MitsuStyle - Mystery Pop

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- - Mystery Pop (http://www.mitsustyle.com/forums/showthread.php?t=28031)

Basic run down.
91 Talon awd
EvoIII 16G (PSI Vary (8-16 PSI) Does it no matter what the psi is at)
sidemounted intercooler
Recirculated BOV
Open Downpipe
Spark Plug Gap .28 (last time adjusted aprox 3-4months ago)
FIAV Deleted
560cc evo injectors
190 lph fuel pump
Rotella T6 5W-40 Full synthetic

Approx 1 week ago. After a little heavy highway cruise. My car started idling at 2300RPM in neutral at any given speed. I adjusted the BISS, (which is not suppose to be touched) But Due to the FIAV being deleted on my throttle body, and temperature varying it needed slight adjustment. I got it to idle at a perfect 800-850RPM which was odd since it never liked idling at that speed. However shortly after the adjustment. Approx an hour or two later, On the way to an appointment i noticed a popping noise under decel usually between 3000-2000rpm.

I figured it might just be some unburned fuel. ( I dont have a wide band or safc so i cant monitor my fuel trim etc However i do have a logger which might help) When i got on a highway ramp i tried re creating the noise. At WOT it pulled like usually but between shifts huge bang. I would shift bang shift bang. I let of the accelerator to make sure nothing was wrong and it sound like slow western cow boy gun battle. (sorry for the bad description) It sounded nice for the first day or two.

I started getting concerned when it popped really loud behind a cop. My full exhaust will be done soon. Yet i am getting worried because it never did this before, and i dont want no flames shooting out my exhaust neither. Considering selling the car, I dont think it will be a desirable trait having these backfire like noises. If its a big enough pop you can feel it in the undercarriage. Not to mention i dont want my 02 sensor getting obliterated from the vibration or shock waves.

Potential causes i came across while researching:
Spark plug gap
spark plug wires
timing problem
run rich issue

One person suggested it could potentially be oil being burned from getting blown passed my rings? The car is due for an oil change.

Few test i was planning on running.
Compression test
Boost leak test (Boost seems to be a little sluggish)

Any hints and suggestions will be appreciated. I will try and get a sound clip.

Thank you

Is the car tuned? sounds like afterfire, it's usually caused by running too rich

If you aren't tuned, do so :) By Shane. You can also check for boost leaks to start with, that could cause you to run rich.

^ No its just a chipped eprom, if it a tuning issue why has it not done this since day one?

It could be a boost leak causing you to run rich and make the exhaust pop from unburned fuel. Do a boost leak test.

i say boost leak, idle dosnt just change for no reason

sounds like a vacuum/boost leak. my pcv line popped off and i had similar symptoms.

Can you log if your idle switch is still functioning?

Quote:

Originally Posted by Halon (Post 359529)

Can you log if your idle switch is still functioning?

By log do you mean if it is still functioning? i dont quite understand? I doubt it self adjust since the motor isnt there. . . not sure if it still sends voltage signals and what not to the ecu. I can try to check and see.

Seriously its never just changed its idle like that before. . .im convinced the after fire and idle issue must be related.

There is an idle switch (on/off). It's on your throttle body, usually a single green wire going to it.

When you let off the gas, that idle switch signal is sent to the ecu. It's either on (1-indicating idle) or off (0 - indicating not idle). If that switch went bad, or the wire broke, or it became misadjusted for whatever reason, then that signal may be stuck at a constant Off / 0 / not idle. So if you have a logger, can you see the idle switch on/off signal? If so, see if that signal changes between on/off when you press the gas pedal, and let go of the gas pedal? If it never changes, than that may be a reason your idle suddenly changed overnight.

Quote:

Originally Posted by Halon (Post 359590)

There is an idle switch (on/off). It's on your throttle body, usually a single green wire going to it.

When you let off the gas, that idle switch signal is sent to the ecu. It's either on (1-indicating idle) or off (0 - indicating not idle). If that switch went bad, or the wire broke, or it became misadjusted for whatever reason, then that signal may be stuck at a constant Off / 0 / not idle. So if you have a logger, can you see the idle switch on/off signal? If so, see if that signal changes between on/off when you press the gas pedal, and let go of the gas pedal? If it never changes, than that may be a reason your idle suddenly changed overnight.

This. Check it, this wire has broke on my 1g before.

Goodhart never over look the obvious boost leak EPIC FAIL side mount had a big hole in it. System held 15psi with the compressor producing 80PSI funny. The noise was hard to hear while driving due to wind etc. (If curious how i couldn't hear such a big leak)

Halon thank you for that suggestion. I checked the wire and the terminal was attached so i moved on to check other things. Yet when carpsycho took a look. He said yeah youre idle wire isnt there. I was confused and took another look the terminal was still there with no wire attached. (Silly mistake)

We reattached the wire and the idle sits at about 1000rpm now instead of the nice 850 or horrible 500. Boost leak must be to blame for the popping noise. Sticked a bolt in the side mount to temporarily seal the leak till i arrived at home. Surprisingly it helped. The Pops have decreased but if they do pop its really loud now. Just finished installing a new side mount, but the battery is dead so im waiting to jump and test drive it. Since the back fire is still there.

Nice, good to hear you found some possible causes. If you have a logger though, I'd still log to make sure that the idle switch is working. I've seen plenty of idle sensors that have the wire hooked up, but still for whatever reason the ECU doesn't get the signal. So I'd still check that if you your logger can do that. Or just let the car idle and detach that wire and see if rpm's change based on whether or not that wire is hooked up.

Cliff note = wire hooked up to idle switch does not mean idle switch signal is good :)

Thank you very much! Ill make sure to log it tomorrow and check, as well as perform the wire test.

Glad to see you got it figured out.

Seriously! I found some other stuff that needs attention, one problem at a time. Just got the car started pulls nice with the better conditioned side mount. Thought i was driving a new car. :D

Thanks again guys.

This popping is after you let off the gas at like higher than 3500rpm... It has a shorty pipe, but my car has a shorty pipe too and doesn't do this. I think it might be the idle switch not working so its still injecting fuel but its not burning in the engine cause its at full vacuum...