Misfires
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Misfires P0300 P030X Rough Idle
Misfires or rough idle
Misfires and rough idle have a lot of the same diagnostic process and symptoms so I have combined them here to avoid two long overlapping articles.. Use some judgement to decide where to focus your efforts depending on your symptoms.
Misfires or the “dreaded P0300” as it’s often called. Dreaded because it has many potential causes and can be a tough challenge to locate this problem, but this is solvable if you are methodical and persistent. Misfires can be felt as a stumble in the engine, seen as a change in rpm or it can be measured and sometimes reported as an ODBII trouble code. P0300 means random misfire, the computer is seeing more than one cylinder report a misfire and P030x are misfires reported in that respective cylinder. I think that most people just assume that a misfire is an electrical problem but this is not always true. The misfire counter function actually monitors the crankshaft rotation speed and when there is a small change in rpm near when a particular cylinder is firing then the ECM reports a “misfire” or more correctly a momentary stumble in engine speed. The implied cause of this is some type of operational problem related to that cylinder. That is if the ECM has properly interpreted which cylinder is causing the problem.
A few misfires does not mean that you have a problem that needs the parts cannon. Most of our trucks have over a hundred thousand miles on them and there is wear and tear everywhere so we have many parameters in our engines that are not up to original factory spec and many of those accumulated minor issues could cause an occasional misfire to be reported. Chasing perfection on the misfire counters on a 20 year old engine can be a waste of time and money. I recently had a scan and arthritis in some joints was observed to which I thought, ya no @$&#. I have had pain there for years but I’m 66 years old and very active so I have a body full of wear and tear. I’m still getting around OK so I would not even think about surgery at this point. That said, the misfire counters can be a valuable diagnostic tool if there is a problem that needs to be resolved.
Most people find out about misfire diagnostic codes when the check engine light (CEL) comes on and they hook up an ODBII code scanner and see a P030x code. Then the Google explains what a misfire code is. Most codes are set when a problem occurs enough times or reaches a certain threshold to be considered a persistent problem worthy of attention according to the ECM rules so if you have a misfire code it has probably happened quite a few times. BTW, if your CEL is flashing then you have a severe misfire condition that requires immediate attention, That many misfires means your engine is running really bad and you can have terrible fuel economy, high emissions (what ODBII cares most about), can get engine damage or can actually kill your catalytic converter if you have a persistent rich fuel condition.
Some people see misfires when they have a scanner with a misfire counter that shows the actual number of misfires reported regardless of trouble code status. This is where you have to use some judgment because again, a low level of misfires reported with no trouble codes and an engine that is running good enough for 250,000 miles might need to be ignored. That said, we have a few members here that provide really good guidance for comprehensive code scanning and live data on the cheap. One such solution is a Bluetooth dongle in the ODBII port reporting to inexpensive Android software showing live misfire counting and accumulated misfire counts on dashboard gauge displays. I have used such capability myself and a persistent misfire problem observed can be a valuable tool for diagnosing a problem. My best example of using this tool was on a Toyota with 4 coil on plug modules. The truck started running like crap but no codes yet. The misfire display in live data showed many misfires on one cylinder. I swapped coil packs with the adjacent cylinder and the misfire counts followed the coil pack so I replaced that coil and the problem was solved. 15 minutes, one part, back on the road. I know that we have a single old school ignition coil on our trucks but this is a good example of how this parameter can be a useful diagnostic tool. This same strategy can be used for swapping plugs or ignition wires for instance on your Blazer. High levels of misfire counts on a single cylinder is way more useful that an occasional random misfire which can be difficult to interpret.
So you have actual misfire codes and a problem with your truck that needs to be resolved. Maybe it’s running really rough, doesn’t accelerate properly under a load or your getting poor fuel economy. Maybe your CEL is flashing and so you really need to figure out what’s happening. Misfires can be caused by one or more items from four main areas:
Fuel delivery
Ignition
Electrical
Engine mechanical issues
There are over a dozen possibilities so we will go through them in order of prevalence to help get a "hit" early on. If you know the history of the truck or have codes then by all means change up the order if the clues send you there.
1) Codes
Are there any codes? Don't forget pending codes and freeze frame data to learn as much as possible about the conditions that cause the problem. I have an article written by Les on scanners in my signature.
2) Work history
Where have you been on the truck, what has been replaced or moved around. Are there any related symptoms that get you started?
3) Misfire counter
In live data on your scanner or BT code set up, is it all cylinders randomly or individual cylinders?
4) Fuel trims/O2 sensors
Is the engine running rich or lean? I have a fuel trim article in my signature. If so, look up the causes for lean or rich codes to modify the order that you go through this guide. Are the O2 sensor waveforms OK? I have an article here. Does it run better or worse at the moment of transition to closed loop operation when the O2 sensors take over fuel control?
5) Fuel pressure
Perform the test in the sticky. Also, could your fuel be contaminated? Replace the fuel filter.
6) Check sensors
Check 5V ref and 12v ref on at least one pink sensor power wire. Check the static values of ECT, IAT, MAP, TPS and the output of the MAF sensor in live data.
7) Injector function
At the harness connector, check for proper battery voltage at the power feed for the injector (pink for most modules). Check for pulse control with a noid light or test probe on all cylinders. Consider an injector balance test with a pulse box where you watch the pressure drop/fuel delivery of each injector. Look for less tan a 1.5 psi spread with a target around 3X psi. Consider measuring injector coil resistance and look for a target of 11-14 ohms with a range of less than 3 ohms across the injectors.
8) Cylinder ignition
If your misfires are random or you have say a rough idle with no misfire counters then jump to more generic steps starting in 8), if you have high individual misfire counts then start here. On the affected cylinder check the condition of the spark plug. If its running rich or lean? Test the strength and cadence of the spark at the plug, look for 1" of regular strong spark. What's the resistance and condition of the spark plug wire, look for 625 ohms per foot resistance. Repeat for all misfiring cylinders. Check the distr cap for arcing/carbon traces. Measure CMP retard and adjust to with +/-2. Is your distr gear worn out? Check your ignition wire order and the seating of all the boots. You can verify poor cylinder performance with a drop cylinder test.
9) Arcing
Check for arcing in the dark by misting over the ignition components with a water bottle
10) Vacuum leaks
Check hoses, gaskets, intake connections. Do your LIM gaskets need replacing? Look for lean fuel trims that improve at 2500rpm
11) Power and grounds
Check the pink wire power and grounds for the PCM, ICM and coil
12) Test the ICM and coil
Look at the ICM control output and grounded coil spark, or have then tested.
13) Exhaust restriction/leaks
Check exhaust back pressure at an O2 sensor port. Check cat performance at the rear O2 sensor and check temps with an IR gun. Check for exhaust leaks
14) Check the EGR valve and PCV
15) Check the IAC valve and idle speed
16) Does it run better or worse after everything gets hot. Common components that can fail after they warm up are ignition coils and ICM's
17) Do a compression test to rule out cylinder mechanical issues like valve problems or rings. Listen for bearing knock or other mechanical sounds. If you are at this step and none of the previous tests have found the problem then it may be time for a dynamic compression test to pinpoint the cause of your misfire:
https://www.alldata.com/us/en/suppor...kId=4010940812
BTW, if you don’t have a Snap On scanner, a EE degree and tons of time to burn, It’s not classic diagnostics and problem solving but sometimes and educated guess after considering the list above makes sense. That’s what I did in my Toyota coil example. Other “known good” or adjacent parts swapping can help get to the bottom of this if necessary.
Good luck, misfires can be a wild ride but it’s fun when you solve these harder problems. Well at least it is for me, I love working on difficult problems! LOL. Let us know if you need help performing any of these tests.
George
Misfires and rough idle have a lot of the same diagnostic process and symptoms so I have combined them here to avoid two long overlapping articles.. Use some judgement to decide where to focus your efforts depending on your symptoms.
Misfires or the “dreaded P0300” as it’s often called. Dreaded because it has many potential causes and can be a tough challenge to locate this problem, but this is solvable if you are methodical and persistent. Misfires can be felt as a stumble in the engine, seen as a change in rpm or it can be measured and sometimes reported as an ODBII trouble code. P0300 means random misfire, the computer is seeing more than one cylinder report a misfire and P030x are misfires reported in that respective cylinder. I think that most people just assume that a misfire is an electrical problem but this is not always true. The misfire counter function actually monitors the crankshaft rotation speed and when there is a small change in rpm near when a particular cylinder is firing then the ECM reports a “misfire” or more correctly a momentary stumble in engine speed. The implied cause of this is some type of operational problem related to that cylinder. That is if the ECM has properly interpreted which cylinder is causing the problem.
A few misfires does not mean that you have a problem that needs the parts cannon. Most of our trucks have over a hundred thousand miles on them and there is wear and tear everywhere so we have many parameters in our engines that are not up to original factory spec and many of those accumulated minor issues could cause an occasional misfire to be reported. Chasing perfection on the misfire counters on a 20 year old engine can be a waste of time and money. I recently had a scan and arthritis in some joints was observed to which I thought, ya no @$&#. I have had pain there for years but I’m 66 years old and very active so I have a body full of wear and tear. I’m still getting around OK so I would not even think about surgery at this point. That said, the misfire counters can be a valuable diagnostic tool if there is a problem that needs to be resolved.
Most people find out about misfire diagnostic codes when the check engine light (CEL) comes on and they hook up an ODBII code scanner and see a P030x code. Then the Google explains what a misfire code is. Most codes are set when a problem occurs enough times or reaches a certain threshold to be considered a persistent problem worthy of attention according to the ECM rules so if you have a misfire code it has probably happened quite a few times. BTW, if your CEL is flashing then you have a severe misfire condition that requires immediate attention, That many misfires means your engine is running really bad and you can have terrible fuel economy, high emissions (what ODBII cares most about), can get engine damage or can actually kill your catalytic converter if you have a persistent rich fuel condition.
Some people see misfires when they have a scanner with a misfire counter that shows the actual number of misfires reported regardless of trouble code status. This is where you have to use some judgment because again, a low level of misfires reported with no trouble codes and an engine that is running good enough for 250,000 miles might need to be ignored. That said, we have a few members here that provide really good guidance for comprehensive code scanning and live data on the cheap. One such solution is a Bluetooth dongle in the ODBII port reporting to inexpensive Android software showing live misfire counting and accumulated misfire counts on dashboard gauge displays. I have used such capability myself and a persistent misfire problem observed can be a valuable tool for diagnosing a problem. My best example of using this tool was on a Toyota with 4 coil on plug modules. The truck started running like crap but no codes yet. The misfire display in live data showed many misfires on one cylinder. I swapped coil packs with the adjacent cylinder and the misfire counts followed the coil pack so I replaced that coil and the problem was solved. 15 minutes, one part, back on the road. I know that we have a single old school ignition coil on our trucks but this is a good example of how this parameter can be a useful diagnostic tool. This same strategy can be used for swapping plugs or ignition wires for instance on your Blazer. High levels of misfire counts on a single cylinder is way more useful that an occasional random misfire which can be difficult to interpret.
So you have actual misfire codes and a problem with your truck that needs to be resolved. Maybe it’s running really rough, doesn’t accelerate properly under a load or your getting poor fuel economy. Maybe your CEL is flashing and so you really need to figure out what’s happening. Misfires can be caused by one or more items from four main areas:
Fuel delivery
Ignition
Electrical
Engine mechanical issues
There are over a dozen possibilities so we will go through them in order of prevalence to help get a "hit" early on. If you know the history of the truck or have codes then by all means change up the order if the clues send you there.
1) Codes
Are there any codes? Don't forget pending codes and freeze frame data to learn as much as possible about the conditions that cause the problem. I have an article written by Les on scanners in my signature.
2) Work history
Where have you been on the truck, what has been replaced or moved around. Are there any related symptoms that get you started?
3) Misfire counter
In live data on your scanner or BT code set up, is it all cylinders randomly or individual cylinders?
4) Fuel trims/O2 sensors
Is the engine running rich or lean? I have a fuel trim article in my signature. If so, look up the causes for lean or rich codes to modify the order that you go through this guide. Are the O2 sensor waveforms OK? I have an article here. Does it run better or worse at the moment of transition to closed loop operation when the O2 sensors take over fuel control?
5) Fuel pressure
Perform the test in the sticky. Also, could your fuel be contaminated? Replace the fuel filter.
6) Check sensors
Check 5V ref and 12v ref on at least one pink sensor power wire. Check the static values of ECT, IAT, MAP, TPS and the output of the MAF sensor in live data.
7) Injector function
At the harness connector, check for proper battery voltage at the power feed for the injector (pink for most modules). Check for pulse control with a noid light or test probe on all cylinders. Consider an injector balance test with a pulse box where you watch the pressure drop/fuel delivery of each injector. Look for less tan a 1.5 psi spread with a target around 3X psi. Consider measuring injector coil resistance and look for a target of 11-14 ohms with a range of less than 3 ohms across the injectors.
8) Cylinder ignition
If your misfires are random or you have say a rough idle with no misfire counters then jump to more generic steps starting in 8), if you have high individual misfire counts then start here. On the affected cylinder check the condition of the spark plug. If its running rich or lean? Test the strength and cadence of the spark at the plug, look for 1" of regular strong spark. What's the resistance and condition of the spark plug wire, look for 625 ohms per foot resistance. Repeat for all misfiring cylinders. Check the distr cap for arcing/carbon traces. Measure CMP retard and adjust to with +/-2. Is your distr gear worn out? Check your ignition wire order and the seating of all the boots. You can verify poor cylinder performance with a drop cylinder test.
9) Arcing
Check for arcing in the dark by misting over the ignition components with a water bottle
10) Vacuum leaks
Check hoses, gaskets, intake connections. Do your LIM gaskets need replacing? Look for lean fuel trims that improve at 2500rpm
11) Power and grounds
Check the pink wire power and grounds for the PCM, ICM and coil
12) Test the ICM and coil
Look at the ICM control output and grounded coil spark, or have then tested.
13) Exhaust restriction/leaks
Check exhaust back pressure at an O2 sensor port. Check cat performance at the rear O2 sensor and check temps with an IR gun. Check for exhaust leaks
14) Check the EGR valve and PCV
15) Check the IAC valve and idle speed
16) Does it run better or worse after everything gets hot. Common components that can fail after they warm up are ignition coils and ICM's
17) Do a compression test to rule out cylinder mechanical issues like valve problems or rings. Listen for bearing knock or other mechanical sounds. If you are at this step and none of the previous tests have found the problem then it may be time for a dynamic compression test to pinpoint the cause of your misfire:
https://www.alldata.com/us/en/suppor...kId=4010940812
BTW, if you don’t have a Snap On scanner, a EE degree and tons of time to burn, It’s not classic diagnostics and problem solving but sometimes and educated guess after considering the list above makes sense. That’s what I did in my Toyota coil example. Other “known good” or adjacent parts swapping can help get to the bottom of this if necessary.
Good luck, misfires can be a wild ride but it’s fun when you solve these harder problems. Well at least it is for me, I love working on difficult problems! LOL. Let us know if you need help performing any of these tests.
George
Last edited by GeorgeLG; Jun 26, 2022 at 07:57 AM.
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