Help With P0300 03 2500hd 6.0

whtailhunter319 · March 17, 2010

In the fleet I maintain we have several 2500HD's with 6.0 liters. A handfull of them keep throwing the P0300 DTC. Up to this point the shop managers fix for this was to clear the code and send them back out, only for the DTC to come back later on. Naturally the guys in the field could care less as long as the truck starts and moves so when the CEL comes back on they just keep on driving. Recently I had one come in for state inspection that had 2 DTC's P0300 and P0135. As this truck has been throwing the p0300 consistantly for the last 40,000 miles I made the assumtion that the O2 code was seperate and have replaced the 02 sensor which has made that DTC go away. Now i need to figure out the p0300 code as im sick of having several DTC's that are unknown because the light is always illuminated for the p0300 code. Being im a heavy equipment mechanic by trade and only get into the fleet vehicles on occasion Im here to look for some input. Ive scoured the internet for information on this issue but havent been able to isolate the problem. I was able to pull the freeze frame data for the P0300 last time it set and was hoping by putting that information on here someone can help me in the right direction. I noticed immediatly that the ST and LT FTRM3 were not even close to the others should that be an idicator? Not knowing buch about how the syustem works id appreciate any and all inputs.Here goes!

TPS 25.8

RPM 2065

Cacullated Load 11.7

MAF 33.04

MAP 41

Coolant temp 174

STFTRM1 5.4

LTFTRM1 7.0

STFTRM2 4.6

LTFTRM2 8.5

STFTRM3 14.8

LTFTRM3 45.3

STFTRM4 5.4

LTFTRM4 7.0

27 MPH

Fuel system1 Closed

Fuel system2 Closed

59ih340 · March 30, 2010

thank you for the info. I havent been able to get my hands on this truck again as of yet but as soon as i do i will get into it more with the info you provided and im sure ill be back with more questions. Thank you so much its greatly appreciated.

I have a 2003 2500HD, had the same problem. Only set the P0300 at 70MPH, dealer diagnosed vacumm leak. Crank relearn fixed it. Had it done at a shop, $60.

+1, Crank relearn

whtailhunter319 · March 25, 2010

thank you for the info. I havent been able to get my hands on this truck again as of yet but as soon as i do i will get into it more with the info you provided and im sure ill be back with more questions. Thank you so much its greatly appreciated.

minibike132 · March 20, 2010

Thank you for getting back to me. The scanner im using is an ACTRON CP9180. This is the only scanner i have access to that is freeze frame data capable. I brought in my vaccum gauge today and hooked that into the intake manifold. Let the engine warm up to operating temperature. At idle from 551RPM-593RPM it was indicatingslowly between 19 and 20 inches.
I don't use a vacuum gauge for this kind of diagnosis, it's not a very reliable way of showing anything other then stuck open valves IMO.

When i increased the throttle to approx 2000 RPM the gauge held at about 22 inches but the CEL began flashing as long as I held the throttle there.

The SES light will flash in a dead miss condition, there are levels of codes. Hard light on is something that will effect emissions, flashing light is something that will contaminate the cat, a dead miss is something that will dammage it.

I began to record data with the scan tool and it was showing a CEL light on even though it didnt set the light on steady. Also if I tapped the accelerator the vacumm gauge would jump significantly over 25. Does any of this information help? I can also give you the FUEL TRIM for that recorded data as well if that will help. Im not a "swap" type mechanic that is why i came here to look for some help to do a real diagnosis instead of shooting into the dark. At idle you can notice a lope in the engine. It does not throw any specific misfire codes per cylinder.

I know that everyone here thinks that P0300 means that it is a random miss and a specific cylinder miss will set a P0301-308. This is simply not the case. In 15 years at GM dealers and another 4 on non dealer shops I have only seen a P0301-308 a handfull of times in a GM vehicle. A dead miss on a cylinder will usually produce a P0300.

I did preform the TSB that called for cleaning the engine wiring harness grounds but this had no effect. As for using all the same fuel, that is not the case as they fuel up where ever the jobs are so they are getting random fuel, from what ive seen this P0300 code seems to be a common issue with these GM's. Initally when i first started getting my hands into these trucks I would do a tune up as they would be at 100K plus milage anyw3ays but this seems to have no effect on the ones that consistantly throw the P0300 code. Thank you for your help and hope we are able to straighten this out and able to began diagnosing all the "problem" children in the fleet. Thanks again and I look forward to your inputs!

More in next post

minibike132 · March 20, 2010

Does the scanner show live data. All the tools I use have a live missfire data screen that, on an 03 will show what cylinder is missfiring. These are super simple engines to diagnose missfires on if you have the equipment to do so. Everything can be swapped cylinder to cylinder for shotgun repairs without spending money if you don't have the equipment to diagnose it correctly.

Look for the missfire data, if it isn't there figure out a way to manually disable the cylinders one at a time. If the injector connectors are accessable to you this is where I would do the disable. Get it missfiring and kill cylinders one at a time. There should be an obvious change in the way it runs if you kill a cylinder that was working properly. When you get to the cylinder with no change you have your miss. Ignition parts are an easy swap on these so you can swap coil wire and plug cylinder to cylinder and see if the miss moves.

The fuel trims help diag if you look at them live. Ho2s 1 is on the 1,3,5,7 bank Ho2s 2 is the 2,4,6,8 bank. Ho2s 3 is the post converter on the odd bank and Ho2s 4 is the post converter on the even bank. The trims you are concerned with will be bank 1 and bank 2. If you are showing a bunch of trims like in the freeze frame just pay attention to shor term trims 1&2. 0 is the optimal trim, it will vary =/-5% on a normally running engine. Negative trims mean the PCM is requesting less fuel meaning the bank is rich. Positive trims mean that the PCM is trying to add fuel because the bank is lean.

If your trims are showing lean on the side that has the miss then forget about swapping the ign parts, no spark = rich exhaust. No fuel = lean exhaust.

minibike132 · March 18, 2010

In the fleet I maintain we have several 2500HD's with 6.0 liters. A handfull of them keep throwing the P0300 DTC. Up to this point the shop managers fix for this was to clear the code and send them back out, only for the DTC to come back later on. Naturally the guys in the field could care less as long as the truck starts and moves so when the CEL comes back on they just keep on driving. Recently I had one come in for state inspection that had 2 DTC's P0300 and P0135. As this truck has been throwing the p0300 consistantly for the last 40,000 miles I made the assumtion that the O2 code was seperate and have replaced the 02 sensor which has made that DTC go away. Now i need to figure out the p0300 code as im sick of having several DTC's that are unknown because the light is always illuminated for the p0300 code. Being im a heavy equipment mechanic by trade and only get into the fleet vehicles on occasion Im here to look for some input. Ive scoured the internet for information on this issue but havent been able to isolate the problem. I was able to pull the freeze frame data for the P0300 last time it set and was hoping by putting that information on here someone can help me in the right direction. I noticed immediatly that the ST and LT FTRM3 were not even close to the others should that be an idicator? Not knowing buch about how the syustem works id appreciate any and all inputs.Here goes!

TPS 25.8

RPM 2065

Cacullated Load 11.7

MAF 33.04

MAP 41

Coolant temp 174

STFTRM1 5.4

LTFTRM1 7.0

STFTRM2 4.6

LTFTRM2 8.5

STFTRM3 14.8

LTFTRM3 45.3

STFTRM4 5.4

LTFTRM4 7.0

27 MPH

Fuel system1 Closed

Fuel system2 Closed

What kind of scanner are you using? The reason I ask is that a valid pid for fuel trim is -20 to +20. 45 isn't even a number that the pcm should put out. Also it should only put out a bank 1 fuel trim and a bank 2 fuel trim. Here you have 4 pid's for fuel trim. You are going to need to get a different type of scanner on this to know if fuel trim is helping out your diagnosis. Right now I think it's just a number that doesn't look right because it just shouldn't be there.

Any time there is a miss fuel trim should help you out, sometimes it will help you pin point the issue sometimes just rule something out.

Example: yesterday I had an 04 2500 with a 5.7 with a p0300, this is a pretty close engine set up on a diagnostic scale for missfire. I had a dead miss on #1 (which a good scanner can show on any GM vehicle since 1997). I looked at my fuel trim and I could see that it was lean, very very lean. Now if you have low compression for any reason it should be rich because you have unburnt fuel. Same thing for an ignition issue. Same thing if you have an injector dumping fuel. But if you are lean and it is only one cylinder then you know you have a clogged injector or huge vacuum leak on only one cylinder. Vacuum leak of that size on one cylinder, I don't think so. It has to be stuck injector. I knew the problem off of the data because it was lean in this case, if it was rich I would have had to do further testing to get it nailed down.

The pid you have shows lean, but it is lean off the scale of what is a number that is valid making me think the number you have there is a miscommunication between the scanner and the vehicle. If it was full lean you would have a dead miss that would be un mistakeable by anyone that is around an engine on any regular basis. Try a different brand scanner, if you can't find one then send the vehicle out. You are only going to shotgun parts at this thing to hope to find an intermittant miss without one. On that engine it can get more expensive then a fleet manager wants to see.

Come back with different numbers that make sense and I will be glad to help you as best as the internet allows.

minibike132 · March 18, 2010

Forgot to mention, you have intermittant missfires across the whole fleet. If this vehicle is intermittant and not a repeatable miss, where is the fuel coming from? Is the whole fleet using the same fuel. If so switch it up and see if that makes a difference. It doesn't take much of an issue with the fuel to cause an intermittant miss. If you fill up at the same place all the time and there is 1% too much ethenol or a little water in the mix you will have issues.

If you have your own tanks check the gas.

bmgmike · March 28, 2010

thank you for the info. I havent been able to get my hands on this truck again as of yet but as soon as i do i will get into it more with the info you provided and im sure ill be back with more questions. Thank you so much its greatly appreciated.

I have a 2003 2500HD, had the same problem. Only set the P0300 at 70MPH, dealer diagnosed vacumm leak. Crank relearn fixed it. Had it done at a shop, $60.

whtailhunter319 · March 18, 2010

Thank you for getting back to me. The scanner im using is an ACTRON CP9180. This is the only scanner i have access to that is freeze frame data capable. I brought in my vaccum gauge today and hooked that into the intake manifold. Let the engine warm up to operating temperature. At idle from 551RPM-593RPM it was indicatingslowly between 19 and 20 inches. When i increased the throttle to approx 2000 RPM the gauge held at about 22 inches but the CEL began flashing as long as I held the throttle there. I began to record data with the scan tool and it was showing a CEL light on even though it didnt set the light on steady. Also if I tapped the accelerator the vacumm gauge would jump significantly over 25. Does any of this information help? I can also give you the FUEL TRIM for that recorded data as well if that will help. Im not a "swap" type mechanic that is why i came here to look for some help to do a real diagnosis instead of shooting into the dark. At idle you can notice a lope in the engine. It does not throw any specific misfire codes per cylinder. I did preform the TSB that called for cleaning the engine wiring harness grounds but this had no effect. As for using all the same fuel, that is not the case as they fuel up where ever the jobs are so they are getting random fuel, from what ive seen this P0300 code seems to be a common issue with these GM's. Initally when i first started getting my hands into these trucks I would do a tune up as they would be at 100K plus milage anyw3ays but this seems to have no effect on the ones that consistantly throw the P0300 code. Thank you for your help and hope we are able to straighten this out and able to began diagnosing all the "problem" children in the fleet. Thanks again and I look forward to your inputs!