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Old 11-30-2011, 09:05 AM   #17 (permalink)
Diesel_Dave
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Quote:
Originally Posted by euromodder View Post
Why don't they keep the EGR valve somewhere under the hood, but not connected to the exhaust or intake ?
I.e. working, but not functional, so it won't throw fault codes.

There's usually no EGR flow measurement, or is there ?
In some cases that can be done, but it depends on the sophistication of the fault detection software. Some ECM's probably just look to see if the valve is moving, others look at intake pressure, intake temp, exhaust pressure, etc. and see if it's consistent with the amount of EGR it thinks it's sending.

In some cases there's a deltaP sensor in the EGR loop that's used to estimate EGR flow. In some cases EGR flow is estimated based on a MAF sensor (fresh air flow) - total charge flow (estimated from speed, IMP, & IMT).

In some cases it has a "primary" estimate based on one set of measurements, and a "secondary" estimate based on another set of measurements. The primary estimate is used for controls, but if the secondary estimate isn't within a certain amount of the primary, it trips a fault code.

It all depends on how the software is set up.
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