Engine: B5244: S80 OBD Code P0172

[KBB]

So are you going to look ate freeze frame data and then the STFT at idle and part load? It will give you a clear understanding as to what the nature of the ECUs fuelling command.

[Bernard46]

OK here goes. I'm using a Creader VI and the freeze frame readings were as follows:-

Fuel System Status 1 - OL-Drive
Calc Load - 27.1%
Engine Coolant Temp - 179F
RPM - 935
Speed - 37MPH
Inlet Manifold Abs Pressure - 9.0 inHg
LTFT - -23.4%
STFT - 0%

I should perhaps stress the P0172 code lit on the dashboard at engine startup (it always does), not while the vehicle was running - this seems to conflict with the 37 mph above but maybe I'm misinterpreting this data?

In addition after a 6 mile run and while the engine was idling at 750 rpm the LTFT was around the -20% mark with the STFT varying between 0 and -10% with the Manifold Abs Pressure pretty steady at 12 inHg.

Can you draw any conclusions from this data?

[Bernard46]

Since the last message I cleared the code and went for a 15 mile drive - stopped half way and the code came up again. From memory the Freeze frame details were

Fuel System Status - CL
Calc load - 37%
RPM - 749
Speed - 0 MPH
Inlet Manifold Abs Pressure - 11.7 inHg
LTFT - -22%
STFT - -10%

I went out to the car to confirm my memory and started it up only to find it had cleared the code itself!! So I cannot confirm the above.

[Bernard46]

Just on the off chance that everyone hasn't got bored out of their mind now that there is some real data around I've got another couple of points to add.

I reset the code and drove several short trips - no more than 6 miles or so for each one. I came back from one this morning and having nothing better to do decided to take a look at the code reader and what it might tell me about how the car was running - I had no message on the dash and no lights on at this point. I mused around with it for a few minutes and then got bored so switched off and went inside for lunch.

Came out after 90 minutes (engine still warmish) and switched on - I was amazed to see the "Emission system service message" and the engine light on the dash. Remember this car had been nowhere since I had been looking at it one and a half hours earlier when it had shown no sign of any problem on the code reader or the dash.

The message had come up either just before or within a second or so of me starting the engine. Even more surprising the freeze frame data showed the problem as occurring when the car was being driven at 38 mph!!! This vehicle had been totally at rest for one and a half hours and prior to that there had been no DTC codes visible. Is it possible it only reports the codes some time after they have occurred - surely not?

Anyway for completeness the pics from the code reader are attached.

The STFT figure of 0 is higher than I had seen on previous occasions when it has tended to be -10 or so. The LTFT is about the same as on previous occasions at -20, but the Intake MAP is somewhat lower than previous occasions at 6.6 inHg (it was around the 12 figure before). The mph figure is amazing (but not inconsistent with the rpm) - the car was actually stationary!

Can anyone shed any light on what's going on here - why is the code reported late and what exactly is triggering the P0172?

[KBB]

The use of live data in diagnostics is a very useful tool. Your first issue may well be the quality of the scan tool itself. Cheap ones have been known to give out poor info and with a poor response time. Live data is processed data in short the ECU reads all the various sensor inputs which will vary from digital signals to analogue signals, processes them and the puts them out on the K Line as various measurements. For example a MAF sensor dose not measure the mass of air entering an engine alone, it measures the flow of air entering the engine, the ECM receives two other critical inputs from the MAP sensor at key on which gives the barometric air pressure and the MAF will also give a analogue signal for the inlet air temperature. The Ecm will perform a calculation to establish the mass of air entering the engine and put this out on the K line as a specified measurement of the mass of air either in gms/second, kg/hour, lbs/min or lbs/hour.

For live data to be trust worthy the ECU must go into Closed Loop Control. Some fault codes will prevent the ECU from entering into CL and will display the status as Open Loop (OL). O2 sensors and Dead MAFS will trigger and OL status and under this condition the will be no STFT or LTFT commands, instead the data put out by the ECM will be substituted values which at best are no good for diagnostics. It is at this point where technician's will evaluate the various sensor out puts with an oscilloscope to see exactly what signal values are being sent the ECM.

[Bernard46]

I'm a mathematician by background and have 35 years experience in the computer industry so I'm reasonably familiar with data, sensors, computing devices and how they work. What I'm looking for is some insight into what the normal readings for this vehicle should look like and therefore where the deviations are - that way one can start to home in on the problem. I accept the tool may be defective but again I've no reason to suspect that - its a Creader VI by the way. Also I don't know whether it would be reasonable for this particular model (MY2001 S80 petrol) to hold off showing a DTC until it had done some additional testing (perhaps the problem has to occur several times and codes are only shown on engine start-up?).

Are you able to tell me whether the ECU will hold off displaying diagnostic codes until it has conducted several tests and is likely to shown them only at engine startup?

Would you care to comment on the readings I have shown above and earlier in terms of whether they look 'normal' or are they all 'wrong looking' or just partially odd?

Any specific help would be appreciated.
Thanks

[KBB]

Whilst in closed loop you have a total fuel correction facto of -32%. In short the 02 sensors are reporting a rich condition and are requesting 30% less fuelling command than what the ECU is pre calculating on input signal data.

Fuel command is based on MAF signal input, Air temperature input, barometric pressure and drivers pedal position command.

With scan tool connected key on engine off look at manifold pressure value and air temp value.

With the engine running at idle and with a DVM back probe the MAF signal wire and measure the signal voltage being produced by the MAF.

This will be a good start.

[Bernard46]

Thanks for that feedback - the car was back in the garage today since they had an opportunity to speak with a Volvo technician and although I wasn't able to speak with the guy who did the work I was told they cleaned the ETM plus some other things, didn't find any vacuum leaks and believe they may have resolved the problem. As a result they have asked me to drive it for a few days and see if the problem returns (in the past it has returned within 24 - 36 hours of a code reset). Right now I don't feel in any shape to take it out since I have a stinking cold but hopefully I'll be feeling better tomorrow. I'll do as they ask and then whether or not the problem returns I'll go back and have a chat with the guy who did the work to find out what he actually did or else to pass on your feedback. Either way it will probably be a day or two before I can report back further. Thanks for your continued interest.

[Bernard46]

Well 2 weeks on and lots of data later plus several trips back to the garage and I'm no nearer a solution. To summarise -

The code always comes on when the engine is started, although I believe it is a pending code prior to this.

The code does occur in Closed Loop ocassionally as well as mainly in Open Loop.

The car passed the UK emissions test with flying colours again this afternoon despite having had the code come up after every single journey following a reset (20 times or so now) - all emissions values are excellent.

On two long (> 2 hour) high speed trips this past weekend the code came up after each journey, although fuel consumption was excellent, no mis firing, spluttering or other reasons for any concern.

Duting both journey (which involved nearly 2 hours of 80ish mph cruising) I had a live scan data recording going to my iPad - my passenger was able to see that when accelerating or climbing hills the car was asking for more fuel 12% LTFT, while decelerating or descending steep hills and no throttle the car was trying to reduce fuel (-21% LTFT). While crusing with steady accelerator pressure LTFT tended to be in the 8% range. LTFT would change instantly when the throttle position was changed, rather than after any delay.

Overall on both journeys only 25 - 30% of LTFT readings were within the -5% - +5% acceptable range.

The O1B1 was tightly clustered in the 0.4 - 0.6 volts range with most values in the 0.47 - 0.52 range. The O2B1 sensor was mostly in the 0.75 area although it did fluctute wildly betwen 0.1 and 0.8 regularly.

I was only able to monitor 3 sensors at a time, so that is all the data I have.

I had one strange occurrence this afternoon when after a 4 hour delay I started up and the car struggled to keep going - it seemed like it was firing on only 3 or 4 cylinders and when I put it in drive (auto) it stopped. I restarted and went through the same performance but after 10 seconds it cleared and ran perfectly except that after a further few seconds there was an phenomenal stench of petrol inside the car (from the air con vents).

So in summary I have a more or less permanent Emissions service required light on, the car runs perfectly apart from the ocassional start, fuel consumption is fine and just like the previous 15 years, state emissions limits are easily met, when accelerating it says it needs more fuel, when decellerating it says its needs less fuel. All in all its a total mystery as to why the light and message are displaying.

[KBB]

I have taken a little time to read through your post here and the other one and looking at the lived data provided. With a P0172 it can be a very easy conclusion to blame the MAF sensor, for me at this stage their is not enough evidence. Contaminated MAF sensors often exhibit an over calculation of air at idle leading to over fuelling with high negative trim values but under load they under estimate air flow and under fuel leading to high positive fuel trim values.

Looking deeper at your live data at idle their is a severe over fuelling but under cruise condition the LTFT is sitting around 8% which is well under the threshold of +20% that would trigger a P0172 DTC.

The other key clue is that under deceleration when the fuel system goes into over run and would reduce or shut off fuel injection delivery you are seeing LTFT -21%.

Idle and deceleration conditions produce maximum low manifold pressure (vacuum) conditions. There are two specific faults that will exhibit over fuelling conditions specifically at low manifold pressures and that is a leaking fuel injector or an open EVAP vent valve in the fuel tank evaporative control system.

To quickly eliminate the EVAP system locate on the manifold where the EVAP pipework fits, disconnect it and plug the manifold side of the connection point. Run the vehicle at idle and look at the fuel trim values and see if the come down.

To test for the leaking injector the simplest way is to fit a fuel pressure test gauge to the shrader valve test port, turn key to position 2 to prime the system. System pressure should hold for 20 minutes. This fault would also support your noting of a heavy smell of petrol in the engine bay at start up.