# System Too Lean - MIL ON - where to start troubleshooting?



## Phaetonlvr (Feb 26, 2013)

About a week ago the MIL came on and a VCDS scan (below) shows DTC P0171 & P0174 System Too Lean - MIL ON. I cleared the codes and the MIL came back on twice over the last week, both times with the same codes. I cleared the codes each time and it takes a while for the MIL to come back on. The car runs very good at speed but it has what feels like a little misfire at idle. This is especially noticeable when in gear, it seems the rpm drops a little bit every few seconds, but the time between those drops is not consistent. I made two videos to show this, see below.

I looked up the codes in the Bentley manual and it suggests the following:
SAE VAG Description 
P0171 16555 System Too Lean Bank 1 

Exhaust system
Exhaust system components, removing and installing

Fuel injectors
Fuel injectors, checking

Fuel pressure regulator and residual pressure
Fuel pressure regulator and residual pressure, checking

Fuel Pumps (FP)
Fuel Pumps (FP), checking

Intake system
Intake system, checking for leaks (false air)

Secondary Air Injection (AIR) system
Secondary Air Injection (AIR) system, checking for proper seal


Can somebody tell me what to check first? Is there a Schrader valve on the fuel line were I could measure the fuel pressure? Or could it be an injector issue?

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Chassis Type: 3D (3D - VW Phaeton (2002 > 2006))
Scan: 01 02 03 05 06 07 08 09 11 13 15 16 17 18 19 23 27 28 29 2E
34 36 37 38 39 46 47 55 56 57 65 66 68 69 71 75 76 77

VIN: WVWBF03D758005095 Mileage: 253780km-157691miles
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Address 01: Engine Labels: 4D0-907-560-BGH.lbl
Part No SW: 4D0 907 560 DD HW: 
Component: 4.2L V8/5V G 0030 
Revision: 00000000 Serial number: VWZ3Z0E5322002
Coding: 0007873
Shop #: WSC 01065 000 00000
VCID: 6BDBC2AA49697F79AB2-5120

4 Faults Found:
17544 - Fuel Trim: Bank 1 (Add) 
P1136 - 001 - System too Lean
16555 - Fuel Trim; Bank 1 
P0171 - 004 - System Too Lean - MIL ON
17546 - Fuel Trim: Bank 2 (Add) 
P1138 - 001 - System too Lean
16558 - Fuel Trim; Bank 2 
P0174 - 004 - System Too Lean
Readiness: 0000 0001

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Address 02: Auto Trans Labels: 09L-927-760.lbl
Part No SW: 09L 927 760 G HW: GS1 9.0 4.1 
Component: AG6 09L 4,2L V8 1903 
Coding: 0000102
Shop #: WSC 22503 444 59177
VCID: 3B7B32EA79892FF97B2-5120

No fault code found.
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rpm drops at the following time in the video: 3, 8, 11, 13 seconds





rpm drops at the following time in the video: 2, 7, 17, 28, 37, 39


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## invisiblewave (Sep 22, 2008)

There have been quite a few MAF sensor failures, but you seem to have problems reported on both banks. Still, the first thing I'd do is swap the MAFs over and see if it makes a difference to the scan.

http://wiki.ross-tech.com/wiki/index.php/16555/P0171/000369


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## Phaetonlvr (Feb 26, 2013)

I haven't yet studied how the system detects a lean condition but I'd assume it's the oxygen sensors that would generate that DTC, one of each bank. I could swap the two MAFs to the opposite side but doubt it would make a difference because it looks like the ECU combines both MAF readings into one. I could swap in the MAFs from my other Phaeton to see if the idle changes.

I tried logging some Measured Value Blocks but couldn't detect anything unusual in the data, will have to study that a little more. Which MVBs should I be logging for this issue? 

Thanks!


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## invisiblewave (Sep 22, 2008)

I've no idea, but the Ross Tech wiki will probably tell you.


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## cbh123 (Aug 25, 2013)

Firstly I would suspects intake air leak.... Somewhere after the MAF 

Too lean is same as too much air. 

O2 sensors repost the leanness, but unusual for them to read too much O2 without other faults. 

There is a low chance that the too lean is due to not enough fuel, uncommon but a quick fuel pressure check is not a bad idea.


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## Phaetonlvr (Feb 26, 2013)

*Lean Condition Issue resolved!*

Time for an update. It turned out that there were actually two different problems at play. The drop in the rpms was caused by a bad coil pack which I determined by monitoring the misfire counters for each cylinder. Put a new coil pack (surprisingly cheap!) on cylinder #7 and the rpm drop went away.

However, I was still getting the MIL with Lean Condition on both banks, usually after about 40 miles of driving and usually after I got off the freeway and was sitting at the first red light while the engine was idling. Then one time when I started the car it had a hard time idling at all, felt like it wanted to stall. I opened the hood and could hear a massive air/vacuum leak. Turns out the crankcase vent valve (also known as PCV) was sucking in outside air. This is the big black valve at the rear of the intake manifold of the V8. I held the vent opening closed with my finger and the idle immediately smoothed out. Put a new one in ($96 for what's basically a plastic check valve!) and all is well, no more Lean Condition codes.


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## invisiblewave (Sep 22, 2008)

Where's the valve located??


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## Phaetonlvr (Feb 26, 2013)

invisiblewave said:


> Where's the valve located??


Under the plastic cover at the rear of the intake manifold. It's the big black valve in the center of the photo.


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## invisiblewave (Sep 22, 2008)

Cheers! I'm becoming increasingly convinced that mine's going to turn out to be the fuel pump/s, but I may as well check the simple things first.


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## Phaetonlvr (Feb 26, 2013)

I was afraid that it might be the fuel pumps on mine also but mine was running fine even at high rpm and hard acceleration, only at idle was it an issue, which makes sense because that's when the manifold vacuum is the highest.

I just took that old crankcase vent valve apart and sure enough the diaphragm has a tear in it. It's a rather simple device, just a plastic cover with a metal spring and a diaphragm inside. How can this valve be $96 when a coil pack is only $19 (both original VW parts)?


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## invisiblewave (Sep 22, 2008)

I'm guessing it's to do with production numbers. Even at that price it's a fraction of the cost of two new fuel pumps! I don't have any symptoms yet other than the CEL.


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## Paldi (Sep 4, 2004)

My VW service tech claims it is due to leaking camshaft seals. My MIL tends to come on at idle. The codes generated are said to be a lean condition on both banks of my V8. Cost to replace these seals was quoted at $1,200. My car has 145,000 miles. I had the T-80 purge valve replaced and my entire engine was removed to fix the water leak behind the oil cooler.


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## invisiblewave (Sep 22, 2008)

Stephan, how does the valve come off? Mine has what looks like a heat shield cover over it with two poppers on it, and it extends down the rubber hose so I can't even see how to get the cover off. It's a fabric thing that wraps right around the valve.


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## PowerDubs (Jul 22, 2001)

I've got that same valve on one of my non Phaeton cars, but it should be the same part #. It just pulls out of the manifold- it is a press fit with a simple o-ring.


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## Phaetonlvr (Feb 26, 2013)

invisiblewave said:


> Stephan, how does the valve come off? Mine has what looks like a heat shield cover over it with two poppers on it, and it extends down the rubber hose so I can't even see how to get the cover off. It's a fabric thing that wraps right around the valve.


There are two hose connections on that valve. One on the bottom and one to the side, both are held on by hose clamps. There was nothing covering that valve on my V8 except for the plastic engine cover. Here's a photo with the valve removed.


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## invisiblewave (Sep 22, 2008)

Thanks for the picture. I stuck my head against it and there's a whirring sound that might be a leak, but it's hard to tell.


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## Paldi (Sep 4, 2004)

Great photo. Mine is also covered with a mesh cloth-like heat shield. 

I wonder if one of those hose clamps is not tight, could that cause enough of a vacuum leak to "lean" both banks?


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## Phaetonlvr (Feb 26, 2013)

Just checked my other V8 and it doesn't have the heat shield cover either.

The hose clamps, if they are original, are those crimp-style ones. I used a screw type hose clamp when I re-installed the valve. I guess it's possible that it could leak air past the hose clamps but even without the clamps it's a pretty tight fit.


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## invisiblewave (Sep 22, 2008)

On mine, I still strongly suspect the fuel pumps, but I'm going to check the valve and clean the MAFs before spending the time & money on fuel pumps (and I won't be doing it at all if I can't fix the transmission clunk first).


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## twgin (Apr 25, 2012)

I fought lean codes for a long time (not the same codes, mine were P1128, P1130, lean both banks) and after everything it turned out to be the primary fuel pump. I found a used set on eBay for under $200 (both pumps and ALL the connecting hoses, level sensors, etc. from a complete fuel tank tear down). During the process a local Audi shop found a small leak in the intake manifold which cut the VCDS fuel trim errors in half but not enough to make the problem go away. Changing the primary fuel pump finally solved the CEL light; car ran fine throughout but couldn't be smogged in CA due to the CEL fault. Anyway, Invisible, still have the secondary pump and all the hoses. Think the pumps may be identical part numbers with different hosing ? If so and you are interested PM me ...


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## invisiblewave (Sep 22, 2008)

Thanks for the offer but the part numbers are different for left & right. I've been looking at used ones, but I think I've come to the conclusion that if I'm going to do the job myself, I might as well do them one at a time starting with the main pump and use new, discounted parts, which can be had for about $280 each.


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## Paldi (Sep 4, 2004)

Mine was finally diagnosed as a broken crankcase vent valve diaphragm. (Not leaky cam seals as posited by the dealer who wanted to do them for $1,200.)

The valve and labor was about $360.00.


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## cbh123 (Aug 25, 2013)

Paldi said:


> Mine was finally diagnosed as a broken crankcase vent valve diaphragm. (Not leaky cam seals as posited by the dealer who wanted to do them for $1,200.)
> 
> The valve and labor was about $360.00.


that is a lot of labour for 2 hose clamps, considering the part is already expensive at 96 dollars. 

but very good to hear the issue is resolved (due to help from here?!)


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## invisiblewave (Sep 22, 2008)

Finally about to get round to hopefully fixing my lean codes now the inspection is due! I just re-read this thread with interest since the dealer also recently diagnosed leaking valve cover gaskets on mine, although I see no sign of an oil leak. My symptoms are a lean condition (now bank 2, but previously both banks), CEL, P0174 ODBII code, humming/vibration from the PCV valve that stops when I lean on it, and (only once) a puff of smoke from the RH side of the engine while reversing. Just called the dealer and they have one in stock, $109 after haggling.


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## invisiblewave (Sep 22, 2008)

Ok, I appear to have fixed the PCV problem by replacing it. EVAP readiness is set. For anyone else doing the job, the only tricky bit was the weird jubliee clips (hose clamps), which appear to be single use. To get them off, you have to lever up the tiny tab then lift the end of the clip. I replaced them with standard jubliee clips. It appears that the dealers are unaware of the PCV since they seem to diagnose the P0174 code as bad valve cover gaskets.

I'm not in the clear quite yet, however, since the catalyst check is still showing failed or incomplete.


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