Leaking Exhaust Helps Pass Smog???

Ok, this is weird, I know.

I'm the original owner of a 1980 Nissan (Datsun) pickup truck with about 200,000 miles. It always passes smog with midrange numbers since new. When I replaced the muffler and installed a custom bumper I cut the exhaust pipe back a couple feet or so. After a couple years the smog testers complained they couldn't get the sniffer up the pipe well enough so I installed hunk of 30" 1-1/2" (?) electrical flex tubing, jamming it into the end of the pipe just aft of the muffler and put a clamp attached to the bumper on the other. Naturally it leaks like crazy with a loud whistle at higher RPMs.

At the next smog test they had no problem with the sniffer fitting the flex.

But amazingly, the HC, CO and NOX (in grams per mile) dropped to near zero. The truck now measures almost pollution free!

I've checked on-line and found that if the sniffer detects an excessive O2 level it aborts the test so the leaks are obviously below that level.

I'm just wondering if the leaks out the end of the exhaust might have pulled in some fresh air or diluted the gas mixture slightly to reduce the readings and the smog sniffer didn't know the difference.

Is this possible?

 

x pogo
keeping the heat in the exhaust system longer for the other gases - HC & CO - to complete the burn process and the catalyst will reduce NOx more effectively, one stipulation, in a zero oxygen environment. leaks and device or system failures can allow oxygen introdction, in your case the short pipe caused a premature cooling of the gases causing some HC &CO to be left to burn in the reduction bed intended for NOx reduction.

 

Thanks, but I didn't describe this very well.

The pipe in question is past the muffler, about the last 30" to the exhaust tip -- I think its called the "tailpipe." No tip was installed.

With the original pipe, for about 15 years, the smog numbers remained about in the same range. With the pipe cut back, the numbers also remained about the same.

When a ventilated pipe was installed, the smog numbers dropped dramatically with no other change to the car. I would imagine the flex pipe, with all the leaks might run a tiny bit cooler rather than warmer since there is likely less exhaust actually getting to the very end of the pipe.

Still a mystery?

 

the amount of temperature loss from from no pipe is much greater even if your replacement pipe leaks, the amount of sample available is less at the sample probe during testing with a leaking pipe however the amount of temperature retained in the muffler forward is substantially higher, the cooling off of the pipe,gas and catalyst is critical to performance, temp required for a cat is 1000-1200 degrees farenheight, absent pipe will allow a rapid cooling and poor performance as far as the cat is concerned. sample dilution will factor in but would not erase the presence of oxides of nitrogen, I am a smog tech/shop owner if would like reference material i would recommend a school book "modern Automotive Technology written by James E. Duffy published by goodheart-wilcox company tinley park, illinois. not everything in the book is current ISO but is a very good fundamental read.

 

Thanks for the tip.

Well, the CAT is a little old. An IR check at 2500 RPMS shows it is not much higher in temp than the inlet/outlet pipe on each side. Not surprising after 28 years and 200,000 miles, plus a half tank of leaded went through sometime back in 1984 or so during an emergency. I doubt it is working at all. But NOX levels were almost zero. There was no CO reading at all, as in ZERO, both at idle and load (don't have the numbers handy) Yes this has the original (rebuilt twice) one barrel carb - no FI, no O2 sensor, only an EGR. and some simple two stage auto spark control (I think). And this was not a one shot test. It ran near zero numbers two years in a row. Crazy, I know.

There was no significant change in the original emission levels between the no pipe and the original solid pipe conditions so whatever the upstream temperature differences you mention didn't have much effect on the numbers -- not surprising with a on-functioning CAT.

It was only when the leaky flex conduit was installed that the smog levels dropped to near zero. It might make upstream temps higher than no pipe but lower than a solid pipe? Or maybe somewhere in between?

Anyway, was curious if the leaky pipe could affect the numbers. I hate the whistle out of the pipe so have been thinking of ditching the flex and welding in a solid pipe again. Then again I **REALLY** like to brag about the new smog numbers.

Thanks again

 

if I didn't have abusiness to run I can get very scientific regarding the hundreds of variables that could participate in your situation if I have more time i will shoot a post with a link that will break down LAMBDA and all the reactions also some believe that there is a new relationship to fuel balance and emmissions