I started up my Monte this morning to go to work and immediately noticed a vibration caused by a misfire. The P0300, random/multiple cylinder misfire code is what I got when I used my scanner. I noticed a slightly off sounding exhaust last night, but nothing like a misfire. The most recent thing I did was repaint the front bumper two days ago. I checked three things, the PCM, spark plugs, and vacuum lines. I have a PCM from ZZP, and I checked the connectors to it. I found dust from the painting process. I cleaned them with electronic connector spray, and plugged it back in. This didn't solve the problem. I also tried replacing the aftermarket PCM with the original. Still misfires. The next thing I checked was the two vacuum lines coming off the throttle body. The merge for the two hoses was cracked so I tried wrapping it in tape and zip ties. That didn't work either. I don't suspect that to be the problem though, because the two things the vacuum lines feed are the evap purge, and fuel pressure regulator. The engine will smell like unburnt fuel while running, and won't stall. So I don't think fuel is the problem. Next, I checked the park plugs. They are copper plugs only about 3,500 miles, and looked fine. Slightly lean maybe. The gap hadn't changed, but I regapped them anyway, from .065", to .060" Does anyone know what the problem could be?

 

P0300 can be a real pain as there are so many possible causes. Best bet is to break out the scan tool and take a look at the sensors to see if you can determine anything from that.

If it truly is randomly jumping around, a single fouled plug likely wouldn't be the cause. An ICM issue could do it, as could a small vac problem as you noted. Could also be a MAF or EGR issue, fuel pressure, etc.

You can narrow it down more looking at the data though.

 

I did scan it, but I wouldnt have known what to look for in the sensor data. I ended up taking it to a mechanic. They said the main cause was the catalytic converter. The cat on the ZZP downpipe I've had for only three months definitely went. Chunks of the thing fall out when shaken. So I took the car home and took it off. The mechanic also said that the IAT sensor and EGR may be causing the misfires. He mentioned the IAT sensor because I had the cover of my CAI off, which shouldn't make a difference, since the CAI is taking in ambient temp air anyway. The EGR was deleted, and the car functioned fine that way, so I don't believe that to be an issue. The car is still misfiring, however, it sounds a lot more tame after taking off the cat. It no longer shakes as violently, so I don't think it's as many cylinders. Still don't know what the issue is though. Maybe the spark plugs being flooded? Although idk how, since it's the stock computer. Should I let it idle and lean out since the computer uses more fuel for startup?

 

It sounds like you have a fueling issue. What's your MPGs been like lately?

 

Just for reference because I dont know if this will help any but on my 05 SC I have the EGR removed and as for the IAT sensor.. I have messed with that thing so much, moved it around 5-6 different spots in my intake, accidentally left it unplugged, accidentally left it just hanging on the side of the intake. Never had any issues with those two..

Fun fact: if the iat sensor is unplugged, your car will think its bringing in -38*f air and youll foul your plugs.. Lol learned that one the hard way.

 

The last I recall it was 19mpg, which has been normal. The car smells like unburnt fuel when I run it, and it won't stall out.

 

The converter may have been the cause of the extreme issues, but it's just a symptom of the real problem. Something is dumping raw fuel into the exhaust- a dead cylinder, stuck open injector, etc. Dumping raw fuel into the exhaust will way over temp the cat and cause it to come apart like that.

Taking off the cat may have fixed the extreme symptom, but there is still another issue to fix.

Honestly, it sounds like it's time to find another mechanic. I get they can't really diagnose it with the jacked up cat and they can't legally delete it, but the thing with IAT and CAI lid makes me question if they really understand the fundamentals here.

Now that your cat is off, you should be able to get a mechanic to actually diagnose the issue. I'd definitely suggest parking the car until you can do that so you don't damage the engine anymore. If you've got a dead cylinder, the walls are getting washed down from the raw fuel that isnt being combusted. Itll also throw off the O2 readings as you're dumping lots of straight air into the exhaust which will globally impact fueling for the other cylinders.

No offense, but if you dont know how to interpret scan tool data, I'd take it to a better mechanic and let them properly diagnose this. It isnt super complicated, there are only maybe 10 or 15 things that could cause it, and only maybe 5 of which are the likely suspects. They need to identify the cylinder that's misfiring, identify the frequency of the misfire, and then start diagnostic tests on the related components working from most to least likely.

 

That sounds about right with the EGR delete. TBH, I'd probably hook it back up once you get the problems fixed as it will improve the gas mileage. If I remember right, you've got a NA L36, so there's really no gains to be had by deleting it.

The unburnt fuel smell is what concerns me most. Its clearly dumping a ton of raw fuel to burn up a cat that quick. I'd imagine it's a lot more noticeable now without the cat - before it was probably burning some of it up inside the exhaust.

 

While I was trying to diagnose it myself, before taking it to the mechanic, idk what I was in the middle of trying, but I started the car with the cat on, and there were several after fires that probably did the most damage to the cat, and made it pretty obvious that it wasn't burning all the fuel it was receiving.

I didn't delete the EGR for any gains really. I just wanted the oil vapor, carbon, etc, out of the engine.

If possible, I'd like to fix it myself. I don't want to loose any more money on a mechanic diagnosing the car. What should I try? I have a spark/ignition tester, so I'll start there. Since it's getting more fuel than it needs, and if spark isn't the problem, a bad injector, or bad signal to an injector maybe?

 

There shouldn't be any oil vapor coming through EGR. Oil vapor would only be coming through PCV and you certainly wouldn't want to delete that. I could certainly see it helping to reduce carbon a bit, but for the drop in gas mileage you get from deleting EGR, itll add up to a lot of money lost in the end. Honestly, I think the only setup worth deleting it on is an intercooled setup as it can plug the tiny channels over a long period.

Do you have a scan tool? I'd start by learning to read the common sensor values - MAF, MAP, O2, IPW, etc. You can also see what the individual cylinder misfires are showing. The goal is to see what the inputs are vs the outputs to see if there is anything that helps point to something there.

If it is a totally dead cylinder, you should be able to find it with a IR temp gun on that runner. Once you've found the cylinder, there are only a few components involved.

If it's a pair of cylinders, it could be a coil pack. If it's all over the place, it could be a ICM, the PCM, or a bad sensor.

The hard part is that the diagnostic path varies greatly depending what each step tells you. Trying to map out every possibility is too tough to do here, hence why I suggested looking at the scan data first to at least get you headed in the right direction.