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?

It doesn't happen more in different years that I know of- it seems to be broadly related to the intake design of the HR motor (both 35 and 37). I wouldn't say it's bad detection logic as much as the engine just sensing more air (lean mixture) that could cause a possible misfire condition.

IMO, this code is much less severe than the actual misfire code that specifies a cylinder. In that case, I would look at the connectors for the coil packs as well as the coil packs themselves for the specific cylinder specified in the code.

It personally doesn't make sense to me that the Stillen (or really any) intake triggers these codes so people automatically assume something is suddenly damaged or that a sensor has failed somehow.

IMO, in these cases where the intake is installed and the code just starts popping up, there's just more air getting into the engine which passes a pre-programmed threshold which then trips the code. That's just my guess- please don't take it as a definitive conclusion for why so many people experience this issue.

I'm having the exact same issues after getting my Stillen CAI installed this weekend. How did you fix it or does anyone have any suggestions? The car sits at 1K RPM, hammering off the line in first; very slow off the line and the car hesitates for a moment before kicking in, and Service Engine Soon light is constantly on.

I checked all the pipes and tightened all the clamps. My engine also has a fast ticking sound coming from the engine. I don't remember it sounding like that when I first bought it last month. It is still under warranty and I highly doubt within a few weeks that it would be spark plugs. It definitely seems like the Stillen is at fault. I'm debating taking it off bc my car is definitely slower >3K RPMs.

The ticking is just your injectors. You can reset the ECU and clear the code with the following:

1. Push start button twice to get - Ignition "ON", count to 3.
2. Depress/Release pedal 5 times - hard and fast.
3. Count to 10 (not 7!) and fully depress pedal.
4. Count to 12 (SES should be blinking), release pedal.
5. Count to 10, depress pedal.
6. Count to 10, release pedal.
7. Turn off ignition, then start engine. ECU reset is done!

Did that procedure. First time it worked but next day it came back on, idles high, slow hesitant start off line, feels sluggish. Tried procedure next couple of days over 50x, light will not go off even after a successful procedure.

what do you mean cleaned it???like took the whole intake off and cleaned the inside and the filters????or you mean like the maf sensors???

I'm having the same issues..... its only happening now since its cold..... i think I'm just gonna get her tuned and stop worrying about it haha...