HC gotcha along with NOx...

I need some advice....

Failed a smog check today on a 113K mile, 1999 boxster.

There are 3 things I can think of straight off that may have contributed and I would like to get feedback from you guys on the best course of action or if I should just take it to a mechanic.

a) I have a non-approved aftermarket exhaust and headers. I have the old headers and could re-install.
b) I have removed the air scoop and could re-install.
c) The idler valve on the car is a bit sticky especially when the car is cold.

At my previous smog check item a) and c) were unchanged. I cannot remember if I put the air scoop back in for that one or not. The numbers I receive on the test are below.

Do you think that the above 3 items would cause these high values? Is it worth re-installing at least a) and b) to see if it works? Or should I just take it to a mechanic?

Test RPM %CO2 MEAS %O2MEAS HC MAX MEAS CO MAX MEAS NO MAX MEAS
15mph 1753 14.2 0.5 58 142 0.52 0.21 451 458
25mph 1926 14.3 0.3 42 53 0.50 0.08 738 179

any advice will be appreciated.

thanks

Jason

Quote
Jason_DW
I need some advice....

Failed a smog check today on a 113K mile, 1999 boxster.

There are 3 things I can think of straight off that may have contributed and I would like to get feedback from you guys on the best course of action or if I should just take it to a mechanic.

a) I have a non-approved aftermarket exhaust and headers. I have the old headers and could re-install.
b) I have removed the air scoop and could re-install.
c) The idler valve on the car is a bit sticky especially when the car is cold.

At my previous smog check item a) and c) were unchanged. I cannot remember if I put the air scoop back in for that one or not. The numbers I receive on the test are below.

Do you think that the above 3 items would cause these high values? Is it worth re-installing at least a) and b) to see if it works? Or should I just take it to a mechanic?

Test RPM %CO2 MEAS %O2MEAS HC MAX MEAS CO MAX MEAS NO MAX MEAS
15mph 1753 14.2 0.5 58 142 0.52 0.21 451 458
25mph 1926 14.3 0.3 42 53 0.50 0.08 738 179

any advice will be appreciated.

thanks

Jason

Was the engine nice and hot at the time of the test? That the numbers are so off and imbalanced suggests that the engine's temp is not a contributing factor still one always wants to make sure he drives the car long enough to get the engine fully up to temp and not let the engine be shut off until after the test is run. For my 02 Boxster's smog test I try to pick a warm to hot day and I do not turn on the A/C as this causes the engine to run cooler than it does otherwise.

Assuming the engine is in reasonable tune (good plugs, coils, injectors not leaking, air filter fresh, fresh oil of the proper type and not overfilled, several good tanks of a top tier gas run through the engine recently and the current load of fuel fresh fresh fresh, etc. but I would get that idle control valve fixed) my advice would be to replace the aftermarket exhaust with the stock exhaust.

If possible have the car pre-tested at a shop to determine if the car will pass when it comes time to test it for real. This will help you know the exhaust (or any other change) was for the better.

Now, if you don't want to go to the trouble swapping exhaust then re-install the factory air intake. Especially if this is as I suspect allowing the engine to breathe in engine compartment air (hot air).Then have the car pre-tested.

What to do next depends upon the results of the pre-test.

If the factory intake doesn't help then you're looking at the exhaust as the cause. Time to swap the exhaust and repeat the pre-test.

If the pre-test still fails the car with the factory intake and exhaust then you've got to look deeper.

When you change something use an OBD code reader (with OBD data viewing functionality too) to erase any error codes (even if none are present) and drive the car around in a variety of scenarios to ensure all of the readiness monitors are complete -- you can use the code reader to view the readiness monitor states and note when they are all flagged complete -- and the engine controller has plenty of time to adapt to the changes you have made.

Do not reset any error codes before you take the car in for another test. This sets all the readiness codes to incomplete and the car will fail. Only 2 are allowed to be incomplete IIRC.

Use the code reader/data viewer to note the long term fuel trims. If they're imbalanced the car probably won't pass. You can help perhaps by before you change anything get the engine hot and note the long term fuel trims and the O2 sensor voltage readings (both #1 and #2 sensors and for both banks) to build up a baseline of what they look like when the engine failed smog.

#1 sensor readings want to swing from 0.2 volts or less to 0.6 volts (or a bit more) at idle about once every few seconds.

#2 sensor readings want to remain about 0.6 or higher. If the readings dip low to under 0.45 volts (and I've seen my car's sensors drop even lower... converters are worn out) the converters are not storing oxygen and could be worn out. Or the sensor readings affected by the exhaust system or the sensors could be just old or as I touched upon above the engine could be in need of some serious tuning.

Sincerely,

Macster.

Thanks Macster for that detailed response.

The car was reasonably warm however it had sat for a few minutes while I removed the engine cover. Additionally he stalled the engine twice before / during the test which probably was not good for the readings.

I think I will replace at least the headers and the air intake...the air intake I am referring to is the just the little scoop piece that is just inside the body work...there is a better term (more recognized) but it escapes me right now.

In researching it seems like the car could be getting too much air (can cause high HC and NO) which would make sense with the intake scoop removed and the bigger diameter headers / exhaust system. I like your recommendation regarding getting a pre-test and reviewing the OBD II readings......might finally be time to buy a reader.

The car is probably due for a good tune up but the oil, coil packs and spark plugs should be all good....it's my bad for leaving the test for the day my registration was due!

thanks again.

Jason

here.

Anyhow, if you want to replace the headers and intake with the stock items that's a bit of work but probably will be required,eventually. Aftermarket headers and converters are based on what I've gathered from the various posts are generally no damn good. A good exhaust system that does all it is supposed to do is a complicated piece of hardware. There's more to a good exhaust than just a bit more sound or a small bump in HP.

Keeping the O2 sensors bathed in the exhaust gases from the cylinders and for the converter providing sufficient surface area with the necessary coating of platinum, pladium and rhodium to effect the necessary conversion of exhaust gases into something acceptable at the smog test station requires a very complex and well designed system.

Sincerely,

MarcW. (I hope I have the right signature name now....)