A little guidance on 0172 "too rich" error, svp?

[jonathan909]

Finally took the '01 Forester out for a proper spin, first since the bearings+rings job.  I haven't fully characterized it yet, but it was pretty damn cranky.  Idle-hunting, stalling, surgy when accelerating at low speed and revs, etc.  Finally popped the "too rich" error - along with 3 misfires, which are clearly the effect of the former.  Fwiw, the dash temperature reading was normal.

Now, I get that this is a closed-loop control system, and I'm pretty good with those - I just need a back-of-the-napkin sketch of what the relevant inputs and outputs are, and a little on the nature of the transfer function.  Can anyone direct me to something reasonable and succinct and understandable to the slightly dense reader?

Worth mentioning that it was running fine before the teardown and nothing was replaced other than some bad pieces of metal, so it wouldn't surprise me at all for the odd slightly crappy connector/connection to be implicated.

[jonathan909]

I'm finally returning to this problem - I've been able to ignore it all summer, but now I have to run it to ground, as this is one of our winter drivers.

Basically, it's running badly at low revs and low load, as described previously.  Too lean is easy - almost always a vacuum leak.  But too rich is a new one for me.  I took a look at live data using one of the freebie android apps and the only thing that looked out of line was the TPS, so I adjusted it.  Other than that, obvious stuff like the ambient and manifold air temp readings looked sane, but beyond that I don't have a baseline understanding of what "in whack" (vs. "out of whack") looks like.  So I'm going through the whole air/fuel path to try to figure out what might cause it.

I replaced the air filter.  This thing has no MAF, so nothing to do there.  Never messed with Sea Foam, so at the suggestion of the parts store guy I blew a can of their "Top Engine Cleaner & Lube" through it.  My expectations were low and I wasn't disappointed.  No change so far.

Injector rail pressure looks a little high compared to the values given in Haynes, though:  They call for 30-34psi at idle, and I'm seeing more like 38.  When I rev it, the pressure jumps to 42-43, falls back to 36, then returns to 38-40 - and it appears that the higher the pressure moves within that range, the crappier (lower) the idle sounds.  Does this sound out of range enough for the regulator to be the problem, or should I keep looking?  Manifold vacuum looks good at 19" Hg - until the rail pressure goes up and the revs drop and it starts running rough - but I get that there's a chicken+egg element here, as the regulator needs vacuum.

If not the regulator, about the next thing I'll be looking at is injectors and the O2 sensor (which isn't throwing any errors, but could still conceivably be misbehaving i.e. out of range).

Any thoughts?

[jonathan909]

Hmm... I'm not sure I'm learning anything here.  I tried swapping in a fuel pressure regulator from the junkpile - the same style as the one in the car.  The result was that the rail pressure actually went up - consistently over 40psi now, though more stable - but it didn't seem to alter the misbehaviour.  As soon as the engine warms up and switches out of cold idle, it runs rough.  One interpretation of this could be that the problem is that the pressure is still too high, but I'm skeptical about that, since I've never had a regulator problem before.  Am I on the right track?

[GeneralDisorder]

Subaru's target standard GM pressure across the injector which is 43.5 psi. 19" of vacuum is 9.3 psi. So you should be seeing 34.2 psi fuel pressure at idle. It will jump momentarily as manifold vacuum drops on a throttle stab.

If the regulator isn't able to regulate down then that would seem to indicate potentially a restricted return line. 

GD

[jonathan909]

Ah, okay... so the regulated pressure is simply the difference between the unregulated and manifold vacuum.  Easiest way to check, I'd guess, is to pull off the return hose and direct it into a bucket.  If that's the problem, can I just blow out the return line, or will something at the tank interfere or be damaged?

 

[jonathan909]

I'd put yet another junkbox regulator on to see what I got, and it's behaving much like the original one, though the idle has improved.  With the return line disconnected and dumping to bucket, there's good flow and 36psi on the rail, which didn't change when I reconnected to the return line to the tank (making a restriction there unlikely to be the problem).  So I'm sensing that maybe the problem here is marginal, perhaps from a slightly low manifold vacuum.  If the book's calling for 30-34 psi, and I'm seeing 36 (via an admittedly cheap and uncalibrated gauge), would the takeaway be that once one gets much over 36 the ECU can't shorten the injection time enough and it starts to run too rich?

[GeneralDisorder]

Well the ECU gives fuel for the measured airflow (on cars with a MAF it's measured directly, on cars without it's calculated from MAP and IAT). It only knows how long to open the injector based on it's assumption of fuel injector flow rate and known fuel pressure of 43.5 across the injector. If the O2 sensor says it's too rich on the exhaust side the feedback loop starts to "trim" the fuel by applying a percentage reduction to the injector pulse width. The ECU is perfectly capable of reducing the pulse width all the way to the minimum allowed, which would probably be more than a 50% reduction in idle pulse width (depends on the injector size and characteristics that are generally not published with OEM injectors). The code is thrown once the trim value exceeds the limits set in software - which is generally some period of time above 25% reduction (so -25% fuel trim). This will then trigger the P0172 code indicating the computer believes something is dramatically wrong. 

GD

[jonathan909]

Time to close the loop on this - I had too much going on last fall and it sat over the winter.

In retrospect, I'd run down a rabbit hole and gotten lost, thinking that out-of-range manifold vacuum might be affecting injector rail pressure, and that bad vacuum may be a product of maladjusted valves - all of which was just getting too arcane.  So I backed up, considered that the only source of parametric combustion feedback is from the O2 sensors, and that (despite them not generating any error codes) it would be stupid not to replace them both on spec - fastest and cheapest thing to try.  So I just did that and now it's as happy as a baby in a barrel of t!ts.

Edited May 28, 2022 by jonathan909

[heartless]

sometimes it is the simple things

and O2 sensors can quite often not throw a code right away.. been there