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Mischievous Misfire

New remand Motor has exact same misfire as previous

Mischievous Misfire

Postby ffejrey [OP] » Feb 21 2021, 5:08am

I'm looking for any and all the help I can get.

Story starts with a rough drive home. CEL flashing and I can feel the misfire. I get it home, pull codes and its p0307, 7th cylinder misfire. I move coil pack, doesn't follow. I change plug and injector, still misfiring. At this point I decide to take it to the dealer. I DRIVE it to dealer, they have it about a week, do all the same things I did and then call me to pick it up. They said they couldn't find anything definitive but they believe that 7 cylinder valves aren't seated well and that I should get a valve job. I go to drive off the lot and the truck barely drives, stalls twice before getting out of dealer lot. Its Friday after work for me (5pm) so they tell me there's nothing they can do, mechanics gone for the weekend. I have my truck towed back to my house and start again. I get it running better by changing MAP sensor. CEL stays off for about 20 miles and then comes back on. p0307. At this point the truck runs good, can tell its still misfiring but only a little more shaky during idle. Runs fine at highway speeds. So I continue to drive it until mid January. I have to have it smogged for registration so I decide to take head off for valve job. Machine shop says the heads look like they have 200-300K on them even though truck only has 107K. He tried to machine but not enough material and advised me to look into new heads. I find heads locally for $1600. But not wanting to go through all this work only to find that wasn't the problem I decided to buy a remand motor with heads for $3200 off Rock Auto. I replace old motor with new, change both knock sensors, crank shaft sensor, crank position sensor every sensor on the block, a new water pump and thermostat, new belts and replace all the fluids. After all that work I start it up to find that Im still having the same p0307 code. this time there's also a P0300 code that also come up every once in awhile.

If it helps I also have a Diablo Trinity computer chip. I bought it before the new motor to turn off DOD or VFM and ungrade tire size to get an accurate MPH reading. I have since changed the layout to show cylinder 7 misfire counter and it seems to count constantly, even after new motor went in. The only thing I can think of now is the PCM must be defective but that seems very strange to me.

and lastly, I've been doing so much research recently and have read that GM trucks can sometimes have issues with non GM electrical parts, IE most of my sensors. Ive tried to buy ACDelco when I can but cannot always get them so have resorted to BOSH and Standard at times. My dad, a retired mechanic, thinks this is ****, he says there manufactured parts for my truck, "they spend million of dollars of these parts and people all over use them, there's no way that can be an issue"
Original Poster
ffejrey
Firing on 1 cylinder [L1]
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Re: Mischievous Misfire

Postby B52Bombardier1 » Feb 21 2021, 3:31pm

Check cylinder 7 coil wiring and spark plug cable. Check cylinder 7 for compression and a cylinder leak down test.

Check cylinder 7 with the engine running and a timing light attached to cylinder 7 - does the light flash consistently?

Rick

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1970 Chevy El Camino 5.3 liter LM7 Engine and 4L60E Transmission
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Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 21 2021, 6:24pm

I tried to upload a video of the timing light.I might have to post it to YouTube first. But its not consistent. The plug wires are all new. I haven't done a leak down test on the new motor. The dealer did it at the first trip around with the old motor and I did it again with the old motor and there was a slight leak but well within working limits. after the first go around I put new wire loom/electrical tape around the wires as the old was disintegrating but ill re check that
Original Poster
ffejrey
Firing on 1 cylinder [L1]
Firing on 1 cylinder [L1]
Posts: 14
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Images: 11
Joined: Feb 21 2021, 4:31am
Location: Cherry Valley CA
Year: 2008
Make/Model: GMC Sierra
Variant: SLT
Extra Info: Flex fuel 5.3
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Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 21 2021, 6:30pm

Original Poster
ffejrey
Firing on 1 cylinder [L1]
Firing on 1 cylinder [L1]
Posts: 14
Topics: 1
Images: 11
Joined: Feb 21 2021, 4:31am
Location: Cherry Valley CA
Year: 2008
Make/Model: GMC Sierra
Variant: SLT
Extra Info: Flex fuel 5.3
Group Membership:
Registered users

Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 21 2021, 6:38pm

These are the codes that were common before I replaced the motor, Now it just seems to be P0300 or P307
Attachments
IMG_1323.jpeg
Original Poster
ffejrey
Firing on 1 cylinder [L1]
Firing on 1 cylinder [L1]
Posts: 14
Topics: 1
Images: 11
Joined: Feb 21 2021, 4:31am
Location: Cherry Valley CA
Year: 2008
Make/Model: GMC Sierra
Variant: SLT
Extra Info: Flex fuel 5.3
Group Membership:
Registered users

Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 21 2021, 6:50pm

Here is the other side, Cylinder 2 with test light

https://youtu.be/A9bep6Pb2qk
Original Poster
ffejrey
Firing on 1 cylinder [L1]
Firing on 1 cylinder [L1]
Posts: 14
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Images: 11
Joined: Feb 21 2021, 4:31am
Location: Cherry Valley CA
Year: 2008
Make/Model: GMC Sierra
Variant: SLT
Extra Info: Flex fuel 5.3
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Re: Mischievous Misfire

Postby ChrisMcP » Feb 21 2021, 10:07pm

You don't mention what the O2 up stream sensors are doing.

Since you have a new engine one would think compression is not a part of the issue. That leaves spark and fuel.
Since the code is the same I guess it's still Cyl 7 so have a look at O2-1 on Bank 1.
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Re: Mischievous Misfire

Postby SpeedyGonzalez » Feb 22 2021, 5:17am

Looks like you have dirty and stuck open injectors and leaking orings, did the reman come complete, or did you end up reusing parts?
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Firing on 5 cylinders [L1]
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Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 22 2021, 8:59pm

I replaced all injectors about a month prior. The motor came with heads and oil pan, thats about it. Ive replaced all the seniors on the block, water pump and harmonic balancer with new parts with install. I used the same intake manifold with new gaskets and O rings for the injectors.I read a post yesterday talking about warped intake manifolds? Think I might go there next. We tried spraying around the cover with carb cleaner but didn't get results that suggest a leak. I don't know, its really blowing my mind. Ive spent about 5K now with the motor, which is still worth it to me, the trucks paid for and nice. It shouldn't be this to diagnose an issue.
Original Poster
ffejrey
Firing on 1 cylinder [L1]
Firing on 1 cylinder [L1]
Posts: 14
Topics: 1
Images: 11
Joined: Feb 21 2021, 4:31am
Location: Cherry Valley CA
Year: 2008
Make/Model: GMC Sierra
Variant: SLT
Extra Info: Flex fuel 5.3
Group Membership:
Registered users

Re: Mischievous Misfire

Postby Used2baFordDrvr » Feb 22 2021, 9:19pm

Your frustration with the problem is very understandable. If you could fabricate some way to perform a smoke test that would help you prove or disprove the possibility of a vacuum leak.

Hang in there and keep us posted on your progress.

Mike
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