How Do I Do a Vacuum Diagnostic

The vacuum test will tell you heaps about the
engine: bad valves, bad rings, bad ignition, bad timing, etc

You need to put the engine back to together for these tests. The vacuum readings are taken from the manifold so:

Step 1 Find a connection point, like the fuel canister line or brake booster.

Step 2 Beg, borrow or steal a vacuum gauge (they're fairly cheap from car accessory places)

Step 3 Connect gauge and start the engine and let idle

Step 4 observe the gauge needle: It should be steady and somewhere between 14" and 22" Hg (mercury column). If you have a steady low reading of 8" to 12" you may have low compression due to ring wear (see step 10).

Step 5 if the needle is fluctuating 1" - 2" Hg (for example between 18" and 20" HG) then you've probably got bad leads, bad dizzy cap, faulty coil, bad plugs or something else in the ignition giving problems.

Step 6 if the needle is fluctuating 3"- 4" Hg then you've probably got dicky valves (sticking, burnt or rockers over adjusted). For example the guage fluctuates between 14" and 18" Hg.

Step 7 If the needle is fluctuating 13" -15" Hg it indicates weak or broken valve springs. For example between 10" and 25" Hg.

Step 8 if the needle floats aimlessly 4" - 5" you have a fuel problem.

Step 9 if the needle drops sharply to about 10" or 12" Hg before rising back to the general vacuum, then you have a head gasket leak on one of the cylinders. If it drops much more than 10" Hg you have a blown head gasket between cylinders:- for example it may fluctuate between 7" and 20" Hg.

Step 10 a steady low idle speed reading (less than say 10 " or 12" Hg) indicates leakage through brake booster, manifold gaskets, etc. If these are ok then it also indicates late valve timing and/or bad piston rings

Step 11 give the throttle a quick blip and the gauge should drop to about 5" Hg and rise up to a peak of about 23" Hg before settling back to step 4 reading. If it doesn't peak about 5" Hg above normal vacuum before settling back down, you probably have worn rings.

Step 12 gradually increase the motor speed to about 3000 rpm. The vacuum should be the same as in step 4. If it is lower the exhaust system is probably clagged and needs to unrestricted. Blip the throttle and see that the vacuum retruns to normal vacuum quickly; a slow return to normal indicates a blocked exhaust. A slow decrease in vacuum to near 0" at idle is also an indicator of restricted exhaust

Step 13 increase engine speed, if the reading vibrates rapidly ranged over 4" Hg, you may have a leaking intake manifold gasket or head gasket, weak valve springs, burned valves, or ignition misfire. If you have noticeable exhaust smoke with the vibration your valve stem guides are probably too worn.

Step 14 general observation of the needle should give you a quick idea of what's going on. A lazy or aimless drifting of the needle indicate poor idle fueling or leaky manifold/carby gasket. A vibrating needle at idle that steadys as revs increase indicates worn valve stem guides. A needle vibrating at all engine revs indicates a leaky head gasket. A fluctuating needle as revs increase indicates worn valve stem guides and/or weak valve springs. Reading gradually dropping to around zero points to a blocked exhaust.