Vacuum Gauge reading

[hilltopfarm]

Hi all,

I am having some issues with my 88' 2.9L truck. When I first start it up it revs waaay up there to about 3K RPM, then drops down to around 1K. The engine then has a really low pitched intermittent "booming" sound, it is only heard at idle, and when listening to the exhaust. I went to my tailpipe and put a piece of paper over it and about every 20-30 sec. it would rapidly flick in then continue to blow outward. I also have a vacuum gauge and the needle at idle is NEVER steady (going between 20-23 Hg) and it also has a very fast flicker, probably on the order of 10 times per second. Do you have any ideas what this may be an indication of?

Thanks,
- Hilltopfarm

 

[Tedybear]

How To Interpret Vacuum Gauge Readings

Contributed By: Gordon McKenzie

I'm using the Haynes Fuel Injection book as it seems to give the longest description of the readings. I doubt this is a copyright infringement as this is public knowledge, but I'll give them credit anyway as they put it together well.

Normal vacuum: All motors regardless of number of cylinders all have about the same vacuum. 15 to 20 inches of mercury (in-Hg) At wide open throttle (WOT) it will approach 0. On deceleration it can briefly go as high as 25 to 28 in-Hg. I think higher compression motors are at the high end while our big lumbering 360;s are middle to low end with 454's being the lowest. Don't quote me on that. For every 1000 feet above sea level, normal readings will drop 1 in-Hg
Cranking vacuum: 1-4 in-hg when cranking with the ignition disabled and throttle at WOT.
Operating vacuum: 15-20 in-Hg at idle. 19-21 in-Hg at steady 2500 rpm. About 0 at WOT. 25-28 while decelerating. In all cases the needle should be fairly steady.
Low stead reading: Leaking gasket carb base, leaky vacuum hose (biggest FSJ problem), late ignition timing or late crankshaft timing (jumped timing chain?)
Low fluctuating needle: Needle fluctuates three to eight inches below normal. Suspect a leak in the intake manifold gasket.
Regular drops: Two to four inch consistent drop. Likely a leaking valve. Perform a leakdown and compression check to isolate the bad valve.
Irregular drops: Two to four inch occasional drop. Sticking valve or ignition misfire. Do a leakdown and compression check as well as inspecting the plugs for fouling.
Rapid vibration: A rapid 4 in variation at idle combined with exhaust smoke indicate worn valve guide. Perform leakdown test to confirm. If rapid vibration occurs with an increase in engine speed, check for a leaking intake manifold gasket or head gasket, weak valve springs burned valve or ignition misfire.
Slight fluctuation: A slight 1 inch fluctuation may mean ignition problems Check all tune up items and try an ignition analyzer.
Large fluctuation: Perform compression or leakdown test to find a weak or dead cylinder or blown head gasket.
Slow Hunting: Needle moves slowly through a wide range. Check for clogged PCV, incorrect idle mixture or carb to manifold gasket leaks.
Slow return after revving: Snap throttle to WOT until engine at 2500 rpm. then release. Vacuum should drop to 0 then go to 25-28 then back down to normal. If vacuum returns slowly and doesn't peak the rings may be worn. If there is a long delay, look for a restricted exhaust. If the exhaust seems restricted, disconnect at manifold and repeat the now loud test.

http://www.ifsja.org/tech/motors/vacuum.html

Nice thing about vacuum and engines. It works across the board. <G>

 

[PanamaExpat]

Without reading that long post from teddybear: Sounds like a sticky valve or a lifter that is bit soggy. Put some Marvel's Mystery Oil in it and see if it clears up after a couple of days. Can't hurt anything to try it.

By the way 10 times per second would then mean your motor was idling at 600 rpm. That makes me think it might more be a lifter. Do you have the classic valve train tick?

 

[tmcalavy]

Good post on interpreting a vacuum gauge readings...Google "reading a vacuum gauge" and one of the return hits will show you animated gauges with different readings and what they mean...just like the post above does in writing. If you're a visual person, it's a great tool to have next to you when you have the gauge hooked up and are watching the readings.