1.8t is throwing p0507 and p0171 codes. Tried a couple things, but issue persists

[joshpeck]

Hi everyone,

my 04 1.8t quattro is throwing p0507 and p0171. Used a code reader at auto parts store and they diagnosed as front o2 sensor.

I figured I would look for obvious vacuum leaks before springing for a new one. Found a damaged check valve on the airbox and a leak on the pancake valve attached to the valve cover breather. Replaced both and cleared codes. Check engine came back shortly after. Got them read again, and diagnosed as faulty CPS by another auto parts store.

Research here suggests it may be the PCV valve below the intake manifold. I got one as well as a new j plug and thermostat, as well as needed gaskets to throw in while I'm down there.

Any ideas before I dive in on that? Could it be the o2 sensor still?

A couple other notes that may help - oil consumption has seemed a bit higher, and last time I changed the plugs one of them (2nd or 3rd) had oil on it on the coil side.

Thanks!

[old guy]

The high idle code (P0507) and lean code (P0171) both indicate a vacuum leak somewhere after the throttle body. My recommendation would be to first run a boost leak test.

Another possibility would be a cracked "L" breather hose connecting the lower block breather to the PCV valve. This would allow unmetered air to enter the intake stream.

I doubt it's the O2 sensor.

[joshpeck]

any tips for doing a boost leak test if I don't have the specialized hardware?

Is the carb cleaner method safe for the car? would MAF cleaner do the same or is it not misty enough?

I appreciate your input!

[old guy]

Are you experiencing a higher than normal idle speed as your code indicates?

[joshpeck]

yes.

When the check engine first came around it was idling around 1100. Now it's closer to 1000 after replacing the pancake valve and check valve.

[old guy]

yes.

When the check engine first came around it was idling around 1100. Now it's closer to 1000 after replacing the pancake valve and check valve.

Try this. There are several vacuum lines connecting to the intake manifold. With the engine at idle clamp then off one at a time. If clamping one of the lines causes a decrease in idle speed you will have found your source of unmetered air that is causing the high idle and lean code.

Once you determine the vacuum line causing the issue just trace the line until you find the leak. Let me know which line drops the idle and we can go from there.

[old guy]

Bedtime for old guys. I'll check back in the a.m.

[caluma4x4]

Bedtime for old guys. I'll check back in the a.m.

X2 on the pcv valve, also change the block breather pipe, its almost certain to be broken. Good access to that when you're doing the thermostat.

Sent from my ANE-LX1 using Tapatalk

[joshpeck]

X2 on the pcv valve, also change the block breather pipe, its almost certain to be broken. Good access to that when you're doing the thermostat.

I've already done the breather and the dipstick. Wishing I had done the PCV while I was at it back then. Hopefully they haven't gotten too brittle in the past 3 years.

[caluma4x4]

Don't worry pcv valve is easily reached once coolant tank is moved out of the way

Sent from my ANE-LX1 using Tapatalk

[joshpeck]

Don't worry pcv valve is easily reached once coolant tank is moved out of the way

Was thinking I would just do a coolant flush and pull the intake manifold and get at everything under it.

I've been having a mystery coolant leak that might be the j plug.

Do you think that's not worth it?

I have all the gaskets and a new thermostat too, but I could always return them.

[caluma4x4]

Well if you have everything you need there you might as well. Don't forget the o ring for the coolant pipe that's connected to the thermostat housing. Does the J plug show any signs if leaking? Another common leak point is the coolant flange at the rear of the cylinder head, it leaks onto the gearbox then evaporates so you'll never see it but you will see the pink stain from the coolant.

Sent from my ANE-LX1 using Tapatalk

[Ol Dirty Noodle]

I just had the same exact problem, 034 PCV kit and billet injector cups with new o-rings and seals fixed it right up

[joshpeck]

Well if you have everything you need there you might as well. Don't forget the o ring for the coolant pipe that's connected to the thermostat housing. Does the J plug show any signs if leaking? Another common leak point is the coolant flange at the rear of the cylinder head, it leaks onto the gearbox then evaporates so you'll never see it but you will see the pink stain from the coolant.

Sent from my ANE-LX1 using Tapatalk

I replaced the coolant flange a little while back but still am getting some coolant loss (seems like it happens overnight maybe).

Which o ring are you talking about on the coolant pipe? I should probably snag one if it isn’t included with the housing.

[caluma4x4]

There is a metal pipe that is secured to the thermostat housing by a single 10 mm bolt, that's the one that has the o ring on it.

Sent from my ANE-LX1 using Tapatalk