Help! Intermittent problem with stalling and poor idle after slowing/stopping.

[Cornwall-Bow]

Hello guys looking for some advice on an issue that's plagued me for a good year now and I'm struggling to get to the bottom of. 

 

Background on my car: 2004 JDM import automatic 350z on 80k miles

 

The codes I get, although rare they pop up I have had the codes:

-p0172 System Too Rich Bank 1

-p0175 System Too Rich Bank 2

 

 

The problem: I keep either stalling out completely or going into an extremely rough idle after stopping (usually at traffic lights) or slowing down to bellow 15 miles an hour usually when slowing to give way. 

 

I'd say 80% of the time the issue causes the car to complete cut out and I have to put it into park and restart it. This often causes the car to restart with the rough idle problem (extremely low idle that feels like it's struggling to get any higher than 500 rpm without help from the accelerator) it also causes the car to drive extremely badly as the power seems to surge or be non existent (almost limping) which feels quite dangerous when trying to find somewhere to pull over or get off a roundabout. It also seems to kick out black smoke when the engine is in this lumpy mode. Probably causing the O2 sensors to kick out the two codes mentioned earlier (p0172 and p0175)

 

What I've found kinda works: by no means a fix but a somewhat odd 'fox' I've found is that if I unplug and plug back in the MAF sensor it almost always starts back up ok and will drive normally for a while although the problem always seems to come back. Either the next journey or the next day. 

 

What I've tried changing on the car so far: 

 

-I've tried cleaning my original MAF sensor

 

-I've bought two more 2nd hand OEM MAF sensors and tried them with no real changes from my original one

 

-Changed both camshaft position sensors with Delphi ones but not the crankshaft position sensor. 

 

-changed the PCV valve with an new OEM one 

 

-Changed the fuel pump with a new OEM one 

 

-Cleaned the throttle body and the inside of my plenum 

 

-Replaced the Injectors with 2nd hand OEM ones

 

-Changed the rocker covers on the left and right side (where being to weep a little oil but nothing crazy) 

 

- I've changed all spark plugs for NKG iridium tipped ones

 

- I've changed one of my coil packs that previously had an issue with missing with an new OEM one and check the condition of the others with no real signs of issues.

 

-I've also cleaned the VVT sensors on the left and the right of the engine

 

-I've cleaned all four of my O2 sensors but not replaced them.

 

Just looking for any advice on what to approach next as the lack of codes is really making my life difficult. Anyone had a similar problem? Was tempted to change my crankshaft position sensor next but the lack of a code is keeping my scratching my head, as surly there whoud be a code of it was the crankshaft position sensor that was failing? 

 

Any help is much appreciated! 

 

 

 

 

 

 

 

 

 

 

 

 

[Ekona]

Sounds like a potential post-MAF air leak to me. The car is dumping crazy amounts of fuel in to try and compensate for the extra air, hence the too rich codes. Check and double check all connections after the MAF, as that’s the cheapest and easiest thing to sort. 

[Cornwall-Bow]

Thanks for the reply Ekona, I'll take a look today and see if there's anything obvious. Could a leak in the exhaust manifold cause an issue like this? Just thinking that's one thing I haven't checked out yet either. 

[Ekona]

Very unlikely, far more likely you’ve got a leak on either the intake manifold (since you took it apart) or the intake pipe work to the throttle body (ditto).

[Cornwall-Bow]

Ah fair enough, I'll take a good look the intake manifold. It's had this problem before and after I changed the injectors and rocker covers. I also replaced the upper and lower intake manifold gaskets as the old ones where looking a little tired. But potentially I've missed something when reassembling, would a bad throttle body throw a code? I've carbon cleaned it a good few times now and it's almost spotless but could it potentially have an issue electrically?

😔

Edited October 3, 2022 by Cornwall-Bow

[Ekona]

Not impossible but highly unlikely, honestly this screams of an air leak to me more than anything. Dodgy throttle body would give different codes, how careful were you when you cleaned it? They’re notoriously sensitive to any kind of touch. 

[Cornwall-Bow]

Fair enough, the only noticeable hissing from near the upper plenum comes from the PCV valve but I thought that was 'normal'. Is there a way I can check if this is excessive? I was warned by a previous post on here to be super careful when cleaning it and I did take lots of care not to move the butterfly valve. So I just took the whole throttle body assembly off and sprayed it with carbon cleaned from both sides. So I'm hoping it's ok?

[Ekona]

Yup, perfect. 
 

Best way to check for air leaks is to spray some carb cleaner around every single possible connection and listen for the idle going up. 

[Cornwall-Bow]

Just tried the carbon cleaner trick around the intake manifold and nothing noticable with the idle or hissing noise. Considering buying a separate throttle body and seeing if I've agitated my current one when cleaning it. But of a weird one but would the crank shaft position sensor always throw a code if it was on its way out? 

[ZMANALEX]

If you have not replaced the front oxygen sensors then I would suggest that you do that, or at the very least get them tested, cleaning would have no effect.

 

Note that as your car is a JDM it will be narrowband.

 

Also I have found that aftermarket cam sensors is not a good plan, and I only fit genuine OEM.

[Cornwall-Bow]

22 hours ago, ZMANALEX said:

If you have not replaced the front oxygen sensors then I would suggest that you do that, or at the very least get them tested, cleaning would have no effect.

 

Note that as your car is a JDM it will be narrowband.

 

Also I have found that aftermarket cam sensors is not a good plan, and I only fit genuine OEM.

Ok O2 sensors will probably be next on my list, I've ordered a secondhand working throttle body so if that doesn't help the issue I'll return/re-sell that and grab some replacement sensors. The cam sensors are Delphi which I thought whoud be ok but I guess the JDM equivalent would be Hitachi or NKG?