Smog test results indication of engine condition

[yummy88]

Oil analysis is often used to detect condition of engine as well as life of engine oil. I was just wondering if smog test results can also give a good indication of the condition of an engine. I am sure the equipment used is also pretty sophisticated.

Yes, I know a compression test is the way to go but some buyers might not want you poking into their cars and also for some engines, a compression test alone can run $300 or more? What about the smog test results? Here in CA, a smog test is required when a car is sold.

I just did a smog test on my Lexus 1992 LS 400 with 200K miles. Results seem to say that engine is in good shape.

And here are the results:

HC (PPM) CO(%) NO(PPM)
15mph 2 (102 max allowed) 0.02(0.68 allowed) 45 (748)
25mph 0 (77) 0.01(0.55) 68 (687)

 

[mechtech2]

It will not show if a head gasket is weeping, water pump failing, rod or main bearings worn out and knocking, or bad cam lobes.
Etc...
It shows that the fuel loop controls and catalytic converter are decent.

 

[MrCritical]

Agreed. I've seen new cars fail the smog test.

 

[yummy88]

But neither will a compression test show failing water pump or conrod or bad cam lobe problems.

Wouldn't a weeping head gasket show up in the exhaust check? After all a weeping HG would casue a slight misfire and if your smog test shows very very low figures it probably means no such misfire.

I was just thinking that any problems with valves and rings will show a high HC level.

One of the commonly specified ways of detecting condition of engine is the leakdown test. Just wondering if the CA smog test can detect problems that the leakdown test can.

 

[BarkerMan]

It might be useful to consider how your car will do on a smog test because that is something you can't duck. At smog time I have to re-tune my wife's car to pass smog, then set it back were it runs well and gets good mileage. I wish there was an easier way to get a test a little before the cut off date so problems could be handled without a last minute panic.

 

[andyd]

My recently retired 88 BMW 528e breezed through MA roller emissions testing with 345K miles on its original drive train and cat. Judging from its replacement, the identical car with 1/3 the miles, it was tired. I mean to do a compression test,I expect to find low numbers across the board.

 

[mechtech2]

A head gasket may be weeping externally.
Or, it may be a minor leak into the combustion chamber or oil that will be undetected by normal emission testing.

 

[yummy88]

Passing emissions does not mean engine is in great shape (in CA you can fail smog check if your gas cap gasket is not in tip top shape). The reason is because the state allows for a certain level of HC/CO/NOX to be present. If the bar is set too low then too many cars will fail.

For your BMW that just passed the tests, what were the levels of HC/CO/NOX. My point is whether one can look at the results of the smog test (not just whether you passed or not). Many people can pass the DMV driver's test but that does not make them truly safe or proficient drivers - the DMV test is just the general standard.

However if the state standards for HC is say 102ppm and your result is 2 would - that might reflect that the engine is running well. If you got 60ppm and thus pass the test, it just shows that your engine is putting out a lot more HC than one that has 2ppm. That might show that one engine is in better shape than another.

Of course there are many many reasons for a high HC that might not be related to condition of the engine (maybe bad tune?)

But I was just wondering if in general a very good smog result across the board HC/O2/NOX would at least mean that car is in good tune and perhaps it might reflect on condition of a car.

After all a car with poor compression will probbaly be burning oil and that is bound to be picked up by the smog test.

 

[Gary Allan]

I'd say that you can't get a truly tired engine to pass emissions. It doesn't have to be tired to fail. I would imagine that stuck rings would increase blow-by and promote stuff like cylinder imbalance that could send the fuel management systems on many vehicles a little wacky.

 

[BarkerMan]

The same politicians that are suing the EPA for the privilege of establishing their own standards for passenger car emissions also want to ditch tail pipe testing in favor of readings from an ODB port. If this new testing becomes the standard a lot of hot rodders will have a free pass to have at it with the engine modifications.

 

[Gary Allan]

It's not that simple here. Your PCM has your VIN integrated into it ..your ODO reading too. Your engine displacement is in there (determined by the VIN)..etc..etc. and it's all LIVE WIRED to the DMV. It's not some simple "plug in a $100 scanner and throw on the badge" type deal. When my pal does the emissions testing, he can see what the mileage was the last time he tested it ..all of them actually ..from when the program was started.

So, you have to jump through some hoops to "cheat". OTOH, if you've got OBDI, which uses the dyno-sniffer test, it doesn't KNOW what vehicle is on the dyno or had the sniffer up its butt. It ONLY knows what PCM it's plugged into. The data that it collects is very limited.

 

[javacontour]

I don't think it proves much of anything about mechanical condition.

If you have a worn engine, but a really good catalytic converter, much of that oil will be burned in the converter.

You may still have some blue smoke, but if the converter is handling any left over HC, then it's likely to pass in all but the most severe cases.

Failing for NOX or CO may or may not indicate a mechanical condition. It could be something like an EGR failure or a bad catalytic converter not converting CO to CO2.