# vacuum/smoke test--going crazy



## 42willys (Sep 19, 2013)

Hi Again
I know the vacuum this has been covered to death but I have read everything I can find and I have no answers to my problem
So here is my problem, When I test for vacuum the needle bounces very very rapidly between 5-10.
I have check all my vacuum lines and found no leaks
I did a smoke test and there is no smoke leaking from the engine at the top:
intake, carb, rubber hoses, heads etc
BUT the smoke seem to poor out of the transmission inspection cover on the bottom rear of the motor.
I have not had the time to pull to cover YET but, whats back there that could be leaking? can there be a vacuum leak there?? 

:banghead::banghead::banghead::banghead:


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## Bensjammin66 (May 12, 2009)

A regular fel-pro oil pan seal leaves much desired on the lip that seals to the main cap. Other than that the rear main is back there and freeze plugs. Are you seeing oil leakage? Depends on your tester too. Ive had leak free engines bounce cheaper vacuum gauges.


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## ALKYGTO (Mar 29, 2010)

Can I ask why you are checking vacuum? What is the problem you are troubleshooting?


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## 42willys (Sep 19, 2013)

thanks you for the replys
1 yes there is a minor oil leak but only a random drip the engine is not even broken in yet

2 just rebuilt the engine and I was checking vacuum initally to set the carb adjustments

the engine was and still is just not running right!!!!!!

i checked the vacuum gage on another car and it was rock steady at 14

engine has 2-3 hours on it, 29 miles to be exact
runs rough and sounds odd when i come to a stop light
i have replaced the plugs, wires, cap rotor, checked the timing, vacuum lines, distributer(for vacuum diaphram)etc
even did the propane thing but may not have the "ear" to her the rpm change

some one told me to block off all the vac lines and cehck it then too...?
there are no power breaks so thats out


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## geeteeohguy (Feb 2, 2008)

A rapidly fluctuating vacuum gauge is indicative of a mechanical engine problem: sticking or burnt valves, or one or more cylinders with zero to low compression. Do a compression test and a leakdown test to verify the engine condition.


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## ALKYGTO (Mar 29, 2010)

Yeah, I was going to say mis-adjusted valves. Probably hanging open.


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## 42willys (Sep 19, 2013)

thank you all for the advice. Ill check the compression tomorrow
what should the readings be?
pretty much stock engine but it is +.30 on the rebuild
test it cold or warmed up?
I understand I'm looking for a low or dead but where should I be if normal 
like I said I have no time or miles on the engine


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## Lemans guy (Oct 14, 2014)

If your compression and leak down is OK make sure the carb and choke settings are right. I have had a bouncy vac gauge and suspected valve issues like GTOguy said, but the carb can make some strange vac fluctuations as well.

Remember the gauge is only reflecting a quick change in vac pressure. Is it on Ported vac hole that you think is manifold vac? Sorry just double check. That vac hole in the carb with a weak throttle spring worn bushings etc can make the plate irate just a hair. Pull the throttle real tight does the vac smooth out?

Sometimes a weak throttle return spring gives you symptoms that are weird. normally though, valve timing or some problem there with what you are saying


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## 42willys (Sep 19, 2013)

ok so.....compression test as follow
1 160
3 157
5 160
7 160

2 160
4 157
6 160
8 160
had some one help me with the leak down test (neither are professionals) but cylinders held pressure at appx 98 lbs


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## chuckha62 (Apr 5, 2010)

Low vacuum is usually indicative of late ignition timing. When you're seeing a low vacuum signal, a slight bounce at 6hg is more obvious than a bounce at 15hg. What is your initial timing set at? Try using your vacuum gauge while setting the timing and see what happens.

If you're running a cam with an inherently low vacuum profile, you're going to see it even worse with late timing.


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## geeteeohguy (Feb 2, 2008)

Leakdown is in percent, not PSI. But with your compression, you don't appear to have a mechanical problem. As others have said, check for late timing, vacuum leaks, and crossed plug wires.


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## 70gto400587 (Apr 13, 2015)

what cam is in this engine ?


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## Rukee (Feb 8, 2007)

When I first got my GTO it passed compression tests, but one piston was broken between the top and second ring groves. :frown2:


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## 42willys (Sep 19, 2013)

i put a q-jet from an 87 pick up on it and although it did not fit 100% correct the car ran w/ 14 in of vacuum

rebulding the carb again to check for cracks or somthing.....hope thats it


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## PontiacJim (Dec 29, 2012)

42willys said:


> i put a q-jet from an 87 pick up on it and although it did not fit 100% correct the car ran w/ 14 in of vacuum
> 
> rebulding the carb again to check for cracks or somthing.....hope thats it


The carb could be your problem. I am thinking the 1987 carb is electronic and it is set up for emissions. It is also most likely not jetted correctly coming off a 1987 engine to work with a hotter cam/Pontiac engine.


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