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hello a friend loaned me his vacumm gauge to check out my 71 240z...well im no expert on this and this is what i got from an idle..maybe some of you guys /gals who are good at this type of equipment reading can help me out...okay at idle i got a reading of 20, from installing the vacumm hose at one of the intake vac. ports..now the thing is it fluctuates ( the needle on the gauge) rapidly at a drop from 20. to 18 or so ( hard to tell because it flucuates so fast) it never goes over 20 at idle just drops down to 18-19 and back to 20 real fast...what does this indicat??? also when i bump up the rpm's the gauge drops toword zero real quick and then up to a higher reading depending on the rate of rpm's is this normal to have such a large drop on initial rpm increase ??? the car does idle alittle rough but has good power thruogh the gears when i drive ..any advise to whats going on.?? thankyou michele
 

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Re: vacuum gauge

Most Chilton books have a section on vac. readings.

The gauge s/b steady at idle conn. to the manifold port. A range of 18 to 20 is an OK reading BUT any movement means that something could be wrong in the head, like a leaky/sticking valve. or worn valve guide.

The opening of the throttle kills the vac. level until the engine 'catches up' with the vac. level. So, what you saw was normal.

If you took the engine vac. reading while the car is coasting with the throtle closed, you'd see the opposite - a higher than normal vac. level.

I have an old vac. ga. instruction sheet that I may be able to scan and send to you. If you'd like me to, let me know your email address if diff. than above. My scanner has not been too reliable. tho.

A VOM and a vac. ga. are two essential tools for diagnosing car problems.
 

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Re: vacuum gauge

the flucuations could be from a leaking valve or on one that needs to be adjusted that is too tight . Valves are like kids when you cant hear them , then you wonder what their doing. Gary Portland Or. The best to ya , Gary Portland
 
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