Why would changing motor oil to fresh oil before inspection be helpful?

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6,640
Location
Suburban Washington DC
The tailpipe test checks the tailpipe, not the oil pan. Does the emissions test facility stick their probe down the dipstick tube?

That's the part I struggle with, what connection there is between emissions testing of the exhaust and how that's influenced by the oil. Even "bad" oil.

You heard of the PCV system? Where crankcase vapors get recirculated back into the intake to get burned in the engine. Since the engine isn't 100% efficient, some of that is unburned and goes out the tailpipe. The more you start with, from old oil, the more will get sniffed in the tailpipe test.
 
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17,271
Location
Upper Midwest
Wouldn't the PCV contribution be caused by blow-by?

I'm struggling with how much oil is being burned in this hypothetical engine as opposed to the tailpipe sniffer being influenced by the products of combustion.
 
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1,692
Location
VA
Somebody on here once suggested Redline 10W-40 to help my Saturn pass Nevada's visible smoke test. I tried it and did pass, but I think that had more to do with taking a long drive before the test. When it failed, the engine wasn't hot because I drove straight to the testing station. I also did the retest on a windy day.

I was interesting that in Nevada, the shop that did your emissions test was not allowed to do any emission repairs if you failed. In Virginia, a shop that does your safety inspection can also repair what they find defective.
 
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2,053
Location
Sequim, WA
Wouldn't the PCV contribution be caused by blow-by?

I'm struggling with how much oil is being burned in this hypothetical engine as opposed to the tailpipe sniffer being influenced by the products of combustion.

What's going to provide a better ring seal, fresh clean oil or dirty fuel diluted oil? Ring seal effects both blow-by and oil left on the cylinder walls.

There have been numerous threads on here over the years where people commented that their vehicle used more oil later in the OCI than earlier.

Ed
 
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