Failed emissions test

[RHSZR1]

Took my 91 in for its emissions test today and to my surprise it failed. Problem is with high NOx on driving test, all other tests are O.K. Took the car to Vette dealer and had them check her over - cat's are O.K. and no problem with the O2 sensor, they flushed the injectors and then had it tested again - no improvement.

Car is pretty much stock, only use Mobil 1 and Sunoco 94 and it passed with flying colors last year. The Vette dealer wants me to drive her hard this weekend and then bring it back on Monday and they will test it again.

Any suggestions as to what I could do to improve my chances, or ideas as to what else I should check?

Ron

[Hotrod]

Originally Posted by RHSZR1

Took my 91 in for its emissions test today and to my surprise it failed. Problem is with high NOx on driving test, all other tests are O.K. Took the car to Vette dealer and had them check her over - cat's are O.K. and no problem with the O2 sensor, they flushed the injectors and then had it tested again - no improvement.

Car is pretty much stock, only use Mobil 1 and Sunoco 94 and it passed with flying colors last year. The Vette dealer wants me to drive her hard this weekend and then bring it back on Monday and they will test it again.

Any suggestions as to what I could do to improve my chances, or ideas as to what else I should check?

Ron

Ron... FYI, the 91 does not have an EGR valve which cools the combustion to lower the NOx emissions. I would check that the car is meeting the GM spec for it's model year ( that's the best the car will do). Lots of states keep lowering the NOx levels to points where some cars can no longer meet the new levels (never designed or built to meet these levels). Check that your state hasn't lowered the level since your last smog test.

[RHSZR1]

Originally Posted by Hotrod

Ron... FYI, the 91 does not have an EGR valve which cools the combustion to lower the NOx emissions. I would check that the car is meeting the GM spec for it's model year ( that's the best the car will do). Lots of states keep lowering the NOx levels to points where some cars can no longer meet the new levels (never designed or built to meet these levels). Check that your state hasn't lowered the level since your last smog test.

Thank you for the info, I am located in London Ontario Canada, I will find out if the specs have been changed. I asked the technician if there was any possibility of incorrect info being entered into the Dyno when they did the test? He told me that they can't do that as they enter the vehicles VIN # and everything is automaticaly set from that. Two different shops did the back to back tests and they both had similar results.

The dealer flushed the injectors after the 1st test and took her for a run, they then had it retested at another shop, results were actually a bit worse after the injector flush. They feel that this may not be too much of a surprise and want me to drive her hard this weekend and they will retest on Monday.

The car is mostly stock - Borla exhaust system and an aftermarket chip - but I can't remember whose chip, I would have to take a look. The car had this chip last year though and it wasn't a problem.

Ron

[Z Factor]

Hello and welcome to the forum.

Actually there is some truth in what the dealership said in so far as getting on it. How often do you really open it up?

[RHSZR1]

Originally Posted by Z Factor

Hello and welcome to the forum.

Actually there is some truth in what the dealership said in so far as getting on it. How often do you really open it up?

I try to open her up whenever I can, i.e. most times that I take her out. I don't put a lot of miles on her, but probably 2 - 3 thousand per summer. The car has about 19K on it.

Ron

[Ccmano]

Take the aftermarket chip out....If it's a "piggyback" board design simply remove the original chip from the board and reinstall. Obviously if it's a replacement chip you'll have to reinstall the original (if you have it) Had similar situation recently (although mine was HC related not Nox) replaced the chip and it passed.

NOx is the result of high combustion chamber temps. ZR-1 tuners typically lean out the mixture 3 to 5 % (rather than enrichen like on regular V8's) This results is higher combustion temps and higher NOx in non-egr cars like ours.

It is also possible you have carbon build up behind the intake valves and in the combustion chamber (a common issue on low mileage older cars) The carbon absorbs fuel (especially at startup, causing hard starting) and leans out the mixture again causing those high combustion chamber temps. This is why "driving the crap out of it" is recommended. It simply blows the carbon out. I've have success with "Techron" and "SeaFoam" additives in removing carbon. I'm not going to recommend the "direct" method but a large bottle of either in a quarter tank of gas and some aggressive driving can work wonders.

Good luck...

Hans

[RHSZR1]

Originally Posted by Ccmano

Take the aftermarket chip out....If it's a "piggyback" board design simply remove the original chip from the board and reinstall. Obviously if it's a replacement chip you'll have to reinstall the original (if you have it) Had similar situation recently (although mine was HC related not Nox) replaced the chip and it passed.

NOx is the result of high combustion chamber temps. ZR-1 tuners typically lean out the mixture 3 to 5 % (rather than enrichen like on regular V8's) This results is higher combustion temps and higher NOx in non-egr cars like ours.

It is also possible you have carbon build up behind the intake valves and in the combustion chamber (a common issue on low mileage older cars) The carbon absorbs fuel (especially at startup, causing hard starting) and leans out the mixture again causing those high combustion chamber temps. This is why "driving the crap out of it" is recommended. It simply blows the carbon out. I've have success with "Techron" and "SeaFoam" additives in removing carbon. I'm not going to recommend the "direct" method but a large bottle of either in a quarter tank of gas and some aggressive driving can work wonders.

Good luck...

Hans

Hans,

Thank you for the info! I took the car for a drive today and let her go several times - hopefully this will help. Looked at some of the additives that are available that claim to "guarantee" you will pass the emissions test, but I am not crazy about using that stuff - sounds like snake oil to me. I am going to take a look at the chip and if as you state it is a piggy back style, I will install the stock chip and see what happens.

If I need a stock chip, any idea of how difficult that will be to get?

Thanks,

Ron

[Ccmano]

I highly recommend the the Techron it works well and is a Chevron product.

As for the chip, ask around on the forum and over at the CF ZR-1 forum, someone will have one...I'd let you borrow mine but mines a 90'. They are specific to the model year.

Hans