Forester 2002, OBD code P0172

[DK]

P0172 "Fuel trim malfunction (A/F too rich)"

The car is misfiring intermittently. Sudden loss of power,
hesitation, jerking, rough idle. The code came up twice
after clearing but it takes weeks before the CEL comes up
(the car is misbehaving even when the CEL is not there).

The symptoms are intermittent but present themselves almost
every day. The problem seems to be more prominent with
cold engine and less prominent at highway speeds.

A local car mechanic wasn't able to diagnose. Things I tried:
changing air filter, using fuel injector cleaner in two tanks,
cleaning ground cable contact that leads to intake manifold
(that was a fix for someone with the same code).

Things Subaru suggests that might cause the problem
(based on http://subaru-car.ru/for1/for_01_sec2_12.pdf ):
- Holes in the exhaust; <-- nothing obvious
- Holes in the air intake system; <-- nothing obvious
- Clogged fuel return line or bent hose
- Improper fuel pump discharge
- Clogged fuel supply line
- Faulty pressure regulator
- Faulty engine coolant temperature sensor
- Problems with mass air flow and intake manifold
pressure sensor.

All of these don't sound like they would cause intermittent
problems only (???)

I will be bringing the car to a dealer but my experience
with this dealer (the only one in town) is that they are
not very competent and tend to replace expensive parts
that fail to fix the problem.

In case they won't fix it, any ideas what might be wrong?

Thanks,

Dima

 

[Carl 1 Lucky Texan]

P0172 "Fuel trim malfunction (A/F too rich)"

The car is misfiring intermittently. Sudden loss of power,
hesitation, jerking, rough idle. The code came up twice
after clearing but it takes weeks before the CEL comes up
(the car is misbehaving even when the CEL is not there).

The symptoms are intermittent but present themselves almost
every day. The problem seems to be more prominent with
cold engine and less prominent at highway speeds.

A local car mechanic wasn't able to diagnose. Things I tried:
changing air filter, using fuel injector cleaner in two tanks,
cleaning ground cable contact that leads to intake manifold
(that was a fix for someone with the same code).

Things Subaru suggests that might cause the problem
(based on http://subaru-car.ru/for1/for_01_sec2_12.pdf ):
- Holes in the exhaust; <-- nothing obvious
- Holes in the air intake system; <-- nothing obvious
- Clogged fuel return line or bent hose
- Improper fuel pump discharge
- Clogged fuel supply line
- Faulty pressure regulator
- Faulty engine coolant temperature sensor
- Problems with mass air flow and intake manifold
pressure sensor.

All of these don't sound like they would cause intermittent
problems only (???)

I will be bringing the car to a dealer but my experience
with this dealer (the only one in town) is that they are
not very competent and tend to replace expensive parts
that fail to fix the problem.

In case they won't fix it, any ideas what might be wrong?

Thanks,

Dima

Did you buy the car used? Does it have a K&N style air
cleaner?(sometimes those are over-oiled and might compromise a MAF sensor.

It does sound a little like a MAF failure. that
s gonna be $$$ if so. Though that code just points to a 'circuit' or
'system' and you could have something as simple as a corroded connector.
Was any work done prior to the onset of problems?

It doesn't really sound like a coolant temp sensor failure.

who read the code? was that the only one?

A decent mechanic should be able to diagnose the sensors without
'shotgunning' the problem. There are procedures for that. Not that there
aren't occasional vehicles with very odd/confusing failures. Probably
2-3 folks that monitor this newsgroup know the specifics, but the dealer
should DEFINITELY be able to confirm a good sensor.


good luck

Carl

 

[DK]

Did you buy the car used? Does it have a K&N style air
cleaner?(sometimes those are over-oiled and might compromise a MAF sensor.

No, we bought it new and it had no real mechanical problems
up to this point.

It does sound a little like a MAF failure. that
s gonna be $$$ if so. Though that code just points to a 'circuit' or
'system' and you could have something as simple as a corroded connector.
Was any work done prior to the onset of problems?

A body shop of that same Subaru dealer fixed some damage
from my daughter's accident past winter (bumper, hood, fender
and front lights replaced). Other than that, nothing.

It doesn't really sound like a coolant temp sensor failure.

who read the code? was that the only one?

Yep, the P0172 was the only code both times the CEL came up.

 

[bigjimpack]

Dont blame K&N filters. They are the best thing you can add to your
car for performance and efficiency. The car is 7-8 year old. Things
go bad. Find a good dealer and if you pay use a credit card. If they
dont fix the problem take it back and tell them to fix it right. Or
drive a little and find a good subie dealer.

 

[Carl 1 Lucky Texan]

Dont blame K&N filters. They are the best thing you can add to your
car for performance and efficiency. The car is 7-8 year old. Things
go bad. Find a good dealer and if you pay use a credit card. If they
dont fix the problem take it back and tell them to fix it right. Or
drive a little and find a good subie dealer.

Did you buy the car used? Does it have a K&N style air
cleaner?(sometimes those are over-oiled and might compromise a MAF sensor.

It does sound a little like a MAF failure. that
s gonna be $$$ if so. Though that code just points to a 'circuit' or
'system' and you could have something as simple as a corroded connector.
Was any work done prior to the onset of problems?

It doesn't really sound like a coolant temp sensor failure.

who read the code? was that the only one?

A decent mechanic should be able to diagnose the sensors without
'shotgunning' the problem. There are procedures for that. Not that there
aren't occasional vehicles with very odd/confusing failures. Probably
2-3 folks that monitor this newsgroup know the specifics, but the dealer
should DEFINITELY be able to confirm a good sensor.

good luck

Carl

Are saying an OVER-OILED aftermarket foam filter could never cause a
problem?

Jeez Jim, don't be so defensive!


;^)


Carl

 

[bigjimpack]

It' hard to "overoil" a K&N filter. Ya spray oil filter is red let
soak in for a bit reinstall. Oversugared gas can cause damage too.

 

[JD]

Not that hard to over-oil them at all.

BTW: oversugared gas clogs the fuel filter. It doesn't actually do damage
to the engine unless the fuel filter is clogged to the point that the fuel
mixture is lean. Its a wives' tale

It' hard to "overoil" a K&N filter. Ya spray oil filter is red let
soak in for a bit reinstall. Oversugared gas can cause damage too.

 

[Carl 1 Lucky Texan]

It' hard to "overoil" a K&N filter. Ya spray oil filter is red let
soak in for a bit reinstall. Oversugared gas can cause damage too.

Are saying an OVER-OILED aftermarket foam filter could never cause a
problem?

Jeez Jim, don't be so defensive!

;^)

Carl

You may be right, in a way, about oiling the K&N filter. (other 'foam'
filters using oil may be a different beast.) Though I have read of some
folks cleaning/replacing MAFs after they experienced problems from
oil-type filters, I am gonna go back to neutral(as regards K&N products)
on this issue after reading this;

http://www.knfilters.com/MAF/massair.htm

and some of the related links.

While K&N themselves have a section warning against over-oiling,

***** Over-oiling

When servicing a K&N filter, take care not to over-oil the element.
Besides impeding air flow, excess oil can migrate into the intake system
where it can coat electronic sensors, which some OEM’s claim may hinder
the sensors’ operation and result in a repair that will not be covered
under warranty. Although K&N disagrees with such claims, as explained
in more detail on this web site, in order to avoid a dispute with an OEM
over the denial of a warranty claim, we suggest that you be careful not
to over-oil your K&N air filter. Never saturate the filter. If oil
drips from the filter, wash it and start over. Use only K&N oil. For
example, an E-1500 filter has 92.4-inches of surface area requiring
1.707 fluid ounces of oil. Follow oiling instructions included with
your filter or refer to the instructions listed here.*****

it appears their testing shows, no oil will leave their filter AND
purposely contaminated MAFs do not fail.

BUT, here; http://www.bobistheoilguy.com/airfilter/airtest1.htm

K&N said BOTH that oil had left their filter and deposited on the test
filter, then reversed themselves and said it hadn't.

Certainly the manufacturer is unlikely to admit to a problem, but it
does appear they have 'stepped up' and are willing to back the point.

sorry for any confusion on my part.

Carl

 

[DK]

P0172 "Fuel trim malfunction (A/F too rich)"

The car is misfiring intermittently. Sudden loss of power,
hesitation, jerking, rough idle. The code came up twice
after clearing but it takes weeks before the CEL comes up
(the car is misbehaving even when the CEL is not there).

The symptoms are intermittent but present themselves almost
every day. The problem seems to be more prominent with
cold engine and less prominent at highway speeds.

A local car mechanic wasn't able to diagnose. Things I tried:
changing air filter, using fuel injector cleaner in two tanks,
cleaning ground cable contact that leads to intake manifold
(that was a fix for someone with the same code).

Things Subaru suggests that might cause the problem
(based on http://subaru-car.ru/for1/for_01_sec2_12.pdf ):
- Holes in the exhaust; <-- nothing obvious
- Holes in the air intake system; <-- nothing obvious
- Clogged fuel return line or bent hose
- Improper fuel pump discharge
- Clogged fuel supply line
- Faulty pressure regulator
- Faulty engine coolant temperature sensor
- Problems with mass air flow and intake manifold
pressure sensor.

All of these don't sound like they would cause intermittent
problems only (???)

I will be bringing the car to a dealer but my experience
with this dealer (the only one in town) is that they are
not very competent and tend to replace expensive parts
that fail to fix the problem.

Well, as I expected, the dealer said it is front oxygen
sensor that is bad. They will replace it for a total
of $243 (OEM is listed $129.95 so it seems they are using
1.5 hours labor to change it... ) I insisted that if that won't
fix the problem, I get the money back.

They did not say why the problem would be intermittent
if it is bad. Subaru manual doesn't even list oxygen sensor
as a potential trouble for just P0172, so I have my doubts.

Will post back once I know how the car behaves after
sensor replacement.

DK

 

[Todd H.]

Well, as I expected, the dealer said it is front oxygen
sensor that is bad. They will replace it for a total
of $243 (OEM is listed $129.95 so it seems they are using
1.5 hours labor to change it... ) I insisted that if that won't
fix the problem, I get the money back.

They did not say why the problem would be intermittent
if it is bad. Subaru manual doesn't even list oxygen sensor
as a potential trouble for just P0172, so I have my doubts.

Will post back once I know how the car behaves after
sensor replacement.

Sounds like you've got a handle on it. What was their response to
your money back guarantee request?

Over the years though, I've observed many anecdotes of shotgunning in
a new o2 sensor actually fixing the issue curiously often, so be
prepared for them possibly being right.

 

[bigjimpack]

K&N stands behind their product and will pay for repairs caused by a
k&n filter. I bet once you say to the dealer that you want the old
part sent to k&n
you'll get warranty service.

 

[DK]

Sounds like you've got a handle on it. What was their response to
your money back guarantee request?

First a surprise then, after hearing my hypothetical example,
agreement but "only if it is the exact same problem again".

Over the years though, I've observed many anecdotes of shotgunning in
a new o2 sensor actually fixing the issue curiously often, so be
prepared for them possibly being right.

Right. It seems that the O2 sensor was indeed a culprit!
Been 1.5 days and no signs of trouble so far. If the replacement
did not change anything, I would have expected several instances
of misfires/power loss. Good!

DK