Thread: Code P0300 , mutiple random missfires, no fix yet

ok I'm getting a code P0300 which is mutiple random missfire. well it's still missfiring and heres what i've done so far.

-Checked for vaccum leaks
-changed CPS (crankshaft position sensor )
-new wires
-new plugs
-cleaned intake filter
-cleaned butterfly behind the MAF
-new coil packs
-had the Ignition Control Module tested (GOOD)
-I've ran 2 bottles of seafoam, 1 bottle of lucas fuel injector cleaner in the past 6 fillups...( I always run 89 or 93 fuel )
-New Cat converter ( after my old one shredded which is probably due to overheating ( hince, missfiring ) )

I have no idea what to do next, any suggestions? does an '06 grand prix even have a Camshaft sensor??

Im having this code pop up now, but i am going through an endless battle to solve a backfiring issue im having, backfires if I let off the gas before it shifts in second. Now finally 8 months later im getting this same code so it might offer insight to the problem.

Also I can feel the engine shake at idle along with the rpm needle jumping a small amount. heres a list of things ive tried which made no difference in my case;

My father has the same car so I borrowed parts to try.

-Cleaned MAF sensor
-Replaced MAF sensor with used from 09 impala
-Replaced spark plugs (ACDELCO Gapped 060)
-Replaced plug wires
-New air filter
-Swapped all three coils with known good coils
-Swapped ignition control module with known good
-Cleaned throttle body completely( Removed from engine and cleaned)
-Swapped Throttle body with known good ( Assembly includes throttle position sensor)
-Got catalytic Converter tested for clogging ( Converter OK)
-Had a new Coolant temp sensor so put that in.
-Inspected PCV valve and found to be not sticking
-Tested fuel pressure and all within specifications **Does decrease during WOT normal???**
-Swapped EGR valve with known good
-Tested EGR valve for leaks, Not leaking
-Swapped CAM Sensor with known good
-Checked for the presence of CAM sensor magnet
-Injector leak test
-Replaced o2 sensor front with NTK BRAND(was beginning to go bad)
-Reset PCM by battery disconnection for ten minutes
-Intake air temp sensor replaced (Serviced with MAF sensor)
-Did visual check for exhaust leak, (no noise found no soot around joints)
-While doing the converter check the dealer said there was a code for the Baro/Map Sensor,
I never seen this code when I checked a week before nor did I see it a few weeks later so I gave a try at changing this sensor anyway and still didn’t solve
-I switch the map and Baro sensor around and no change.
-Tried driving without the supercharger belt connected and this made no difference in the backfiring

Last week I thought id get lucky by leaving the car with the mechanic for a day but he could not solve the problem, he stated 5 cylinders were misfiring, very small amounts but still so. He gave a shot at cleaning the injectors by taking them off the car and cleaning individually) No difference.

Brought it to a second garage however he said it was normal, even though im sure he seen the misfires on the live data( he was just lazy to do real mechanic work or something)


I will post what I find to solve this code on mine as soon as I can track it down, At this point Im going to test the compression and test the fuel pressure again...

I must also add the car sucks the gas right up since the backfiring began, Like around 20-24 Canadian MPG

yeah dude, if this Cam position sensor doesn't fix it...I'm at the point of turning her over to the Stealership, I can't get my dam car inspected...or atleast pay them to put it on that machine they have...

I have an aeroforce gauge and it displays missfirs by cylinder. When I had a p0300 code I was able to see that it was a specific cylinder even though it was random code. I don't no if any of the code readers display misfires by cylinder but it is worth a shot.

Aeroforce also let's you display live sensor data which might help you with trouble shooting sensors.

Check your LIM gaskets. All thedifferent ports you could be leaking something into the cylinder.

Ok..I'm assuming you both have other threads here? Plaese link them because most of us don't want to tell you to do the same stuff we might have already suggested. It'll just aggrevate everyone. Please link em if ya got them.

Ok.. Misfires eh.. P0300 hmmm
Keeping in mind this is for a rear wheel drive GTO, use the diagnostic aids to think of things in our cars that might be similar.
GearChatter.com :: View topic - DTC P0300

Like bad motor mounts etc etc...

Then look at your O2 wiring, make sure it's not touching any plug wires. Sounds like you've both gone through the usual suspects of tune up and vacuum. Check the elbow at the brake booster, it's a check valve. When bad it's a vacuum leak. Being P0300 it's random multilpe which usually means it's something else goofy.

Did it start happening after a battery cable unhook? Try and case learn, it's a 1 in a 100 chance, but when the pcm loses it's position..it does all sorts of f'd up stuff.

nah this is my first P0300 thread, although I have posted in other ones with my issues...Yes, Threadjacked lol

Yeah I have that same page printed out already, guy at the place printed it off for me after it failed inspection.

Ok, you have the page, anything stand out from the suggestions or what stuff from it have you tried?

BTW that page shoudl be slightly different. Not sure if the place had the GM specific page.

I had a look from that printout for my specific year from alldata and it's pretty much the same thing, as for that elbow im going to check that next as I had no idea that was a check valve.

Only other thing on the page that catches my mind is my belt tensioner pulley has just started chirping.

I tried a case learn and did nothing for me unfortunatly.

As for the threads I havnt made one on the issue yet, alwys on the go, haha.

*just checked the check valve and mine is good, I wouldnt get that lucky i guess, haha

Originally Posted by michaelbm

This would show something on the compression test too right?

Yes.

one thing i dont see mentioned yet is checking all the ground wires. making sure you have a good chassis ground. that was a good point made about making sure the plug wires dont touch any other wires like the Kr sensor on the block or O2 sensor.

deff check those KR sensor wires on the block

I dunno, I'm lost...I bought some more plugs, A/C delco's this time and a cam sensor...I'll try that...but I'm stumped on this car...

Isn't there a ground under the ICM too?

have you tried swapping a known good PCM? It worked for me, maybe it will work for you? The PCM could have something wrong with it where resetting it isn't fixing it.

ummm yeah PCM's are very expensive and they just don't pass by my desk everyday lol...so thats pretty much an impossible thing to do here for me...

A/C delco plugs put in today gapped at .60, no fix...Next task is replacing the camshaft position sensor...Maybe Monday or so..

Originally Posted by offroadfury6

ummm yeah PCM's are very expensive and they just don't pass by my desk everyday lol...so thats pretty much an impossible thing to do here for me...

A/C delco plugs put in today gapped at .60, no fix...Next task is replacing the camshaft position sensor...Maybe Monday or so..

You could buy a modded one for around $100 and send them yours. Should all work out as they will flash your old one anyways.

Im thinking on trying a pcm, Rare they go but always a chance for sure.

Tonight I got a pending code for cylinder 4 misfire P0304, Guess I know what cylinder I will test first.

I bought a compression tester today so I'll be able to rule some stuff out

lucky you. atleast you know what cylinder it is...