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Well, I did get the in-tank pump fixed, and I did replace the O2 sensor. The retest had NOX around 495 - just under the limit (although some of the other items actually got worse, but not bad enough to fail).
Subsequent to all this, I noticed one day that the vents for the heating system didn't seem to be engaging predictably. Bingo, maybe? A leak in one of the servo diaphrams would allow unmetered air to be constantly pulled from this system and into the intake manifold.
When I tested for vacuum from the line under the hood that goes through the firewall and on to the heater control system, I did get a leak in all but the defrost-only position. I took apart the dash and tested each of the vacuum lines from the heater controls. All were airtight, with the exception of the yellow line. As per the FAQ, this is the line that activates full floor flow (and thus turns off defrost). By detaching that line from the servo and plugging it, I no longer get a leak when testing from under the hood. As pointed out in the FAQ, the downside is that defrost never gets fully turned off, hence there will never be as full a flow to the floor.
Is it reasonable to think that such a leak would cause enough airflow into the intake manifold to impact significantly on the NOX reading?
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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto
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