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IP vacuum w/dead LP

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I measured the vacuum the IP pulls with a dead lift pump. I plugged the combination pressure/vacuum gauge into the fuel bleeder outflow hose, so this is measuring vacuum at the dirty side of the fuel filter, attenuated by about 18 feet of hose and the bleeder valve. Next step is to repeat these measurements on the IP side of the filter, where the vacuum will clearly be higher.


4 HG @ 725 RPM @ 0 MPH -- Idle
3 HG @ 1350 RPM @ 10 MPH -- Steady
5 HG @ 1750 RPM @ 37 MPH -- Steady
6 HG @ 1700 RPM @ 48 MPH -- Steady
9.5 HG @ 2800 RPM @ 60 MPH -- Accelerating


Since these pressures can clearly go negative, I can't understand why folks would install a pressure-only gauge here. It seems to me to call for a combination vacuum/pressure gauge like PN 56480809 at mscdirect.com, an Ashcroft 30-HG to 15-PSI gauge. 30-HG to 10-PSI would be better, if you can find one. The one I'm using in the picture is the right range, but it's more a tool than a permanent gauge.
 

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Well, do you need to know how much vacuum there is, because any vacuum is bad. You really just need to know how much pressure you have. And if its zero you need to fix something. If its low you need to check filters, etc..

I wouldnt worry about testing it more, get that LP running so the IP doesnt fail prematurely.
 
I agree with Buddy ... you have established that it's broke. Figuring out exactly HOW broke isn't really germane to the issue...

The system is supposed to be pressurized at all times when the LP is working - there should always be positive pressure up to the inlet of the IP. If there isn't, then Houston, we have a problem.
 
I plan to fix this as soon as my new Walbro arrives. While I wait, I took a brief spin to measure the pressure with a dead LP.

Given that I'm going to install a pressure gauge, it seems wise to install one that covers the full range of pressure/vacuum that can happen.

I was thinking that low pressure means "investigate when you get home", and high vacuum means "pull over NOW". Is there really no difference in how a driver should respond to low pressure vs vacuum?
 
DO NOT run it while you are waiting for the new Walbro LP even for curiousity.

It will ruin your IP soon and that is way more expensive than the LP.

Just a warning.
 
Yes, but I guess we just assume if we have zero pressure that we have vacuum and need to fix it right away. Low pressure would be like less than 6psi at idle and less than 3psi at accelaration. The low is kind of relative to how it performs nominally.
 
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