CIS - AAR, AAV, Decel Valve - LONG
04-28-2006, 10:35 AM
#1
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CIS - AAR, AAV, Decel Valve - LONG
Looking for some education on the interaction of these 3 components. I just got my car back from my independent Porsche shop after a transmission transplant. Good news is the rebuilt transmission from Gary Fairbanks is great. The bad news is that my CIS is acting up. Here is the story, and my thoughts after some research here, on Pelican, Bentley, and Adrian's book:
Before engine drop:
Cold Start - Started immediately with initial "vroom" (AAV doing it's job); then a few seconds of stumbling with occasional intake pop before settling into a nice 1200-1400 RPM idle (AAR slow to react?), followed by a gradual RPM decrease to a normal idle of approx. 850 RPM. Could drive off after the initial stumbling with no hesitation or popping.
Warm Start/Running - No problems, car started immediately and idled normally at approx. 850 RPM. Decel seemed fine with revs hanging initially and no exhaust popping (Decel Valve doing it's job).
During engine drop:
It was discovered that the metal air pipe connecting the AAV, AAR, and Decel Valve to the Cold Start Valve was cracked and had been epoxied, so this was replaced with a good one.
After engine re-install:
Timing and CO was checked and found that the timing and mixture settings were way off (to compensate for air leak??) and were brought back to spec.
Current running state:
Cold Start - Starts immediately, but without initial "vroom" (AAV not doing it's job?) and runs on the verge of stalling, with significant intake popping, for approx 1 minute before settling into a low rough idle of approx 500 RPM (AAR not doing it's job?), then eventually settling into a normal idle of approx. 850 RPM. Only then can I drive off without any intake popping.
Warm Start/Running - Starts immediately and idles fine. Decel produces exhaust backfiring or popping, and the revs do not hang momentarily as they did before (Decel Valve not doing it's job?).
So, after much reading, I see that these 3 devices all deliver extra air to the intake for better combustion during cold start and decel situations. It also appears that all 3 devices get this air upstream from the same source (air box?). I have read about vacuum leaks causing high idle during warm running, but have not found anything covering my specific symptoms. I am thinking that somehow the air source for these 3 devices is not working. Could it be as simple as a disconnected hose? If so, where to look? I have no guages to measure pressures, etc., and since it was running fine before, I doubt that could be control pressures, fuel supply, etc. If it is as simple as tracking down a disconnected hose, I would like to try to fix this before taking the car back to the shop. Anyone BTDT? Is my understanding of how these things work sound? Thanks for any help.
Before engine drop:
Cold Start - Started immediately with initial "vroom" (AAV doing it's job); then a few seconds of stumbling with occasional intake pop before settling into a nice 1200-1400 RPM idle (AAR slow to react?), followed by a gradual RPM decrease to a normal idle of approx. 850 RPM. Could drive off after the initial stumbling with no hesitation or popping.
Warm Start/Running - No problems, car started immediately and idled normally at approx. 850 RPM. Decel seemed fine with revs hanging initially and no exhaust popping (Decel Valve doing it's job).
During engine drop:
It was discovered that the metal air pipe connecting the AAV, AAR, and Decel Valve to the Cold Start Valve was cracked and had been epoxied, so this was replaced with a good one.
After engine re-install:
Timing and CO was checked and found that the timing and mixture settings were way off (to compensate for air leak??) and were brought back to spec.
Current running state:
Cold Start - Starts immediately, but without initial "vroom" (AAV not doing it's job?) and runs on the verge of stalling, with significant intake popping, for approx 1 minute before settling into a low rough idle of approx 500 RPM (AAR not doing it's job?), then eventually settling into a normal idle of approx. 850 RPM. Only then can I drive off without any intake popping.
Warm Start/Running - Starts immediately and idles fine. Decel produces exhaust backfiring or popping, and the revs do not hang momentarily as they did before (Decel Valve not doing it's job?).
So, after much reading, I see that these 3 devices all deliver extra air to the intake for better combustion during cold start and decel situations. It also appears that all 3 devices get this air upstream from the same source (air box?). I have read about vacuum leaks causing high idle during warm running, but have not found anything covering my specific symptoms. I am thinking that somehow the air source for these 3 devices is not working. Could it be as simple as a disconnected hose? If so, where to look? I have no guages to measure pressures, etc., and since it was running fine before, I doubt that could be control pressures, fuel supply, etc. If it is as simple as tracking down a disconnected hose, I would like to try to fix this before taking the car back to the shop. Anyone BTDT? Is my understanding of how these things work sound? Thanks for any help.
04-28-2006, 10:40 AM
#2
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Thread Starter
One critical ommission from my summary above - prior to this the O2 sensor was disconnected. During re-install a new one was fitted and connected. Could I have issues with my O2 control unit? I'm still leaning toward something relating to the AAV, AAR, and Decel Valve since the sypmptoms all point to those 3 devices not working......
Sorry for the long winded post(s).
Sorry for the long winded post(s).
04-28-2006, 10:58 AM
#3
Instead of speculating on parts not doing their job, I would recommend doing the relatively simple tests to give you some unambiguous information.
I just posted about chasing down a fast idle on my 78 SC (No O2 sensor). I've come to believe it is a malfunctioning auxiliary air valve (not closing off all the way), but I first had to rule out the auxiliary air regulator, which wasn't difficult to check at all.
https://rennlist.com/forums/911-forum/269178-is-this-my-deceleration-valve-and-which-line-to-pull-to-check-fast-idle.html
First, check fuel pressures (primary, cold control, and warm control pressures), then the various other components. Don't forget about the frequency valve in your O2 sensor car.
Sounds like your decel valve might not be functioning since you are getting the backfires upon throttle release. Not sure, but may also point to a generally rich mixture especially since you say you have some idle issues? That's all I'll say now until you have more info. Others know more than me for sure, but I've been doing quite a bit of reading about Bosch CIS myself lately.
Another informative CIS web page:
http://members.rennlist.com/jimwms/CIS/CIShome.html
good luck,
Brett
I just posted about chasing down a fast idle on my 78 SC (No O2 sensor). I've come to believe it is a malfunctioning auxiliary air valve (not closing off all the way), but I first had to rule out the auxiliary air regulator, which wasn't difficult to check at all.
https://rennlist.com/forums/911-forum/269178-is-this-my-deceleration-valve-and-which-line-to-pull-to-check-fast-idle.html
First, check fuel pressures (primary, cold control, and warm control pressures), then the various other components. Don't forget about the frequency valve in your O2 sensor car.
Sounds like your decel valve might not be functioning since you are getting the backfires upon throttle release. Not sure, but may also point to a generally rich mixture especially since you say you have some idle issues? That's all I'll say now until you have more info. Others know more than me for sure, but I've been doing quite a bit of reading about Bosch CIS myself lately.
Another informative CIS web page:
http://members.rennlist.com/jimwms/CIS/CIShome.html
good luck,
Brett
04-28-2006, 11:28 AM
#4
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Thread Starter
Thanks Brett - I did read your post, and the other CIS page. These and others have provided me with a very quick study - Thanks. The car will go back to the shop unless I find something simple. I realize that it could be many things, and systematic troubleshooting is the way to go. What leads me to believe that it is something simple is that all was basically good before the engine drop (with the exception of a slow to react AAR). The fact that my symptoms point to these 3 devices not working all at once, and that the pipe connecting the 3 to the cold start valve area was replaced leads me to believe that my problem is immediately upstream or downstream from them. It seems to me that the cold start valve is not getting any auxiliary air at any time.
04-28-2006, 12:36 PM
#5
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Brian: If the car came to my shop with your symptoms the first thing that I would do is test/adjust the cold control pressure - overnight stay required. Congrats on the trans - I've been told that GF is a fine rebuilder...
Pete
Pete
04-28-2006, 01:20 PM
#6
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Thread Starter
Thanks Pete. Car is going back to the shop. Funny thing is, it didn't go in with those symptoms, but came out with them! Since the timing and mixture were so far off, and with the air pipe leak, disconnected O2 sensor, I suppose other settings could be messed up just to get the car to run in it's previous state. Back to square one.
The GF deal worked best for me, as I was able to drive my old box to his shop and pick up the rebuilt unit. Saved some time, and gave me a chance to meet him.
The GF deal worked best for me, as I was able to drive my old box to his shop and pick up the rebuilt unit. Saved some time, and gave me a chance to meet him.
04-28-2006, 02:44 PM
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Cold Start - Started immediately with initial "vroom" (AAV doing it's job); then a few seconds of stumbling with occasional intake pop before settling into a nice 1200-1400 RPM idle (AAR slow to react?), followed by a gradual RPM decrease to a normal idle of approx. 850 RPM. Could drive off after the initial stumbling with no hesitation or popping
Hello Brian,
The initial stumble after firing is usually caused by the vacuum retard. The car fires at 5 BTDC on the CSV and as soon as there is vacuum in the manifold, the ignition timing is pulled back creating the stumble. The vacuum retard is only relevent to the idle emission test, plug the line and reset the idle, your cold start performance will improve greatly. The problem with the final version of 911 CIS is that it was calibrated very lean for emissions and any leaks make the situation worse. Once the engine wears, the system cannot compensate, so the settings must be changed. Forget the CVS, AAV, Decel, 02 sensor, they have nothing to do with your problem. You cannot hear the AAV working, it is just a controlled vacuum leak and slow to react. Most of these cars were sold into warm weather markets, there is little experience with actual cold starts. If you live in the NE and use the car in cold weather, eliminating the vacuum retard is the first thing to do. The basic cold start mixture on Lambda CIS is too lean and is supplemented by the frequency valve enrichment, but the factory switch is too high temp and nothing happens unless the temp fall below 59 F. Stoddard sells a replacement VW switch that triggers at a higher temp which also help get rid of that start up stumble, especially if the car is in a heated garage. I suspect the timing and mixture was correctly set for the engine to run properly, and they "improved" it by setting it to run poorly.
Paul
Hello Brian,
The initial stumble after firing is usually caused by the vacuum retard. The car fires at 5 BTDC on the CSV and as soon as there is vacuum in the manifold, the ignition timing is pulled back creating the stumble. The vacuum retard is only relevent to the idle emission test, plug the line and reset the idle, your cold start performance will improve greatly. The problem with the final version of 911 CIS is that it was calibrated very lean for emissions and any leaks make the situation worse. Once the engine wears, the system cannot compensate, so the settings must be changed. Forget the CVS, AAV, Decel, 02 sensor, they have nothing to do with your problem. You cannot hear the AAV working, it is just a controlled vacuum leak and slow to react. Most of these cars were sold into warm weather markets, there is little experience with actual cold starts. If you live in the NE and use the car in cold weather, eliminating the vacuum retard is the first thing to do. The basic cold start mixture on Lambda CIS is too lean and is supplemented by the frequency valve enrichment, but the factory switch is too high temp and nothing happens unless the temp fall below 59 F. Stoddard sells a replacement VW switch that triggers at a higher temp which also help get rid of that start up stumble, especially if the car is in a heated garage. I suspect the timing and mixture was correctly set for the engine to run properly, and they "improved" it by setting it to run poorly.
Paul
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04-28-2006, 03:06 PM
#8
Paul, are you implying that the idle and mixture was simply set incorrectly by Brian's shop because they left the distributor vacuum plugged in during the adjustment?
I would call the AAV and AAR actions a controlled throttle opening rather than a controlled vacuum leak. The air that is bypasses the throttle in these cases is metered air rather than unmetered air from an intake vacuum leak to the outside.
Brett
I would call the AAV and AAR actions a controlled throttle opening rather than a controlled vacuum leak. The air that is bypasses the throttle in these cases is metered air rather than unmetered air from an intake vacuum leak to the outside.
Brett
04-28-2006, 03:38 PM
#9
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Thread Starter
Thanks Paul - your comments may be applicable to the way the car used to run before the recent engine drop a few weeks ago for tranny work. I did drive the car over the Winter here in MA on clear, cold days (temps in the teens), and it always started right up. In fact, it always started the same, with the initial stumble, whether 20 degrees or 60 degrees. When I was referring to cold start, I meant as in first start of the day, or after the engine has cooled. As for the current state, the intial start is very rough, lots of intake popping (while not giving it any gas), never acheiving fast idle, and I have decel exhaust backfiring after warmed up. I know there are many interrelated things that it could be, but my minimally informed guess is that my current state problems have something to do with the supply of auxiliary air from all 3 of the interconnected devices (AAV, AAR, and Decel Valve), especially since that pipe was replaced during the engine drop. Or, maybe as Pete suggested, it is now getting enough air and not enough fuel (cold control pressure), although the decel exhaust backfire would indicate not enough air. Who knows....
At this point, it is going back into the shop Monday. Hopefully, I will have a favorable outcome to post!
At this point, it is going back into the shop Monday. Hopefully, I will have a favorable outcome to post!
04-28-2006, 04:45 PM
#10
Brian, your warm control pressure may be fine. It's possible that it's just the cold control pressure that is too high (too lean on fuel). Ya just gotta get out the gauge and check it.
Brett
Brett
04-28-2006, 04:47 PM
#11
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BRIAN: I know what's wrong (maybe)! They neglected to plug in the wire harness just inboard of the top of the left rear shock. You will find a multi-pin plug on the end of the harness, and a receiver for the plug mounted on the frame rail near the shock. You might have to move the heater blower a little bit to see it/reach it. Good luck!
Pete
Pete
04-28-2006, 04:55 PM
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Brian,
Yes, the initial stumble after the engine fires is often caused by the vaccum retard kicking in. The factory calibration for a cold start is on the lean edge, necessitating the frequency valve enrichment. CIS is more like a carburetor than EFI, there is no active idle control or timing map to compenstate for the stumble and you have to use experience to adjust the settings for the best compromise. The AAV starts open and slowly closes, it cannot jump back to catch a stalling motor. The factory CIS spec was just OK on a new engine to meet emissions, but once the engine is worn the system has no ability to compensate on a cold start. Since the air plumbing and the mixture and timing were all disturbed, it is true you are guessing at what they did to "create" the problem. Your car has a test connector that will give you a rough indication of whether the system is operating correctly and if it is compensting for an incorrect mixture adjustment once it goes closed loop. I would check the timing with both vacuum lines disconnected and plugged, and check the mixture with a dwell meter hooked up to the test connector once warm. Reconnect the advance hose, and try another cold start the next day.
Paul
Yes, the initial stumble after the engine fires is often caused by the vaccum retard kicking in. The factory calibration for a cold start is on the lean edge, necessitating the frequency valve enrichment. CIS is more like a carburetor than EFI, there is no active idle control or timing map to compenstate for the stumble and you have to use experience to adjust the settings for the best compromise. The AAV starts open and slowly closes, it cannot jump back to catch a stalling motor. The factory CIS spec was just OK on a new engine to meet emissions, but once the engine is worn the system has no ability to compensate on a cold start. Since the air plumbing and the mixture and timing were all disturbed, it is true you are guessing at what they did to "create" the problem. Your car has a test connector that will give you a rough indication of whether the system is operating correctly and if it is compensting for an incorrect mixture adjustment once it goes closed loop. I would check the timing with both vacuum lines disconnected and plugged, and check the mixture with a dwell meter hooked up to the test connector once warm. Reconnect the advance hose, and try another cold start the next day.
Paul
04-28-2006, 05:12 PM
#13
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Brian: Follow up thought: If the harness is not plugged in, it will be in one of two places - (1) lying on top of the engine down near the throttle pull rod, or (2) trapped by the engine bay seal when the engine was raised into position. If it's #2, don't try to pull it through. You will damage the seal, or the multi-pin plug, or both. The engine should be lowered a couple of inches to get it loose...
Pete
Pete
04-28-2006, 07:01 PM
#14
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Paul
That is very interesting news abut the start up stumble. My car does it everytime ( I live in Canada ) and once in a while it stalls right out. You sid you can some how stop this . I did not quite understand, is there something I can disconnect or plug up ?
Thanks
PS I have a 78SC
That is very interesting news abut the start up stumble. My car does it everytime ( I live in Canada ) and once in a while it stalls right out. You sid you can some how stop this . I did not quite understand, is there something I can disconnect or plug up ?
Thanks
PS I have a 78SC
04-28-2006, 08:38 PM
#15
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Pete - I was really hoping it would be something so simple, but I took a look, and it appears that the harness is connected. That is, if I found what you are referring to - directly behind the heater blower motor? I couldn't see that well, but did reach my arm back there and felt a harness and a squarish plug plugged in facing the front of the car, right next to the shock tower. Didn't have time to remove the blower motor. I may take the blower motor out for a look-see tomorrow if I have some time....
What exactly does that harness control?
What exactly does that harness control?