter Damage Due to Installation of Alarm Systems

2006 and Prior GM Passenger Cars and Light Duty Trucks
2006 and Prior HUMMER H2, H3
2006 and Prior Isuzu Light Duty Trucks


This bulletin is being revised to add model years. Please discard Corporate Bulletin Number 02-06-05-004A (Section 06 - Engine/Propulsion System). General Motors Engineering, in an effort to determine the root cause of catalytic converter damage, has determined that aftermarket alarm systems incorrectly installed in vehicles have the potential to cause misfire codes and damage to the converter. These alarm systems use a circuit interrupt which utilizes the ignition circuit on the vehicles.

These alarm systems utilize mechanical relays and normal vehicle movement can trigger these relays to engage and disengage the ignition circuit while the vehicle is in motion. These disruptions of the ignition circuit, which occur in milliseconds, may cause more fuel to be commanded. Overtime, this dumping of fuel on and off again can cause misfire codes and ultimately damage the converter assembly.
Important: Engineering could not identify any alarms that utilize solid state circuitry that would eliminate this concern. Because of this, it has been determined that all alarm systems must be routed through the starter circuit in order to avoid this condition.

Dealers must be aware of this issue and take note of the wiring on vehicles with alarm systems that come in for repair, particularly for catalytic converter damage that seem to have no known root cause.

Just 2 other question:
Have you been able to do an intake leak test?
Do you use E-85 Ethanol?
And more: (sorry, just keep thinking) or the transmission removed?
Was the Crank Position Sensor replaced or just removed lately?
When the SES light comes on at 59Mph, do you feel any performance issue - even for a second or two - like a hesitation?
on October 13, 2010
No aftermarket alarm, never use e85, have not done intake leak test yet, Crank Position Sensor untouched, am always checking for the feel of hesitation but never notice it. I'm curious as to if after I clear code and drive under 60 mph for a while and then at a rolling speed of 30+ mph I go all out, it will run to 85 or 90 with no hesitation and no CEL indicator light. If it was vacuum issue I would think this would be when it would be most prominent.
on October 20, 2010
Sound like you may have a problem with the fuel injector. They have been known to have problems and i have seen where cleaning them doesn't work. Also a possibility of debris in the fuel rail. Your computer is adding fuel to the system. Normally fuel trim will be -6 to 6%. The design of the fuel rail allows debris to be trapped in several injectors. I am currently chasing a similar problem although my fuel trims are good and my missfire counts on cylinders#1 and #6 increase evenly when problem occurs. Problem occurs intermitantly above 45mph at a steady cruise. Do you know what cylinders you are dropping. I don't feel the engine running rough and believe that only one cylinder is actually dropping and the other is a phantom reading since it fires on the same stroke. i'll keep you posted what i find.
on October 27, 2010
After using beter scanning tool found cylinder misfires on #1 37,285 #6 34851 and #5 736. Without having replaced any components we did a crankshaft position system variation learn procedure and ran the vehicle hard for about 40 minutes. No troube codes shown and for about a week now under all types of driving conditions no sign of po300 code. Being my vehicle has 215,000+ mile the relearn procedure seems to have been the best way to resolve my problem before replacing unneccessary components. Special Thanks to ZeeTech for all his input in resolving my problem..
on October 27, 2010
I really appreciate the update! We are thankful when clients responses back with the fix.
I'm glad it runs well now, I like to see problems disappear. You can see the difference between scanners now :) Being an Euro tech I also learned few things about GM systems by going through loads of TSBs and wiring diagrams. Sorry to communicate through e-mails, but there was just way too much info to post it here.
The other thing is, it's a lot easier to help someone if the person gives us enough information to work with, and you did. Nobody likes a post like: " I have a misfire, what should I do?"
Enjoy your truck now and you know, if there is a problem, you can always visit


on October 28, 2010
I have been following your posts with interest because I am having the same problem, and my truck has 217,000 miles. The minor difference is mine trips at 65 to 70 MPH. Can you tell me what exactly is a "crankshaft position system variation learn procedure" that you performed?
on October 28, 2010
If you post an e-mail, I can send the info for you also, however it requires an OE or equalent
diagnostic computer.


Actually, here it is:

The crankshaft position 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 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 sensor has been replaced.
CAUTION: Set the vehicle parking brake and block the drive wheels when performing the Crankshaft Position System Variation Learning Procedure in order to prevent personal injury. Release the throttle immediately when the engine starts to decelerate. 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.

IMPORTANT: 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 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 24X reference circuit.

If the scan tool indicates a problem with the Cam signal, refer to DTC P0341 Camshaft Position (CMP) Sensor Performance.
If the scan tool indicates a problem with the 3X crank signal, refer to DTC P1374 Crankshaft Position (CKP) High to Low Resolution Frequency Correlation.
If the scan tool indicates a problem with the 24X crank signal, refer to DTC P0336 Crankshaft Position (CKP) Sensor Circuit.
The scan tool crankshaft position system variation learn function will no be enabled until engine coolant temperature reaches 70°C (158°F).

Selecting the crankshaft position system variation learn procedure on the scan will command the PCM to enable CKP system variation learn fuel cutoff and allow the crankshaft position system variation compensating values to be stored in the PCM. The PCM must detect an engine speed of 5150 rpm (CKP system variation learn fuel cutoff) during the crankshaft position system variation learn procedure to store the crankshaft position system variation compensating values and complete the procedure.
Set the parking brake.
Block the drive wheels.
Ensure the hood is closed.
Start the engine and allow engine coolant temperature to reach at least 70°C (158°F).
Turn OFF the ignition switch.
Select and enable the crankshaft position variation learn procedure with the scan tool.
Start the vehicle.
Apply and hold the brake pedal firmly.
Ensure the transaxle is in park.
Increase accelerator pedal position until the fuel cutoff is reached at 5150 rpm. Immediately release the accelerator pedal after fuel cutoff is reached.
The crankshaft position system variation compensating values are learned when the rpm decreases back to idle. If the procedure terminates.
Observe DTC status for DTC P1336.
If the scan tool indicates that DTC P1336 ran and passed, the crankshaft position system variation learn procedure is complete. If the scan tool indicates DTC P1336 failed or not run, check for other DTCs. If no DTCs other than P1336 are set, repeat the crankshaft position system variation learn procedure as necessary.
on August 29, 2016
Questions I have 2005 Chevy express ran great just wanted to tune it up I did so an know I have code po300 an u can hear wire ark bad an missing bad at 65
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