: P0300 Misfire - My problem and solution + more problems...

12-06-13, 09:03 PM
A few weeks ago I purchased a 2000 STS for $800 Canadian... It was supposedly fine except for the driver side caliper and rotor. The rotor was missing, and the caliper was hanging by the hose. There was an engine light, which he said was due to a bad seal on the gas cap (and showed me the bad seal on the gas cap). I started the car from cold, and it fired right up, smooth as butter, so I didn't take the check engine light too seriously. I also noticed grease all around the caliper, which he said was brake fluid, but I knew better, and figured a CV Axle. The PO said he was going to fix it, but bailed due to lack of funds, and time (he had a lot of projects, so it was believable). For $800 I figured I could make a couple dollars no problem, even if there were more issues.

I got it home, and pulled the wheel. Sure enough, the CV boot was broken. I checked the CV joint, and it was in excellent shape, so I just got a new boot. I replaced the caliper, rotor, and brake pads(both sides) as well. I then decided to move onto the check engine light. Not knowing that it had a built in scanner, I hooked up my laptop to the OBD port, and read the code....just one....but the worst one......P0300 Misfire. I warmed her up for the first time, and more and more white smoke started coming out of the exhaust. Smelled real sweet. I checked the coolant level, and it was mostly gone. I figured bad head gasket, but wasn't ready to give up. I've done a few head jobs in the past couple years, a Ford Ranger, MGB, and a bunch of dirt bikes, but I have absolutely no interest in busting open a Northstar lol, what a nightmare. I figured I was pretty much screwed, but wasn't willing to give up for another day or two. I first tried...Don't shoot me... Rislone's Head Gasket Fix, figuring that if it did the job, it would save me a decent loss, and worst case scenerio, I lose $30 and return the product.... Well I am going to have to return it, because it didn't do a damn thing. I really had no faith in the first place, but read some good reviews from people. Once that was a bust (yes I thermacycled it like 30 bloody times) , I decided to try a couple other things to get rid of the code, as the motor ran real smooth, and refused to move past the 1/2 way mark on the temp guage.

- Running without belts (hoping it was an accessory that was causing the misfire code)
- Cleaning the injectors, MAF, Air filter, Throttle body

These things did nothing, and the code would almost instantly reappear once I cleared it. I then realized that the MAP sensor was really loose, seems the seal has given way a bit. I unplugged the MAP, booted her up, and took her for a spin. NO CODE!!!!!!! I let it idle for half an hour, just to be sure, and sure enough, NO CODE!!!!!!
Tomorrow I plan on replacing the MAP, and hopefully she will pass emissions.
I also noticed there is much less smoke coming out the exhaust, like a normal amount (~ freezing here now), which is weird, because it really seemed like a HG issue before, the smoke was really sweet, and I clearly saw the coolant coming out the pipes. Really can't explain that one.....

I have one other issue that I am trying to solve as well. When I took er for a spin, I noticed a constant grinding noise, based on vehicle speed, and it's fairly loud. I put the car in neutral, and even shut off the engine while moving, same noise, unchanged.

Not used to working on Auto's, or FWD for that matter, so I'm a little behind in that regard. I am seriously hoping it is not a tranny issue though, if so I will probably be floating face first in lake Ontario tomorrow. My hope is that it is a wheel bearing or something else that's simple and cheap, but I guess we will see shortly. I am waiting till tomorrow to tackle that issue, as it is bloody cold here once dark.

I wanted to post my experience here, as I didn't find any page talking about the P0300 code with relation to the MAP sensor. Hope this helps someone else.

12-07-13, 11:32 AM
Did u tow car home? U said u pulled wheel to fix it but I can't see how wheel is mounted properly with a missing rotor?

12-09-13, 11:37 AM
Lol, I got a good laugh out of that one. It wasn't mounted properly, indeed. The caliper was hanging by the hose, and the wheel was mounted without a rotor. There was a wheel weight on the inside of the wheel, and it actually was half rubbed off. I was pretty amazed myself. I did tow it home, as I wasn't about to drive with a hanging caliper.

So to update everyone, the MAP sensor was not my issue unfortunately, I wasn't getting a misfire code without it, but it must have put it in limp mode and stopped counting misfires. When I replaced it, the P0300 came right back. I used a nice scanner that I borrowed from a neighbor, and it seems I'm getting an identical number of misfires on cylinder 5 and 8, no misfires on the other cylinders. I tried switching the coil packs, as well as cleaning the plugs on 5 and 8 (which weren't too bad), cleaning the connection to the boot, and switching boots. No difference.

I'm wondering if anybody knows of something relating cylinder 5 and 8, because they are also diagonally placed from one another, so it doesn't seem to be a gasket issue. I searched, but couldn't find a post with both 5 and 8 as a problem....

12-09-13, 05:01 PM
I notice that your STS is now being parted out in the Seville Classifieds.

12-09-13, 06:37 PM
Lol, yup. Have given up a bit, as I think it is a head bolt issue, or something of that nature. Not something I want to do, as I am in the process of moving mid month, and just don't have the time. Got a little hosed on this one it seems. Still up in the air, if someone has an idea regarding the misfire on 5 and 8 at the same time then I would give it a shot, but I'm done spending time trying things, and am certainly done putting money into her. Everything else is really good on this car, that's why it is so frustrating as well. Would sell it as a complete project, or just part er out and take the rest to the scrap yard. Lesson learned... Don't buy a northstar that has more than....actually just never buy a northstar lol. JK, but I will definitely examine the next Northstar I get much more carefully.

I've seen enough cars with misfire problems, but let me tell you, this is 100% the smoothest running motor with a misfire I have ever seen, and it has 2 cylinders tripping! Gotta say something for the motor I guess.

12-09-13, 08:44 PM
Have you ruled out the coils in the ignition cassettes?

12-09-13, 10:42 PM
Thanks for the advice, I did switched the coil assemblies, and same thing, same cylinders.

12-10-13, 11:44 AM
Before you decide to part out check this post if you havent already!

I am getting a pcm reprogrammed tomorrow for same issue. misfire has me stumped

12-10-13, 12:05 PM
Thanks! Read the whole thread. That's interesting that you have exactly the same issue as me. I have EXACTLY the same number of misfires on 5 and 8 as well. I may hold off, and see if that PCM works for you. That sure would be nice. Please update us! May try messing with the injectors today. Haven't got any calls (hard to sell anything this season other than christmas gifts), so I mine as well keep trying a few things here and there.