P0300 Misifre where to go from here
#1
10-08-2020, 07:39 PM
P0300 Misifre where to go from here
I have been fighting the dreaded P0300 misfire since I changed the timing chain, timing cover, motor mounts, front diff seals and the tie rods.
It first appeared as I was driving the truck to get a CASE relearn done. The P0300 prevented a relearn from being done so the ignition coil, cap and rotor were changed out. The misfire code was gone long enough to get the relearn done.
Change the #! cylinder plug wire and plug. No real issues noted so I drive the truck to Montana and back while getting 16.6 mpg. 2001 ZR2 automatic BTW.
So as I approach KC on may way back the Code popped back up. I had cleared it twice before on the trip but thought it was a carboned engine with 85 octane mountain blend gas.
Now that I have more time I start digging into the issue. Changed the remaining plugs and wires and find #2 wire has a burn mark on the boot. Misfires reduced but #1 count still goes up but the misfires show up on 2 which is why I found the wire. Later professional scanner reveals that #1 is responsible for both the counter going up and the current misfire.
Find CMP retard is about almost -10 degrees replace distributor with new one that is aluminum billet from Summit.
CMP retard drops to -1 +/- degree at 1100 rpm. Misfires reduces but become more consistent. Number leads the count followed at about 1/2 rate by #4 and #6. Still getting good fuel mileage between 15-16 mix of city and highway driving.
Didn't do a CASE relearn with new Distributor and used the Summit cap,rotor and Crank position sensor. No codes besides C0265 and P300.
Have been checking the fuel pressure and getting 52 psig on my gauge and my mechanic got 54 psig. Change fuel filter after 7000 miles on fuel pump from last year. pick up 3-5 psi but no change on misfire rate.
So I have been watching the fuel trims and have them at idle and at 2000 rpm.
Idlle at 575 RPM
-0.8% Long Tern Fuel Trim Average Bank !
0.0% Long Tern Fuel Trim Average Bank 2
-1.6% Short Tern Fuel Trim Average Bank !
-0.8% Short Tern Fuel Trim Average Bank 2
At 2000 RPM @ 68 mph
-0.8% Long Tern Fuel Trim Average Bank !
0.0% Long Tern Fuel Trim Average Bank 2
-0.8% Short Tern Fuel Trim Average Bank !
0.0% Short Tern Fuel Trim Average Bank 2
These don't strike me as an issue but the low fuel pressure is concerning. Delphi pump with Lifetime warranty so I can pull it out and get a new one.
Oh prior to my purchase the CPFI was converted to MPFI and the fuel pressure hold for about 10 minutes losing 1-3 psig at most usually less.
Have run fuel injector cleaner through the tank without a change
Checked engine vacuum and have 18" at idle and it's steady.
Checked the compression and have 162-185 range stone cold on cylinders 1 thru 5. The pacesetter headers make checking #6 without pulling the the back impossible
So do I drop the tank and install a replacement fuel pump?
Or
Try to get an injector balance done to see if I have a bad injector?
Or
Replace the cap and rotor with the AC Delco units from my old distributor?
Install the Crank position sensor from the same?
Or
When I replaced the extra long bolt I had to use for the crank position sensor with a stock one the threaded fitting come out with it. New bolt tightened up but will add JB Weld to hold it. yes there was an oil leak but the senor was tight against the timing cover
Sea Foam the engine?
The misfire is worse when idling, when cold before the closed loop operation and going uphill.
Normally I would say replace the fuel pump but the fuel trims are pretty good unless I am not understanding them.
It first appeared as I was driving the truck to get a CASE relearn done. The P0300 prevented a relearn from being done so the ignition coil, cap and rotor were changed out. The misfire code was gone long enough to get the relearn done.
Change the #! cylinder plug wire and plug. No real issues noted so I drive the truck to Montana and back while getting 16.6 mpg. 2001 ZR2 automatic BTW.
So as I approach KC on may way back the Code popped back up. I had cleared it twice before on the trip but thought it was a carboned engine with 85 octane mountain blend gas.
Now that I have more time I start digging into the issue. Changed the remaining plugs and wires and find #2 wire has a burn mark on the boot. Misfires reduced but #1 count still goes up but the misfires show up on 2 which is why I found the wire. Later professional scanner reveals that #1 is responsible for both the counter going up and the current misfire.
Find CMP retard is about almost -10 degrees replace distributor with new one that is aluminum billet from Summit.
CMP retard drops to -1 +/- degree at 1100 rpm. Misfires reduces but become more consistent. Number leads the count followed at about 1/2 rate by #4 and #6. Still getting good fuel mileage between 15-16 mix of city and highway driving.
Didn't do a CASE relearn with new Distributor and used the Summit cap,rotor and Crank position sensor. No codes besides C0265 and P300.
Have been checking the fuel pressure and getting 52 psig on my gauge and my mechanic got 54 psig. Change fuel filter after 7000 miles on fuel pump from last year. pick up 3-5 psi but no change on misfire rate.
So I have been watching the fuel trims and have them at idle and at 2000 rpm.
Idlle at 575 RPM
-0.8% Long Tern Fuel Trim Average Bank !
0.0% Long Tern Fuel Trim Average Bank 2
-1.6% Short Tern Fuel Trim Average Bank !
-0.8% Short Tern Fuel Trim Average Bank 2
At 2000 RPM @ 68 mph
-0.8% Long Tern Fuel Trim Average Bank !
0.0% Long Tern Fuel Trim Average Bank 2
-0.8% Short Tern Fuel Trim Average Bank !
0.0% Short Tern Fuel Trim Average Bank 2
These don't strike me as an issue but the low fuel pressure is concerning. Delphi pump with Lifetime warranty so I can pull it out and get a new one.
Oh prior to my purchase the CPFI was converted to MPFI and the fuel pressure hold for about 10 minutes losing 1-3 psig at most usually less.
Have run fuel injector cleaner through the tank without a change
Checked engine vacuum and have 18" at idle and it's steady.
Checked the compression and have 162-185 range stone cold on cylinders 1 thru 5. The pacesetter headers make checking #6 without pulling the the back impossible
So do I drop the tank and install a replacement fuel pump?
Or
Try to get an injector balance done to see if I have a bad injector?
Or
Replace the cap and rotor with the AC Delco units from my old distributor?
Install the Crank position sensor from the same?
Or
When I replaced the extra long bolt I had to use for the crank position sensor with a stock one the threaded fitting come out with it. New bolt tightened up but will add JB Weld to hold it. yes there was an oil leak but the senor was tight against the timing cover
Sea Foam the engine?
The misfire is worse when idling, when cold before the closed loop operation and going uphill.
Normally I would say replace the fuel pump but the fuel trims are pretty good unless I am not understanding them.
#2
10-08-2020, 08:10 PM
There is a lot here.
Misfires are registered when the crank sensor sees a subtle momentary change in crank rotation speed near the time of that cylinder firing. Each crank sensor has a slightly different waveform and the relearn process calibrates the output of that crank sensor for the VCM. Sometimes you can swap a crank sensor without issue but since your reporting misfires, that needs to be taken care of. You are spending a lot of money and want to be involved, you might want to consider a capable scan tool.
Your fuel trims are excellent, no problem there. So your fuel pump does or does not meet the spec?
Do the lower compression numbers correlate to the misfires?
Does the MIL ever blink?
A cylinder balance test is a good diagnostic process. You can do it yourself with one of these:
Look at the ignition system in the dark, then mist water around everything.
Your going down the rabbit hole pretty deep, I think I see you down here - do you have an O scope?
George
Misfires are registered when the crank sensor sees a subtle momentary change in crank rotation speed near the time of that cylinder firing. Each crank sensor has a slightly different waveform and the relearn process calibrates the output of that crank sensor for the VCM. Sometimes you can swap a crank sensor without issue but since your reporting misfires, that needs to be taken care of. You are spending a lot of money and want to be involved, you might want to consider a capable scan tool.
Your fuel trims are excellent, no problem there. So your fuel pump does or does not meet the spec?
Do the lower compression numbers correlate to the misfires?
Does the MIL ever blink?
A cylinder balance test is a good diagnostic process. You can do it yourself with one of these:
Look at the ignition system in the dark, then mist water around everything.
Your going down the rabbit hole pretty deep, I think I see you down here - do you have an O scope?
George
#3
10-08-2020, 08:20 PM
Additionally:
The fact that the truck runs better after closed loop that says that either something is just warming up or the O2 sensors are modifying fuel delivery based on the exhaust and correcting for any bad assumptions during open loop when the VCM has to depend on a set of sensors and the VCM lookup tables. Do you know how to look at the live sensor values to see if they make sense - ECT, MAP, MAF, TPS? Does it run better as soon as the scanner shows the change to closed loop or generally when the truck warms up?
George
The fact that the truck runs better after closed loop that says that either something is just warming up or the O2 sensors are modifying fuel delivery based on the exhaust and correcting for any bad assumptions during open loop when the VCM has to depend on a set of sensors and the VCM lookup tables. Do you know how to look at the live sensor values to see if they make sense - ECT, MAP, MAF, TPS? Does it run better as soon as the scanner shows the change to closed loop or generally when the truck warms up?
George
#4
10-08-2020, 08:49 PM
Fuel pump should have 60-66 psi per information on this board but that was for the poppet valves so was wondering if the injectors were less finicky.
My son has a 2005 Trailblazer that ran great with 38 psig at the fuel rail. Except for the huge fuel trims and P0174 code the previous owner drove on for 2 years. The way the ECM can compensate for bad parts is impressive.
#1 162 psig with misfires generally is highest when code set
#2 185 psig minimal usually less than 20 counted when code is set
#3. 183 psig minimal
#4 165 psig usually 3rd highest count when code is set
#5 164 psig minimal
#6 unknown header blocks access
MIL doesn’t blink sequence is pending code then set code. I know this bad misfire can damage cats.
No O scope just compression tester, vacuum gauge, fuel pressure gauge and an ELM327 OBDII reader talking to Dash Command via WiFi
Will redo wet test but with old cap, rotor and wires no issue observed.
Had wear observed on old cap and rotor after 30k miles. Changed when I did the LIM.
changed out plugs on suggestion of mechanic and it was easy to replace when I did the compression test.
Did fuel filter and air filter based on Captain Hooks post fuel pressure or misfire forget which
My son has a 2005 Trailblazer that ran great with 38 psig at the fuel rail. Except for the huge fuel trims and P0174 code the previous owner drove on for 2 years. The way the ECM can compensate for bad parts is impressive.
#1 162 psig with misfires generally is highest when code set
#2 185 psig minimal usually less than 20 counted when code is set
#3. 183 psig minimal
#4 165 psig usually 3rd highest count when code is set
#5 164 psig minimal
#6 unknown header blocks access
MIL doesn’t blink sequence is pending code then set code. I know this bad misfire can damage cats.
No O scope just compression tester, vacuum gauge, fuel pressure gauge and an ELM327 OBDII reader talking to Dash Command via WiFi
Will redo wet test but with old cap, rotor and wires no issue observed.
Had wear observed on old cap and rotor after 30k miles. Changed when I did the LIM.
changed out plugs on suggestion of mechanic and it was easy to replace when I did the compression test.
Did fuel filter and air filter based on Captain Hooks post fuel pressure or misfire forget which
#8
10-09-2020, 06:56 PM
I have decided that I will address the bolt issue on the crank sensor. Drove to work and there were so few misfires that the counters didn't keep them.
Cylinder 1 was the only one with more than 2 per drive cycle but....
On the way home I had over 1200 in cylinder 1 counter and between 100-250 on cylinders 4 and 6.
Messed with the crank sensor the check SES light turned off in about 100 feet and saw few misfires.
Think I have issues but getting the crank sensor sorted will allow me to identify and address the other.
Cylinder 1 was the only one with more than 2 per drive cycle but....
On the way home I had over 1200 in cylinder 1 counter and between 100-250 on cylinders 4 and 6.
Messed with the crank sensor the check SES light turned off in about 100 feet and saw few misfires.
Think I have issues but getting the crank sensor sorted will allow me to identify and address the other.
#9
10-22-2020, 08:28 PM
I took advantage of the warm day and warrantied my fuel pump. The initial fuel was 60 versus 52-55 of the old one. I will double after driving a bit and after I filled up the tank. Put in 18.2 gallons on a 19 gallon capacity.
Drove it and not seeing misfires on cylinder 1 and 6 like before 2-8 per drive cycle. Now cylinder 4 throw up 51 on an early drive cycle but cleared quickly.
Real test is tomorrow when it sits overnight and the temp is in the 40s. Still about 80 at 7 pm
Drove it and not seeing misfires on cylinder 1 and 6 like before 2-8 per drive cycle. Now cylinder 4 throw up 51 on an early drive cycle but cleared quickly.
Real test is tomorrow when it sits overnight and the temp is in the 40s. Still about 80 at 7 pm
Last edited by 2001ZR2; 10-22-2020 at 08:29 PM. Reason: Missing word
#10
10-23-2020, 11:53 AM
Originally Posted by 2001ZR2
I took advantage of the warm day and warrantied my fuel pump. The initial fuel was 60 versus 52-55 of the old one. I will double after driving a bit and after I filled up the tank. Put in 18.2 gallons on a 19 gallon capacity.
Drove it and not seeing misfires on cylinder 1 and 6 like before 2-8 per drive cycle. Now cylinder 4 throw up 51 on an early drive cycle but cleared quickly.
Real test is tomorrow when it sits overnight and the temp is in the 40s. Still about 80 at 7 pm
Drove it and not seeing misfires on cylinder 1 and 6 like before 2-8 per drive cycle. Now cylinder 4 throw up 51 on an early drive cycle but cleared quickly.
Real test is tomorrow when it sits overnight and the temp is in the 40s. Still about 80 at 7 pm
If you are not getting SES light and are just expecting zero counts all the time on a scanner - that is unrealistic IMHO. My Blazer counts up some on certain cylinders, but not enough to turn on the light and then the PCM periodically clears them. It's been doing that for 50K miles.
Also some scanners confuse cylinder #1 with another. You can pull a plug wire to make sure you are dealing with the correct cylinders. But maybe you already have it fixed after all!!
Hope this helps
