PO300 Random Misfire

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MGregoir

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Sounds like it's coming from the area of the number 2 cylinder, GM 3100 SFI. I know the vacuum fuel pressure regulator bleeds off when it sits overnight, and it only misfires under load when leaving a light but it's a bad one.

I checked the plugs and wires. Replaced the plug with a new OEM one.

Test the coils? DIS module was replaced at 60,000 miles, right now the car is at 120,000 miles.
 
I had a wicked misfire that only happened under load. There was a break in one of the ignition wires that only short circuited under load. Under normal conditions, the spark plug was the path of least resistance. But under load, the short circuit became the path of least resistance.

I found it by revving the engine and listening to the engine bay. When it misfired, I could hear the spark.
 
How can you tell where a misfire is coming from [#2]? Why in the world would you change one plug?
This still could be ignition, but also a fuel injector, throttle position sensor, or MAF sensor.
Hopefully it is not a mechanical problem affecting one cylinder.
 
My car will tell you which cylinder on a PO300 code. I'm sure his does too.

Work it out by swapping injectors out, then swap coils out, etc. I've never seen a MAF or a TPS affect one specific cylinder.

If you don't have a proper test for the wires I'd replace them anyway, just to eliminate that.

This has been the "angel of death" code in the Chrysler community since the 5.7 Hemi came out. It can be very bad news!
 
The code reads random misfire, I think it can't detect which cylinder because it's pretty severe. I looked at it with a mechanic I know and he thought it was coming from the #2 pot as well.

I pulled the #2 plug and swapped it with a known good on that cylinder to rule out the first and most obvious potential source of the problem, I am going to put all matching OEM plugs in.

The plug I did pull from the number 2 cylinder (where the noise seems to be eminating from) showed some browning on the ceramic near where the wire connects and there were signs of arcing on the connector for that cylinder's coil. I took out other plugs (cylinders 1, 3, 5) and they appear to be fine. The plugs I had in are Denso Iridiums with less than 15,000 miles. The plug was clean but showed signs of fuel wash.

I am really confused because I started it and drove it in the morning and it ran absolutely fine, let it sit parked all day then as soon as I started it again there was a major misfire.

I will change the fuel filter, change the plugs, check the wires (wires were new with plugs), and start troubleshooting from there. I think/hope it is just a bad coil.
 
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P0300 means random misfire as in thru-out more than 1 cyl. P0301, 302 etc indicated exactly which cyl misfired..P301 for cyl 1, P302 for cyl 2 and so on. Misfires can be from plus/wires, coils or injectors - they are the biggest culprits. More unlikely, but a possiblity according to GM TSB's I read, is a bad CKP (crank position sensor). I always do a full tune-up and clean the injectors and that has always worked. Cheap enough to do and I agree with the poster above - why only change one plug??
 
Also one more thing - if the check engine light flashes than you got a real bad random misfire! You could damage your cat converters by driving it.
 
I am changing all of the plugs!!! ****!

I never would change just one and leave a mismatched set in. It's just that it seemed like the location of the knock, and then looking at the plug pointed to the number 2 cylinder, so I swapped it with a new one to see if the miss would go away.

When troubleshooting, only change one thing at a time so you can actually locate the problem. We changed the plug and it didn't change anything, so all of the plugs are now changed to match so it has new OEM plugs in. I was going to change the full set anyways because I bought all six because when I needed one to test with.

I changed the fuel filter and checked all the hose connections and EGR valve. Crank position sensor is possible - it went once before and cooked the catalytic converter if I remember right. It was before it was my car.
 
Originally Posted By: MGregoir
I am changing all of the plugs!!! ****!

I never would change just one and leave a mismatched set in. It's just that it seemed like the location of the knock, and then looking at the plug pointed to the number 2 cylinder, so I swapped it with a new one to see if the miss would go away.

When troubleshooting, only change one thing at a time so you can actually locate the problem. We changed the plug and it didn't change anything, so all of the plugs are now changed to match so it has new OEM plugs in. I was going to change the full set anyways because I bought all six because when I needed one to test with.

I changed the fuel filter and checked all the hose connections and EGR valve. Crank position sensor is possible - it went once before and cooked the catalytic converter if I remember right. It was before it was my car.



I hear ya.
 
Quote:
I am changing all of the plugs!!! ****!



lol.gif
This is one of those "Serpico" moments. You were caught in the bathroom with the light out. Nothing you said made a difference after that
lol.gif
 
I had a similar problem, scan tool identified it as missfire detected #2 cylinder. At times especially under load then engine would actually stall on the freeway. Checked all the usual stuff, then pulled the vacuum line off the fuel pressure regulator and fuel came out. Replaced the regulator and fixed it all.
 
as for the single spark plug replacement, if there is a misfire under warranty, Ford and Mazda will only authorize replacement of one spark plug and either the single wire for that cylinder or the coil if COP.
 
Originally Posted By: mechtech2

How can you tell where a misfire is coming from [#2]? Why in the world would you change one plug?
This still could be ignition, but also a fuel injector, throttle position sensor, or MAF sensor.


Exactly.

When you get a P0300, you need to look for a COMMON cause, some problem that effects all cylinders. Swapping one plug is pointless.

Focusing in on one plug, one coil, one injector etc is wasting time.
 
It's going to the dealer.

I don't have the equipment, skill or willingness to troubleshoot it fully, I just looked to see if there was a real obvious problem first, like bad plugs, wires, bad fuel or poor fuel flow, an EGR leak or vacuum leak. Throttle position sensor and MAF readings are correct on my scan tool, and there are no other codes except P0300.

I'd just end up changing a bunch of parts and scratching my head if it still doesn't work.

Fuel pressure regulator sounds likely to me knowing that it was sort-of marginal to begin with, I just really don't want to drain the coolant (this system is TERRIBLE for trapping air when trying to refill it) and pull the upper intake manifold to get at it. I also don't really feel like checking each individual injector.

The bill is gonna suck hard for diagnostic time ($125 an hour). I'm going to have to leave a note taped to the inside of the hood saying that the engine is full of G-05 and not to refill it with Dex-Cool as well as on the work order and a couple of gallons of the correct coolant on the passenger seat.
 
Originally Posted By: mechtech2
'Sounds like', and an engine code, are very different.
Good luck.



Yes. An engine code is your start point for diagnostics. I hate when you talk to people and they think a code means a bad part and just replace stuff when a code is just a symptom reporting to you via a scan tool. This is why everyone should have an OEM service manual so you can follow the diagnostic tree and fix it right the first time....no need for internet "experts".
 
I had the factory manuals and looked at the tree, I made it to step eight when I decided it was going to the dealer...who can't look at it until Friday and nobody else in town has time to look at it any sooner.
 
Originally Posted By: MGregoir
Ignition control module. $500 parts and labour. They just got to it today.


Do you know if the dealer actually tested the old module before replacing it?
 
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