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2005 5.3 p0300 misfire I cant track down

2005 5.3 p0300 misfire I cant track down

Postby Chubs [OP] » Jan 24 2020, 7:40am

Hate for my first post to be the "beg for help" post, but I am at the end of my knowledge around diagnosing this issue and after everything I have read on the site, it seems to be a very informative place, so hoping to get some insight on maybe something I have overlooked.

Some background the truck is a 2005 Silverado 1500 with the 5.3 Vin Z (which I believe is Flexfuel) the truck has 164k miles.

Purchased the truck from a buddy once the CEL came on and he didnt want to deal with it after he tried a few fixes. When the light came on he did some research and decided to replace the fuel injector in cylinder 1. From then on the code switched to a P0300.

He took it to a shop and they said the compression was good, but the running compression on cylinder 1 was 45 PSI and cylinder 2 was 65. I assume they only did cylinder 2 for relative compression. I didnt know this information about the running compression until I called the shop after I bought the truck, but no worries as I didnt pay much for it.

When I got the truck I changed out the plugs, to the double platinum AC delcos that were recommended which didnt help. I also did a compression test and the compression was 170-180 across all 8 cylinders. The shop mentioned without taking it apart there guess was it could be a valve spring failed, I pulled the drive side cover and all the springs looked good with no damage that I can see, I have yet to pull the passenger side cover.

I also have one of the ELM327 scan tools with Torque pro and pulled up the misfire data it was stating the cylinders 2 and 8 are misfiring, but after reading about Torque Pro and misfire data online I am not sure it is accurate information to go off of.

I appreciate any help on this issue.
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Re: 2005 5.3 p0300 misfire I cant track down

Postby zbkeebler » Jan 24 2020, 3:31pm

Welcome. I've used the misfire counter on the torque app several times and it has helped narrow down the issue. Never really cared if the misfire counts were accurate as long as it gave me a direction so as long as you can decipher the PID's naming scheme it should be good enough. Have you checked for a intake manifold leak? You could also try swapping the fuel injectors around to see if the misfires move to another cylinder.

When you replaced #1 is it because there were also readings on that cylinder? Did you inspect it to see if it was clogged? could be crud clogging the other 2 as well.

Just some thoughts from the things I checked when I had the same code on a 2010
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Re: 2005 5.3 p0300 misfire I cant track down

Postby Chubs [OP] » Jan 24 2020, 7:33pm

zbkeebler wrote:Welcome. I've used the misfire counter on the torque app several times and it has helped narrow down the issue. Never really cared if the misfire counts were accurate as long as it gave me a direction so as long as you can decipher the PID's naming scheme it should be good enough. Have you checked for a intake manifold leak? You could also try swapping the fuel injectors around to see if the misfires move to another cylinder.

When you replaced #1 is it because there were also readings on that cylinder? Did you inspect it to see if it was clogged? could be crud clogging the other 2 as well.

Just some thoughts from the things I checked when I had the same code on a 2010


I appreciate the response.

So that was one of my questions when the Torque app says cylinder 1-8 does that map correctly to the cylinders on the engine i.e. cylinders 1 3 5 7 on driver side and 2 4 6 8 on passenger from front to back? I loaded the GM PIDs in torque pro to get the misfire data, but wasn't sure if the cylinder mapping was accurate to the engine.

I sprayed some starter fluid all around the intake manifold just now and did not notice any change in idle.

However I did notice when I was unplugging all the injectors that every cylinder had a very noticeable change in idle when unplugged which I would assume means that cylinder was contributing for the most part. However the fuel injector for cylinder 1 did not have a very noticeable change in idle and there was a ticking when it was plugged in, that went away very clearly when I unplugged it.

I took a video that I hope is clear enough to hear that ticking.



This is the injector that is brand new, so I am wondering if I should just get a set of re manufactured injectors and replace them all.
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Re: 2005 5.3 p0300 misfire I cant track down

Postby MIKE91B » Jan 24 2020, 10:23pm

Test the scanner first. Pull up the misfire data and unplug the coil on a good cylinder. If it shows that cylinder is misfiring then you know your data is good. Do that and report back. Also that noise sounds top end as in valve train but the fact it goes away when combustion is stoped screams bottom end. That’s odd.
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Re: 2005 5.3 p0300 misfire I cant track down

Postby esox07 » Jan 25 2020, 12:18am

I think a great percentage of members here came to this site for the first time to ask a question. I hope you get it figured out.
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Re: 2005 5.3 p0300 misfire I cant track down

Postby SpeedyGonzalez » Jan 25 2020, 5:24am

What are your fuel Trims looking like? It's super easy to pinch the injector orings if you put them in wrong and that's a super hard leak to find that'll result in a P0300.Ive found the starting fluid trick only changes speed if you have a huge leak, but other than that the O2s will pick up the extra fuel and you can see the PCM lean the short term Trims for that bank out a bit. I recently had the same issue in my flex converted truck and it turned out to be cheap injectors, but I wouldn't be surprised if the truck sat for a while with fuel in the rails for some varnish to clog the injectors.
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