[event3horizon]

My 2008 Eos, 2.0TFSI, 58K miles, (engine code BPY) has been having some idling problems, system too lean condition that I thought I had fixed but it came back.

Vag codes:

ODB II:
P0171 System Too Lean Bank 1

I have no misfires, but I am seeing short term fuel trims peg at +12 % to + 18 % or more during closed loop and a bit rough idling and sometimes high 1000 rpm at idle. MAF sensor seems too low for me, it's around 2.2g/s to 2.8g/s normally I think it's 3 or 3.5??

What's interesting is that I suspected the PCV so I replaced (with cheaper aftermarket, not oem) and reset the codes and they didn't come back for about 2 weeks. Before I would clear them and they would come back within a day or shorter.

But now the problem is back. I'm wondering if I have another air leak somewhere or maybe I shouldn't use aftermarket PCV valve? I also suspected the oxygen sensors but the voltage was around 0.625 on Bank 1 sensor 2 which I believe is the downstream. It seems to be just reacting to the situation and not an issue within itself.

Is ambient air temp a factor? For some reason my ambient air temps are way too high (it was 75f outside), bad sensor?

Here's some more parameters during idle:

 

[voxmagna]

Start with what you know and get the temperature sensor readings correct first. Temperature is very important to determine the correct fuel mapping. Not sure where it is on your engine. They often combine MAF and temperature in the same sensor because both are critical and the MAFsensor is using temperature as its method of measurement. Mixture ratio is leaned off when air temperature is high and made richer (ECU choke equivalent) when it is low.

I haven't looked at what sensor they use for the dash, but what temperature does that show?

 

[event3horizon]

Start with what you know and get the temperature sensor readings correct first. Temperature is very important to determine the correct fuel mapping. Not sure where it is on your engine. They often combine MAF and temperature in the same sensor because both are critical and the MAFsensor is using temperature as its method of measurement. Mixture ratio is leaned off when air temperature is high and made richer (ECU choke equivalent) when it is low.

I haven't looked at what sensor they use for the dash, but what temperature does that show?

Just checked, dash temperature is ok. I found out that the ODB II sensor 'ambient air temp' reading maybe bogus. I found the VW-specific PID for ambient air temp and it's fine so I'm ruling that out.

I did more probing, I found that voltage data for the upstream oxygen sensor and it looks good. It is around 1.45 volts. These aren't the legacy oxygen sensors that should be below 1V and oscillate rather they are more advanced 'wideband' sensors that have higher than 1v voltages. I compared with My passat and Audi and the EOS doesn't seem any different.

The short term fuel trims are still around +11 or +12 , not enough to trip the codes again as I cleared them although oddly the long terms aren't that out of whack. I'm reading trims need to exceed +/- 20 percent to throw the codes. One interesting thing that I did find that may be the actual culprit (or red herring) is the PCV valve again. I made a makeshift stethoscope to listen in on hissing noises around the engine and it's definitley hissing from the PCV. I sprayed it with brake cleaner and I think the fuel trims and o2 sensors react. The $12 aftermarket PCV that I replaced it with may be garbage.

 

[voxmagna]

Since the dash display ambient temperature isn't critical and probably only used for climate control they could use another sensor. But the MAF air temperature and engine coolant temperatures are critical and used by the ECU to set fuelling compensation. Coolant sensors may also be duplicated, what's on the gauge isn't always what the ECU is looking at. What temperature is diagnostics showing - engine air temperature or something else because it seems a long way off and digits shouldn't lie.

You have to be careful drawing conclusions from sensors when they are in the ECU mixture control loop. Something else can be wrong and what you see measuring the sensors is after compensation or corrections have been made. As you suggested, perhaps you have a bad PCV?

I don't know if you already know this but the simple PCV valve going faulty (stuck closed) causes crank case vacuum to build up pulling in the crankshaft oil seal and causing a massive oil leak and headache. It's not worth penny pinching for something the Chinese made from poor quality plastic with molding deformities.

I had a fright on a later V.W after finding a huge pool of oil under the car. Fortunately I discovered the dipstick had lifted up and the PCV system had forced oil out! This link describes symptoms and some simple checks you can do on a closed loop system PCV valve:

PCV Valve - How Does It Work - Failure Signs - Ways To Test The System

Knowing how the PCV valve actually works, is the first step in diagnosing a bad or failing one. Helping stop, highly toxic and harmful gases.

dannysengineportal.com

 

[Paddy OPlastic]

I'd bet any money it's the PCV. Had exactly the same codes. VW redesigned it and it really isn't expensive from the dealer.

 

[voxmagna]

That's interesting and something to be aware of. The aftermarket and Chinese clones (where many aftermarket items come from) aren't always up to date. They copy something not knowing how it should work or whether an original V.W design has been improved, then churn out thousands all the same. It must be a nightmare for V.W dealer shops these days not knowing what parts have been put in cars they have in to repair.

 

[event3horizon]

Thanks , I will replace with OEM PCV and report back soon. I don't notice any issues with oil leaks, not even misfires. Just sometimes high, erratic idles and over-compensated fuel trims (system adding fuel due to lean condition) The fuel trims get worse (+20) when I turn off the A/c compressor. I'm thinking the engine needs to pull more air when a/c compressor is on therefore making whatever vacuum leak I'm having have less of an effect on the fuel trims.

 

[voxmagna]

The fuel trims get worse (+20) when I turn off the A/c compressor.

The operation of an Efi engine is a finely balanced thing. When all the planets are in alignment and feedback from sensors is within range on an engine without mechanical faults, all works well. But anything wrong that was not designed for or engine wear causes the engine management to move away from automatic compensation and its ability to control it. Efi engines on the plus side can maintain performance and emissions as the engine wears. The downside is there's a cliff edge where the control breaks down. Yours isn't an old engine, but any fault like air leaks or bad components does a similar thing. Most engineers will struggle to resolve these kind of faults when feeback controls are active. The classic fault finding method I've only used a few times is to simulate a sensor normal value with a voltage or resistor. The feedback loop is now open without control and the engine should run like an older pre efi ECU engine. Fault finding without the complication of feedback compensation is lot easier. For example, lets say you simulated sensor responses for a normal closed loop emission. At low speed idle you would expect the engine to idle smoothly without any feedback compensation at one speed and no load. If there's an air leak, bad valve, or fuel injector, the idle might be wildly erratic or even stall. When the ECU is compensating, faults can be masked and show smaller efffects.

Most ECUs have compenation for AC (and alternator loading) when it's turned on. In it's simple form the engine could die on idle when the AC comp cuts in so the ECU detects when the AC is on and compensates the idle to maintain a constant rpm. There's a clue to your problem there because a low stable idle rpm with a small load and correct mixture balance is hardest to achieve. I suspect if you touch the gas pedal a little on idle moving rpm up to say 1200 and back, it won't be stable, hunt or may be slow to react? That's usually the problem caused by air leaks, which only need to be small to have a big effect at idle with low air and fuel volumes entering the engine.

 

[event3horizon]

Update: I replaced the PCV with VW original part. At first I thought I had everything fixed, the fuel trims looked fine, better than they were, but of course, after 10-15 miles of driving, check engine light back on. This is frustrating.

The good thing is the new PCV may have fixed P2279 intake air system leak as that code has not come back, however, the P2187 system too lean at idle bank1 and perhaps unrelated I have a P2181 Cooling system performance code tripped although I don't see any cooling problems and temps are fine on the gauge. The fuel trims are still looking bad at 15-18 percent positive. I don't know what's going on, halppp

 

[voxmagna]

perhaps unrelated I have a P2181 Cooling system performance code tripped

Why ignore temperature warnings that could mean expensive engine damage ahead? Search the code with Google and everywhere it tells you: Do nothing and it could lead to major engine damage if ignored. I'll say it again: Most efi engines sense engine coolant temperature in the cylinder head for exclusive use of the ECU mapping. In fact and curiously, you can have a cylinder head coolant problem where coolant gets converted to steam, causes vapor locks and the cylinder head temperature sensor shows a temperature no higher than steam, when the cyl. head is melting! The temp sensor used by the ECU for fuelling is usually better quality and more linear than the others.

The dash gauge with a less accurate sensor is usually near the water pump or bottom hose and would not measure coolant temperature inside the cylinder head, it's for approximation and visual use on the dash only and not to control the engine. It's important to understand this because you can have air locks, a cracked cylinder head or a bad cyl. head gasket and that sensor if it is correct, is giving you a big warning. You should learn how to use diagnostics real time measurements properly. Fault code trips tell you nothing, but reading the cyl. head temperature sensor output as the engine warms up and idles can tell you a lot. Fuel trim won't be correct if the engine cylinder head is overheating and those high temperatures are sent to the ECU.

Is that print from V.W compatible diagnostics? What does it mean by 'Coolant Temperature' and which sensor if there are two is being measured? The sensor you should be looking at is the the one that gives the P2181 fault, because it is reading temperature outside the range V.W expects. Many have been fooled seeing the dash guage looking normal when the cylinder head is too hot. You can try a contact thermometer on the cyl. head or paint a black patch near the sensor if fitted to use an IR thermometer. Then see if the head temperature is similar to the dash gauge?

If you concentrate on the cooling system warning first, it might lead to a solution for the other problems? What makes you think a fuel trim error is putting on the MIL light? It could be serious over temperature in the cyl. head?

Has anybody drained and replaced coolant? These engines are quite tricky to fill as per procedures in the shop manual.

 

[event3horizon]

Vox, as much as I appreciate your responses, I think you may be over diagnosing a bit or jumping the gun. I have no noticeable cooling issues at all and the coolant is fresh. I looked at the coolant temp parameters on the computer (Vag data) and it shows G52 sensor temps even lower what the gauge shows me. Gauge shows 190F and the sensor data shows 170F.

Regardless of the cooling thing, I FINALLY found the air leak and I am certain about it. I used a makeshift 'stethoscope' and honed in on the vapor canister check valve hose, it is CRACKED on both ends and it is what is causing my bad fuel trims, system too lean codes. When I wiggle it I hear it hiss and the fuel trims go crazy so finally found my problem there. I will be replacing them with part 06F 133 781 L https://www.ecstuning.com/b-genuine-volkswagen-audi-parts/vapor-canister-check-valve-kit/06f133781l/

That will get the vacuum leak problems out of the way. I don't regret using OEM PCV valve because I think the car idles and feels perfect when I changed it with the original part. I am going to try to hunt down the cooling system code I'm pondering whether to just replace the thermostat.

 

[event3horizon]

Correction, G62 sensor readings are 170F at idle with warmed up engine.

 

[event3horizon]

It's likely this, the thermostat may just not be opening and closing in a timely manner as the system expects it to:

 

[voxmagna]

I replied to what you posted in #9 4 hours ago:"Fault code P2181 Cooling system performance" - which shouldn't trip on it's own. I've never seen it on MY07. I've got a car stethoscope - very useful because making audio recording with a phone just plays back lots of noise you can't make much of.

On an old 2008 the thermostat could be opening on idle, but the radiator core is corroded and bad? Run your fingers over it. If you get dust it's bad. MY07 is going, I looked up the aftermarket prices which weren't too bad and I'll change it on the next coolant drain and flush.

A split hose is hard evidence then. I think they make them out of epdm? But whatever it is, it seems to get attacked by oil, heat, then go hard and brittle, particularly on the molding joins if they are tees or there is movement.

EPDM is cheap but has poor resistance to oil and fuels. Nitrile or Viton elastomers are much better for breather hoses (silicone is no good). I'd choose that for straight hose and use copper for Tees and bends if it needs them. Do it once and the problem never comes back.