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Old 11-17-2018, 05:37 PM   #1
jmrich56
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Join Date: Sep 2018
Location: San Dimas
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98 1500 5.7 vacumn check - internal leak

I have a 98 1500 w/5.7 - need to resolve a PO305 -cylinder 5 misfire.
Having checked out the typical suspects - spark, spark plug, plug wire,
distributor cap / rotor, fuel pressure, fuel pressure bleed down, & compression,
2 mechanics suggested an internal vacuum leak. Didn't think to ask how to
perform a check for that. Came home and googled - found a test that makes
sense but would like to confirm. I pulled the air intake hose out of the right bank valve cover and pulled the pcv valve out of left bank valve cover. Started engine and put my thumbs over both valve cover inlets. There was noticeable
vacuum at both valve cover inlets.
Can someone confirm that there should not be vacuum at the valve cover inlets with the pcv hose removed from the valve cover?
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Old 11-18-2018, 01:18 PM   #2
Mr_Rich
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Location: Santa Maria, Calif.
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Re: 98 1500 5.7 vacumn check - internal leak

Wouldn't that kind of hint that you have a leaking intake manifold gasket? I would think without the PCV hooked up you might have positive crankcase pressure than negative which the PCV system would take care of.
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1989 K1500 Chev. EC
Original owner; some of the aftermarket parts I've installed are Borla headers, Hypertech chip, Edelbrock water pump, and a Stillen rear disc brake conversion kit.
2009 Hummer H3T 3.7L 5M.
1997 K2500 Chev. EC 7.4L/ 4L80E
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Old 11-18-2018, 02:13 PM   #3
jmrich56
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Re: 98 1500 5.7 vacumn check - internal leak

Yes, gasket most likely suspect. A local mechanic instructed me to alter the
check by sealing the valve cover inlets with duct tape along with tape on inlet of the PCV valve. Then remove oil dip stick - start engine - check for vacuum at dip stick. I confirmed vacuum at dip stick. I believe my 5.7 is a 1st. gen. vortec and GM only produced them for a few yrs. Curious if cracked intake manifolds were a common problem.
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Old 11-18-2018, 04:49 PM   #4
crsgmc
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Re: 98 1500 5.7 vacumn check - internal leak

I have a 96, and yes the intake gaskets are known to leak. Had one that was leaking out of the back of the engine and the oil would run down to the pan and on to the floor. I replaced the gasket and it still leaked. I did not see any cracks in the intake manifold, but the gasket surface was pitted quite a bit. I gave up and purchased a new intake manifold and it sealed up perfect. Replacing the gasket is not that hard but the heater hose fittings usually fall apart when you try to remove it. I have not heard of any cracked intake manifolds but that does not mean much. It would be best to get the felpro steel backed gasket, they are supposed to last longer. I used this gasket so time will tell.
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