P0300: Intermittent Misfire
Sep 12, 2014 | 08:48 PM
#21
The misfire is/was not isolated to a single cylinder so compression doesn't seem to be the issue.
Pushrod length was measured during engine build.
Cylinder 4 is no longer the only misfiring cylinder and doesn't seem to be even the primary misfiring cylinder anymore.
Pins and injector connectors have been checked.
Intake gaskets were new with the engine rebuild. I cannot find an air intake leak.
I replaced the MAF sensor and fuel pressure regulator and since then the P0300 has not come back but was replaced by P0101, P0172 and P0175. I have a new MAP sensor on the way although since ordering the tuner has told me the old one is functioning properly. Tuner sent me a new tune to account for new MAF. Hopefully that will help the rich fuel situation. When the new MAP arrives I'll load the new tune and see what happens.
Pushrod length was measured during engine build.
Cylinder 4 is no longer the only misfiring cylinder and doesn't seem to be even the primary misfiring cylinder anymore.
Pins and injector connectors have been checked.
Intake gaskets were new with the engine rebuild. I cannot find an air intake leak.
I replaced the MAF sensor and fuel pressure regulator and since then the P0300 has not come back but was replaced by P0101, P0172 and P0175. I have a new MAP sensor on the way although since ordering the tuner has told me the old one is functioning properly. Tuner sent me a new tune to account for new MAF. Hopefully that will help the rich fuel situation. When the new MAP arrives I'll load the new tune and see what happens.
Update: Problem still not resolved. Time, money and parts thrown at this thing with no success.
New plugs, wires, MAF sensor, MAP sensor, O2 sensors, fuel filter, fuel pressure regulator, new grounds for the coil packs, swapped coil packs around, swapped fuel injectors around, New tune, crank relearn, additional grounds run to the ECM and additional ground strap run from the block to the chassis.
Still having the same problem. Low RPM misfire from idle to 3000RPM. When the truck hits 3000RPM the misfire stops and it takes off like a rocket. It's like flipping a switch at 3000rpm. What would explain this?
New plugs, wires, MAF sensor, MAP sensor, O2 sensors, fuel filter, fuel pressure regulator, new grounds for the coil packs, swapped coil packs around, swapped fuel injectors around, New tune, crank relearn, additional grounds run to the ECM and additional ground strap run from the block to the chassis.
Still having the same problem. Low RPM misfire from idle to 3000RPM. When the truck hits 3000RPM the misfire stops and it takes off like a rocket. It's like flipping a switch at 3000rpm. What would explain this?
No mention that P0300 was the only one you are still fighting.
From here, I would still say the tune is the issue.
But Mechanically, I would check and verify compression. Make sure its not low. You could also do a cylinder leak down test. I would also inspect the Push rod length and verify spring pressure.
What about Fuel pressure? IS the correct PSI Being supplied?
Once you are 100% confident that it is mechanically a sound, I would go back to the tuner. If at all possible have him look at it in person. Is this a mail order tune or a local shop?
Are you sure the Injector data is correct in the tune?
Sep 15, 2014 | 08:10 PM
#22
Thread Starter
Teching In
Joined: Aug 2014
Posts: 21
Likes: 0
Discovered something new today. Tuner requested that I record a data log while driving the truck in second gear. It seems the misfire is related to throttle position. I previously though that at 3000rpm it smoothed out, but when running in second gear it misfired at all RPM under light throttle. Once I throttled past about 40% the misfire stopped and truck ran great.
Anyone have experience with a misfire that is related to throttle position?
Anyone have experience with a misfire that is related to throttle position?
Sep 26, 2014 | 04:58 AM
#28
Launching!
Joined: Jul 2012
Posts: 199
Likes: 0
From: Dallas, GA
Hopefully its fixed by now but it sounds like it needs to be tuned. You have a cam and the misfire is happening in all cylinders rather than just one which points to tuning issues. Either way I feel your pain I had all sorts of codes and my trucks was surging etc and I tryed everything and when it was all said and done tuning was the little gremlin!
Sep 26, 2014 | 08:27 PM
#29
TECH Enthusiast
Joined: Apr 2013
Posts: 551
Likes: 1
From: eastern nc
mis fire
this may sound crazy.i was debating with my self about posting. but I fix a suburban with same problem. with a bottle of seafoam. I pour some in spray paint can top .brought truck up to about 3000 rpm .had hose ( soft hose ) so I could squeeze to slow suction .it clean THE INTAKE and I guess to pistons also p300 went away. I am think that the gunk in the intake was causing the problem.
