Reading a vaccum guage - The 1947 - Present Chevrolet & GMC Truck Message Board Network

lukecp · 07-18-2003, 11:23 PM

Awhile back i put a vaccum guage on my truck, and at idle it pulled between 18.5 and 19.5 inches of vaccum. The needle on the guage was vibrating very fast between the 2 marks, but it never went about 19.5" or below 18.5". I have read on vaccum guages in a few books i have and on the net, and they all say that a variation like that is a result of "igniton troubles". What could be the troubles? The truck runs like a top, no misses, hesitation, starts great, ect. My plugs and wires (AC Delco R43S and Taylor Hi-Energy wires) have about 1000 miles on them, but the cap and rotor are over a year old and have 13k miles on them. Any ideas on what would be causing this? Excess play in the distributor shaft? Worn out cap and rotor (they looked OK....)?

Thanks for any help

wxman1 · 07-19-2003, 12:34 AM

Found this on a web page on diagnosing engine
problems with a vacuum gauge.

The gauge needle fluctuates from 15-20in./Hg
rapidly at idle. This would indicate a
sticking valve or ignition miss.

The gauge reading is low but steady and reads
between 15-20in./Hg Indicates a late ignition
or valve timing, low compression, stuck
throttle or a vacuum leak.

The needle fluctuates as the RPMs increase.
Weak valve springs, worn valve stem guides, or
leaking headgasket. A clogged catalytic
converter may also cause this condition.

Gradual drop of vacuum at idle. Choked muffler
or obstruction in exhaust system. A choked
muffler will exhibit a slow vacuum decrease
to ZERO at idle.

Vacuum gauge needle drifts. Minor intake leak
of the manifold or bad fuel injector O-rings.

The needle stays steady but then drops
regularly and recovers only to repeat. Burnt valves or improper valve clearance
adjustment.The needle will drop when the
defective valve operates.

The needle vibrates excessively at idle but
steadies out as the engine RPM is increased.
This would indicate worn valve guides.

Jim M.
Mt Juliet

JayDubBlazer · 07-19-2003, 03:53 AM

Wow, that is a nice piece of information. I am going copy and paste that into my "file of knowlege"

**GMCPaul** · 07-19-2003, 07:10 AM

We give a complete rundown on trouble shooting and tuning with the vaccum gauge at the following link I'm sure you will find helpful.
http://www.gmcpauls.com/Tech%20Tips1.htm
GMCPaul

07-18-2003, 11:23 PM	#1
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Reading a vaccum guage Awhile back i put a vaccum guage on my truck, and at idle it pulled between 18.5 and 19.5 inches of vaccum. The needle on the guage was vibrating very fast between the 2 marks, but it never went about 19.5" or below 18.5". I have read on vaccum guages in a few books i have and on the net, and they all say that a variation like that is a result of "igniton troubles". What could be the troubles? The truck runs like a top, no misses, hesitation, starts great, ect. My plugs and wires (AC Delco R43S and Taylor Hi-Energy wires) have about 1000 miles on them, but the cap and rotor are over a year old and have 13k miles on them. Any ideas on what would be causing this? Excess play in the distributor shaft? Worn out cap and rotor (they looked OK....)? Thanks for any help __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

07-19-2003, 03:53 AM	#3
JayDubBlazer go cyclones Join Date: Jun 2003 Location: Ames, IA Posts: 1,883	Wow, that is a nice piece of information. I am going copy and paste that into my "file of knowlege" __________________ Wes PROJECT OLE GREEN 1972 Blazer 4X4 350/700R4 2017 Silverado LT

07-19-2003, 07:10 AM	#4
GMCPaul Senior Member Join Date: Mar 2000 Location: Rockville,In. USA Posts: 3,722	We give a complete rundown on trouble shooting and tuning with the vaccum gauge at the following link I'm sure you will find helpful. http://www.gmcpauls.com/Tech%20Tips1.htm GMCPaul __________________ Secure Online Catalog for-GMCPauls Truck Parts Email- GMCPAULS@GMAIL.COM

07-19-2003, 12:34 AM	#2
wxman1 Senior Member Join Date: May 2002 Location: mt juliet, tennessee, usa Posts: 237	Found this on a web page on diagnosing engine problems with a vacuum gauge. The gauge needle fluctuates from 15-20in./Hg rapidly at idle. This would indicate a sticking valve or ignition miss. The gauge reading is low but steady and reads between 15-20in./Hg Indicates a late ignition or valve timing, low compression, stuck throttle or a vacuum leak. The needle fluctuates as the RPMs increase. Weak valve springs, worn valve stem guides, or leaking headgasket. A clogged catalytic converter may also cause this condition. Gradual drop of vacuum at idle. Choked muffler or obstruction in exhaust system. A choked muffler will exhibit a slow vacuum decrease to ZERO at idle. Vacuum gauge needle drifts. Minor intake leak of the manifold or bad fuel injector O-rings. The needle stays steady but then drops regularly and recovers only to repeat. Burnt valves or improper valve clearance adjustment.The needle will drop when the defective valve operates. The needle vibrates excessively at idle but steadies out as the engine RPM is increased. This would indicate worn valve guides. Jim M. Mt Juliet



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