Change oil before getting a smog test?

[berniedd]

I was just wondering why many in the forums opine/recommended to get an oil change prior to a smog test? I understand the benefits if an oil change is already due, but if I have still-good oil in the engine, how would that influence the smog test results? And didn't I just read somewhere in BITOG that a new oil in the engine actually may increase wear rates slightly till it has aged a little?

Maybe it's just a precaution so that folks won't forget that scheduled oil change?

 

[yonyon]

Some people will recommend changing your oil to cure a hangnail or a headache or genital warts or pretty much anything. I guess it might help with the headache.

Realistically, this is a valid thing to do if you've got a carburetor and a PCV system.

 

[Danh]

If your car is new enough to have an OBD port (around 1996 or later), a smog test typically consists of plugging into the port to see if any emissions codes are present, pending or have been recently erased. If you don't have a check engine light illuminated you should pass; an oil change would make no difference.

 

[Chris142]

It does make a difference. Seen it myself several times

 

[Chris142]

Originally Posted By: Danh
If your car is new enough to have an OBD port (around 1996 or later), a smog test typically consists of plugging into the port to see if any emissions codes are present, pending or have been recently erased. If you don't have a check engine light illuminated you should pass; an oil change would make no difference.
depends on the state. Here all cars get ran on the dyno regardless of age

 

[Bamaro]

I cant see how it will make any difference.

 

[eljefino]

Used oil always has about 1% gas floating around in it, the HCs it gives off about equal the new HCs it absorbs.

New-that-day oil soaks up more than it lets out. At least that's the theory, which seems plausible.

 

[Mr Nice]

Luckily we don't have any emission testing in Florida.

Nor do we have yearly inspections or taxes on the value of our vehicle to renew tags.

 

[mikered30]

Just drive it really hard before a smog test, no oil change necessary.

 

[147_Grain]

Fresh oil creates a better seal between the piston rings and cylinder wall, resulting in lower emissions.

 

[earlyre]

it's magic!
if you go in after a fresh oil change, the oil fairy will wave her magic wrench, and make the computer pass your car on the first try!*






*not valid outside mystical realms.

 

[martinq]

Originally Posted By: Chris142
It does make a difference. Seen it myself several times

Same here.

I use the test as a marker for maintenance. Before I go in I change fluids, check/swap the plugs, fresh gas, etc.

 

[spk2000]

Would be interesting to see evidence presented on same vehicle results before and after an oil change using the same equipment. This would then solve the theory. Someone should contact Mythbusters.

 

[yonyon]

My "new" car saw the HCs drop from 71 to 68 after an oil change two weeks ago. I then used the car for a week (my commute is just over 2 hours) and the HCs came down to 27. 27 is less than the applicable limit of 300, so I ran it and passed without an issue.

There was a change when the oil was replaced, but I can't say whether or not it's enough to matter. This was a no-load test as we no longer check emissions on the dyne in NJ.

 

[martinq]

Originally Posted By: spk2000
Would be interesting to see evidence presented on same vehicle results before and after an oil change using the same equipment. This would then solve the theory.

Used to have a used-car dealer bring over cars for testing every week. If one failed, but was close, they'd do an oil-change and bring it back in. 90% of the time it'd pass second time with the fresh oil.

Originally Posted By: spk2000
Someone should contact Mythbusters.

That's a perfect one for them to test. Easy, straightforward and can be done in a day. Could also test other things like air-filters and spark-plugs, etc.

 

[yonyon]

...and at the end they can see if parking the car in the desert on top of 50 lbs. of explosives does anything for the emissions.

 

[martinq]

Originally Posted By: yonyon
...and at the end they can see if parking the car in the desert on top of 50 lbs. of explosives does anything for the emissions.

Favorite episode ever! When is it on!??

 

[Errtt]

If used high mileage oil in the crankcase contains byproducts saturated, some fumes routed through/into the intake, wouldn't that have an effect.
Fresh oil change, less byproduct, unsaturated byproduct fumes routed to intake, better results. No?

 

[crazyoildude]

put in something in the gas that guarantees it to pass if its pre 1996 i saw it work plenty of times.. its illegal in some places it works so good.

 

[bdcardinal]

Originally Posted By: Chris142
Originally Posted By: Danh
If your car is new enough to have an OBD port (around 1996 or later), a smog test typically consists of plugging into the port to see if any emissions codes are present, pending or have been recently erased. If you don't have a check engine light illuminated you should pass; an oil change would make no difference.
depends on the state. Here all cars get ran on the dyno regardless of age


Santa Barbara County still gets the TSI test. But ya, any way you cut it your cars tailpipe is getting violated every couple of years.