Quote:
Originally Posted by Cd
I guess I'm not understanding how this all works. A sniffer gets placed at the end of the tailpipe and measures the amount of pollution right?
If there is a pretty good size hole somewhere in the exhaust system, the reading will be off because a lot of the pollution is spewed out the hole - is that right ? 
Since the test is supposed to be done over*, why are the results absolutely identical ? I wanted to stress that even the RPM was identical in the second test - is this just a generic number or were they really testing both times at exactly 2,591 RPM ? |
There's no reason to assume there was a "pretty good size hole". A pin-hole leak would be a failure. (The pin-hole leak would become a serious leak long before the next annual inspection.)
The lower rpm - 690 rpm - is just measured warm idle. The manufacturer's specification is no doubt something like 700 rmps +- 20 rpm. Baring gigantic holes in the exhaust or a plugged exhaust, no change is almost to be expected.
The upper rpm no doubt meets another specification. Maybe 2600 rpm +-50 rpm. The shop would use a tool to set the engine to temporarily idle within the specified range. It's a little surprising that it would come out to be the exact same number, but if you had the shop do multiple tests one right after the other the margin of error probably wouldn't be all that large. eg. Do a hundred test and the upper rpm might be 2,587 +-17 rpm or it might be 2,603 +- 19 rpm.
The car failed the inspection for safety reasons. The safety problem was fix and the car passed. With out more info, I'd give the shop the benefit of the doubt.