P0300 code random misfire...

**Fedge** · 12-20-2006, 05:35 AM

having a similar issue, slight random miss on cylinders 1 and 6...HPTuners and a few runs found it...I noticed that under light load and idle, i was having this miss, albeit a slight one. I get no codes at all...I have no EGR or cats...I checked the springs, rockers, fuel pressure, MAF, wires, plugs, vacuum, did a compression test (140 is a little low, no?) only thing left is coils and pulling the heads. when my father bent a valve, the miss was very evident and constant, there was nothing random about it like mine...kinda sucks. i was gonna start buying MSD coils one at a time (75 bux each!!) till i had a full set...

hope you figure it out...when you do, post up! i'll do the same if i fix it

**gen3performance** · 12-20-2006, 06:48 AM

i work at a gm dealer(16 years) and deal with this same issue every day but mostly on 5.3 engines because we work on more suburbans and tahoe than ls cars.usually when you loose a coil it goes completely.the crank sensor is a good suspect.the things that i find the most are 1)wires that are broken inside the insulation to the injector. 2)restricted catalytic converters or exhaust. 3)partially clogged injectors. and 4)very rarely slightly flat camshaft lobs.this usually causes a miss between 1800-2000 rpm but not noticable at idle.we use a tech 2 scan tool which allows us to see exactally what cylinder is missing.it would be very difficult to diagnose this concern without that.with the tech 2 we can also see what the fuel trim is doing and the o2 readings at the same time.i know we charge about 85.00 to diagnose something.it may be worth it to try to get someone to diagnose it for you instead of throwing parts at it.parts are not cheap which i'm sure is something you already know.just my 2 cents.

**GatorSS** · 12-20-2006, 11:51 AM

Thanks for your info. I might go out and try a few other things, but I'll probably just wait for my new convertible notebook computer (I had an "accident" with my current notebook) and the new HP Tuners software to come, which might be slightly delayed due to the holidays.

I miss driving my SS since it's my daily driver...just because I enjoy driving it. All my other vehicles are totally different animals....and much less fun.

Obviously, your experience is much more extensive than mine, but if my cats are restricted, wouldn't it throw a different code? Same with the crank sensor?

**GatorSS** · 12-20-2006, 11:54 AM

Oh, forgot to mention, I have almost 107k miles on it and this is the first problem I've had. I am the original owner...it had 7 miles on it when I bought it.

**GatorSS** · 12-21-2006, 09:10 AM

Cleaned the fuel injectors yesterday, just to see if it might be a clogged injector, and it was inexpensive since I had a cleaning kit and only needed a can of cleaner.

No change.

Still intermittent misfire. It will run right for a few minutes, misfire a few times, run right a little, then steady misfire for a minute or two, then fire right for about 10 seconds, then back to a misfire every few seconds, then fire right a little, then several minutes of constant misfire...and so on.

**Chad97z** · 12-21-2006, 03:50 PM

Originally Posted by Loco

Thats what my car is throwing.

I have changed the plugs a few times and it is still there. I also changed the wires & still there. I had it scanned at Autozone & the scanner did not say which cylinder(s).

I can feel the car is down on power, but at 70+ it'll shake pretty bad.

I know the wires are all on properly and the plugs are good. That leaves coil packs & what else?

Anyone in South NJ with software that says which cylinder(s) it is want to help?

Could need a rocker arm adjustment. My car did the same thing after I did a head gasket install. And I got one of the rocker arms too tight. I only had one cylinder doing it. So it was not random. I was not getting enough compression in the #3 cylinder because the rocker arm was sticky from over tightening. So if possible, check your compression on each cylinder. That will narrow it down.

**tblu92** · 12-21-2006, 04:32 PM

Do you have an aftermarket camshaft ??? 90% of all camshaft installs will throw a random misfire code--The misfire file to disable the misfire detection is C5601--enter "1" and that will eliminate the codes.If you believe it is a false code as in a camshaft install it is OK to do---or in any case where you feel it is a false code--Just be aware from now on--you will have to use your ears to detect a misfire--which should be easy for a car guy. Also in any misfire sittuation it would be a good idea to do a "crank position sensor re-learn"---these procedure are included in most camshaft install instructions--or by doing a search in most forums

**GatorSS** · 12-30-2006, 08:53 AM

Stock camshaft. It is not a false code, it started misfiring before it threw the code. Now it is a constant misfire, but still only the P0300 code. Plugs look virtually identical. Exhaust has the rotten egg smell, which indicates cat problems. However, the cat problem could either be a symptom of the misfire, or the cause, since it started smelling a little bad before the misfire problem. But I thought I would have a cat code as well.

Still waiting on my software. I don't have time for more troubleshooting.

**9t8z28** · 12-30-2006, 09:17 AM

I had the same code with my LT1. It was running in open loop because the Bank one, Sensor one was disconnected. It came up random Misfire. Im not saying that is your problem, but give it a try. Running in open loop can cause some serious problems.

**mrr23** · 12-30-2006, 04:01 PM

Originally Posted by Phlash_riot

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

remove starter and you'll see the crank sensor.

**Phlash_riot** · 12-31-2006, 04:32 PM

Well i guess i could try to replace taht sehnsor. I need to get her hooked up to a computer first I think to see how things look. Might as well since i have to get eh !EGR and !AIR before i leave in the 23rd.

**GatorSS** · 01-02-2007, 03:01 PM

Originally Posted by gen3performance

i work at a gm dealer(16 years) and deal with this same issue every day but mostly on 5.3 engines because we work on more suburbans and tahoe than ls cars.usually when you loose a coil it goes completely.the crank sensor is a good suspect.the things that i find the most are 1)wires that are broken inside the insulation to the injector. 2)restricted catalytic converters or exhaust. 3)partially clogged injectors. and 4)very rarely slightly flat camshaft lobs.this usually causes a miss between 1800-2000 rpm but not noticable at idle.we use a tech 2 scan tool which allows us to see exactally what cylinder is missing.it would be very difficult to diagnose this concern without that.with the tech 2 we can also see what the fuel trim is doing and the o2 readings at the same time.i know we charge about 85.00 to diagnose something.it may be worth it to try to get someone to diagnose it for you instead of throwing parts at it.parts are not cheap which i'm sure is something you already know.just my 2 cents.

I've been tinkering some more with it. In your experience and in regards to number 1) above, where have you seen the broken wires to the injectors? Close to the injectors, or further up in the wiring harness? In those cases, did it show another code in addition to the P0300?

Considering number 2), I thought that might be my problem until I pulled both cats for inspection. Too my surprise, both were super clean, like brand new as are all four of my 02 sensors.

So, I suspect my problem is number 3), but I think I have one that is totally clogged since now that the misfire is constant, I no longer smell any unburnt fuel (rotten egg smell) in the exhaust like I did when it was internittent. After removing all of my injectors, one looks like it is clogged, while the other seven look relatively clean. Of course, I know appearances can be deceiving, so I'm going to send all eight off for injector service.

So far, I've thrown no parts at it, only some time and a little money for the rail-connected injector cleaner, which didn't clear the clog. The whole process has been educational, of which I'm all in favor.

**badazta** · 01-02-2007, 04:31 PM

I'm having the same code... Bought new msd wires and ngk plugs today, I woulda never connected the exhaust til it was mentioned. Now thinking about it maybe a month ago my girlfriend notice something in my drivers side exhaust tip when I looked it was a piece of honeycomb I'm geussing from one of my cats. I have long tubes and aftermaket cats taking them off and cleaning them will be next. My car is lowered 2+ inches and the exhaust rubs everything thinking that might of jarged something loose. Can anyone help me understand the connectoin between a misfire and the exhaust?

**badazta** · 01-06-2007, 01:24 PM

I just ripped apart my exhaust.. cats are plugged I also found a kinked coil wire (red) that was bare after I took off the coil covers I had on. Scratching my head on what to do now... 350 for the catted y that has maybe 3000 miles on it and its plugged. Toys...

**tblu92** · 01-06-2007, 04:35 PM

If you had a bad wire for long time with a dead miss--then all that raw unburnt fuel would plug the cat up eventually----and you could have had a humungous backfire that would have blown the converter apart or even have blown a hole in it--

**badazta** · 01-09-2007, 05:16 PM

Finally got all the plugs changed along with wires all in all about 4 hours. fixed the kinked coil wire, cleaned out the other cat since the the drivers side was empty anyways. Went for a ride!!! Same problem!!! check engine light blinking and misfiring bad. I did have a cracked plug #2 cylinder. I guess I bought the extended warrenty for a reason, I'll throw it on the trailer and bring it in. I also changed the fuel filter, I'm pullin my hair out.

**9t8z28** · 01-09-2007, 05:24 PM

I don't think that this was asked, but what mods to you have done to the car? What is the mileage?

**badazta** · 01-09-2007, 05:43 PM

just over 50k hp tune qtp lt and catted y corsa exhaust slp maf cold air kit lid bellow now msd wires and ngk plugs for the nx wet that isnt installed yet.... I was reading previous threads and seen that 440 rwhp ta had issues with his and it was said that it may be the computer because of his exhaust being welded on. well a month or so ago I had my y pipe welded to the lt I know we didnt disconnect the battery. But that was a month or so ago heck maybe longer maybe even three. I can order a crank sensor but when do you throw in the towel??

**badazta** · 01-09-2007, 05:47 PM

I'm also noticing a strong gas smell when I start it, and the number 7 plug had a little gas on it even after sitting for three days.

**Too Fast** · 01-10-2007, 03:02 AM

Now it sounds like a leaking injector, badaz. #7.
Just because the exhaust is welded should not be a PCM issue, unless it was MIG or ARC welded. Sometimes the sparking during the welding can ruin PCM(s) unless the PCM is removed during the weld.