1998 S70 P0171 vacuum leak, bad hoses 9155862 Topic is solved

[erikv11]

We're not making it up, by far and away a vacuum leak is the most common cause of the p0172. So with that in mind, this is the explanation I have seen, it makes sense to me:

Yes, the ECU detects a difference between the MAF and O2 signals (a lean condition due to extra, unmetered air), and dumps in more fuel in to compensate. But the compensation is not always instantaneously stoichiometric, you are giving it too much credit, the overall result is overcompensation and ultimately a p0172. I would guess the ECU is set up to err on the side of too much fuel rather than too little, since too lean will kill the engine a lot faster than too rich.

[misha]

Actually no, the ecu adds fuel to reach stoichiometric. It's a closed loop system.

I think that it works this way:
Ecu have a default values of how much air and fuel it needs to maintain proper air fuel mixture.Also it have a default values on how long injectors can be opened and how much fuel they can inject according their capacity and time given.
Every deviation of those programmed values in it's memory is treated like a fault.When it detects a fault for emissions it disables the fuel trim,trying to run the engine at optimal efficiency for the parameters he have at the moment.At this point it turns on check engine light and reports a fault in engine management.

So...it turns on CEL because the engine is not performing,according to parameters from various sensors he gets,like it's programmed to.

Those sensors can give unproper readings and can send wrong signal to ecu because of a bad wiring or a bad sensor itself.Ecu doesn't know if the wiring or the sensor is bad,but it knows what signal it gets from them.

[draser]

The code in question, p0172 pops up after LTFT is past negative 20-25%. For this to happen the STFT needs to be double digit negative for a predetermined time and miles. STFT is driven by the O2 sensor which detects the rich mixture. Now the high negative LTFT means that ECU actually pulls fuel... pretty neat process. Not sure how to reconcile that with cracked elbow, air getting in and ECU overcompensating. And how to explain this code's brother, P0171 for example. Hopefully post author can shed some light with progress on this and maybe a set of live data.

[erikv11]

Yep, that's the summary. Hard to reconcile but true.

If you can grab live data, just unhook the elbow on your car and watch what happens, would be interesting I agree.

[draser]

Why this causes exhaust oxygen too little, hence "bank 1 fuel too rich"? What else to look at? If do smoke test, which hose I should blow smoke into?

Kinda hard to explain, isn't it? Especially because LTFT actually pulls (to get the p0172) fuel and does not add as the consensus seems to be. To your question, if you suspect an air leak just hook up the scanner on live data and take a reading at idle and another at 1500-2000 rpm. If STFT goes from positive to about 0 then you have an air leak. No smoke test needed.

Other causes of rich mixture are ECT and thermostat no working properly and tricking ECU to think engine's still cold. ECT pretty easy to test. The fuel delivery like pressure and leaky injectors would be at top of list. Fuel pressure I can read with scanner on my car (S60).

[draser]

Looking at the PCV tubing diagram, this "elbow" coming from the PTC routes blow by gases from PCV back into throttle body and intake. It it an actual return line, so the flow is from PTC and into TB, having the pressure allowed by the valving of the PCV. So the explanation for the rich code, P0172 is no mistery now. Basically that quantity of air/gases (coming from this pressurized line) does not make it into intake, so the quantity of air is less hence the rich fault. No ECU overcompensating anything.

[erikv11]

You are probably on to something regarding an explanation that makes more sense (remember, I openly state mine was speculation), but the details aren't quite there for me:

The elbow is on the intake manifold (IM), not the PTC. It is on the IM on both turbo cars and on NAs, which do not have a PTC. When disconnected it throws the same codes, NA or turbo. The main function of that line is stated as to provide vacuum at idle to help route crankcase gases through the PCV piping. When the elbow is disconnected at the IM, it is no different from having a hole in the IM, it is a vacuum leak.

Maybe I am not getting it, but this seems different from your explanation?

[draser]

The rerouted blow by gasses have a certain pressure to them (part of the crankcase pressure). This mass of air, missing now due to cracked elbow, is greater than the open port air sucked in hence triggering the rich code. Not to mention that the air sucked in thru such a small port becomes negligible past idle. Past idle the O2 sensor only misses the blow by gases.

[erikv11]

I see, so you're saying a small hole/leak doesn't seem to matter under vacuum, only under positive pressure. Could be, that makes sense.

[erikv11]

Could be, but maybe not. According to VIDA, p0172 actually indicates a lean condition:

From VIDA:

The control module receives information from the HO2S about the fuel/air mixture in the idling and partial load ranges. If the fuel/air mixture deviates from λ=1 the short term fuel trim will compensate this by adjusting the injection timing so that λ=1 is achieved. When the short term fuel trim carries out this adjustment, its mid-point must be adapted by the long term trim. When the long term fuel trim has adapted the short term trim to a maximum value, DTC 2-3-2 (idling range - 0.48 ms to +0.40 ms) or DTC 2-3-1 (partial load range 0.773 to 1.227) and status message Upper Limit (lean fuel/air mixture) or Lower Limit (rich fuel/air mixture) will be set.

So ... P0172 actually indicates a lean condition by advising that the ECU has reached it's upper limit of compensation: It has added as much fuel as it's willing to add to rich up the mixture, but it still seems lean according to O2 sensor feedback.

That is from http://volvospeed.com/vs_forum/topic/16 ... fuel-trim/, the poster, mercuric, knows his stuff.