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post #1 of (permalink) Old 02-05-2007, 01:21 PM Thread Starter
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Vac leak, propane test question

A long story but I want to prove that my intake manifold isn't cracked or leaking from the under side.
This is a 1990 Chevy 350.
I believe the intake is a full seal type and I can plug off or seal the PCV and flood propane into the area via the oil fill cap in the valve cover to test for a vac leak.
Any one ever tried this? Hazards? Or another way, short of removing the intake?
I've posted this on several BBD's.
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post #2 of (permalink) Old 02-05-2007, 02:31 PM
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Re: Vac leak, propane test question

Yes, fairly safe - I've done it many times. There's not "SUPPOSED" to be any spark in there.


Another way - juryrig a vacuum gauge on the crankcase somewhere, plug off the PCV. You should see slight pressure, not vacuum, unless the bottom side is leaking.
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post #3 of (permalink) Old 02-05-2007, 03:56 PM
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Re: Vac leak, propane test question

Just out of curiosity, what makes you think you might have a leak? Low vacuum reading?
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post #4 of (permalink) Old 02-06-2007, 11:01 AM Thread Starter
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Re: Vac leak, propane test question

Thanks RRich

JeepDawg, I have a idle problem and we (lots of folks) have elimitated everything fuel & electrical and the only place we haven't checked for a vac leak is under the manifold ... we are just trying to elimitate all possiblities before spending more money or taking something apart.

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post #5 of (permalink) Old 02-06-2007, 11:27 AM
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Re: Vac leak, propane test question

I have a 93 5.7L in my jeep...basically the same animal...
What is your vacuum reading?

RRich is the expert in this area so hopefully he will chime in.

Be aware timing can cause low vacuum....there are lots of things that can cause it. I would need to go out an measure mine, but I think it is around 22" or better at idle. Unless your running low vacuum, look at other things.

BTW...if the vacuum is low, have you checked the brake booster to make sure it's not leaking?

Also...vacuum leaks tend to cause a high idle...the computer will see the lower manifold vacuum and adjust the fuel to compensate (up to a point).

I would suggest putting a OBDI scanner on it and reading the actually sensor values.....that is how I found a bad connection with my primary temp sender.
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