My CEL light came on and when I checked on durametric it had 2 faults po171 and po174. After clearing it keeps coming back on after about 100 miles of driving. Car is running fine though with normal idle and able to maintain boost at 1.3 bar (tuned) even with these fault codes.
Any ideas guys?

 

Most likely cause is the MAF.... unplug it while it's running and see if there's any change, if not then you need a new MAF. Is the car completely stock?

Other possible, but less likely culprits.... Intake/vacuum leak, bad O2's, bad seal on gas tank lid, short in MAF wiring harness, bad gas, carbon deposits on Cat....

Start at the MAF and work through it.

 

Here is the manual page for P0171. The entry for P0174 is the same, just for bank 2. Given that you have both, it would seem to point to the MAF (as suggested above) or an intake air leak. Or, if you are really lucky, the oil filler neck isn't closed tightly.

Jon

 

Before buying a new MAF Sensor ($200), I'd do a leak test. I'm getting similar codes, and discovered my Bailey DVs were leaking. I ordered some Bosch 710N's and will be installing them (this weekend, i hope!).

But from my research, it's usually an intake leak. Those are fairly common especially with the DVs and the F hose.

 

+1 I found an intake leak last weekend at the turbo inlet. These cars are plumbing nightmares.

It's really easy to do. There's a DIY (not mine) somewhere, but here it is again:
Buy a 3" ID (3.5" OD) PVC pipe cleanout with a screw-in end cap. Drill a hole in the end can for an air line quick disconnect fitting. Screw in the end cap and air line with teflon tape to seal. Remove air box and slip the pipe into the rubber coupling and tighten the hose clamp. Place some towels between the PVC pipe and the body to discourage it from popping out. Pressurize and listen for leaks.

Jon

 

Jon, I did nearly the same thing. The odd thing is that 3" pipe is nowhere near 3" on the ID or OD. My dad explained this to me as just being a standard and to think of the sizes more as names than representing an actual measurement. I got a 3" plug with a 1.5" hole in it and then got a 1.5" plug with a .5" thread and threaded a nipple in there with some teflon tape. Then used PVC glue to glue the 1.5" plug in to the 3" plug. That sounds complex, but it's not and I'll post pictures when i can.

The other thing is that I don't have a compressor with a regulator, so I used one of those pressure sprayers and cut off the spray nozzle and plugged it into the nipple. Again, I'll post pictures when I can.

Finally, the one other thing I'd encourage is get a small paintbrush (3/8" would be great) and paint a mixture of 25% dish soap, 75% water onto the various joints where you hear the leak coming from. Everything is so tight, it can be a nightmare identifying exactly what the piece leaking is (if it isn't obvious).

Every car has its things that drive you nuts. For Evos it's clutches and trannies, for Subies it's ringland issues and for us it's intake and vacuum leaks (and clutch accumulators). I'm a new owner of an old 996 turbo, but I imagine my leak tester and i will become fast friends into the future.

Best of luck to the OP in figuring this out.

 

I've attached a photo of mine. It's 3" ID and 3.5" OD. Some guys either T-in or provide a separate pressure gauge. My compressor has a regulator, so I just used that. Soapy water as a tell-tale is a good idea. Works nicely to find leaks in tires.

Jon

 

Do the test from the turbos... 2.25" coupler... block one off and pressure test the other...
your problems will be solved...

 

I've meant to ask this before, why would you test from the turbos and not the intake? It seems like testing from the intake is certainly easier to get at and also tests more of the intake system. Moreover the FSM tells you to test from the intake.

 

Thanks guys for all the great advice, will check them out. I notice it only comes on in city driving and not when I have been driving it hard on track onon freeways. Why is this?

 

It is a 'fixed volume leak'... when you are driving hard, you are getting a large volume of air in the MAF where it is being metered- the contribution from you leak is a small % of the overall air flow, and therefore the amount of compensation the DME is doing to correct the AFR is small...this stays below the level that sets a CEL.

In city driving, highway cruising, low RPM stuff, you have much lower airflow into the engine- the impact of the fixed leak is therefore a much larger percentage if the total airflow and the DME has to adjust more to keep the AFR correct. Over time the cumulative impact of adjusting the short term fuel trims will build up and bing- CEL.

Look for a small vacuum line disconnected.

My 2 cents

A

 

Hi guys, just a quick update. Pressure tested...no boost leak. Checked vacuum hoses, nadda, all ok. I had changed to the 997 intercoolers just before the CEL came on, could this be a factor? Oxygen sensors 6 months old.
Thanks to all for their ideas.