Vacuum Readings

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On the back of the engine near the firewall you'll find a vacuum tree. Normally these have one or two ports that have a cover over it. Remove one of these covers and attach a vacuum gage. Compare your readings to these:

Engine Speed=Reading=Indication of Engine Condition
Smooth and steady idle (800 to 1200 RPM)=Between 17 to 21 inches=Engine is in Good Condition, but perform next test to be sure.

Open and close throttle quickly=Jumps from 2 to about 25 inches=Engine is in Good Condition.

Smooth and steady idle=Steady, but lower than normal reading=Worn rings, but perform next test to be sure.

Open and close throttle quickly=Jumps from 0 to 22 inches=Confirms worn rings.

Steady idle=Intermittent dropping back 3 or 5 divisions and returns to normal=Sticky Valves. If injection of penetrating oil into intake manifold temporarily stops pointer from dropping back, it's certain the valves are sticking.

Steady 3000 RPM=Pointer fluctuates rapidly, faster engine speed causes more pointer swing=Weak valve springs.

Steady idle=Fast fluctuation between 14 to 19 points Worn intake valve stem guides.

Excessive pointer vibration at all speeds indicates a leaky head gasket.

Steady idle=Constant drop=Burnt valve or insufficient tappet clearance holding valve partly open or a spark plug occasionally miss firing.

Steady idle=Steady 8 to 14 inches=Incorrect valve timing. It must also be remembered that vacuum leaks and/or poor compression can result in a low vacuum reading.

Steady idle=Steady 14 to 16 inches=Incorrect ignition timing.

Steady idle=Drifting from 14 to 16 inches=Plug gaps too close or points not synchronized.

Steady idle=Drifting 5 to 19 inches=Compression leak between cylinders.

Steady idle=Steady below 5 inches=Leaky manifold or carburetor gasket, or stuck manifold heat control valve.

Steady idle=Floats slowly between 12 and 16 inches=Carburetor out of adjustment.

Blipping engine speed=Quick drop to zero then return to normal reading=Muffler is clear.

Blipping engine speed=Slow drop of pointer then slow return to normal reading=Muffler is choked or blocked.

A good vacuum gage and this chart can go a long ways to helping solve a lot of engine troubles.

Vacuum Readings

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.

Vacuum Pressure levels

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At idling speed, an engine at sea level should show a steady vacuum reading between 14" and 22" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 23" or more.

With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, spark plug gap, primary ignition circuit, high tension cables, distributor cap or ignition coil. Fluctuations of 3 to 4 inches may be sticking valves.

Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, manifold-to-carburetor gaskets, vacuum brakes or the vacuum modulator. Low readings could also be very late valve timing or worn piston rings.

Cylinder Head Gasket Leakage:

With the engine Idling, the vacuum gauge pointer will drop sharply, every time the leak occurs. The drop will be from the steady reading shown by the pointer to a reading of 10" to 12" Hg or less. If the leak Is between two cylinders, the drop will be much greater. You can determine the location of the leak by compression tests.

Fuel Control System Troubles:

All other systems in an engine must be functioning properly before you check the fuel control system as a cause for poor engine performance. If the pointer has a slow floating motion of 4" to 5", you should check the fuel control.

1995 Chevrolet Blazer with a 4.3 W vortec with CMFI. It has 17 inches of vacuum at idle so I don't think that it has a leak.

[ ] Proper engine operation should result in an even vacuum reading from 15 inches to 20 inches of vacuum with 15 inches of Mercury being more appropriate for high performance type engines. A vacuum reading that cycles up and down slowly could indicate a fuel mixture problem from improper carburetor or fuel injection operation.

Low fluctuating reading If the needle fluctuates about 3 to 8 inches below normal, suspect an intake gasket leak at an intake port, or possibly a faulty injector on port-injected engines.