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

Messages
4,709
I have heard of this. To change the oil to fresh oil before you get the car inspected. I can't for the life of me think of why that would help.

What is the deal with that? (And has anyone else ever heard this advice. Same as, say, put Premium gas in it before you go.)

All they do on 1996 and newer cars is read the OBD2 so I am suspicious of why changing the oil would help... This is gonna be on a 2004.. I am locked in familial debate about this as we speak, thought I would get an answer.

Let's assume emissions check.
 
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7,116
Location
Roanoke Virginia
I don’t know. We don’t do emissions tests in my part of the state I’m in. Only the northern part of the state does the emissions tests. That’s the only reason I could think of it.
 
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1,670
Location
California
I could see a difference if the new oil is a lower viscosity (0/5w20) and the vehicle requires a sniffer (emissions) test. But even then, this would only be needed if there was a reason to believe the vehicle will not pass the sniffer test. And in that case it needs proper repairs, not thinner oil.
 
Messages
319
Location
Atlanta, GA
If this is just a OBD2 plugin emissions check its not gonna make a lick of difference whether oil is fresh or has 20k miles on it (well unless the neglected car gets a CEL due to neglect).

We have a similar annual emissions check in our area and it involves an OBD2 plugin, they test the gas cap separately then mirror under the car to make sure the cat is still intact. Its been a few years since I have had to get one done (diesels are exempt) but honestly I don't think any of them even popped the hood.
 
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6,640
Location
Suburban Washington DC
Old wives tail. People who did it and it failed will never admit it. Irrelevant.

If you ever do an oil change on an older car or one that hasn't had the oil changed in a long time, take a whiff of the drain pan. Likely get a strong gas smell. Good old hydrocarbons. And what does a tailpipe test check for? Excess hydrocarbons.
 
Messages
17,271
Location
Upper Midwest
If you ever do an oil change on an older car or one that hasn't had the oil changed in a long time, take a whiff of the drain pan. Likely get a strong gas smell. Good old hydrocarbons. And what does a tailpipe test check for? Excess hydrocarbons.
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.
 
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5,532
Location
down in the park
All cars here get a physical sniff test. Lambda value has to be withing the 0.997/1.003 range and CO has to below a threshold depending on the first registration date. 2% for old non catalysed cars I think. 0.35% for recent cars. some at idle, some at higher rpm...

Diesels get revved to redline and the opacity of the exhaust gas gets measured.
 
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