2004 mazda 6 v6 surging idle
 

My sports wagon has a cel code of 0171 & 0174 ( lean bank 1 & 2 ) for several months. It just recently developed a surging idle.
No problem accelerating or cruising though. Its got 63k miles, still on the original plug & coils. Tried cleaning the throttle body & maf sensor, neither made a difference. I see people mentioning the idle air control ( iac ) valve. This sounds like the most likely cause, but if this engine ( 3.0 v6 ) has one, i can't locate. I also seems to be running rich ( i call smell the exhuast even while inside the car ).
Anybody have any thoughts?

Could be a leaking vaccuum line or cracked intake accordian tube (before the throttle body).
Could be an exhaust leak near the engine.
I would also change your PCV Valve and replace that hose: PCV's are known to a) get stuck open b) hose collapses c) hose cracks
Put in a good Fuel System Cleaner: Red Line Synthetic Oil - Gasoline Fuel Additives - SI-1 Complete Fuel System Cleaner

That's conflicting: you have lean codes and you think it runs rich?
I think you may have 2 problems. One is a vacuum leak and Noggin spoke to that.
The other one could be a miss fire or a fuel leak because you can smell fuel.
You need to find out where that smell comes from. Do the exhaust fumes smell like gas? Please, be careful! If so you have a miss fire.

Do you get the fuel smell inside the car with all windows closed and the vents and blower turned off?
If no: does the smell appear when the vents and blower turned on?
If yes, you likely have a fuel leak under the hood.

Does the smell depend on how full the tank is? I. e it smells after filling up but not when the tank is near 1/4. Your fuel filler hose may be leaking.

pcv hose is pretty common with that car. I would check for a misfire as well, there are many ways of doing this. Just google it. I have seen a lot of these coils go bad, but that does not make sense with the 0171 & 0174 codes. Hows the air intake boot from the top air filter housing to the throttle body look? Any cracks or dry rotted?

[quote=Ed@Continental;125630]pcv hose is pretty common with that car. I would check for a misfire as well, there are many ways of doing this. Just google it. I have seen a lot of these coils go bad, but that does not make sense with the 0171 & 0174 codes. Hows the air intake boot from the top air filter housing to the throttle body look? Any cracks or dry rotted?[/"
"i just put on a new intake hose & cleaned the maf sensor ( no difference noticed in idle ). as far as a bad coil, would that only cause a poor idle? as soon as i accelerate, no problem. as soon as i back off , the surging starts again. as i come to a stop, it may idle fine for a second or so, then it drops 200-300 rpm's, then starts surging pretty hard ( lights dims as the rev's drop too ). i mentioned it's probably running rich, this is because the exhaust smells like it. i'm not smelling gasoline, just exhaust. i haven't looked at the pcv valve yet. if this car had a iac valve, i would suspect that, but i can't locate one. "

[quote=cgladish;125653"i just put on a new intake hose & cleaned the maf sensor ( no difference noticed in idle ). . as soon as i back off , the surging starts again. as i come to a stop, it may idle fine for a second or so, then it drops 200-300 rpm's, then starts surging pretty hard ( lights dims as the rev's drop too ). [/quote]

I hope you cleaned the throttle body while you were there! Not Certain this will fix all your problems, but it should be done----carbon bits may block throttle plate @ idle causing it to surge. Best way to clean is to unbolt throttle body catching all throttle body cleaner with paper towels or rags.
Rough Idling Issues | MotorWeek : Throtle Body Cleaning Video. You don't need another person if you first disconnect negative terminal before cleaning.
Change the plenum gasket @ throttle body: Mahle W0133-1807640 Throttle Body Gasket | Auto Parts Warehouse

tried most of what you suggested...
 

[quote=cgladish;125653][quote=UseYourNoggin;125654]I hope you cleaned the throttle body while you were there! Not Certain this will fix all your problems, but it should be done----carbon bits may block throttle plate @ idle causing it to surge. Best way to clean is to unbolt throttle body catching all throttle body cleaner with paper towels or rags.
Rough Idling Issues |*MotorWeek : Throtle Body Cleaning Video. You don't need another person if you first disconnect negative terminal before cleaning.
Change the plenum gasket @ throttle body: Mahle W0133-1807640 Throttle Body Gasket | Auto Parts Warehouse["i 've tried cleaning both the maf sensor & throttle body with no difference in the idle. i'll check the pcv valve. i've thought about trying the seafoam method a try, long shot, but i'll give it a try for $12." /quote]

[quote=tanprotege;125611]That's conflicting: you have lean codes and you think it runs rich?
I think you may have 2 problems. One is a vacuum leak and Noggin spoke to that.
The other one could be a miss fire or a fuel leak because you can smell fuel.
You need to find out where that smell comes from. Do the exhaust fumes smell like gas? Please, be careful! If so you have a miss fire.

Do you get the fuel smell inside the car with all windows closed and the vents and blower turned off?
If no: does the smell appear when the vents and blower turned on?
If yes, you likely have a fuel leak under the hood.

Does the smell depend on how full the tank is? I. e it smells after filling up but not when the tank is near 1/4. Your fuel filler hose may be leaking.

"in my saying that it might be running rich, this is based on the exhaust smell, not that i'm smelling any gasoline. as far as the possible misfire, the only problem i'm having is the idle. as soon as i accelerate, the car runs fine. but when i back off or coast, then the rough/surging idle issue starts. i would probably focus on it being a vacuum leak or the pcv valve."

The IAC valve is on the throttle body.

Exhaust smell: It is normal to get some exhaust smell on a cold start because the catalytic converter is not yet operating. Once it has warmed up the exhaust smell should be much reduced.
If you are getting exhaust smell in the cabin there could be a leak in the exhaust system. close to the engine. The fumes are likely picked up by the blower and presented to your sensitive nose. Not good! Not healthy!
I don't know if your car has an EGR system. If it does have one and it has a broken line that could explain a lean condition and the exhaust smell all in one.
So I suggest to check for exhaust leaks and EGR leaks first.
An exhaust leak could also explain rough or noisy coasting and idle.
When you get off the gas it will cause a sudden drop in pressure in the exhaust system .This could pull in air and thus present O2 to the sensors. Presence of O2 would indicate a lean condition.

problem solved! it turned out to be the top hose from the IM that connects to the
second hose ( that in turns connects to the pcv valve ) had a hole in it.
weird in that it looked like the hole was a result a something rubbing against it, but nothing was close it. the hose also felt to be pretty thin too.
anyways, swapped it out & back in business!
thanks for the help!