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Newbie looking for guidance: P0300 Misfire at idle/Supercharged

 
Old 02-12-2019, 08:39 PM
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Question Newbie looking for guidance: P0300 Misfire at idle/Supercharged

Hello everyone,


New guy checking in and looking for some guidance. I am finally starting my build on my boosted Tahoe. What I got is a 2000 Tahoe 5.3L with a Magnuson supercharger (MP112). I picked it up pretty cheap but it needs a little work. I've been around mechanics my entire life but I don't claim to be one myself. I know my limits and I try to stay within them.


Currently it has a significant misfire at idle. I would say it is present about 75% of the time but the other 25%, it runs smooth. The previous owned said he did plugs, wires and a coil on cylinder #6. I've having some trouble getting it pass emissions barely missing the NOx reading. The limit is 2.0 and I am registering at 2.9 but all the other emissions readings pass with no problems. I did an injector balance test on all the injectors and they are all reading about 38# across the board. I took temp readings on each manifold and #6 is running a bit cooler than the others. All cylinders were roughly about 190*+/- and #6 was about 170*. The misfire seems to go away as the RPMS rise as it seems to run perfect past 1000rpms. Can anyone point me in my next direction? Forgive me for being such a noob but we all have to start somewhere right? I look forward to learning about it and fixing it myself. Appreciate any help.
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Old 02-12-2019, 09:48 PM
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It needs a tune, find a tuner in your area to help you get it running,
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Old 02-12-2019, 09:57 PM
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Originally Posted by qweedqwag View Post
It needs a tune, find a tuner in your area to help you get it running,
We had some really good ones in the area but the one closed shop and the other wasnít sure if he could fit the Tahoe in his dyno bay. I think I found at least one of the problems as the fuel pressure gauge is only showing 35psi of pressure Iíll start there..
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Old 02-12-2019, 10:33 PM
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That WOULD cause your misfire issue. Lack of pressure means lean running at idle and possibly other areas,
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Old 02-12-2019, 11:40 PM
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Originally Posted by G Atsma View Post
That WOULD cause your misfire issue. Lack of pressure means lean running at idle and possibly other areas,
Fuel pressure regulator or pump? Like I said, I’m learning but I assumed 35psi was plenty of pressure till I started researching what normal was. I was still in the world of 24v Cummins that only needed 17psi.

Last edited by SouthPaw; 02-13-2019 at 08:28 AM.
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Old 02-12-2019, 11:56 PM
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Either/or. Check pressure AHEAD of the regulator. It better be above 60psi. If it is, the regulator is the culprit.
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Old 02-13-2019, 08:27 AM
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I feel really dumb now since it was right in front of my face but I think I found the problem. I am hoping to get some time today at work to see if just needs to be snugged up or what might be going on that is causing that leak.





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Old 02-14-2019, 06:53 PM
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That's not causing low pressure or a misfire. It does need to be fixed.
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Old 02-14-2019, 07:38 PM
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Originally Posted by dw456post View Post
That's not causing low pressure or a misfire. It does need to be fixed.
Yeah, but Iím wondering if itís making my fuel gauge off? I am dropping in a Walbro 450 tomororw. I put a new filter and regulator with no change. Under load, itís at 40psi and idle is 36-38psi
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Old 02-14-2019, 07:40 PM
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That's not even a leak, more like weeping; Would not affect pressure readings.
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Old 02-14-2019, 09:05 PM
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What alternator and pulleys are you running? It does sound like youíre running lean at idle but itís interesting that it would clear up once engine speed increases above 1000 rpm. Makes me suspect the fuel pump isnít getting enough voltage at idle but fuel delivery improves as the engine rpms go up and the fuel pump gets more volts with more alternator output. It would be easy to check alt output with a volt meter.
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Old 02-14-2019, 09:48 PM
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Originally Posted by s30.hybrid View Post
What alternator and pulleys are you running? It does sound like youíre running lean at idle but itís interesting that it would clear up once engine speed increases above 1000 rpm. Makes me suspect the fuel pump isnít getting enough voltage at idle but fuel delivery improves as the engine rpms go up and the fuel pump gets more volts with more alternator output. It would be easy to check alt output with a volt meter.
I am going to check everything out tomorrow. As busy as works been, I have only been able to spend a few minutes on it here and there. I just bought it a few days ago for pretty cheap. I figured a new fuel pump wouldnít be a bad idea anyways.
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Old 02-15-2019, 10:22 PM
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Well put in a Walbro 450 today, removed the extra inline pump after the fuel filter and no change. Fuel pressure stays at 38psi but now bleeds off with the key on?

Last edited by SouthPaw; 02-15-2019 at 10:43 PM.
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