Check Engine Light
 

Hey guys,

So last week I started my car in the morning, everything started and sounded normal until I looked at my dash. There was an immediate solid check engine light. The car was idling at about 1100 rpm. Anyways I drove it to work and it seemed completely fine (power and smoothness in the curve were all there just a high idle). I reset the car the light went off and everything went back to normal.

Fast forward a week to today (320 miles driven), I started the car and again got a check engine light upon cold startup with the same high idle when warm. I scanned and got the dreaded P0300 for a random misfire.

Now I have checked for vacuum leaks in the intake track (tightened down all clamps as I installed some post mafs 8 months ago) and check for exhaust leaks (there were some after the cats but I don't think that would code for a p0300). I reset the code and have driven it 50 miles and everything seems normal.

I am wondering if it has to do with the fact I had only a 1/4 tank of gas at the time and didn't give the pump a chance to prime the system to fuel pressure.
:confused:

How many miles on the car? Check your spark plugs as well.
Also the fuel pump primes whenever you put electric power to the vehicle. (Aka turning the key on but not engine)

The car just hit 44k this week. I bought it with 23k on it last year.

cant stress enough looking over the spark plugs. dont ignore the little things. could have a coil pack going bad as well causing a weak spark. i would definitely start there next.

Yeah I think I am just going to change the plugs anyways, I don't think they have been done for the cars life so they could probably use a replacement.

I would just think that at high load/rpm the rpms would break up if there was a coil with weak spark/plug that was no good. Then again electricity does some weird things. haha

If you search the forums you will find that some members that had this code required clutch/flywheel replacement to correct. I'd speculate that the ECU can trigger this code based on vibration and not necessarily knowledge that the plugs are actually misfiring.

I was actually reading about those occurrences, and after the flywheel was replaced all was normal.

I have also recently found that when I drive my car and then shut it off then (within 30 seconds) turn it back on it makes a clunking noise. The noise doesn't happen any other time ie if I did the same drive then let the car sit for 2 minutes.

I'm not sure it that is related, however I just had my transmission out of the car and inspected the flywheel/clutch and everything looks/works great.

P0300 is a pain in the ***. Anything can trigger it. I had a crappy Spec clutch that rattled a lot, and that was enough to trick the ecu into thinking it was a misfire.

Yeah I got the engine light and I was hoping it would be a simple code... Nope I got the shittiest most ambiguous code there is.

Could a post cat exhaust leak or an exhaust rattle cause the code? I have a beluga cat back (y pipe back) and on cold starts it vibrates slightly.

Could be. My tuner just turned off the P0300 code. If there's a legit misfire, there will be a cylinder associated with it with its own code.

Exactly.

Other than the code the car drives great (aside from high idle). Still chirps 3rd gear. haha

Check you oil level. If it is high, this can cause issues.

I had this happen to my car last year and it sent the idle even higher than yours. If I stopped at the store after driving for awhile, then got back in to drive, the engine would idle extremely roughly.

Because I had stupidly overfilled the car, the oil foamed and wreaked major havoc.
I had to completely drain the oil, then refill.

Once I did that, everything was fine.

Thats crazy. haha

You just made me check the oil. It is luckily (or unluckily) right in-between the markers.

Back to the drawing board.

Another good place to start is to figure out if the car is running rich or lean when throwing the misfire codes. This can elimate some possisbilites depending on rich or lean. Grab a scan tool and look at the short term fuel trims. (Looks up a video on what STFT if you don't already know). Good luck man!

I have the exact same thing happening to my car. I had it checked out by the dealer before I reset the code and everything checked out fine. My exhaust had come loose out of the left hanger but that was an easy fix. Fast forward 3 weeks and I started the car for the first time in a couple of days and the code is back on. I am convinced it might be an exhaust leak or a bad sensor. I tested for vacuum leaks and checks the MAF sensors as well and they all checked out. Maybe a bad O2 sensor?

A bad O2 would be it's own code, wouldn't it? P0300 seems to be a catch-all code for any time the ecu sees something weird.

My thoughts as well. All vitals are fine on the car. I just reset the code yet again. Next time if theres a next time I am going to do an exhaust leak test and potentially replace all exhaust gaskets.

I am thinking it may have something to do with my exhaust gaskets near the rear 02 sensor, the ones where the cat meets the y pipe. I have re-used them twice so it might just be a bad leak. haha

I reset the code on Monday and I have driven 100 miles so far...

I made it 700 on the first reset until it occurred again. I think you are right about the gaskets. I have long tube headers to AAM 2.5 true dual and HFCS so I think maybe the dealership loosed the connection past the HFC to re hang the exhaust and didnt tighten it all the way. For now though do the video game cheat to reset the code! https://www.youtube.com/watch?v=95WszTnphPI

Prior to erasing the code you might want to record the freeze frame data and post it here. That data might lead some of the electronic engine management gurus here to point you a little closer to solving the mystery.

What is the freeze frame data and how can you get to it?

Whenever you get a MIL light the ECU stores the conditions in which they. Kind of like when you screen shot something, I think that most half decent scan tools can give you that data.

If I get another light I am going to record the freeze frame and post it to this thread...