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I have two vacuum thermovalves (an old one, and one I bought this winter) with identicle part numbers. Both the Bosche # and the Porsche # are the same.
They work completely backwards from each other. One is closed when cold, the other is open when cold. When installed, one gives me great cold starts, the other causes rough idling and sputtering, but when warm, the engine goes like a scalded ape. My understanding of how the WUR with two vacuum connections works is as follows: The two vacuum connections work on either sides of a baffle and thus work against each other. The vacuum lines come from either side of the throttle plate. The net result is that the difference in vacuum pressure is a function of the postion of the throttle. When the throttle is wide open, the vacuums are the same, and the WUR should provide a lower control pressure to enrichen the mixture. (full throttle enrichment) On cold starts, the thermovalve closes the vacuum line coming from the intake side of the throttle body, which "tricks" the WUR into thinking it's at full throttle to enrichen the mixture to help with cold starting. Anybody still with me? If this is how it is supposed to work, then my old thermovalve is functioning properly, and the new one is working backwards. With the new thermovalve in the car it seems like I get a lean condition on startup, and then I get full throttle enrichment all the time. Can anybody set me straight??? ------------------ Bill Krause '79 911SC Euro |
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Bill,
I hope this helps. I just picked up a new thermovalve for my 78. The part number is 911.606.106.01 If that matches your number my thermovalve is open. I have not yet installed it and can gently blow air through the valve. (Boy if that's not a beautiful visual image) FWIW By the way let me know what it the correct position, open or closed. I don't want to put a part that won't work in the system. This is tough enough without creating new problems. ------------------ 78 3.0L/3.2L CIS 911SC |
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Based on all of my reading, research, and troubleshooting, you're right on the mark. On my '78SC ROW, the vacuum retard is also controlled by the thermovalve.
When cold, the thermovalve should be closed, blocking vacuum to both the WUR and the distributor. This serves to allow the WUR to richen the mixture, and to keep the timing "advanced," both of which aid cold start performance. As the engine warms up and the thermovalve opens, a differential vacuum is routed to the WUR, raising control pressure and leaning the mixture, and the vacuum retard mechanism is allowed to operate. ------------------ Michael '78 911SC Euro |
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By the way, on the WUR with two vacuum connections, it was designed to allow a differential vacuum, which basically works against each other from the two sides of the throttle body. Depending on throttle position, the change in this differential vacuum changes the control pressure on the WUR, effectively "fine tuning" the mixture.
------------------ Michael '78 911SC Euro |
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Michael,
You mention you have a 1978SC ROW and your distributor has a vacuum retard. I have the exact same car and my distributor is a vacuum advance. I have tested it with a timing gun. However I get no vacuum from the nipple located above the throttle plate. Still trying to figure out why. Tony '78 911SC |
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David -- That's the part number that's on both of my thermovalves. My old one is closed when cold, the new one is open when cold. You'd think they would have different part numbers.
Michael, I'm glad you agree with me, because I was starting to go a little nuts with this. I put my old thermovalve back on, because it seems to work the way I think it should, plus it gives me the best starts. Tony -- I'm still working on my distributor issue. I can't get the vacuum port to advance or retard the timing, so I think I need to take it apart and see what's up. Maybe this is one of the subtle differences between ROW and US cars, or maybe even a mid year change. ------------------ Bill Krause '79 911SC Euro |
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