96 850 Turbo - P0172 Won't Go Away

"System too rich (Bank 1)"

I've been fighting this thing for a while and frustration is building.
It can take up to ~240 miles for the code to come back when reset, less
when it's warmer out.

Based on the code description and what the dealer was planning to do I:

(1) replaced the Mass Airflow Sensor.

When that didn't work I discovered this group and tried the following:

(2) Replaced all of the rubber vacuum elbows even though most of them
seemed fine.

(3) Found and repaired a split plastic hose running under the front of
the car to the charcoal canister (looked to be coming from the fuel
tank).

(4) Replaced the rubber vacuum hoses on the driver's side of the engine
which looked pretty ratty but didn't have visible splits or holes.
(Color coded Red, Blue, Yellow and White.) The hard plastic vacuum
hoses that I found all seem to be in good shape.

Any other ideas? Things I can easily check?

If I can find someone with OBD2 software will I be able to learn
anything by looking at the snapshot data when the code was triggered?

Thanks in advance!

 

There's not much else that can cause this code. If the air mass meter
has been changed, and you're sure there's no vacuum leaks (intake
manifold gasket, injector seals etc.) then one other cause can be the
fuel pressure (weak pump or defective regulator). And if there's any
leak in the exhaust between the engine and oxygen sensor, air can get
sucked in tripping this code.

Some garages have a machine that generates smoke. This is connected to
the intake, which is slightly pressurized, and then the leaks are
obvious where the smoke leaks out.

Also, weak fuel pumps generally give a symptom of extended cranking
before starting, especially when the car has not been used for more than
5 hours. P0172 generally indicates high fuel pressure, if that's the
cause.

An OBD tool that captures data may help - it certainly can't hurt.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Even the one behind the power steering pump that you can hardly see?

Any unmetered air entering the intake stream after the mass meter will
drive the system rich, i.e. intake manifold gasket, crack or split in
the intake bellows hose, exhaust crack or leak near the O2 sensor.

Possible cam timing issue.

When checking/replacing the vacuum hose on the fuel pressure regulator
was there any whiff of gasoline? Any trace will justify condemning the
regulator. Hang a gauge on the fuel rail, the regulator can stick
intermittently, and see if you can catch it raising the fuel pressure
when the MIL comes on.

Make sure the coolant temperature sensor is sending a valid reading to
the control unit.

Only if you can identify the bad reading from a component you already
suspect. Typically what you'll see is a collection of all the bad data
that were present when the code set. IOW, O2 readings will be rich, long
term fuel trim will be cranked all the way lean, short term fuel trim
may be adapted lean as well, injector dwell or frequency will be high,
manifold vacuum will be low, barometer may be inaccurate, MAF signal
will be rich, load signal may or maynot agree with vacuum readings
depending on throttle position switch interpretation, and so on and on.

Mixture codes are more typically from the electronics reporting a
mechanical problem.

Bob

 

<snip>

Can't a failing oxygen sensor cause that code? And surely a flaky
engine computer could cause that code or nearly any other code.

 

The ECU can tell the difference between "too rich" and a failing sensor by
the way the sensor works. When cold (or completely dead) the sensor appears
as an open circuit, and the .45 volt bias from the ECU is seen on the sensor
line as a good indication the O2 sensor has gone bye-bye. When the sensor is
working and the mixture is too lean, the output is around 0.1 volt, and when
the mixture is too rich it is around 0.9 volt DC. The ECU uses that
information to dither the mixture rapidly, slightly rich to slightly lean
and back again, hopefully 7 transitions per second or more. The sensors
don't usually die altogether, they just get weak and sluggish so the sensor
is slow to respond.

The ECU could produce the code by being insane, but that would be a pretty
unusual failure. ECU failures are rare enough, and the ones that aren't
caused by the ECU getting wet tend to be the power handling areas going bad.

If the code is "real" the ECU is saying that it is trying to keep the
mixture under control by slaving to the O2 sensor transitions, but the "ded
reckoning" part of the ECU is saying there should be more fuel. This is
often called being "out of trim."

This brings us to Mike's observation that the fuel pressure being high can
cause the code. The ECU calls for a certain amount of fuel to be injected,
but the high pressure forces more fuel through than expected... too rich.
The rest of the possibilities seemed to me to be backward - causing the O2
sensor to think the mixture was too lean. But I get confused about things
like that lately.

But I do wonder if an injector could be leaking into the manifold or slow to
close after a while. I am reminded of the mistake I made in buying
aftermarket injectors for our 85 765T. The first set was the wrong ones
(they were for a non-turbo) and the second set also failed to make the
engine run decently at all... barely idled when cold, perhaps 20 hp max. I
eventually took it to the dealer where it was run through the tests and
parts swaps. New AMM didn't help - the mixture was still way rich. The
injectors flow tested right on the money, but when Volvo injectors were
installed the AMM adjusted right up. Something wrong with them dynamically,
I guess.

Mike

 

I have to agree, I don't see how a vacuum leak would result in a "system
too rich" error, but it does. I suppose it must have something to do
with the way the system reacts to small "test changes" in injector pulse
width, or something like that.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Interesting - do both intake leaks and exhaust leaks do that?

Mike

 

Well according to the book they do, but I've never personally
experienced an exhaust leak that set a mixture code. I have seen intake
leaks that set codes for both too rich and too lean. And on my S70, the
infamous rubber elbow behind the power steering pump was bad, I noticed
it without any codes being set.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)