P0300 code random misfire...

**Phlash_riot** · 12-15-2006, 08:16 PM

Same issue here and i still have it after replacing the plugs ad wires and visual check of the injectors. Happens to me in all gears(M6) at ~1500-2200RPM so i am at a loss. Ed B Vert hooked mt car up to his super wama-dyne computer and showed a misfire on one of my cyl. but not sure if that was due to the bent valve in my heads that i have since replaced with LS6 heads.
Well good luck on your end i will let you know if i find anything else out. I think that i am going to go reset my PCM tomorrow.

EDIT: Can you replace the Crank shaft position sensor with ripping apart the engine?

**Steve Crowe** · 12-16-2006, 12:23 AM

Yes, probably just one and that can be corrected easily.

CPS? You'll get one heck of a hard code on that one and the car will probably quit in a matter of days. I've changed a few over time but it's always been a Chyrsler product (mainly a Jeep) that has been running off-road.

Please learn to read your own PCM, a few $$ here: http://www.obd-2.com and a $100 used laptop gives you the capabilty to chart your car in real time.

Disclaimer: I am not in any way associated with Alex, I just know where to go when I need true PCM diagnostic equipment.

**GatorSS** · 12-16-2006, 06:47 AM

Are you saying that is better than HPTuners?

**Steve Crowe** · 12-16-2006, 07:54 AM

Depends on what you want to do - or LEARN.

If you want to tweak your car - and you know what you're doing - I would recommend these guys: http://65.109.52.43/home_index.htm

Not for the faint of heart. This is in no way a toy as you can can damage your vehicle if you mess up. Buy it, learn it then put it to use.

**GatorSS** · 12-16-2006, 09:01 AM

That software only supports V6 engines, not the LS1.

**Steve Crowe** · 12-16-2006, 09:14 AM

Yes, it will.

It will write to all GM PCMs from '96 up to CAN. You must know what you're doing though.

**GatorSS** · 12-17-2006, 10:40 AM

Sorry, I was just going by what that company claims on the web link you gave. From their own website:

Digital Horsepower Inc currently offers the following products and services :

Digital Horsepower 1.0 PCM Units The 1.0 are our off the shelf performance PCM upgrade. To view the features of this product, click here. We support the following applications :

Pontiac Grand Prix GTP w/ 3800 Series II Supercharged : 1997 - 2003
Pontiac Grand Prix GT/SE w/ 3800 Series II : 1997 - 2003
Chevrolet Monte Carlo Z34 w/3800 Series II : 1998 - 1999
Chevrolet Monte Carlo SS w/3800 Series II : 2000 - 2003
Chevrolet Monte Carlo SS w/3800 Series II Supercharged : 2004
Chevrolet Imapala SS w/3800 Series II Supercharged : 2004
Buick Regal GS w/ 3800 Series II Supercharged : 1998 - 2003
Grand AM GT w/3400 : 1999 - 2002

Custom Programming
If you are looking for a more customized PCM option, we offer custom programming for the following models :

General Motors V6 3100, 3400, 3800 engines : 1997 - 2004
General Motors V8 LS1 : 1997 - 2003
Saturn : 1997 - 2003

According to them, they have no product for sale that supports the LS1, they only will do custom tuning services for the LS1. If they have a product for sale that will support the LS1, then they need to update their website.

I wonder how it compares to HP Tuners?

Thanks for your help.

**Too Fast** · 12-17-2006, 11:19 AM

Not a common problem with our cars, but it could be a sticking EGR. However, that should show up as a code as well. But, a sticking open EGR valve will cause a miss at idle and low load conditions, like a constant speed. At idle sometimes a sticking open egr sounds like the car has a wicked cam, and that would give you the random misfire code. Have you fixed your car yet?

**Steve Crowe** · 12-17-2006, 11:35 AM

They just don't have any full time support to help us write to all the PCMs, that is our job to figure out.

A few $$ gets you into the PCM but I will warn everyone again, you better know what you're doing.

You hook up, pull down your .bin file. Modify it as you please but vary the fuel tables just a tiny bit at a time.

**Steve Crowe** · 12-17-2006, 11:51 AM

EGR? Pull the stupid thing off and clean the carbon off it. Put it back.

I'd rather do an EGR than a thermostat.

**Nasty TA** · 12-17-2006, 02:09 PM

If you are getting a random missfire code after changing plugs and wires you have a broken valve spring or a hole or crack in a cylinder. If a coil pack shorts out it will blow the 15 amp fuse and short out that entire bank. It can be caused by carbon buildup but not at all rpm's. I have had five different cutomers come to me with this ame problem. The only way to know that it is a spring is to pull the rockers on the cylinders that are missing the most. Most of the time it will make two miss if only one spring is broken. It will backfeed into the next in the firing order or the one before it. most common ones to do this are five and six. I also had this happen to my TA and i just chnged the springs and away i went.

**GatorSS** · 12-17-2006, 07:27 PM

My misfire is intermittant, which has increased in frequency, but still intermittant. It begins within a few minutes after a cold start-up, almost immediately after a warm start-up, and is evident at all rpms, but much more so under load.

It's been parked for the last few days. When my new notebook computer arrives, I'll be able to plug in and figure it out.

**Phlash_riot** · 12-17-2006, 09:44 PM

Originally Posted by Nasty TA

If you are getting a random missfire code after changing plugs and wires you have a broken valve spring or a hole or crack in a cylinder. If a coil pack shorts out it will blow the 15 amp fuse and short out that entire bank. It can be caused by carbon buildup but not at all rpm's. I have had five different cutomers come to me with this ame problem. The only way to know that it is a spring is to pull the rockers on the cylinders that are missing the most. Most of the time it will make two miss if only one spring is broken. It will backfeed into the next in the firing order or the one before it. most common ones to do this are five and six. I also had this happen to my TA and i just chnged the springs and away i went.

I have a new (18000 miles) heads and have the same symptoms as when i Had my old heads with bent valves. also while i had my heads off i looked at the pistons and cleaned them up of the carbon deposits and there were no holes or cracks that i could see.

Also can't be the EGR for me cause i ripped it off and put in blockoff plates

**SSTODD** · 12-18-2006, 07:30 AM

Do you have factory or aftermarket cats??? may want to check out O2 sims if not.

**GatorSS** · 12-18-2006, 10:40 AM

I still have my factory cats...but they will be gone soon after I get the new notebook computer and the new HP Tuners software. Also, I checked my EGR valve and it is not sticking.

I keep thinking it's a coil or two, since my current symptons virtually mirror those I've experienced on outborad motors with failing coils over the years. But I'm just a hobbyist and not an engine expert. I'll figure it out soon and let y'all know.

Thanks for all the input here.

**Phlash_riot** · 12-19-2006, 10:16 AM

I have heard that the exhaust can rattle sometimes and send that noise into the engine therefor tripping the knock sensors wich then retards the timming... Worthwhile looking into that?

**GatorSS** · 12-19-2006, 03:17 PM

But timing retard will not cause misfire.

**Too Fast** · 12-19-2006, 03:29 PM

So, another thing to check is your MAF sensor. Try disconnecting it and see if it runs good then that should be your problem. Are you using an oil-based air filter? (K&N or equivalent?) This is a no-no for our cars. The oil skews the MAF sensor.

Something else that can happen is the rocker arm studs can get loose, on more than 1 cylinder, and that can cause the random misfire code. Good luck and keep us informed....

**GatorSS** · 12-19-2006, 08:44 PM

MAF sensor and IAT sensor are cleaned regularly, even cleaned them again after the misfire first appeared.

Haven't checked the rocker arm studs, at least not yet, that is. Thanks for the info.

**Too Fast** · 12-20-2006, 02:58 AM

Still, try disconnecting the MAF. Then drive the car. Of course the MAF code will appear. It is an easy check to do. The MAF can get skewed and tell the PCM there is more or less air than there really is and not set a MAF code.