# Worn main engine bearing and emissions **URL:** \ **Category:** Factual Questions\ **Created:** [August 30, 2017, 3:42am UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075 "2017-08-30T03:42:49Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [August 30, 2017, 3:42am UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/1 "2017-08-30T03:42:49Z")
I have a truck that I need to register. Problem is it has a worn main bearing and it needs to pass emissions before I can register it. Assuming nothing else wrong with it, would a worn main bearing affect the emissions test? Note: I’m going to replace the engine but I need to get it registered before I can get the new engine. ---
**Author:** ![mind\_the\_gap](https://avatars.discourse-cdn.com/v4/letter/m/ea666f/32.png) [@mind\_the\_gap](https://boards.straightdope.com/u/mind_the_gap)\ **Post date:** [August 30, 2017, 3:53am UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/2 "2017-08-30T03:53:43Z")
If it were me, I’d do a compression or leakdown test, look at the plugs, and if all else looked ok, I’d go for it. Thing is, worn mains usually don’t happen without other things being worn, like valve guides and seals. Those can leak some oil but that’s normally visible upon acceleration after stopping. Pistons, rings and cylinders all may be worn to a fair degree. Again, any oil burnt is usually visible when driving. I’ve seen cars burn oil and pass smog as weird as that seems. All told, it’s a bit of a gamble. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [August 30, 2017, 4:06am UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/3 "2017-08-30T04:06:05Z")
If it’s just a worn main seal, that won’t affect emissions - it’s in the crankcase, which is after any of the emission control stuff (other than EGR, which shouldn’t be affected). ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [August 30, 2017, 4:09am UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/4 "2017-08-30T04:09:14Z")
> [@Saint\_Cad](#): > > snip Assuming nothing else wrong with it, would a worn main bearing affect the emissions test? snip Worn main bearings would have no effect on an emissions test. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [August 30, 2017, 9:43am UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/5 "2017-08-30T09:43:05Z")
> [@ZurBob](#): > > Worn main bearings would have no effect on an emissions test. Agreed. ---
**Author:** ![Tired\_and\_Cranky](https://avatars.discourse-cdn.com/v4/letter/t/a87d85/32.png) [@Tired\_and\_Cranky](https://boards.straightdope.com/u/Tired_and_Cranky)\ **Post date:** [August 30, 2017, 12:58pm UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/6 "2017-08-30T12:58:01Z")
In my state, it’s really easy to know if you’ll pass the emissions test with any car or light truck made since 1996: Is your check engine light on? If not, you’re fine. My state relies on the car’s on-board diagnostic system. To test it, they plug a scanner into the computer. If the scanner shows that the emissions monitors are all active and the computer doesn’t have any emissions codes, the car is in compliance. You can clear an emissions codes using your own scanner or by disconnecting the battery for a few minutes but this won’t help you pass the test if you have an underlying emissions problem. If you clear the emissions codes, you will also reset the emissions monitors. It will take some time before they all return to active status. If there are not enough active emissions monitors during the test, you will fail even if your check engine light is off. If you drive it long enough to reset all the emissions monitors, they will detect the emissions problem and trip the light again. You also can’t just pull out the check engine light. That won’t fool the computer. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [August 30, 2017, 2:13pm UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/7 "2017-08-30T14:13:27Z")
> [@Saint\_Cad](#): > > Problem is it has a worn main bearing … How was this determined? ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 30, 2017, 2:53pm UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/8 "2017-08-30T14:53:00Z")
> [@ZurBob](#): > > Worn main bearings would have no effect on an emissions test. By themselves, no, but if an engine has logged enough hours to wear the main bearings, it’s likely that the components of the emissions control system are also well aged and may not be performing to spec. That said, if the check engine light isn’t on, you’re probably fine. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [August 30, 2017, 3:24pm UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/9 "2017-08-30T15:24:58Z")
> [@Machine\_Elf](#): > > By themselves, no, but if an engine has logged enough hours to wear the main bearings, it’s likely that the components of the emissions control system are also well aged and may not be performing to spec. > > That said, if the check engine light isn’t on, you’re probably fine. if the check engine light is not on, then _in theory_ it should pass. ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [August 30, 2017, 3:35pm UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/10 "2017-08-30T15:35:53Z")
> [@Saint\_Cad](#): > > Assuming nothing else wrong with it, would a worn main bearing affect the emissions test? > [@Machine\_Elf](#): > > By themselves, no, but if an engine has logged enough hours to wear the main bearings, it’s likely that the components of the emissions control system are also well aged and may not be performing to spec. The OP was specific. Thus a specific answer. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [August 30, 2017, 7:34pm UTC](https://boards.straightdope.com/t/worn-main-engine-bearing-and-emissions/795075/11 "2017-08-30T19:34:41Z")
How old is the truck and how will they perform the emissions test? The second part can be found if know what state you are in.