Need help interpreting vacuum/HVAC tests
11-09-2013, 05:51 PM
#1
Drifting
Thread Starter
Need help interpreting vacuum/HVAC tests
Hello all...
Today I followed the simple initial vacuum tests Dwayne describes on his top end refresh and HVAC vacuum leak troubleshooting guides. Recently, a dull tap every 5 to 10 seconds occurs behind the center vent. I suspect leaking actuator, but the behavior is different than previous experience with another 928. And, its behavior during the testing described below is confusing to me.
The intake flappy valve works fine. Revving fast to 4000 rpm the flappy opens fully. Applying vacuum to the line to the flappy will move it at 8 inches of Hg and a couple more pumps of the MytiVac opens the flap fully at 10 inches Hg. I didn't test the line to the flappy solenoid because the rev test seems normal.
Removed the line from brake booster check valve to blue/black static system check valve and tested it. Holds vacuum. Pulled the blue/black valve and sucked air through it, making a slight kazoo sound in one direction. Not able to suck air the wrong way. Seems to be working, though maybe not as free flowing in the right direction as it should be.
Then removed the black line to the HVAC system from the four-way splitter and heard the long hissing sound of air flowing into the vacuum canister in the fender. Canister good (car had been parked for a week...I was impressed)
Then things get more interesting: (I did all of the following 5 times)
Set climate control slider to the left O position.
Pulled a vacuum on the black line to HVAC with ignition off. Holds vacuum.
Turned ignition key until cluster lights, HVAC fan, etc. come on. Vacuum dropped to zero. Applied vacuum and it leaked as fast as I could pump it, then suddenly it holds, more pumping vacuum increasing on the gauge, then a dull single tap sound in center of firewall (center vent) and it drops. Continue pumping, suddenly it holds vacuum again and vacuum rises with each pump and then drops again at some threshold.
This seems to be mimicking the once every 8 to 10 second dull tap I hear in the center vent while driving. The time interval between taps varies sometimes, but it is not the higher frequency tapping (almost fluttering) I had with an 86.5 a few years ago. But is this the beginning of that symptom?
Moving the climate control slider to the main vents position (white rectangle) gives the same behavior as the O position (except the fan is running).
Moving slider to the down pointing triangle (floor) held vacuum, and all other slider positions held vacuum.
During all tests the vacuum actuators do their job (except maybe the center comb flap).
Reconnected the vacuum line to the 4-way splitter and started the car. Pulled the center vent louvers to observe the comb flap. I could hear the dull tap and vaguely see something move, but not really sure if it was the comb rapidly opening slightly and then closing (blink-and-you-missed-it-fast), or if a door behind the comb flap was similarly opening slightly and closing rapidly to produce the tap.
BTW...during all of this the comb flap looks like bars from top to bottom Not sure if this is an open or closed position, but it never moved.
Then after observing and feeling (finger lightly pressing on the comb flap) about 6 more taps....no more tapping. This tapping behavior started only since temperatures have dropped (northern Virginia). Coincidentally, the engine reached operating temperature (cooling fan came on) a few minutes before the dull tap every several seconds stopped.
My guess is that the vacuum system is mostly functioning fine, but the comb flap actuator has a small leak that for a mysterious reason makes the loss of vacuum loss intermittent, but more frequent in the colder weather (perhaps cold temp contraction causing a small tear in an actuator to open up a bit more).
Has anyone had some similar experience with their HVAC system? Suggestions?
Thanks
Today I followed the simple initial vacuum tests Dwayne describes on his top end refresh and HVAC vacuum leak troubleshooting guides. Recently, a dull tap every 5 to 10 seconds occurs behind the center vent. I suspect leaking actuator, but the behavior is different than previous experience with another 928. And, its behavior during the testing described below is confusing to me.
The intake flappy valve works fine. Revving fast to 4000 rpm the flappy opens fully. Applying vacuum to the line to the flappy will move it at 8 inches of Hg and a couple more pumps of the MytiVac opens the flap fully at 10 inches Hg. I didn't test the line to the flappy solenoid because the rev test seems normal.
Removed the line from brake booster check valve to blue/black static system check valve and tested it. Holds vacuum. Pulled the blue/black valve and sucked air through it, making a slight kazoo sound in one direction. Not able to suck air the wrong way. Seems to be working, though maybe not as free flowing in the right direction as it should be.
Then removed the black line to the HVAC system from the four-way splitter and heard the long hissing sound of air flowing into the vacuum canister in the fender. Canister good (car had been parked for a week...I was impressed)
Then things get more interesting: (I did all of the following 5 times)
Set climate control slider to the left O position.
Pulled a vacuum on the black line to HVAC with ignition off. Holds vacuum.
Turned ignition key until cluster lights, HVAC fan, etc. come on. Vacuum dropped to zero. Applied vacuum and it leaked as fast as I could pump it, then suddenly it holds, more pumping vacuum increasing on the gauge, then a dull single tap sound in center of firewall (center vent) and it drops. Continue pumping, suddenly it holds vacuum again and vacuum rises with each pump and then drops again at some threshold.
This seems to be mimicking the once every 8 to 10 second dull tap I hear in the center vent while driving. The time interval between taps varies sometimes, but it is not the higher frequency tapping (almost fluttering) I had with an 86.5 a few years ago. But is this the beginning of that symptom?
Moving the climate control slider to the main vents position (white rectangle) gives the same behavior as the O position (except the fan is running).
Moving slider to the down pointing triangle (floor) held vacuum, and all other slider positions held vacuum.
During all tests the vacuum actuators do their job (except maybe the center comb flap).
Reconnected the vacuum line to the 4-way splitter and started the car. Pulled the center vent louvers to observe the comb flap. I could hear the dull tap and vaguely see something move, but not really sure if it was the comb rapidly opening slightly and then closing (blink-and-you-missed-it-fast), or if a door behind the comb flap was similarly opening slightly and closing rapidly to produce the tap.
BTW...during all of this the comb flap looks like bars from top to bottom Not sure if this is an open or closed position, but it never moved.
Then after observing and feeling (finger lightly pressing on the comb flap) about 6 more taps....no more tapping. This tapping behavior started only since temperatures have dropped (northern Virginia). Coincidentally, the engine reached operating temperature (cooling fan came on) a few minutes before the dull tap every several seconds stopped.
My guess is that the vacuum system is mostly functioning fine, but the comb flap actuator has a small leak that for a mysterious reason makes the loss of vacuum loss intermittent, but more frequent in the colder weather (perhaps cold temp contraction causing a small tear in an actuator to open up a bit more).
Has anyone had some similar experience with their HVAC system? Suggestions?
Thanks
11-09-2013, 06:55 PM
#2
I hate to admit it but I've now completed 3 complete actuator/diaphragms replacements for self and friends. It's never fun but gets easier each time.
It sounds like you might have a slow leak in the comb flap actuator that only occurs when it is in an expanded or contracted position. You could do a few tests to see if it holds vacuum when manipulating the comb position.
The failures for this diaphragm I've seen are usually just small pin ***** holes. The good news is that that actuator diaphragm is inexpensive(<$25 from Roger IIRC) and relatively easy to repair following Dwayne's instructions. Re-clipping the actuator arm is the only challenging part of the repair.
Good luck.
It sounds like you might have a slow leak in the comb flap actuator that only occurs when it is in an expanded or contracted position. You could do a few tests to see if it holds vacuum when manipulating the comb position.
The failures for this diaphragm I've seen are usually just small pin ***** holes. The good news is that that actuator diaphragm is inexpensive(<$25 from Roger IIRC) and relatively easy to repair following Dwayne's instructions. Re-clipping the actuator arm is the only challenging part of the repair.
Good luck.
11-09-2013, 08:08 PM
#3
Burning Brakes
Jon, I just did this job. I highly recommend that you replace the comb flap actuator. That actuator (in my opinion) gets the most "abuse." When refurbishing my actuators 3 months ago (defrost, comb, and footwell) the defrost and footwell actuators had mere pinpricks (as mentioned in post #2 of your thread), whereas the comb flap actuator was completely torn in my '86.
The comb flap actuator is very difficult to reattach to the diaphragm. I ended up (as Dwayne did) drilling a hole, which made it possible to get leverage.
Once you've done that, test the system again.
Since you've gotten this far, you will want to change that diaphragm. If it isn't damaged now, it will be soon. It sounds like there are pin ****** that don't leak air when the comb flap is in certain positions.
By the way (although I didn't encounter this problem personally) there are sometimes instances of the comb flap actuator arm being damaged. Be very gentle with the arm- it is flimsy plastic.
The comb flap actuator is very difficult to reattach to the diaphragm. I ended up (as Dwayne did) drilling a hole, which made it possible to get leverage.
Once you've done that, test the system again.
Since you've gotten this far, you will want to change that diaphragm. If it isn't damaged now, it will be soon. It sounds like there are pin ****** that don't leak air when the comb flap is in certain positions.
By the way (although I didn't encounter this problem personally) there are sometimes instances of the comb flap actuator arm being damaged. Be very gentle with the arm- it is flimsy plastic.
11-09-2013, 09:48 PM
#4
Drifting
Thread Starter
NC and Scott,
Thanks very much. Just the kind of insight I was hoping for. Looks like this summer I'll be changing the center actuator and while in there, testing the other lines one at a time with the MitiVac...and going from there.
Thanks very much. Just the kind of insight I was hoping for. Looks like this summer I'll be changing the center actuator and while in there, testing the other lines one at a time with the MitiVac...and going from there.
11-09-2013, 10:14 PM
#5
Burning Brakes
Jon, also make sure that you have a few replacement solenoids around. I bought five (as a prophylactic measure) and had to replace three that were leaking.
If you have to change the defrost actuator, I wouldn't disassemble it and use just the rubber diaphragm on the old actuator- I would change the whole assembly. I did this past summer and the damn actuator would not hold vacuum, no matter what.
If you have to change the defrost actuator, I wouldn't disassemble it and use just the rubber diaphragm on the old actuator- I would change the whole assembly. I did this past summer and the damn actuator would not hold vacuum, no matter what.