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Intake idle negative pressure

 
Old 07-03-2017, 06:09 PM
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Smudo
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Default Intake idle negative pressure

I suspect that I might have small vacuum leak under the intake manifold. With idling engine I think that I hear a sound which seems like there is small vacuum leak but I am not sure, whether that is how a vacuum leak sounds or whether that is normal sound of the engine. According to my boost gauge (aftermarket VDO), the negative pressure at idle ir around 0.62 bar or 9 psi. Is this normal vacuum at idle or not? I would like to hear some thoughts before I remove the intake manifold. Thanks.
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Old 07-03-2017, 06:50 PM
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My '83 idles at around 35-40 kPa absolute pressure. 14.7 (nominal) psi atmospheric pressure minus 9 psi vacuum leaves 5.7 psi absolute, which is ~39 kPa. Sounds like you're in the ballpark (ignoring CR differences and so on), or my car also has a vacuum leak. I don't miss having an AFM or MAF to complain about it to me.
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Old 07-04-2017, 01:34 AM
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Thanks, in case there is a vacuum leak, it must be small as othervise it would be hard to reach 16.5 psi intake preassure while accelerating. Will need to test it with so call start gas.
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Old 07-04-2017, 09:22 AM
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At idle I am at -18 on my aftermarket gauge 3500ft elevation. I do have a larger cam in my 3.0L
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Old 07-04-2017, 01:14 PM
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Originally Posted by Smudo View Post
I suspect that I might have small vacuum leak under the intake manifold. With idling engine I think that I hear a sound which seems like there is small vacuum leak but I am not sure, whether that is how a vacuum leak sounds or whether that is normal sound of the engine. According to my boost gauge (aftermarket VDO), the negative pressure at idle ir around 0.62 bar or 9 psi. Is this normal vacuum at idle or not? I would like to hear some thoughts before I remove the intake manifold. Thanks.
It's hard to detect small vacuum leaks using a vacuum gauge alone, and most turbos can compensate enough to create 16.5 psi of boost despite a small leak. Your motor will run leaner on vacuum and richer on boost as a result, however, and the turbo will be operating less efficiently (and therefore make less power), so it's always worth tracking down any suspected leaks. Your best bet is to pressure test the intake track -- see Van's video below.


As for your readings, engine vacuum is normally measured in "inches of mercury" (inHg), and on that scale you're hoping for about 18 inHg with the motor idling at 840+/- at sea level (less at higher elevations, more at higher rpms). Most gauges show vacuum on that scale, so am wondering if you are only seeing 9 inHg of vacuum, which would be very low. If it really is a PSI reading, it depends on whether it's absolute or gauge pressure. If that's 9psi less than atmospheric pressure (i.e., gauge pressure) then that's a little over 18 inHg, which is great. If that's 9psi above absolute zero (i.e., absolute pressure) then that's a little under 12 inHg, which is low.
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Old 07-04-2017, 03:10 PM
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Thanks for the comments. The reading is 9psi below the absolute preassure at around 950 rpms.




Here is video. If you put on the headphones (or even without them) you can here a sound of potential vacuum leak.

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Old 07-04-2017, 07:40 PM
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That gauge is reading about 18.5 inHG of vacuum, which is good. At normal idle speed, you're likely lower, so I wouldn't rule out leaks based on those numbers. Always hard to diagnose from internet videos, but it does kind of sound like there is a hissing sound. I'd pressure test the system like in Van's video above.

(FYI, to say it is 9psi "below the absolute pressure" confuses the concepts a bit, and is kind of like saying it's 10 degrees below the Fahrenheit. If interested, google "gauge v absolute pressure" for a zillion tutorials/explanations.)
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Old 07-06-2017, 05:22 PM
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Today took of the intake manifold.




Did find one damaged hose. It might be the cause of hissing sound under the manifold.


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Old 07-07-2017, 03:46 AM
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Hissing sound can also come from bypass valve if not stock.

Originally Posted by Smudo View Post
Today took of the intake manifold.




Did find one damaged hose. It might be the cause of hissing sound under the manifold.


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Old 07-09-2017, 05:22 PM
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Did replace the damaged hose and did put everything back together. The idle vacuum reading did not change at all but I did notice, that any accelerator pedal application while driving does result in instant and higher positive pressure although also before the repair I was not complaining, but still I see an improvement. The hissing sound also had reduced.
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