4.3 vortec P300 random misfire - Chevrolet Forum - Chevy Enthusiasts Forums

4.3 vortec P300 random misfire


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jguill's Avatar
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July 7th, 2010, 2:22 PM   #1  
4.3 vortec P300 random misfire

My 4.3 vortec in a 96 Jimmy has code of P300 random misfire. It idles rough and has a slight backfire kinda like air popping sound. When i rev the engine its stays about the same up until 2k rpm and smoothes out slightly. Ive replaced cam position sensor, distributor cap,rotor, wires and plugs. Set cylinder 1 at TDC and checked to see if the rotor was aligned up with the piston one mard on the distributor and was perfect. Vacuum guage holds 16-17in with a little needle shake. 2k RPM holds steady at 20in, reving the engine it drops toward 0 and when i let off it jumps to 25-30 and steadly goes back to 16-17. Fuel pressure with key on is 54lbs and idle 55lbs. Fuel pressure holds about 51 with key off and doesnt loose any pressure. Im lost and cant seem to figure it out! The only thing i haven replaced with a new one is the crankshaft postion sensor. I did swap it out with a used one with the same results. Any help would be greatly apreciated.

 
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July 7th, 2010, 11:17 PM   #2  
So was this engine rebuilt or is this just a somewhat "new" issue? Depending on that it could be you just need a tune-up new cap rotor and wires. Other wise it is a diagnosis that is a shot in the dark.

 
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July 8th, 2010, 12:04 AM   #3  
Its a new issuse. A couple of weeks ago i had a missfire in cylinder 1. I replaced the plug and wire on that one and it fixed the problem. All of a sudden a few days ago it started running really cutting out and showing the P300 code. I replaced the pickup coil, cap, rotor, wires and plugs, but still have the same problem. I checked the fuel filter which is less than a year old and noticed the part inside rattled, so im hopeing nothing got through to the injectors.

 
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July 8th, 2010, 9:19 PM   #4  
You might want to run an injection system cleaner through and see if that cleans it up a little. After that you might want to recheck all your ignition components IE Cap, Rotor, Wires, Plugs etc. just to make sure you don't have any carbon fouling or fuel fouling.

 
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July 25th, 2010, 12:57 AM   #5  
Make sure you have the spark plugs in the right firing order, and check them to make sure there is spark

 
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July 25th, 2010, 8:36 PM   #6  
Has it died on you and not start. My mom's 97 Bonneville was having a simular problem and noticed that engine coolant was leaking into her intake manifold and when it would fill up and start getting into the intake runners down to the cylinders it would misfire and throw a code and eventually would flood out the cylinder and die. Ended up being the gasket between the intake and the throttle body were the engine coolant circulates around the throttle body. If you have the 2 piece plastic intake you can remove the throttle body and look into the intake with a light and you will be able to see the coolant pooled up in the valleys. Hope this helps.

 
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August 5th, 2010, 3:32 PM   #7  
This kind of sounds familiar as this has happend to me with an isuzu rodeo. I had multiple random misfires, ran horribly with misfires but ran alright if you stayed on the gas. I replaced plugs, coil packs and o2 sensors before I thought about the head gasket. The computer in my isuzu kept compensating for the extra air in the cylinders by richening the mixture to keep it running so it was hard to tell. It adjusted so quick that the ole starting fluid trick didn't work, but smoke testing will. Your problem sounds real familiar and the symptoms can be really misleading but the head gasket is just a thought. I hope it helps some good luck.

 
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August 5th, 2010, 3:44 PM   #8  
find an autoparts retailer that has a computer to hook up to it, things like this u can trow mega $$$ at it and never fix it, if most things arent fixed with a tune-up then it is time for a diagnosis

 
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September 13th, 2010, 3:41 PM   #9  
Posted By: jguill Its a new issuse. A couple of weeks ago i had a missfire in cylinder 1. I replaced the plug and wire on that one and it fixed the problem. All of a sudden a few days ago it started running really cutting out and showing the P300 code. I replaced the pickup coil, cap, rotor, wires and plugs, but still have the same problem. I checked the fuel filter which is less than a year old and noticed the part inside rattled, so im hopeing nothing got through to the injectors.
You checked the fuel filter but did you replace it? I chased that code in my silverado for a while.. It needed a new fuel filter

 
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January 15th, 2011, 10:04 AM   #10  
95 Blazer Cutting Off

We've got the same problem with our truck. Here is a little bit of whats going on with us and our truck. Please, if you have any kind of information that we can use, let me know.

Our blazer is giving us some problems and we haven't been able to figure out whats going on. We are looking for some feedback from people on what they may think is the problem. We hope that everyone can help.

We have replaced a bunch of parts on our truck and still having problems. What the truck is doing is cutting off while driving down the road. The timing is set right, the vacuums and electricals are right, and the timing chain has not jumped teeth or broken. Right behind the motor, the distributor, has some smoke that comes up once it cuts off. It's not running hot and once the smoke goes away and sits for a minute, it cranks right back up and drives for a couple more miles before it happens all over again. It has some kind of knocking noise, which we think is the 2 lifters that are loose, coming from the bottom of the motor. We know we have a bad cam, which is getting replaced today, hoping that this fixes the issue at hand. Please, send me your advice or what you may think could or would be the problem.

Thank you

 
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