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I recently purchased a 2004 Base model c5 from a dealer (98k miles has Callaway honker, short tube headers only mods we know have been done.) . During test everything was fine. After getting the car home the check engine came on, we read the code and it was P0300, naturally we were thinking probably just needs a tune car had sat for a bit and wasn't driven a ton. The strangest thing is the P0300 is the only code present and it is only active at idle. While driving the code is not current. (Idles around 550rpm.) It also wants to stall on hot starts (not sure if the 2 are related)
The following weekend we did a full tuneup. Plugs, wires, coil packs, upstream O2's, cleaned throttle body, cleaned MAF sensor, new thermostat, coolant flush, replaced the upper and lower radiator hoses because they were original and really soft. Was fine for a day, next day P0300 came back (again, no other codes present only at idle).
We then took it to a shop to see if they could find anything, ran a compression test (checked out) he ended up putting a new MAF on and put the factory tune on the vehicle. Again ok for a day or so then P0300 back on.
The next weekend we did throttle body gasket, valley pan gasket, new knock sensors, intake manifold gaskets and Cam sensor. P0300 again.
We have done smoke tests on the engine (no visible vacuum leaks), Tested fuel pressure (right at 60psi, believe factory is 59?), I've done the screwdriver stethoscope on the fuel injectors they all seem to be operating fine.
When I have it on the OBD2 live data shows that the PCM is adding fuel, when I first checked it, bank 1 and 2 LTFT were both around 11% Yesterday after doing the Crank Sensor Bank 1 was around 8% and bank 2 was around 5%.
But still P0300 only while Idling. And it really didn't look like any specific cylinder missing when I looks with the shops scanner it did not look like any one cylinder missing. They really do seem to be random.
Really just at a loss to what else it could be at this point. Any help is greatly appreciated.
Holy S##T did you fire the parts canon !!…you should have taken the car to a diagnostic shop and you would have only needed to replace 1 part…even though you did a compression test that only tells you the ability of the combustion chamber to seal…something like a sticky valve would not have been able to be seen doing a compression test…with random misfires you may have worn TT couplings…there is a TSB out for a while about this…not sure why your mechanic didn’t look at TSB’s ??…you ALWAYS look at TSB’s first !!…if idle only I’d be looking at possibly ignition possibly coil “control”
intermittently dropping out but you would need a lab scope to see this…if the car sat for a while I’d tell the mechanic to do an injector balance test to see if any injectors are clogged.
Holy S##T did you fire the parts canon !!…you should have taken the car to a diagnostic shop and you would have only needed to replace 1 part…even though you did a compression test that only tells you the ability of the combustion chamber to seal…something like a sticky valve would not have been able to be seen doing a compression test…with random misfires you may have worn TT couplings…there is a TSB out for a while about this…not sure why your mechanic didn’t look at TSB’s ??…you ALWAYS look at TSB’s first !!…if idle only I’d be looking at possibly ignition possibly coil “control”
intermittently dropping out but you would need a lab scope to see this…if the car sat for a while I’d tell the mechanic to do an injector balance test to see if any injectors are clogged.
🤦♂️ it’s only at idle and it is totally random across all cylinders. But it’s at idle in park,neutral and drive which is why I didn’t really lean towards TT I just shut off the injectors one by one using my scanner so I think they’re all working properly.
i did connect a pressure gauge to the fuel rail today. Immediately on power it shot to 51psi (which I think is low) and then after about 10 minutes it went down to 40psi. Not sure if that’s normal. Once I started the engine the psi needle was going wild which makes me think possibly fuel injector damper?
I did run a fuel trim relearn through my scanner and the mis counter seemed to lessen but still randoms recording.
KOEO (Key On Engine Off) you should be seeing 55-62 psi…you are only seeing 51 so that’s too low…can you trust your fuel pressure gauge ??…fuel pressure should be within those limits KOEO and not drop on a snap acceleration…fuel volume is just as important as fuel pressure…even sitting fuel pressure should not decrease and you see 40 psi…if your fuel pump is bad I find it hard to believe it only misfires at idle…if you trust your pressure gauge and can test it on a WOT run see what the pressure drops to…it should NOT drop…even without a pressure gauge installed if you were look at your upstream O2 sensors they should go full rich (800mV to 1 volt) under WOT…you most likely have a bad pump or HIGH resistance in the fuel pump circuit…if you had an amp clamp and look at fuel pump current you should see around 5-6 amps…you could use your DVOM across terminals 30 and 87 of the fuel pump relay with the key ON but most DVOM are limited to 10 amps…over 10 amps you’ll blow the fuse in your DVOM…if you saw low amperage that could point to high resistance in the circuit or a weak pump…if high current flow the fuel pump is struggling to turn to operate which can be a filter clog…being you have a 2004 with the FFS fuel system (look at your RPO codes in the glovebox and see if you see FFS) your pump is at the top of the tank and the fuel filter is internal…”fuel trim relearn” has nothing to do with your fuel pressure…best thing if your fuel pressure numbers are accurate is to do a voltage drop check on both the positive and negative side of the fuel pump circuit with a DVOM…if the “we” in your verbiage means your mechanic he should know how to do the test…if not he is in the wrong business.
This is the circuit…as you can see there are a few connectors (C140,402, and 412)…you will need these to figure out where and if you have high resistance in the pump circuit…C140 is one of the connectors outboard of the battery…C402 and 412 are also on the ground side of the pump circuit along with G401 which is located on the left frame rail and can be seen with the left rear tire removed…I’d check G401 first…you can even check current flow on the pump circuit on the ground side with an amp clamp too.
KOEO (Key On Engine Off) you should be seeing 55-62 psi…you are only seeing 51 so that’s too low…can you trust your fuel pressure gauge ??…fuel pressure should be within those limits KOEO and not drop on a snap acceleration…fuel volume is just as important as fuel pressure…even sitting fuel pressure should not decrease and you see 40 psi…if your fuel pump is bad I find it hard to believe it only misfires at idle…if you trust your pressure gauge and can test it on a WOT run see what the pressure drops to…it should NOT drop…even without a pressure gauge installed if you were look at your upstream O2 sensors they should go full rich (800mV to 1 volt) under WOT…you most likely have a bad pump or HIGH resistance in the fuel pump circuit…if you had an amp clamp and look at fuel pump current you should see around 5-6 amps…you could use your DVOM across terminals 30 and 87 of the fuel pump relay with the key ON but most DVOM are limited to 10 amps…over 10 amps you’ll blow the fuse in your DVOM…if you saw low amperage that could point to high resistance in the circuit or a weak pump…if high current flow the fuel pump is struggling to turn to operate which can be a filter clog…being you have a 2004 with the FFS fuel system (look at your RPO codes in the glovebox and see if you see FFS) your pump is at the top of the tank and the fuel filter is internal…”fuel trim relearn” has nothing to do with your fuel pressure…best thing if your fuel pressure numbers are accurate is to do a voltage drop check on both the positive and negative side of the fuel pump circuit with a DVOM…if the “we” in your verbiage means your mechanic he should know how to do the test…if not he is in the wrong business.
im even more lost now I thought all 04’s had FFS but I don’t have it on the RpO list
It should have it since you have a 2004…see if your fuel pump is at the bottom of the left fuel tank…what is the 10th digit in your VIN ??…it would be a 4 if you have a 2004.
It should have it since you have a 2004…see if your fuel pump is at the bottom of the left fuel tank…what is the 10th digit in your VIN ??…it would be a 4 if you have a 2004.
I didn’t have a chance to get the car off the ground today but possibly have some more info.
after the fuel trim learn the LTFT came down from about 14 on both banks to about 7 on bank 1 and 4 on bank 2.
the misfire counter is legitamately every cylinder missing and the high counts vary cycle per per cycle. They are also really only present under 1000 rpm’s we checked with the scanner at several rpm levels and over 1k the numbers were all at 0.
Probably also worth noting the Evap monitors have not completed once since I got the car. (About a month ago) No evap codes though.
Rechecked fuel pressure, while pump is priming it’s at 60, then once pump shuts off 50. While idling sits right around 60. If you prime the pump and let it sit for a while it does drop to below 40 still. Running pressure is fine though.
It should have it since you have a 2004…see if your fuel pump is at the bottom of the left fuel tank…what is the 10th digit in your VIN ??…it would be a 4 if you have a 2004.
also 10th digit is a 4 but ffs is not listed in the RPO codes and also the we is my brother and I. Usually we’re pretty good at figuring cars out but this one really has us stumped
St. Jude Donor '14-'15-'16-'17-'18-'19-'20-'21-'22-'23
Originally Posted by Sean04C5
also 10th digit is a 4 but ffs is not listed in the RPO codes and also the we is my brother and I. Usually we’re pretty good at figuring cars out but this one really has us stumped
That is actually a picture of a 97-02 fuel pump. Your 04 is on the top of the tank
If you do have a 2004 maybe the glovebox was replaced at some point with a used one…when I purchased my 2008 the PO had the glovebox replaced at a dealership so there was no RPO code sticker when I purchased it…I went to a Chevy dealer in town with my VIN and the parts department ran a report with all my RPO’s….seeing where your fuel tank is located is the important part…the picture I posted was of my old 2001…if you have kept your fuel tank near full or less than 1/4 tank the EVAP monitor will not run and there are other factors which will cause it not to run such as ECT, IAT and some others when you start the car and when it’s running…if you had a factory level scan tool you can do a “Service Bay Test” and bypass all those other requirements…plus or minus 10% is good for fuel trims but you should look at “total fuel trims” which is long terms plus short terms…as far as fuel pressure dropping after the fuel pump primes there are limits for that…a drop of more than 5 psi in one minute is bad…I had someone in my town over yesterday with an 01 who says his car takes a few seconds to start after it sits…I used a Mityvac and was able to diagnose a bad fuel feed line check valve…now as far as your misfires I don’t know if any one or more cylinders are counting up more than others…if you only see a few misfires on every cylinder but one cylinder has 100 misfires I’d worry about that one…the crankshaft position sensor is the engines “misfire monitor” and if that things is wigging out that may be a cause…DO YOU ACTUALLY FEEL THE MISFIRES ??…if the car is running smooth but only shows misfires it can be the crank sensor…if all cylinders were misfiring the car will be running poorly at idle if at all…BTW, if you purchased it recently from a dealership did you buy it with any type of warranty ??…if so I’d bring it back to the dealership and let them fix it.
If you do have a 2004 maybe the glovebox was replaced at some point with a used one…when I purchased my 2008 the PO had the glovebox replaced at a dealership so there was no RPO code sticker when I purchased it…I went to a Chevy dealer in town with my VIN and the parts department ran a report with all my RPO’s….seeing where your fuel tank is located is the important part…the picture I posted was of my old 2001…if you have kept your fuel tank near full or less than 1/4 tank the EVAP monitor will not run and there are other factors which will cause it not to run such as ECT, IAT and some others when you start the car and when it’s running…if you had a factory level scan tool you can do a “Service Bay Test” and bypass all those other requirements…plus or minus 10% is good for fuel trims but you should look at “total fuel trims” which is long terms plus short terms…as far as fuel pressure dropping after the fuel pump primes there are limits for that…a drop of more than 5 psi in one minute is bad…I had someone in my town over yesterday with an 01 who says his car takes a few seconds to start after it sits…I used a Mityvac and was able to diagnose a bad fuel feed line check valve…now as far as your misfires I don’t know if any one or more cylinders are counting up more than others…if you only see a few misfires on every cylinder but one cylinder has 100 misfires I’d worry about that one…the crankshaft position sensor is the engines “misfire monitor” and if that things is wigging out that may be a cause…DO YOU ACTUALLY FEEL THE MISFIRES ??…if the car is running smooth but only shows misfires it can be the crank sensor…if all cylinders were misfiring the car will be running poorly at idle if at all…BTW, if you purchased it recently from a dealership did you buy it with any type of warranty ??…if so I’d bring it back to the dealership and let them fix it.
it is definitely misfiring. You can feel and see it. But it’s not 1 cylinder going up to 30 misses consistently. It will be cylinder 4 30 on one cycle, cylinder 1 27 on the next cycle, cylinder 7 25 on the next cycle and so on. While the rest consistently read 6-10 on each cycle.
Sometimes you’ll see misfires on companions cylinders so the companion cylinders are 1-6, 8-5, 7-4, 2-3….if 1 is misfiring you may pick up a misfire on its companion 6…1 is on the compression stroke and 6 is on the exhaust stroke but both are at TDC…also the cylinder before or after the cylinder in the firing order may be misfiring…if it is a flakey crank sensor you won’t be able diagnose it accurately without a scope and I don’t think you have one of those. Maybe you should find a diagnostic shop in your area and let them look at it…almost all use a scope…I love diagnosing misfires because sometimes they can be a challenge…if the car is under a used car warranty bring it back to that dealership…you said the car “sat for a bit” so I’d do an injector balance test if you have a scan tool capable of doing one…you may have some clogged injectors…a diagnostic shop will look at your secondary ignition waveforms and look at the coil pack current ramps…pics below of the ignition waveform and current ramp…without the right diagnostic equipment you and your brother will only be guessing…if the parts you changed are aftermarket and not OEM that would also be suspect.
Sometimes you’ll see misfires on companions cylinders so the companion cylinders are 1-6, 8-5, 7-4, 2-3….if 1 is misfiring you may pick up a misfire on its companion 6…1 is on the compression stroke and 6 is on the exhaust stroke but both are at TDC…also the cylinder before or after the cylinder in the firing order may be misfiring…if it is a flakey crank sensor you won’t be able diagnose it accurately without a scope and I don’t think you have one of those. Maybe you should find a diagnostic shop in your area and let them look at it…almost all use a scope…I love diagnosing misfires because sometimes they can be a challenge…if the car is under a used car warranty bring it back to that dealership…you said the car “sat for a bit” so I’d do an injector balance test if you have a scan tool capable of doing one…you may have some clogged injectors…a diagnostic shop will look at your secondary ignition waveforms and look at the coil pack current ramps…pics below of the ignition waveform and current ramp…without the right diagnostic equipment you and your brother will only be guessing…if the parts you changed are aftermarket and not OEM that would also be suspect.
Could it be something with the reluctor wheel? And if so how would you check?
Everything we bought was oem, we managed to get a faulty Delco cam sensor so there is a duralast in there for now until the OEM gets here but no codes going for the duralast as of now.
Sometimes you’ll see misfires on companions cylinders so the companion cylinders are 1-6, 8-5, 7-4, 2-3….if 1 is misfiring you may pick up a misfire on its companion 6…1 is on the compression stroke and 6 is on the exhaust stroke but both are at TDC…also the cylinder before or after the cylinder in the firing order may be misfiring…if it is a flakey crank sensor you won’t be able diagnose it accurately without a scope and I don’t think you have one of those. Maybe you should find a diagnostic shop in your area and let them look at it…almost all use a scope…I love diagnosing misfires because sometimes they can be a challenge…if the car is under a used car warranty bring it back to that dealership…you said the car “sat for a bit” so I’d do an injector balance test if you have a scan tool capable of doing one…you may have some clogged injectors…a diagnostic shop will look at your secondary ignition waveforms and look at the coil pack current ramps…pics below of the ignition waveform and current ramp…without the right diagnostic equipment you and your brother will only be guessing…if the parts you changed are aftermarket and not OEM that would also be suspect.
No, you’re looking at injector pulse width (average) which is the time in milliseconds the injector stays open…pic below and with a scope you can measure that time…pulse width is normally around 1.5 milliseconds to 2 milliseconds…the black arrow is pointing to the injector on time.
Could it be something with the reluctor wheel? And if so how would you check?
Everything we bought was oem, we managed to get a faulty Delco cam sensor so there is a duralast in there for now until the OEM gets here but no codes going for the duralast as of now.
Your fuel trims look OK and the car has no issues starting or backfiring so I don't it has shifted radially...to check for a radial shift occurrence a cam-crank correlation with a scope will need to be done...below is a cam-crank from my 2001...cam is a 1X and crank is a 24X...there are 24 pulses between each cam sensor signal...if the cam and crank leading or trailing edges don't line up like this picture the reluctor has shifted radially...red is cam and blue is crank....you can remove the crank sensor and maybe see if the reluctor has moved axially which will diminish the voltage you see...0-12 volt square wave,..you may only see 0-8 volts.
Your fuel trims look OK and the car has no issues starting or backfiring so I don't it has shifted radially...to check for a radial shift occurrence a cam-crank correlation with a scope will need to be done...below is a cam-crank from my 2001...cam is a 1X and crank is a 24X...there are 24 pulses between each cam sensor signal...if the cam and crank leading or trailing edges don't line up like this picture the reluctor has shifted radially...red is cam and blue is crank....you can remove the crank sensor and maybe see if the reluctor has moved axially which will diminish the voltage you see...0-12 volt square wave,..you may only see 0-8 volts.
It actually has been having trouble starting lately. But its really strange. It'll crank but not start and then the second try everytime will fire right up. I will definitely have to get it scoped it looks like.
04 C5 P0300 nightmare
