On cars with no lamdas egr flow is measured with air mass difference only ..... it has no other way really , even if egr valve has built in tracking switch to monitor its position the real flow is measued by air mass difference .
But yes ,with lamdas it can use the difference in oxygen to monitor egr also .
Where you are getting actual egr flow more than desired is correct for your circumstance , because you have no vac to the egr the valve will be closed so egr flow will be higher
Doesn't sound right does it ....it's not measuring flow through the valve it's really measuring air mass difference , open egr lower air mass ..... closed egr more air mass .
So when you are driving on say light load ecm will be commanding egr to open let's say 35 % duty , but because there is no vac to egr it cannot open so air mass will always read as higher than actual or egr higher than actual ..... it's just the wording gets confusing as like you say how can egr be open with no vac and a strong Spring holding it back .
If you say your lamdas are fixed at 99.9 % for all the time you have had it , the ecm must be using the lack of air mass change to work out that egr is not opening thinking about it .
Does your car have dpf ?? lamdas on the diesels are very often used for regeneration control and ensuring pre dpf or oxidation cats are up there efficiency threshold as they assist in getting regen temps up .
Diagnostics is a field I enjoy doing in my job ...... specifically non intrusive diag ....pico scope , in cylinder pressure transducer waveform diag ..... something I'm just getting to grips with .
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