P0300 misfire on 1 3 5 7

I've looked at alot of other post and couldn't find something similar to my current issue.

I have a 2004 Yukon XL 1500 5.3l slt. I was driving home last Friday and had my engine light start blinking at me. I pulled off at a flying j and ran the codes on my scanner. I found a P0300, I didn't do much research as I was worried that cats were clogged. I have since replaced the coils, wires, spark plugs, and both catalytic converters (i also did the oil pan gasket while I was there but that is unrelated).

Once everything was together I started the truck and still had the p0300. I found misfires primary happening on cylinder 1 3 5 7 I had about 3-5 misfires on 2 4 6 8 for everything 1000 misfires on 1 3 5 7.

I checked the injectors and they are working fine as well the fuel pressure seems to be good. I did not find a leak around the intake manifold and even sprayed brake clean around the gasket to see if it would cause any studder and it didn't get in. I tested the purge valve and the MAF. There are no other codes running on the vehicle and before this it ran with no issues.

Vehicle info: 2004 Yukon Xl slt 5.3l 2wd flex fuel.
New parts:
Spark plugs
Crank seal
Valve cover gaskets
Oil pan gasket and pick up tube o ring
Coil packs all 8 and wires
Thermostat
High output alternator

It certainly sounds like a problem with your left cylinder bank, since cylinders 1,3,5 and 7 are all on the driver's side. The only thing I can think of that could affect all four cylinders simultaneously, assuming you've already replaced the usual suspects like the catalytic converter, ignition coils, and so on, is the wiring harnesses for the fuel injection and ignition systems.

These can sometimes cause problems, and if you had a grounding issue in one of the harnesses, you'd definitely experience these kinds of symptoms. The fact that it appeared suddenly supports this theory. For example, a catalytic converter can become clogged over time; this starts at some point and then gets worse.

It could also be a problem with the engine management system, although I consider that rather unusual.

I would check or replace the harness for ignition and injectors. I'm not sure if this is one harness or two separate...

Good luck

nonickatall said:

It certainly sounds like a problem with your left cylinder bank, since cylinders 1,3,5 and 7 are all on the driver's side. The only thing I can think of that could affect all four cylinders simultaneously, assuming you've already replaced the usual suspects like the catalytic converter, ignition coils, and so on, is the wiring harnesses for the fuel injection and ignition systems.

These can sometimes cause problems, and if you had a grounding issue in one of the harnesses, you'd definitely experience these kinds of symptoms. The fact that it appeared suddenly supports this theory. For example, a catalytic converter can become clogged over time; this starts at some point and then gets worse.

It could also be a problem with the engine management system, although I consider that rather unusual.

I would check or replace the harness for ignition and injectors. I'm not sure if this is one harness or two separate...

Good luck

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I should add that when I first start the engine for the first 1000 rotations there seems to be no problems. If I start it and then pull the fuse for that side of the engine it shudders but keeps going, if I wait a minute it does the same thing but without pulling the fuse. I'm gonna try and scope it out later today I'll update with votage and pulse rate for the injectors.

psyphr said:

I should add that when I first start the engine for the first 1000 rotations there seems to be no problems. If I start it and then pull the fuse for that side of the engine it shudders but keeps going, if I wait a minute it does the same thing but without pulling the fuse. I'm gonna try and scope it out later today I'll update with votage and pulse rate for the injectors.

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That sounds exactly like an electrical contact problem, because it often happens when you have, for example, a broken wire and the wire is cold. Then the current flows, but if current flows regularly, the contact resistance can increase due to heating, and that's when the problems start. Since all four cylinders are affected, it's probably the ground cable of the injection or ignition harness.

I don't know if that is one harness or two..

Properly someone knows that...

It can all so be a little bit corrosion in the connector, where it's attached to the engine control. So I would remove this connector, inspect it and connect a few times to remove corrosion.

going through the same thing right now w/ an 07. only things i haven't replaced are the MAP & PCV. it's driving me NUTS!!!!!

nonickatall said:

That sounds exactly like an electrical contact problem, because it often happens when you have, for example, a broken wire and the wire is cold. Then the current flows, but if current flows regularly, the contact resistance can increase due to heating, and that's when the problems start. Since all four cylinders are affected, it's probably the ground cable of the injection or ignition harness.

I don't know if that is one harness or two..

Properly someone knows that...

It can all so be a little bit corrosion in the connector, where it's attached to the engine control. So I would remove this connector, inspect it and connect a few times to remove corrosion.

Click to expand...

I'm going to ohm test all the injector wires and coil wires back to the ecm to see if I have a disconnect. I'll let you know what I find.

psyphr said:

I'm going to ohm test all the injector wires and coil wires back to the ecm to see if I have a disconnect. I'll let you know what I find.

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Have you check the injectors to make sure they are pulsing? Or how did you check them?

I know they say to check with a noid light, but I've used an LED test light for this. With the engine cranking or running, the test light will blink. Test cold and once you know the symptoms occur.

Stop driving the vehicle if you haven't done so. Driving with with a severe misfire will destroy your cats on short order.

strutaeng said:

Have you check the injectors to make sure they are pulsing? Or how did you check them?

I know they say to check with a noid light, but I've used an LED test light for this. With the engine cranking or running, the test light will blink. Test cold and once you know the symptoms occur.

Stop driving the vehicle if you haven't done so. Driving with with a severe misfire will destroy your cats on short order.

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It is parked I have 4 other gmt800's to drive until its fixed.

I mainly listened for injector pulse with a mechanic stethoscope. I'm gonna check with a noid light when it comes in. When the vehicle first starts everything is firing if I pull the injector fuses respectively during the first 1000 rotations I get a response from even bank or odd bank respectively but after about 1000 rotations the driver side losses spark and if I pull the main plug for the coils on the odd bank nothing happens so it's only firing the 4 even cylinders. Unfortunately it will take awhile to get it somewhat safely to operating temp as I have to shut it down and restart it so im not warming up only half the engine.

When I get home I have a few things I'm gonna test:
Ground resistance, individual spark testers for each cylinder on the driver side to see when I'm exactly loosing spark, (I'm probably just gonna add a crap ton of ground just to remove that as an issue I have alot of spare 4ga wire from a dock feed I did recently so I'll use that.) Test continuity on individual feeds from the ecm plug to each coil and injector (maybe a bad wire, but I'd assume since all 4 are gone it would be a 12vlt constant not a 5v reference).

commercialfshrmn said:

going through the same thing right now w/ an 07. only things i haven't replaced are the MAP & PCV. it's driving me NUTS!!!!!

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Did you replace the intake manifold or the gaskets for it? I was debating doing that next but I've already dropped 1200 over the weekend between oem coils and plugs and catalytic convertors. I have found an issue but I do have a smoke machine I was planning to hook it up to see if there are any leaks.

psyphr said:

Did you replace the intake manifold or the gaskets for it? I was debating doing that next but I've already dropped 1200 over the weekend between oem coils and plugs and catalytic convertors. I have found an issue but I do have a smoke machine I was planning to hook it up to see if there are any leaks.

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If the engine runs smoothly until it reaches operating temperature and misfires occur on cylinders of one cylinder bank, I don't suspect an intake leak.

The problem with intake leaks is that the amount of air drawn in doesn't match the reading from the mass airflow sensor. This leads to a lean mixture, but not to misfires on one cylinder bank.

Have you checked or replaced the oxygen sensors before the catalytic converter?

If not, swap them and see if the problem moves to the other bank.

nonickatall said:

If the engine runs smoothly until it reaches operating temperature and misfires occur on cylinders of one cylinder bank, I don't suspect an intake leak.

The problem with intake leaks is that the amount of air drawn in doesn't match the reading from the mass airflow sensor. This leads to a lean mixture, but not to misfires on one cylinder bank.

Have you checked or replaced the oxygen sensors before the catalytic converter?

If not, swap them and see if the problem moves to the other bank.

Click to expand...

All 4 02 sensors are new but I have my parts guy sending a warranty set so I should have another new set of 4.

Do the swap first.

psyphr said:

All 4 02 sensors are new but I have my parts guy sending a warranty set so I should have another new set of 4.

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When the o2 Sensors are new, the remaining suspicion imho is your harness..

Forgot to mention, after changing the O2 sensors be sure to reset the long term fuel trims with a scanner.

Solved Update: so first thank you everyone who had input. First thing I tried was ground checks but I had amazing resistance I got 2ohms from the plug on the coil rail to the hood. So I ruled that out. Then I looked at my trim data and notice very stupid levels. Unplugged the driver side bank 1 02 sensor and bam everything works great.

My parts guy has some waiting on me so I'm gonna get those and do a reset on the computers and call it a day.

I'll update if anything returns.

What month 04? My 04 has a fuel filter.