P0300 / Engine RPM speed @ idle
 

I got a P0300 code (MIL) yesterday (Random/Multiple Cylinder Misfire Detected). I won't have a chance to have it looked at by the dealership until this weekend. The car is mostly stock except for K&N filters and a CBE. I don't know if it's bad gas or what, but I've never had this happen before. I did go to a different gas station the last time I filled up. It could also be the battery, but the car starts without a problem.

Already checked the following:
-air filters
-intake
-oil level
-exhaust leaks
-fuel cap

A few things some of you might be able to help me with:
I've noticed that the idle is sitting arount 1,000-1,200 rpm's when the engine is already warmed up. I can't for the life of me remember what it supposed to be. This is for the manual, in neutral. What's the normal rpm at idle for your car. I thought it was supposed to be around 700-800 rpm (warm).

I'm currently trying to run through this tank of gas and filling up with the place I normally go to (Sonoco). Should I try some fuel injector cleaner?

Should I clear the code and see if it comes back? Only problem would be the dealer giving me a hard time if they don't see it.

Yea that idle is not correct. I got this same code and symptoms last year and could not find the problem. I cleared the code and the CEL went away and the car started to idle fine. Now for the longest time I had a huge throttle delay problem and finally decided to bring the car to RAD (a tuner in my area) to find out the problem. This is all after the CEL problem. They found out the car was running improper AFR's on bank1 and bank2 due to a exhaust leak. I know you said you check for them but just be 100% sure cause I think that's what my problem was.

I found this.


Edit: Google "P0300" for more fixes.

Come to think of it, I did notice a slight hiss whenever I lifted off the throttle last week. It went away so I didn't think anything of it. The Borla CBE has slip joints, maybe one of them came loose a bit. I guess I'll have to double check and re-tighten. Hopefully this fixes it.

On a separate note, do you go to the Wed night meets at RAD? I've been meaning to go but the weather has been crappy and/or I've been too busy to go.

I have not been to them in a while. The Z has just come out of the garage for the year and I recently started a new job so I have not had much free time but if the weather is nice the Wednesday after this coming Wednesday I will probably go since I will be off.

Just finish tightning the bolts and cleared the code. Thanks for the tip. So far so good. Revs at idle are back to normal.

You almost definitely have a vacuum leak somewhere after the MAF sensors or somewhere before (or just past) the primary O2 sensors.

It's causing the car to run lean, and leads to a lean miss (and probably high positive fuel trims).

You can try spraying carb cleaner around the intake area and near the TB's while the car is running to see if the idle changes -- when it does, you are closing in on the leaky spot. Failing that, you'll have to get it on a lift and check for evidence of leaks around the primary O2's. That will be trickier to pinpoint (might see black exhaust residue), so hopefully the leak is before the headers.

Good luck.

I checked the intake. Nothing out of the ordinary. I didn't even get a chance to install the Gen 3's yet so it's currently the stock intake setup. If the code comes back, I'll make sure to check around the 02 sensors. I can't imagine an exhaust leak after the cats could trigger it, can it? Also, the misfire is random, so it could also be something in the fuel or fuel system or the battery. The car is close to 4 years (around 39k miles) with the original battery. It still starts fine, even with the cold weather.

It shouldn't.

I'd look for a rupture along the intake tract itself -- I had stupidly over-tightened the hose clamp on my old Z and managed to bend in slightly the neck of the intake box where it meets the tube causing an intermittent leak and weird idle.

Another possibility is a failing header gasket or problem with the TB's (apparently, not uncommon) or a small pinhole or crack in one of the resonators attached to the intake tubes (I had one of those crack where it clamped to the intake hose).

The only other things I can think of are something screwy with the misfire detection logic, (but the high idle definitely sounds like a vac leak) or a dirty MAF sensor giving misreads.

If you can't pin it down mechanically, I guess you could try doing so via datalogging if you have Osiris or if you can get hold of OBD II cable and software.

The thing is, the rev's are fine now after the reset. That's kind of why I'm mostly ruling out the intake side. MIL hasn't popped up again so far, so I'm hoping it may have just been a fluke.

If you have a Uprev tuning cable you can shut off the P0300 code and it will never show up again.:tup:

The engine moving to around 1100 rpm is completely normal when this code is thrown. It'll stay that way until the code is cleared.

This code really is sort of a mystery with no 100% fix out there short of a tune and auto-clear. IMO, it's just a result of leaner air than expected entering the engine. As long as it's just a 300 code and it doesn't specify a certain cylinder, I wouldn't give it much worry.

It is pretty annoying, though.

I had this code...bad AFR sensor.

I had same issue. Cleaned my Stillen Gen 3's, issue disappeared.

Really?

If so then it's bad misfire detection logic in the ECU.

Is it more common in earlier models?