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P0172 and P0175


wild_one
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1 hour ago, wild_one said:

Running rich at tickover and lean when throttle opens.

Leaky fuel injector? Allowing too much fuel in at low power, the ECU adjusts the fuel trims down to compensate, then it will run too lean under power.

Could also be MAF as you suggest, if it read high at low airflow, the ECU might have to adjust the fuel trims down to match the actual air flow, then again it would run lean as the airflow increased, and the MAF read nearer to correct. MAF sensor elements covered in oil & dirt could cause something like this. Some engines will run ok with the MAF disconnected, and can run better like that if the MAF is faulty.

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1 hour ago, Tdci-Peter said:

Leaky fuel injector? Allowing too much fuel in at low power, the ECU adjusts the fuel trims down to compensate, then it will run too lean under power.

Could also be MAF as you suggest, if it read high at low airflow, the ECU might have to adjust the fuel trims down to match the actual air flow, then again it would run lean as the airflow increased, and the MAF read nearer to correct. MAF sensor elements covered in oil & dirt could cause something like this. Some engines will run ok with the MAF disconnected, and can run better like that if the MAF is faulty.

I was wondering if running with the MAF disconnected might prove the MAF good/bad quickly?

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26 minutes ago, wild_one said:

I was wondering if running with the MAF disconnected might prove the MAF good/bad quickly?

It is not a guaranteed test, it might refuse to run, or be much worse without it. But if it does seem a bit better, then the MAF does become suspect number 1. You will need the code reader to check for the MAF DTCs that should appear, and to reset them after the test, or it could get rather confusing.

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14 hours ago, Tdci-Peter said:

It is not a guaranteed test, it might refuse to run, or be much worse without it. But if it does seem a bit better, then the MAF does become suspect number 1. You will need the code reader to check for the MAF DTCs that should appear, and to reset them after the test, or it could get rather confusing.

I've got rich codes not MAF specific ones. Problem is, that the a MAF can be faulty (giving erroneous signals) but not actually be operating out of its design parameters so codes are not set

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4 hours ago, wild_one said:

I've got rich codes not MAF specific ones.

You will get MAF specific DTCs if you do the disconnection test. The ECU will certainly notice it, that is the idea. It will then revert to some in-built default strategy, which would be less efficient or less accurate than a fully working MAF, but just might be better than a out of calibration MAF.

It is certainly true that error codes saying one thing might be caused by something else. The ECU, and the programmers who coded it, have to make assumptions, and they are not always right. Years ago, my Vauxhall told me (via codes) that its MAF was faulty, I did not believe it, and it was actually the idle control valve. But ECU could not monitor the ICV, so it did not know where the fault was.

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17 hours ago, Tdci-Peter said:

You will get MAF specific DTCs if you do the disconnection test. The ECU will certainly notice it, that is the idea. It will then revert to some in-built default strategy, which would be less efficient or less accurate than a fully working MAF, but just might be better than a out of calibration MAF.

It is certainly true that error codes saying one thing might be caused by something else. The ECU, and the programmers who coded it, have to make assumptions, and they are not always right. Years ago, my Vauxhall told me (via codes) that its MAF was faulty, I did not believe it, and it was actually the idle control valve. But ECU could not monitor the ICV, so it did not know where the fault was.

I had a check this morning with a hot engine at idle my MAF was showing between 0,99 and 1.13 g/s which seems a little low for a 1.6ltr engine?

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On 11/8/2018 at 1:49 PM, wild_one said:

I had a check this morning with a hot engine at idle my MAF was showing between 0,99 and 1.13 g/s which seems a little low for a 1.6ltr engine?

At a quick look, the 1.6l engines do not usually have an EGR. If so, then 1.13g/s does sound low. Assuming a 30kPa abs. manifold pressure (figures from an older mk1 1.6, but it is unlikely to be very different), then at 850 rpm, the intake air flow could be around 4 g/s: (1.6l /2 x 850/60 x 1.2g/l x 0.3Bar). This is quite approximate, it does not account for valve timing overlap, intake restrictions, etc.

I have just recalled that the 1.6l tivvt engine uses its variable valve overlap to perform an EGR function, but even so, it won't be 75% EGR to 25% fresh air. Maybe 50/50 at most. So that would still be about 2 g/s.

 

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  • 3 weeks later...
On 11/9/2018 at 4:15 PM, Tdci-Peter said:

At a quick look, the 1.6l engines do not usually have an EGR. If so, then 1.13g/s does sound low. Assuming a 30kPa abs. manifold pressure (figures from an older mk1 1.6, but it is unlikely to be very different), then at 850 rpm, the intake air flow could be around 4 g/s: (1.6l /2 x 850/60 x 1.2g/l x 0.3Bar). This is quite approximate, it does not account for valve timing overlap, intake restrictions, etc.

I have just recalled that the 1.6l tivvt engine uses its variable valve overlap to perform an EGR function, but even so, it won't be 75% EGR to 25% fresh air. Maybe 50/50 at most. So that would still be about 2 g/s.

 

Thanks

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  • 2 years later...
On 11/9/2018 at 4:15 PM, Tdci-Peter said:

At a quick look, the 1.6l engines do not usually have an EGR. If so, then 1.13g/s does sound low. Assuming a 30kPa abs. manifold pressure (figures from an older mk1 1.6, but it is unlikely to be very different), then at 850 rpm, the intake air flow could be around 4 g/s: (1.6l /2 x 850/60 x 1.2g/l x 0.3Bar). This is quite approximate, it does not account for valve timing overlap, intake restrictions, etc.

I have just recalled that the 1.6l tivvt engine uses its variable valve overlap to perform an EGR function, but even so, it won't be 75% EGR to 25% fresh air. Maybe 50/50 at most. So that would still be about 2 g/s.

 

Im an apprentice mechanic at the moment and trying to find some info on focus with a P0172 code. Curious about your formula to work out MAF but cant figure out the 1.2g/l. Any help would be appreciated 

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56 minutes ago, Thor2371 said:

Curious about your formula to work out MAF but cant figure out the 1.2g/l.

It is the density of normal ambient air.

https://en.wikipedia.org/wiki/Density_of_air

1.2kg/cubic metre = 1.2g/l.

This density reduces proportional to the absolute pressure as it goes through the throttle into the inlet manifold "vacuum" (Boyles Law, and assuming no significant temperature change at that stage), my 0.3Bar is a bit of a guess. There will however be a significant temperature drop when petrol evaporates in the inlet manifold and cylinder during the intake stroke. This will vary a lot, and will be counteracted by heating from the hot cylinder head, piston and cylinder. So a full analysis would be very complex, but in this case I was after a ball park figure to see if the quoted flow was anywhere near reasonable.

The amount of valve timing adjustment available on these VVT engines can retain quite a lot of exhaust gas in the engine, it serves instead of an EGR. Any gas retained / re-circulated like this (either in a real EGR or because of valve timing) counts in the 1.6l/2 total piston displacement per rev, but doe not come from the outside air, so does not go through the MAF.

Some manufacturers have a flow measurement orifice (or venturi) and transducer in the EGR circuit to properly measure the rather critical EGR flow, but Ford don't, at least not on any common UK car. The only way the ECU can estimate the EGR flow is to subtract the MAF flow from this calculated piston diaplacement based flow, and subtrcting two numbers with a significant error or uncertainty in each can lead to a big uncertainty in the difference. But it seems to work reasonably well most of the time, I suppose!

I have used Forscan to plot out the MAF and a calculated EGR flow from MAP & RPM on my Diesel, and it seems to stack up as well as I would expect. Air flows and EGR flows are usually greater on a Diesel due to the lack of throttle, and lean burn conditions (lots of air, and very little fuel at low engine power.)

Unfortunately, all the guesswork and approximation in this type of calculation make the answers so rough that they can only detect gross errors, minor leaks and calibration errors would not show up.

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