Euro versions - why you can't run an air/fuel ratio meter.
03-25-2002, 11:54 AM
#1
Burning Brakes
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Euro versions - why you can't run an air/fuel ratio meter.
So here's the short version:
I installed a Nordskog (formerly Intellitronix) A/F ratio meter in my '85 Euro 928 to try and diagnose why I failed my emissions test for high NOx. My car has a twin cat from a US version, but no other modifications to the engine or engine management. So no closed loop, no O2 sensor, no air injection into the cat.
I put a Bosch 3 wire sensor into the twin cat and built a decent little wiring harness for it and the meter. Once hooked up and working, the meter was telling me the car was running lean all the time. This would explain the high NOx. So after some troubleshooting, I decided the air mass meter was bad. Mainly because I could not adjust it so the car idled stoich. The adjuster did nothing and everything else (throttle switch, temp senders, injectors, etc) tested out fine. Ordered up a new air mass meter, installed it, and now I can set the idle mix. Fine. Super.
But under any non-idle condition, the air fuel meter reads lean. Like off the scale lean. Even under load and high rpm where air leaks have basically no effect (WOT does run stoich to rich).
I finally know why this is happening. It's the damn air injection. The Euros have an air pump too, but it injects air into the exhast ports on the heads. Therefore, the exhaust is dilluted with air when it gets to the O2 sensor, the O2 sensor senses excess air, and tells the meter "lean". On a US car, the air injection is downstream of the O2 sensor, so the O2 sensor reads accurately.
Now that I understand this, I'm going to rig the air injection to always go into the airbox (just while I troubleshoot) and see what my exhaust gas composition is really like. My theory is that I have one or two small air leaks because the idle mix still floats around and drops lean under no load and elevated rpms (highest vacuum except for coast down). And otherwise the engine is in perfect shape now.
Anyone know why Porsche implemented this weird air injection system on the Euro versions? These cars didn't have to meet any emissions requirements. It must serve some other purpose.
And does anyone know if it's vacuum or lack of vacuum that switches the diverter valve to send air into the airbox?
Bryan
I installed a Nordskog (formerly Intellitronix) A/F ratio meter in my '85 Euro 928 to try and diagnose why I failed my emissions test for high NOx. My car has a twin cat from a US version, but no other modifications to the engine or engine management. So no closed loop, no O2 sensor, no air injection into the cat.
I put a Bosch 3 wire sensor into the twin cat and built a decent little wiring harness for it and the meter. Once hooked up and working, the meter was telling me the car was running lean all the time. This would explain the high NOx. So after some troubleshooting, I decided the air mass meter was bad. Mainly because I could not adjust it so the car idled stoich. The adjuster did nothing and everything else (throttle switch, temp senders, injectors, etc) tested out fine. Ordered up a new air mass meter, installed it, and now I can set the idle mix. Fine. Super.
But under any non-idle condition, the air fuel meter reads lean. Like off the scale lean. Even under load and high rpm where air leaks have basically no effect (WOT does run stoich to rich).
I finally know why this is happening. It's the damn air injection. The Euros have an air pump too, but it injects air into the exhast ports on the heads. Therefore, the exhaust is dilluted with air when it gets to the O2 sensor, the O2 sensor senses excess air, and tells the meter "lean". On a US car, the air injection is downstream of the O2 sensor, so the O2 sensor reads accurately.
Now that I understand this, I'm going to rig the air injection to always go into the airbox (just while I troubleshoot) and see what my exhaust gas composition is really like. My theory is that I have one or two small air leaks because the idle mix still floats around and drops lean under no load and elevated rpms (highest vacuum except for coast down). And otherwise the engine is in perfect shape now.
Anyone know why Porsche implemented this weird air injection system on the Euro versions? These cars didn't have to meet any emissions requirements. It must serve some other purpose.
And does anyone know if it's vacuum or lack of vacuum that switches the diverter valve to send air into the airbox?
Bryan
03-26-2002, 02:29 PM
#2
Rennlist Member
Hi Bryan,
The Euro airpump system is a bit of a mystery to us in Europe as well !
Many people just take the thing off, and this may be the simplest solution for you. When adjusting idle CO2, the airpump line into the ports has to be disconnected and blanked off to get a true reading, which is what you discovered.
I think the idea was to "dilute" the exhaust gases, especially at idle (i.e. high vacuum) so I reckon the divert valve puts the pumped air into the airbox at WOT (low vacuum).
Regards,
John (Cambridge, England -'86 S2)
The Euro airpump system is a bit of a mystery to us in Europe as well !
Many people just take the thing off, and this may be the simplest solution for you. When adjusting idle CO2, the airpump line into the ports has to be disconnected and blanked off to get a true reading, which is what you discovered.
I think the idea was to "dilute" the exhaust gases, especially at idle (i.e. high vacuum) so I reckon the divert valve puts the pumped air into the airbox at WOT (low vacuum).
Regards,
John (Cambridge, England -'86 S2)
03-26-2002, 03:02 PM
#3
Burning Brakes
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Wow, so I need to disable the air injection to set the mixture properly at idle. Oops. Right now I've got the mixture adjusted so that the O2 sensor is satisfied (reading stoich - neither rich nor lean) at idle. With the air injection hooked up and working. So that means it's running really rich? It doesn't smell rich.
Wait a second. If it's injecting air at idle, I shouldn't be able to bring the mixture up to stoich at all, should I? O2 sensors sense excess air, and with air injection, there's always going to be excess air no matter what is going on inside the combustion chambers. Or if the idle mix is really rich, will the air injection bring things back to what the O2 sensor thinks is stoich? Dang this is confusing.
So past blunders aside (now I wonder if I really needed that new air mass meter) my plan is going to be to disable the air injection and see what my actual exhaust gas composition is, and go from there.
If my engine isn't running lean, then why were my NOx high enough to make me fail an emissions test? NOx are usually caused by running lean. Of course, I have replaced my air mass meter since the test. And I still suspect I have some small vacuum leaks which would lean the mix at idle. Which is a component (about 1/3) of the emissions test. Steady-state cruising and coast-down are the other 2/3. My NOx were just barely over the limit. So maybe fixing vacuum leaks will do it.
More verbose postings to follow as discoveries are made.
Bryan
Wait a second. If it's injecting air at idle, I shouldn't be able to bring the mixture up to stoich at all, should I? O2 sensors sense excess air, and with air injection, there's always going to be excess air no matter what is going on inside the combustion chambers. Or if the idle mix is really rich, will the air injection bring things back to what the O2 sensor thinks is stoich? Dang this is confusing.
So past blunders aside (now I wonder if I really needed that new air mass meter) my plan is going to be to disable the air injection and see what my actual exhaust gas composition is, and go from there.
If my engine isn't running lean, then why were my NOx high enough to make me fail an emissions test? NOx are usually caused by running lean. Of course, I have replaced my air mass meter since the test. And I still suspect I have some small vacuum leaks which would lean the mix at idle. Which is a component (about 1/3) of the emissions test. Steady-state cruising and coast-down are the other 2/3. My NOx were just barely over the limit. So maybe fixing vacuum leaks will do it.
More verbose postings to follow as discoveries are made.
Bryan
03-26-2002, 03:34 PM
#4
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While Ihave no information that is specific to the 928 ROW air injection set-ups, I can help a bit with air injection in general.
The diverter valve was often called the "glup valve" in the USA. When one of the earlier pollution-controlled engines was cruising, there was fuel "plated" (think condensation, although that wasn't how it got there) on the walls of the intake tract. When you lifted abruptly, the sudden vacuum spike would evaporate that fuel, and since there was very little air going in due to the closed throttle, there would be a very brief rich spike going thru the cylinders. When this rich spike went thru the cylinder, it usually didn't ignite - until it hit the high temp and the injected air in the exhaust, where it did ignite. This would cause a noticeable "afterfire" (explosion in the exhaust) and if the exhaust and intake valves were both open on that cylinder (due to valve overlap),usually an even more noticeable "backfire" (explosion in the intake tract).
The gulp valve or diverter valve's fuction was to respond to a sudden intake manifold vacuum spike by diverting the air injection pump flow from the exhaust to a dump port in the air cleaner. This prevented the explosions when you lifted off the throttle abruptly.
The easiest way to kill the air flow to the exhaust might be to disconnect and plug the air hose at the diverter valve.
The diverter valve was often called the "glup valve" in the USA. When one of the earlier pollution-controlled engines was cruising, there was fuel "plated" (think condensation, although that wasn't how it got there) on the walls of the intake tract. When you lifted abruptly, the sudden vacuum spike would evaporate that fuel, and since there was very little air going in due to the closed throttle, there would be a very brief rich spike going thru the cylinders. When this rich spike went thru the cylinder, it usually didn't ignite - until it hit the high temp and the injected air in the exhaust, where it did ignite. This would cause a noticeable "afterfire" (explosion in the exhaust) and if the exhaust and intake valves were both open on that cylinder (due to valve overlap),usually an even more noticeable "backfire" (explosion in the intake tract).
The gulp valve or diverter valve's fuction was to respond to a sudden intake manifold vacuum spike by diverting the air injection pump flow from the exhaust to a dump port in the air cleaner. This prevented the explosions when you lifted off the throttle abruptly.
The easiest way to kill the air flow to the exhaust might be to disconnect and plug the air hose at the diverter valve.
03-27-2002, 10:05 PM
#5
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The most over looked area of false air, are vacuum leaks below the throttle plate, throttle plate housing gasket, intake manifold gasket, oil dipstick seal, oil filler neck gasket, valve cover gasket.
A vacuum gage is a good diagnostic tool. Connected to the throttle housing, raise the engine to 1700RPMs to disable any idle stabilization valve the car may have, spray carburetor cleaner on or around any potential source of leak (vacuum), the gage will have a higher reading if there’s a leak.
The wrong plugs heat range, advanced ignition timing, vacuum leaks, sticking valves, and a hot running engine can cause a lean fuel condition, carbon on the valves will rise compression = an increase in combustion pressure = lean mixture. If no vacuum leaks are found, adjust the air fuel –mixture richer. Don’t assume a new MAF, or AFM is adjusted right. Even CIS systems have a means of adjustment.
On the early 928’s it is called a diverter valve, on the later 928’s it is called a blow off switching valve. Its function is the same for both. To prevent excessive air pump loads as exhaust backpressure increases, intake vacuum will open the diverter and switching valve, dumping the air into the air box. The vacuum line is connected to the throttle housing.
At idle and partial load, fresh air from the air pump is directed behind the exhaust valve in cars with out a catalytic converter. The purpose is to dilute the exhaust, minimize the emission of CO and HC.
On cars with catalytic converters and a 02sensor the heads are plugged. The air is fed into the catalytic converter.
Disconnect the air pump belt see if you get a rich mixture.
Happy hunting.
Steve C
The Great White
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" /> <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
A vacuum gage is a good diagnostic tool. Connected to the throttle housing, raise the engine to 1700RPMs to disable any idle stabilization valve the car may have, spray carburetor cleaner on or around any potential source of leak (vacuum), the gage will have a higher reading if there’s a leak.
The wrong plugs heat range, advanced ignition timing, vacuum leaks, sticking valves, and a hot running engine can cause a lean fuel condition, carbon on the valves will rise compression = an increase in combustion pressure = lean mixture. If no vacuum leaks are found, adjust the air fuel –mixture richer. Don’t assume a new MAF, or AFM is adjusted right. Even CIS systems have a means of adjustment.
On the early 928’s it is called a diverter valve, on the later 928’s it is called a blow off switching valve. Its function is the same for both. To prevent excessive air pump loads as exhaust backpressure increases, intake vacuum will open the diverter and switching valve, dumping the air into the air box. The vacuum line is connected to the throttle housing.
At idle and partial load, fresh air from the air pump is directed behind the exhaust valve in cars with out a catalytic converter. The purpose is to dilute the exhaust, minimize the emission of CO and HC.
On cars with catalytic converters and a 02sensor the heads are plugged. The air is fed into the catalytic converter.
Disconnect the air pump belt see if you get a rich mixture.
Happy hunting.
Steve C
The Great White
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" /> <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
10-10-2012, 10:23 AM
#6
Nordschleife Master
Wow, so I need to disable the air injection to set the mixture properly at idle. Oops. Right now I've got the mixture adjusted so that the O2 sensor is satisfied (reading stoich - neither rich nor lean) at idle. With the air injection hooked up and working. So that means it's running really rich? It doesn't smell rich.
This thread turned up in my search, as I just ran into this issue on my non-cat '87 S4. I restored the original air pump setup on my non-cat '87 S4 and the car's been running rich despite the evidence of my WBO2. It finally occurred to me today that the airpump system, being a non-cat car, feeds bonus oxygen into the exhaust ports before the O2 sensor.
Page 24-214 in the WSM shows the procedure for adjusting the mixture..
Can I get away with just unplugging the vacuum line off the diverter valve while setting the idle mixture???
The WSM states you need to disconnect the hose from the air pump to the diverter valve at the diverter valve end, and plug or cap it - which is going to be more fiddly than just popping off the vacuum line.
Tomorrow I'm gonna test my spare MAF - I suspect its not actually ready for rebuild, and I was just struggling to get the AFR down to stoich because of all that extra oxygen being pumped into the exhaust ports before the WBO2!
Last edited by Hilton; 10-10-2012 at 10:41 AM.
10-10-2012, 10:59 AM
#7
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As everybody has found out working air pump will totally mess up mixture values on exhaust. Simplest solution is to block diverter valve vacuum line.
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10-10-2012, 08:06 PM
#8
Nordschleife Master
10-10-2012, 08:49 PM
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Putting an o2 sensor into the cat will mess up your a/f ratio depending on how far into the cat you went. I'd imagine the cat's job is to put out as clean as possible air. That might explain the leanness.
edit - Old thread...
edit - Old thread...
10-11-2012, 05:05 AM
#11
Nordschleife Master
For non-cat S4's, there is very different air pump plumbing. They pump air directly into all 8 of the exhaust ports (i.e. before the O2 sensor) on the heads, under high vacuum (idle, and just off idle). See the attached picture - the manifolds are 29 and 30 and are joined to each exhaust port by banjo bolts.
I've removed the complete assembly of manifolds and plugged the ports in two of my 928's. This one however I elected to keep them and make the airpump work
On earlier "Euro" 928's, the air pump feeds ports on just the rear of the heads.
This issue doesn't affect any US cars, or cars with factory catalytic converters, where the airpump feeds air directly into the cats after the O2 sensor.
10-11-2012, 01:23 PM
#12
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On not-cat cars, airpump injected air is used to reduce HydroCarbons (unburned fuel) by afterburning them in the exhaust manifolds.
Both cat and non-cat versions divert airpump air into the airbox -at full throttle- so as to not overheat the exhaust system (cats).
Both cat and non-cat versions divert airpump air into the airbox -at full throttle- so as to not overheat the exhaust system (cats).
10-11-2012, 02:50 PM
#13
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Rusted out air pipes will also mess up engine running fairly well when non cat engine is converted to use lambda sensor. Quess how we found this out. Luckily there is one way valve (part 23 in PET picture) which prevents exhaust gasses moving into wrong direction and killing air pump. It does good enough job on killing itself.
Also in cat versions measuring emissions from the end of the exhaust will give totally wrong oxygen values at idle if vacuum is not blocked. Many local owners have found this hard way in yearly inspections. There is upper limit on oxygen and its exceeded five times if fully functioning pump is not deleted during measurement. This was reason for two pipes from headers to next to intake in early S4 engines. They allow measurement while pump is working and also checking each side of the engine separately without cats having any effect on results.
Luckily there is one way valve (part 23 in PET picture) which prevents exhaust gasses moving into wrong direction and killing air pump. It does good enough job on killing itself.Also in cat versions measuring emissions from the end of the exhaust will give totally wrong oxygen values at idle if vacuum is not blocked. Many local owners have found this hard way in yearly inspections. There is upper limit on oxygen and its exceeded five times if fully functioning pump is not deleted during measurement. This was reason for two pipes from headers to next to intake in early S4 engines. They allow measurement while pump is working and also checking each side of the engine separately without cats having any effect on results.
10-11-2012, 03:14 PM
#14
Rennlist Member
On my 84 Euro, I replaced the original exhaust manifolds with MSDS headers ad removed the airpump and associated plumbing to the back of the heads. I capped the outlet at the back of the head with a vacuum-type plug. It seems my car runs very rich. What additionally do i need to do to level that out?
10-15-2012, 07:16 PM
#15
Nordschleife Master
I forget which of mine I seen it on, but either the US 83 or the Euro 84 has a thermal vacuum switch between the diverter valve (19) and throttle vacuum located on the (US) passenger side of the engine valley.