No Test Pipes
01-30-2002, 07:30 PM
#1
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No Test Pipes
My '91 S4 doesn't have the "test pipes" that I see under the air filter housing in the older S4 models. When were they deleted, and why? Or better: how were they used in the first place?
01-30-2002, 08:15 PM
#2
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Nicole, I'm not really sure what they were for, but I believe that they were only on the '87 model. At least that's what I've been told before. I know I don't have them on my '89 GT. Hope that helps a little.
01-30-2002, 10:14 PM
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Hi, Nicole!
The two test tubes sticking up by the air cleaner were intended to allow sampling of the exhaust gases in front of the catalytic converter, so that you could adjust the idle mixture.
Since the later cars are fully computer-controlled, with little adjustment possibilities, the tubes were omitted. The tubes were found on the '85 - '86 models, but I don't remember about the '87s.
The two test tubes sticking up by the air cleaner were intended to allow sampling of the exhaust gases in front of the catalytic converter, so that you could adjust the idle mixture.
Since the later cars are fully computer-controlled, with little adjustment possibilities, the tubes were omitted. The tubes were found on the '85 - '86 models, but I don't remember about the '87s.
01-30-2002, 10:35 PM
#4
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The test pipes are indeed on the 87 models (vin#2223).
After replacing my cams last summer - and inquireing about other YM's (and discovering there absence) - I removed mine.
As a side note - the test pipes also have a bracket on them which provides an additional bolt for the air housing. There are 2 other bolts already there for that purpose. No big loss. If someone reads this and decides to remove the test pipes - just be ceratin to plug the holes properly to ensure no fumes escape into the engine compartment.
hth-
After replacing my cams last summer - and inquireing about other YM's (and discovering there absence) - I removed mine.
As a side note - the test pipes also have a bracket on them which provides an additional bolt for the air housing. There are 2 other bolts already there for that purpose. No big loss. If someone reads this and decides to remove the test pipes - just be ceratin to plug the holes properly to ensure no fumes escape into the engine compartment.
hth-
01-30-2002, 11:33 PM
#5
My '88 had the test pipes on it, but they were taken off when exhaust work was done. As Jay said, if anyone does remove them be sure to plug the two unused holes in the bottom of the airbox. If you don't you'll be sucking unfilter air in through them and into the engine.
01-30-2002, 11:45 PM
#6
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Thanks for clarifying the purpose of these pipes. Now, how did they analyze the exhaust that came off the test pipes? Using the sensors that usually go into the tail pipe for the smog test?
The newest car I remember seeing them on was an '89 S4. Maybe they were cancelled as the GT exhaust was phased in? I believe, all '90 and '91 models have the same dual tip exhaust as the '89 GT.
I guess I should be glad that I have one less item to worry about on my car...
Thanks, guys!
The newest car I remember seeing them on was an '89 S4. Maybe they were cancelled as the GT exhaust was phased in? I believe, all '90 and '91 models have the same dual tip exhaust as the '89 GT.
I guess I should be glad that I have one less item to worry about on my car...
Thanks, guys!
01-31-2002, 03:10 AM
#7
[quote]Originally posted by Nicole:
<strong>Thanks for clarifying the purpose of these pipes. Now, how did they analyze the exhaust that came off the test pipes? Using the sensors that usually go into the tail pipe for the smog test?
</strong><hr></blockquote>
The pipes make it easier to get a reading from the exhaust before it goes through the cats. They're used when setting the air/fuel. The caps are removed from the pipes and an exhaust gas analyzer is attached to them. The o2 sensor is disconnected, and the mixture is adjusted to specs by turning a screw in the side of the mass airflow sensor. This sets the mixture close to the theoreticly ideal air/fuel ratio, and within the range of adjustment that's possible for the fuel injection computer monitoring the o2 sensor. The mass airflow sensors on the later cars don't allow for any adjustment, so that's probably why the pipes were no longer put on. Some of the cars that came equipped without cats used a tuning potentiometer located near the fuel injection computer instead of on the airflow sensor. Cars that came without cats also and had a different coding plug to let the computer know that.
<strong>Thanks for clarifying the purpose of these pipes. Now, how did they analyze the exhaust that came off the test pipes? Using the sensors that usually go into the tail pipe for the smog test?
</strong><hr></blockquote>
The pipes make it easier to get a reading from the exhaust before it goes through the cats. They're used when setting the air/fuel. The caps are removed from the pipes and an exhaust gas analyzer is attached to them. The o2 sensor is disconnected, and the mixture is adjusted to specs by turning a screw in the side of the mass airflow sensor. This sets the mixture close to the theoreticly ideal air/fuel ratio, and within the range of adjustment that's possible for the fuel injection computer monitoring the o2 sensor. The mass airflow sensors on the later cars don't allow for any adjustment, so that's probably why the pipes were no longer put on. Some of the cars that came equipped without cats used a tuning potentiometer located near the fuel injection computer instead of on the airflow sensor. Cars that came without cats also and had a different coding plug to let the computer know that.