2008 GMC Sierra 1500 5.3l vortec AFM-P0301
#1
2008 GMC Sierra 1500 5.3l vortec AFM-P0301
My truck has a misfire on cylinder 1 at low rpms and only throws the check engine light randomly then goes off. I checked plugs, they looked good, replaced the plug wires, did compression check on 1,3,5 all were near 190. When I rev it up the misfire fades out, I cleaned the injector but I'm wondering if it'* not working right. I haven't pulled the valve cover yet, what else should I try?
#2
Senior Member
What are the fuel trims doing? How many miles?
BTW Looking at a plug is not a valid test although I don't believe your misfire is due to the plug.
BTW Looking at a plug is not a valid test although I don't believe your misfire is due to the plug.
#3
#5
Senior Member
That kind of miss at idle which goes away with throttle is many times from a vacuum leak at the intake.
You man need to replace the intake gasket.
You man need to replace the intake gasket.
#6
Senior Member
True Car Nut
I agree with carfixer007 ^^^^^^^
It can digest the slight bit of extra air at higher engine speeds, and there is less vacuum anyways. A bit of home DIY diagnostics for this possibility:
1. Drive it around for 20 minutes
2. Let it idle for a minute
3. Clear codes
4. Shut it off (hood closed)
5. Walk away for five minutes
6. Come back
7. Start it up
8. Does it have any misfires now?
Steps 4, 5, and 6 are designed to heat soak the intake gasket. A marginal one will expand enough to seal up when it gets warm. Start the engine, and the cool air coming in starts to cool it off but it takes a while. Drive it or rev it up for a bit and it gets cooled off even more, creating a slight gap and therefore a slight leak. I see a few of these here every fall/winter when the weather starts to cool off. I've almost never heard of it in the middle of summer.
It can digest the slight bit of extra air at higher engine speeds, and there is less vacuum anyways. A bit of home DIY diagnostics for this possibility:
1. Drive it around for 20 minutes
2. Let it idle for a minute
3. Clear codes
4. Shut it off (hood closed)
5. Walk away for five minutes
6. Come back
7. Start it up
8. Does it have any misfires now?
Steps 4, 5, and 6 are designed to heat soak the intake gasket. A marginal one will expand enough to seal up when it gets warm. Start the engine, and the cool air coming in starts to cool it off but it takes a while. Drive it or rev it up for a bit and it gets cooled off even more, creating a slight gap and therefore a slight leak. I see a few of these here every fall/winter when the weather starts to cool off. I've almost never heard of it in the middle of summer.
Last edited by CathedralCub; 09-26-2022 at 06:31 PM. Reason: Changed a period to a colon
#7
It'* not throwing the misfire code all the time but it'* throwing P0171 and 0174 now, I'm starting to lean towards fuel pump or filter, maybe a vacuum leak too. I don't have the equipment to diagnose all the possibilities so gonna call the mechanic tomorrow. Thanks for the help though.
#8
It'* also been throwing a lot of false codes, like B2aaa, could this possibly be an ECM issue? I didn't clear the 0171 and 4 codes, checked it again after start up and false codes showed this time.
#9
Senior Member
You talk about false codes. I have no idea why or how you determine they are false.
I doubt it'* your ECM.
I doubt it'* your ECM.
#10
Senior Member
True Car Nut
It'* not throwing the misfire code all the time but it'* throwing P0171 and 0174 now, I'm starting to lean towards fuel pump or filter, maybe a vacuum leak too. I don't have the equipment to diagnose all the possibilities so gonna call the mechanic tomorrow. Thanks for the help though.