Rough idle with strange intake noise
 

I have a 2004 with 85k miles. It has a rough idle with a strange noise that almost sounds like a vacuum leak. I have posted a video . It is idling at about 800 rpm. The car is hard to start, I have to crank more than usual to get it to start especially when it's hot.
Here is what I have done and found so far:

• It has the GM coil retrofit (verified spark with timing light)
• Oil nozzle valves are bad but the nozzles don't seem clogged
• No error codes
• I can't find a leak
• LTFT is running around 12+
• Power seems to be pretty strong especially in higher RPMs
• The car seems to load up while idling. After idling while the engine has fully warmed up the engine revs slower as if it has to un-flood the motor. I then runs fine with a little less power.
• The cat is fine, it looks like new. It has less than 18k miles.
• Plugs have less than 10k miles, I pulled and inspected them

Could a bad injector or oil nozzle make the noise on the video? If not, what else could it be? Any help would be appreciated, I'm at my wits end with this.


WSGator

Good job you did the vid as youd have had lots of timewasting with people (like me) who would have said split intake hose from airbox, etc.

That clearly sounds like and exit noise not an intake, have you checked the plugs to make sure they are all in properly? Could be one has worked loose and is allowing gas to blow by.

Yes, I pulled the plugs and inspected them. They all look great and were well seated. I agree that it sounds like an exit noise, I sounds like it's exiting into the intake.

I worked on the car this weekend. I replaced the oil injector nozzles/valves and performed a few more tests.
Here is what I am seeing:
Vacuum at idle is 11 on front rotor and 9 on rear (taken from the LIM vacuum nipples)
The fuel pressure is pretty steady at 59 psi.
The car takes excessive cranking to start, more when hot.
The LTFT is 11 to 12 at idle.
The STFT will drop to 0 on acceleration and once I let off the throttle and it drops to idle down to rough 500 rpm. After a few seconds the STFT jumps to 25% and the rpm raises to 1500 and will lower back down slowly to 850. It will hold there with a somewhat rough idle, it will cycle dropping down a little and then back to 850 rpm every 10 - 15 seconds. I have had it stall while coming down from a high rpm.
I used a mechanics stethoscope to try to find out where the sound is coming from and it is definitely coming from inside the intake, it's loudest when I touch it to the UIM, not as loud from the plastic LIM.
I also touched the stethoscope to the injectors, One of the lower injectors (rear rotor, center) sounded like a jackhammer, it was excessively loud. Could the injector be causing this noise? I know it could be causing the rough idle. The car seems to be running lean. I can't even give it gas when trying to start it, it stops trying to fire. I have to start it with zero throttle. Are the lower center injectors the primary injectors? Any other ideas I should look for?

Hey wsgator, my car started making this exact same sound about 2 weeks ago as well. I took this eerily similar video:
I'm definitely interested in what you figure out since mine is making an identical sound.

I just found this thread and am wondering if you've checked for the hose the last post mentions being disconnected? I haven't yet because my cars at home and I'm at work currently. Checked it, definitely not the problem.

Edit: Also, it seems we're getting one "puff" for every three revolutions, aka only one puff for every three combustion cycles. I don't have enough knowledge about this stuff to draw any conclusions from that, anyone else? A friend theorized that maybe one of the apex seals is leaking, therefore letting some of the combustion back into the intake, causing the noise.

Bad news: took my car to an independent Mazda shop and they did a compression test. As my friend theorized the sound is blowback into the intake via a failing/failed apex seal.

Taking the car into the Mazda dealer tomorrow to start the warranty process, hopefully that goes smoothly, though I'm expecting it won't.

I was hoping that wasn't the problem also but I do think mine is the apex seal also. I noticed the same timing as you, 1 in every three or four.
My problem is that my warranty has expired. I have an engine from a 2009 with 16k miles but I have to make sure the short block will work in my 2004.