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Mischievous Misfire

New remand Motor has exact same misfire as previous

Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 22 2021, 9:22pm

Thanks Mike!
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Firing on 1 cylinder [L1]
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Re: Mischievous Misfire

Postby SpeedyGonzalez » Feb 23 2021, 2:55am

Spraying the intake will not have an audible effect, you need a scanner that can read live data and display your fuel trims while you spray. If you see a drop in your fuel trims then you know you have a leak, if you're sure your intake gaskets are tight then the next place to look is your injector orings.
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Firing on 5 cylinders [L1]
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Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 23 2021, 5:53am

I torched them down to spec first pass, final pass and in specific order. I did not change intake manifold bolts, I reused the old. I recently ready I should never use old intake manifold bolts as the might have stretched and rubber grommets shriveled. I don’t have a live data track per say, just a simple reader eraser. Although I’ll have to check my Diablo trinity chip. I know there’s all sorts of different gages you can load up, I’ve got a cylinder 7misfire counter on one now counting faster than the seconds. I’ll have to look at that.’

Thank you!
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Re: Mischievous Misfire

Postby SpeedyGonzalez » Feb 24 2021, 4:15pm

Wherever you read the intake bolts are single-use is just wrong, the grommets on the intake are there to give the manifold room to expand and contract and the bolts are not of the torque to yield type. The shriveling is true of the valve cover grommets and that will manifest as an oil leak. I would start by looking at that number 7 cylinder, honestly, I'd say buy the injector O-rings and a squeeze tube of synthetic grease from your parts house, pull the 4 bolts for the rail, and swap the o-rings. Takes half an hour and is cheap insurance, make sure you lube the o-rings well with the grease before pushing the rail back in, I also like to lube the injector caps, as well as they like to get stuck in the intake.
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Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 26 2021, 4:48pm

Thank You SpeedyGonzalez! I agree with you, to me it sounds like a way to get another $100 out of me for 10 intake manifold bolts. Which is INSANE!! At $10 a bolt I might have give up my day job and slang bolts on the corner. But if you're reading or posting here, you've most likely felt my frustration with a mechanical issue and are just about willing to try anything to get results.

When I put the manifold back on the motor and the injectors back I replaced only the top O rings, as I had left the Injectors in the manifold when I removed it from the old long block. I did use WD-40 on the top O ring and still found it tough to get them all seated so I ended up removing the injectors from manifold, putting them in the rail first and then seating them back in the manifold. Is that how you would do it?

Today Ill be off at noon and its nice weather so Im gonna try it again. I have another set of new O rights and Intake manifold gaskets so I think Ill take it apart, inspect and measure the manifold for wrapping, soak the fuel rail and make sure its clean then install injectors with grease as you have suggested.

I have an appointment Monday to have the PCM checked. Labor rates $140 and guy said 2 hour job so hopefully I get somewhere before I spend $300
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Re: Mischievous Misfire

Postby SpeedyGonzalez » Feb 27 2021, 1:55am

That is how I would have installed it, injectors in rail, use the bolts to slowly and evenly pull the rail down onto the manifold. How did it go?
Why are you having your PCM checked?
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Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 27 2021, 4:57am

Well gentlemen, I think I have finally fixed my issue. Thanks to SpeedyGonzzalezs advice I removed the intake manifold again to replace all the O rings and assemble everything on the bench to ensure everything was tight, no leaks. While putting it back together I decided to put the injectors back in random order, not the cylinders I pulled them out of. After everything was put back together I took her for a drive armed with the Diablo Sport Trinity. I had misfire counters pulled up on the digital gauges and saw that number 8 cylinder was now counting like crazy. So I found one of my old injectors, replaced the BRAND NEW one in 8 and sure enough, no more misfire. Drove it around and let it run for about 30 mins and its still going strong. I plan to go get it smogged tomorrow. Ill update again tomorrow. Thanks for everyones input. I really appreciate it
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ffejrey
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Re: Mischievous Misfire

Postby SpeedyGonzalez » Feb 27 2021, 5:10am

Glad you found it, I've had similar problems with injectors, although it's usually from them sitting for a while with no fuel in them, the varnish can cause the pintles to stick and leak. I've also found that if the oil used to store the injectors has leaked out over time while sitting on a shelf or such, the pintles can be rusted to the seats, its best practice to bench test all injectors with some brake clean and an injector tester if the injectors have sat dry for longer than a week. The brake clean will dissolve any varnish, and the injector tester will get the pintle moving.
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Re: Mischievous Misfire

Postby SierraHarley » Feb 27 2021, 7:48am

Arriba arriba for Speedy Gonzales!
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Re: Mischievous Misfire

Postby ffejrey [OP] » Feb 27 2021, 5:24pm

Can you bench test injectors without an official tester? Would like an 18V dewalt battery open them up?
Original Poster
ffejrey
Firing on 1 cylinder [L1]
Firing on 1 cylinder [L1]
Posts: 14
Topics: 1
Images: 11
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Location: Cherry Valley CA
Year: 2008
Make/Model: GMC Sierra
Variant: SLT
Extra Info: Flex fuel 5.3
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