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Join Date: Jun 2006
Posts: 46
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Pop valve lifting w/no backfire?
I've noticed the pop valve lifting (1/4" or so) when I have the air box cover off and advance the throttle. Is this back pressure normal for a stock 2.7 CIS or do I have a valve, head gasket, timing problem? Any ideas/suggestions? This is just a gradual lifting as rpm's increase, no backfire is occurring.
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1974 911 Targa "SPEED LIMIT 70 MPH" + 75 CLARK B. |
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11-10-2006, 08:31 PM
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Now in 993 land ...
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You mean the pop off valve?
Not good! There should be a VACUUM in the intake, no pressure. I can not think of any healthy situation in which there is overpressure in the intake. Yes, when you rapidly floor it to WOT, the vacuum will be very small, but nowhere over atmospheric pressure to lift the pop off lid! Stupid question, but that pop off valve works okay? George |
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11-10-2006, 11:37 PM
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Thanks aigel, I didn't think there was supposed to be any back pressure.The pop off valve works fine (spring/seat are good). If someone has seen this happen before and the cause, please let me know what you found.
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1974 911 Targa "SPEED LIMIT 70 MPH" + 75 CLARK B. |
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11-11-2006, 04:37 AM
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Bump--Any thoughts?
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1974 911 Targa "SPEED LIMIT 70 MPH" + 75 CLARK B. |
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11-11-2006, 12:34 PM
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Now in 993 land ...
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Clark,
It would be interesting if you could put a vacuum gauge on the intake and see what it is, throughout the different scenarios (idle, quick WOT, steady higher rpms). I am surprised the car is running right with this condition. The metering plate of the CIS should be all confused. How is the engine breathing any air if there is overpressure in the intake? I have not had such a condition on any of my cars, Porsche or not. I can only think that compression from the engine gets into the intake. I forget: Does the crank case breather go straight into the intake system? Then crank case overpressure may be the issue (blow by). Take the oil fill cap off and see if you can feel any overpressure when holding your hand over the oil fill tube. Other than that the only way engine compression can make it into the intake system is via the valves. Leaking intake valve or strange timing. I am surprised nobody else is chiming in here. Hopefully my above thoughts will get others interested/ George |
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11-11-2006, 01:13 PM
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Thanks again aigel. Excellent feedback, I didn't think of that (crankcase breather pressure). I'll keep you informed via this post. May be able to check it out tomorrow, lots of irons in the fire today.
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1974 911 Targa "SPEED LIMIT 70 MPH" + 75 CLARK B. |
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11-11-2006, 01:55 PM
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