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Question on Engine Code

#1 User is offline   ratracer49 Icon

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Posted 23 April 2018 - 08:33 AM

This isn't really racing related (unless you count that it's for my tow vehicle) but I have a question on an engine code. I've seen on here in the past some good help from people on daily drives so I thought I'd ask. I have a 2001 Silverado with the 6.0 gas engine. I am getting a P0300 Random Misfire Code. The check engine light seams to trigger mainly when I'm coasting from speed or maintaining speed. I don't notice any actual misfire, at least not enough of one I can discern. My question is, can this code be caused by an exhaust leak? I know I have a leak at the exhaust manifold gasket and was wondering if once I rectify that my misfire code might go away. I guess I'm trying to be lazy and hoping to not have to go through looking for vacuum leaks, checking coils, and checking injectors to track it down.
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#2 User is offline   Verwayne Icon

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Posted 23 April 2018 - 04:08 PM

I asked a friend about that code and he told me:

"If you did a search on that code it could be triggered by numerous issues. Doubt there is a way to find out unless he takes it to have scanned."

I'm not sure who is and around your area, but Auto Zone will generally do a free scan if they aren't waist deep in customers.
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#3 User is offline   DRTmotorsports Icon

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Posted 23 April 2018 - 08:21 PM

View Postratracer49, on 23 April 2018 - 08:33 AM, said:

This isn't really racing related (unless you count that it's for my tow vehicle) but I have a question on an engine code. I've seen on here in the past some good help from people on daily drives so I thought I'd ask. I have a 2001 Silverado with the 6.0 gas engine. I am getting a P0300 Random Misfire Code. The check engine light seams to trigger mainly when I'm coasting from speed or maintaining speed. I don't notice any actual misfire, at least not enough of one I can discern. My question is, can this code be caused by an exhaust leak? I know I have a leak at the exhaust manifold gasket and was wondering if once I rectify that my misfire code might go away. I guess I'm trying to be lazy and hoping to not have to go through looking for vacuum leaks, checking coils, and checking injectors to track it down.


An autozone scan will not reveal anything other than what you already know. A P0300. They may give you a list of things it could be, but the bottom line is that code can be triggered by many things. Could be anything from old plugs, bad wires, O2 sensors, even an ECM. It is a pretty large list of things that can trigger that code. If this was my truck, I would start by looking at spark plugs and wires. If they are old, and just do not look good, change em. If they are newer, and look to be in good shape, it may be more cost effective to take it to a trusted shop and let them do some live scanning. They can actually observe most of the sensors, and what they are doing while the vehicle is running to help pin point the issue. Sorry, but this code is probly the worst one to have, just too many variables. Good luck.
Duane Turner
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#4 User is offline   mod911 Icon

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Posted 24 April 2018 - 07:57 AM

Check the intake gaskets for leaks,they leak under high vacuum conditions such as cruise and decel. Does the light ever FLASH? Be glad its a 6 litre since they are not a DOD motor. A partly clogged injector will give similar results. Look at mode 6 data or watch misfire COUNTER rack up misfires cyl by cyl live. A scanner is of little help mostly,people ask for their codes to be checked and when I say pxxx they say what is that. They already knew since their bud or auto zonnies get them the code. They must be expecting maajic instructions to fix it for free....AS I tell people when they ask me THE question......DO you do diagnostics? I just YES, we do.....NEXT come THE question.....Do you charge for it? I say yes and they say just for checking codes,isn't that customer service? I say whats your name,put in my data base and up comes NOTHING....I say but you are not a customer......well I would be maybe if you give me something free that I already have....I then say, why do you expect the SMARTEST guy in the room to work for free? I pay monthly for information about 4 hundred PLUS the cost of a Snappy top line diagnostics platform + updates and other expensive quickly outdated equipment....

Search for clues is the best advice....coils are rare fix,injectors too unless someone take a probe or power wire to them....clean throttle bore first....spray water with some wiper fluid in it and see if it make a worse misfire there is a good place to look....
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#5 User is offline   ratracer49 Icon

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Posted 24 April 2018 - 08:18 AM

Yes unfortunately this is a very vague code. I have one of the MaxiScan units (don't recall the model) which is how I got the P0300 code. It was the only engine code nothing more specific yet. One thing I probably should have added is that I just replaced the plugs. I had new ones waiting to put them in and once I got the code I replaced them so I could look at the old ones and see what I could find. The plugs were definitely due to be replaced but none of them looked off in color. I cleared the code and it came back again. Like I said it runs fine but obviously something is triggering the code.

Being it's such a vague code is it possible that the exhaust leak I know I have at the manifold gasket could cause this code? I don't want to waste a lot of time and money fixing things that don't need to be so I figure bringing it to a shop will be the way to go if its not the leak. However I also don't want to pay for someone to just tell me I have an exhaust leak that I know I have. So my next plan of action will be replace the gasket, and probably some studs and bust a few knuckles. Then see if that clears it up. If not off to the shop.

Thank you both for your time and response.
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#6 User is offline   ratracer49 Icon

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Posted 24 April 2018 - 08:34 AM

View Postmod911, on 24 April 2018 - 08:57 AM, said:

Check the intake gaskets for leaks,they leak under high vacuum conditions such as cruise and decel. Does the light ever FLASH? Be glad its a 6 litre since they are not a DOD motor. A partly clogged injector will give similar results. Look at mode 6 data or watch misfire COUNTER rack up misfires cyl by cyl live. A scanner is of little help mostly,people ask for their codes to be checked and when I say pxxx they say what is that. They already knew since their bud or auto zonnies get them the code. They must be expecting maajic instructions to fix it for free....AS I tell people when they ask me THE question......DO you do diagnostics? I just YES, we do.....NEXT come THE question.....Do you charge for it? I say yes and they say just for checking codes,isn't that customer service? I say whats your name,put in my data base and up comes NOTHING....I say but you are not a customer......well I would be maybe if you give me something free that I already have....I then say, why do you expect the SMARTEST guy in the room to work for free? I pay monthly for information about 4 hundred PLUS the cost of a Snappy top line diagnostics platform + updates and other expensive quickly outdated equipment....

Search for clues is the best advice....coils are rare fix,injectors too unless someone take a probe or power wire to them....clean throttle bore first....spray water with some wiper fluid in it and see if it make a worse misfire there is a good place to look....



I'm a bit slow on the keyboard and got side tracked so my previous reply was before I saw yours. Thanks for the tips it gives me a couple more tools to self diagnose.

In case you haven't noticed the smartest guy in the room is always expected to work for free. I have never understood though why people feel that someone should provide a service for free. Just because you don't hand them a physical product doesn't mean there is no work or cost of equipment involved.
You have to learn the rules of the game. And then you have to play better than anyone else.

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#7 User is offline   mod911 Icon

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Posted 25 April 2018 - 07:28 AM

Its not likely that the exhaust leak is to blame...soak them in oil for as long as you can and maybe a reverse drill will screw them out...random misfires are fun. just because the plugs look good doesn't mean anything. They can get chemically coated and other things that cannot be seen but measured on a scope. Check burn time of the plugs if you can,its old but look anyway...
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