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Q: Engine Surge and OBD II code P0171 and P0174 on 2001 Subaru Outback

My car is a Subaru Legacy Outback H6-VDC Wagon (2001).
Basically a terrific car..however in 2006 I had the catal;ytic
converter replaced under warranty with a month to spare..LUCKY!!.
Anyhow it could have been before the converter was replaced but
on a 400 mile trip to pickup my wife and the car started losing
power and a hesitation/surge on acceleration under a load ie while
climbing a hill or passing (very dangerous!!!) These problems
only occurs after the car is up to operating temperature. Doesn't
occur after it sits and cools off a a period of time. Once it starts
hesitation/surge it does it regardless, traffic light starts
and general motoring as long as you are trying to accelerate max
speed 20 to 30 mph. Before I took in to the dealer I hooked my Actron
scanner to the datalink to see any codes both "key on engine off" and
"key on engine off" I also hooked my wife's mickey mouse CarMD scanner
as well and all three times got 2 codes: P0171 and P0174 which could
point to the MAF sensor on either bank or both. At no time during these
surge events was the "CHECK ENGINE" light on!!
I took the car to my local Subaru dealer and my wife tried to explain to
female service coordinator the problem and then left. I got a call from
the coordinator that I needed a $500 plug change and a transmission service
and finally a fuel filter in which I agreed on all 3. They couldn't get any
OBD codes and I NEVER reset codes as that would be stupid!!!
Picked up car following day it ran alright for a couple of dayS until friday
rush hour traffic and then the hesitation/surge upon acceleration. I drove
35 miles back to the dealership insisted the coordinator take a ride to observe
the problem she still couldn't comprehend it so I ended the ride and had
her write it up again. The mecahanic can only repair an item if the problem
is presented to him correctly. She has been TOLD twice to check and or service
the MAF sensor we will see tomorrow I doubt it!!! I will see tomorrow....nothing
done I am calling Subaru America after I give the service manager achance
to sort it out and the local BBB. The last bill for plugs, transmission service
and fuel filter cost me $1000 and I am on fixed income. Can you help with
the problem? Sorry so long winded just total frutration. Thanks
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I agree with the previous commenter that vacuum hoses are the most likely culprit but a couple of things you said are really intriguing me. First if it is vacuum hoses than that should be pretty apparent at idle as well as under load. For a severe stumble under load that would have to be a really major vacuum leak. I am assuming you and the shop checked for that pretty throughly. As you probably know P0171 and P0174 are both System Too Lean faults and not necessarily tied to the MAF sensor. I would not be so quick to discount the fuel pump and fuel sending units. there is a TSB (9-38-03) for the fuel pump from Subaru outlining clearly the repair/replace procedures for dealers which leads me to believe its done pretty often and is easy to screw up. Also I wouldn't discount the CarMD tool either. While I don't work for them directly they do contract with professionals such as myself to answer questions just like this. Good luck.
Well you were taken advantage of by that shop, when a customer comes in for a problem and the suggested repairs don't address the issue, this is just poor service and greedy too.
Here is some info on the P0171 and P0174 codes.
These are normally from a vacuum leak, or a MAF sensor can cause it too (not a transmission service or fuel filter!)
Good Luck
Thank you very much for your quick response on problem. I had a frank talk to the service mgr...nice guy but my car is still in the dealership waiting for a fuel pump [unreal!!] my next stop is Subaru America and my lawyer. I advised the service mgr that like you I suspected a vacuum leak, or a MAF sensor problem and was trying to tell me that Subaru doesn't have the standard MAF sensor???????. At any rate I amgoing to wait and see after my $1600 bill is paid and the problem is there all hell is going to happen!!!
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