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2002 GMC Sierra 1500 Question: I have a P0300 code for a random misfire.

 

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big01_dram, 4.8L V8, Pekin, IL, September 09, 2011, 18:42
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 Journeyman

I did a tune up and all was well. Shortly after I developed a random misfire. New plugs wires, fuel filter. It is a coil over system so its not there. The only thing I can find is the fuel pressure is low. Around 48 psi instead of 62 psi. I didnt get this filter on wrong thanks to the little arrow thats points that way ---->. I have wondered it the fuel trim adjusted so far out with the old plugs that it is just relearning but how long should that take? Could my fuel pump really just be putting this low pressure out all of a sudden? I have stumped myself.

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  • Answer #1

    patrick mannion from Greg Solow's Engine Room, September 10, 2011, 01:28
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     Master

    Fuel pressure being low is a concern and could definitely cause a misfire. Is the fuel pump volume OK it should put out a pint of fuel in 10 seconds. Mis-fires could be caused by intake manifold leaks as well. A professional grade scan tool would read live sensor data but also long and short term fuel trim figures which may locate where the problem is. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info. There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online

    I just had a Toyota Sienna in the shop a few weeks ago, it was really hard to start ran awful when it did start, I put fresh gas in it it set various cylinder misfire codes that changed from a few injectors to different injectors. It turned out his kid had put gas in in from cans that were in there garage at home. I put another big inline filter in line (had to cut the fuel line and put it in series) I did an injector rail flush and now car runs great. There is a big difference in scan tools, professional grade scan tool with Mode 6 and freeze frame/live sensor information is a great help in looking at complex problems but as you are trying to do don't over look the obvious simple things. Alldata is great for theory of operation and test procedures.

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    big01_dram, September 10, 2011, 08:47
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     Journeyman

    Thanks Patrick. That is my first problem my scanner doesnt have serial data list. So no misfire chart either. Volume I have not checked yet. I checked for vacuum leaks the best i could by listening and using some brake clean. It made no change in how she is running. I have wondered about bad gas because when I double checked the gap on the plugs they smelled a bit funny. I put some injector cleaner in and now the light is out. Hoping maybe its just bad gas. The tank was filled up after the tune up which is when the light came on. Maybe this is just staring me in the face and I can see it.

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  • Answer #2

    delane 72 garage and body shop September 10, 2011, 23:05
     Master

    could have bad wires us best plug wires that u can buy an ckeck for cracked plug use platum plug autolite or acdelco spark plugs

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    big01_dram, September 11, 2011, 06:57
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     Journeyman

    Further checking has me thinking it is the fuel pump. I got it scanned at the parts store and got a p174 code along with the p300. Great now my scanner is not giving me all codes. So I check bank 2 plugs which are now all fouled. I checked the regulator which did spit a bit of gas out the vacuum port. I have replaced the regulator and its running better but not completely.

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  • Answer #3

    deajr39 September 07, 2014, 15:48
     Rookie

    change rotor and distributor i have 2000 gmc sierra 4.3 I had to change distributor and rotor to stop random misfire

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