# P0300 code, Please help!



## Ashvin (Jun 18, 2008)

I just had the head gasket replaced on my 00 maxima with 217,000 miles. They also sent the heads to be machined. This had to be done twice because I guess they didn't do the heads right the first time and the car was running super rough. I took it back and they machined and adjusted the heads again and this time it was running smooth for about a week. But now I got the p0300 code all of a sudden. Is this something thats related to them changing the head gasket and machining the heads? or is it something different? I've already replaced 4 of the coils before, but this code is random multiple misfire so I don't know what to do. My spark plugs were changed right before the head gasket ordeal.


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## Outkast (Jan 29, 2006)

They could have shaved the head too much and severly changed the compression causing the misfire.


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## Ashvin (Jun 18, 2008)

Thanks for the reply. So the compression should be better right? If so could it be that my fuel filter or injectors need to be changed so more fuel can get to the engine or do I need new heads? As you know the Fuel filter is in the gas tank so It has never been changed.


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## Outkast (Jan 29, 2006)

No, if they shaved it down too much it raise the compression so high that it detonates prematurely and causes a misfire. But don't rely just on that casue it's just one posible senario. Could just be a fouled plug, or bad coil, or even a clogged injector.


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## Ashvin (Jun 18, 2008)

I changed all my plugs and all he coils this weekend. The light came back on right away still. So what are the possible remaining causes? Bad fuel filter, bad injectors or bad head and valve job? can it be the timing?


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## Outkast (Jan 29, 2006)

injectors maybe, get them ohm tested but I'm thinking more over the head job being the problem.


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## Ashvin (Jun 18, 2008)

So I should say that the car is not actually missing. Not that I can notice anyways, or that affects the performance. I does worry me though that SES light starts blinking at highway speeds and then stops blinking when I slow down. Should they have done a head and valve job automatically with the head gasket replacement? Is that standard? My engine never had a problem before.


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## mer1316 (May 13, 2009)

hey dude first off, the fact your heads where machined has nothing to do with a misfire( i am a ASE certified tech and for that extent a very good engine performance tech with a lot of experience on imports) and the fact is that the amount of material they cut to make your heads flat will not affect your stroke, for you fuel mixture to detonate you need a spark with this being said if you said you replace the spark plugs and the code persist then is one of you coils but then you said that you replaced them, to begin you should have gone to a decent place where they have good diagnostic tools, they tell you everything that is going on inside your engine, they will check exactly which cylinder is misfiring because every time there is a misfire the computer saves that data then you check the coil you can either kill the coil or the injector just to be sure, now i do not know what kind of coils you have, if it has 4 wires the information comes stright from the computer, if it only has 2 then there is a external ignition source that commands the coil check from the computer and to be honest save some money and take it to someone who knows what is doing, and your fuel has nothing to do with it because there will be rich or lean codes along with the misfire, i hope this helped you ok. laters.


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## Outkast (Jan 29, 2006)

mer1316 said:


> hey dude first off, the fact your heads where machined has nothing to do with a misfire( i am a ASE certified tech and for that extent a very good engine performance tech with a lot of experience on imports) and the fact is that the amount of material they cut to make your heads flat will not affect your stroke, for you fuel mixture to detonate you need a spark with this being said if you said you replace the spark plugs and the code persist then is one of you coils but then you said that you replaced them, to begin you should have gone to a decent place where they have good diagnostic tools, they tell you everything that is going on inside your engine, they will check exactly which cylinder is misfiring because every time there is a misfire the computer saves that data then you check the coil you can either kill the coil or the injector just to be sure, now i do not know what kind of coils you have, if it has 4 wires the information comes stright from the computer, if it only has 2 then there is a external ignition source that commands the coil check from the computer and to be honest save some money and take it to someone who knows what is doing, and your fuel has nothing to do with it because there will be rich or lean codes along with the misfire, i hope this helped you ok. laters.


Seriously, if you shave a head too much it changes the cumbustion chamber area which in turn raises the compression ratio. That is basic ASE stuff. I never said it would change the stroke. Secondly, if you read the OP then you'll see that they had to do it twice which means they screwed up once they could have screwed up again.


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## Ashvin (Jun 18, 2008)

I did change all the plugs and the coils. The light came right back on. 
First time I got my car back after the head gasket replacement and the head and valve job it was super shaky and felt like a truck instead of the silky smooth engine I was used to. I took it back and they said the heads were not done right and the valves were leaking so they redid the whole job. 
The car ran smooth for about a week and then I got the ses light and the p0300 code. I have one year warranty on the work with these guys so I'm hesitant to go somewhere else when I already spent $2700 with them. 
Could the valves have been screwed up? or the heads messed up?


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## mer1316 (May 13, 2009)

sorry i did not explained myself well enough, if you had a worked performed in the head yes you may have changed the A/f Ratio by cutting too much angle in the valves or the actual combustion chamber, but that will generate a different set of codes because you do not have the right mixture which is 14.7:1 which is the optimal ratio for emission purposes, again you have a P0300 that is a RANDOM MISFIRE DETECTED it does not even have a p03xx which actually points a specific cylinder where the misfire was detected, now if that is the only code you have and the car is runnig good now all you have to do is find which part of the ignition is failing, by default you have the new coils and plugs so what is there left to look into, i could tell you what to do but it will take too long as far as diagnosing but you do need a scanner and a real one not the one in autozone. let me know for any questions. and no if your valves where messed up your car would not even start and if it did it would not have any power cause there would be no compresion to generate power.


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## Ashvin (Jun 18, 2008)

I just picked up the car for the mechanics and they checked the compression and it was at 150 which they said where its exactly should be. They also said they pressure tested all the valves for leakage again and they are not leaking, and they checked the fuel pressure and it is fine.
They told me despite the code there is no misfire. Let me say again the car is running great, with no performance problems at all. I can easily get it up to 100+ mph. 
They said it must be my computer and that I should take it to a auto electric place for better diagnosis and scope the valves. I have never had a computer problem before.
What do you guys think?


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## Ashvin (Jun 18, 2008)

Is 150 compression too low? Is that causing the misfire code?


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## Outkast (Jan 29, 2006)

That number is good for compression but it doesn't say if the head was shaved improperly. The ratio is a mathmatic eqation and can not be measured by a tool.


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## Ashvin (Jun 18, 2008)

The new mechanic checked the compression himself. The front is 150 but the back is at 230! They didn't check the back. The whole thing has to be taken apart and the cams realigned and reset. The guy is willing to redo it so i guess I'm gonna have to give him a third chance.


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## Ashvin (Jun 18, 2008)

*Bad News!*

The first mechanic looked at the car again, after I told him that the electrical mechanic said the problem is not electrical and that he thinks the problem is the cams being out of time.
He took the car somewhere else himself and had a "master mechanic" look at it. Now they say the problem is the rings can't handle the compression from the newly machined heads and there is too much compression for the bottom part of the engine. 
They say it is not fixable. They are refunding me all my money and said just to drive it till it eventually dies. He said it would just run rough with little power and burn gas, and that it could last a whole bunch more miles, but the point is he can't fix it.
How long do you guys think it will last? I went in with a head gasket problem and ended up with a ruined engine. At least I'm getting my money back.


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## Outkast (Jan 29, 2006)

Well it can't be the bottom but as long as you got your money back that a good start. Now just get some used heads from a junkyard have them cleaned but not decked, new valve seals, and new head gaskets. And everything should be good. And if you don't want to deal with all that just get a good whole engine get it dropped in.


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