Hey all- just made a fresh account on here because I’m doing my best to fix my girlfriend’s car! I’m typically an old school ford guy (1987 mustang 5.0!) so im trying to wrap my head around things.

So, it’s a 2008 Mazda3 with the 2.0 and an auto transmission
Car will drive almost perfect, especially on startup, but after a short time driving, the car spits, sputters, shifts weird, has next to no power (foot to the floor, shifts around and hardly accelerates, struggles to even make it to 40 MPH) and develops a loud hiss
Symptoms- no codes whatsoever, though there is an exhaust leak that threw a code prior but no longer does anymore. This drives me crazy because given how the car acts I think it should definitely be throwing a code.
I’ve noticed that it appears to happen after going about 60 and cruising a while, then once you come to a stop and attempt to get back up to speed it just can’t pull itself together at all.

The engine noise noticeably changes and develops a loud hiss that isn’t present when the car drives okay. Sometimes it clears up for a short while but it typically returns. If I give it full throttle and let off it sometimes makes a bad blow off valve sound (mind you, this car has no turbo)
This hiss is also not noticeable at idle, only when driving, so I cannot hop out and try and locate the sound [img]data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7[/img]

What I’ve checked: removed coils while idling and all cylinders bogged down, so i know it’s firing right, I also listened to each injector with a screwdriver and I hear them firing as well.
I checked the purge solenoid valve for proper operation and it works perfectly
Cleaned MAF and this helped idle but not the problem. checked air filter, looks good. Tried spraying carb cleaner near any suspect vacuum leak areas to see change in rpm but no luck.

This feels to me like some valve that kicks open/closed and gets stuck open or closed once it goes into open loop or something, and the hiss makes me definitely think it's vacuum related. Could this be PCV? Intake shutter valve? Tumble shutter valve actuator? This has me stumped. Please advise!

 

What are you describing should have generated a few different DTC let alone NONE????

Check the monitors for readiness state. One or more should be in fault. If not, then the suspect may be the ECU itself.

 

I figured that something was up without codes showing, this is odd. What gets me is that the car is 100% okay sometimes, but others not.
So- I did the monitor readiness check (turned key on for 15 seconds, and the check engine light blinked 8 times, indicating that they are incomplete)
What should I do next? Is this a new ECU situation?

 

Well I would see if your can get the Monitors scanned using a Diagnostic Scanner connected directly to the OBDII connector and see which monitors are in fault. You might start looking into a Rebuild ECU just in case. There is also a possibility that you can have a PASS-Through done on your ECU and then see what happens but most shops that have that capacity do charge for the service.

Lets do a simple test just to see if at least the ECU is reporting? Unplug a coil and start the engine. You should get a DTC relating to that cylinder. This is not a conclusive test for ECU unless it does not report.

 

I have this exact issue on a different car. It was my rear catalytic converter.