There are many diagnostic tools now available, mostly electronic, often in the form of leading edge apps and electronic diagnostic tools or simulations. While this is great for electronic fuel-and-spark system or overall powertrain diagnostics, there is one inexpensive and time honored spot check for the engine long block's* condition—the simple engine testing vacuum gauge.
*Note: The long block is the engine block with all of the reciprocating parts plus the cylinder head installed. If the long block is in good operating condition with normal valve lift and valve timing, the rest of the engine's performance is about spark, fuel and exhaust tuning.
Years ago, when I began working as a truck mechanic, the vacuum gauge was a standard tune-up item alongside a compression gauge. The vacuum test was considered quite useful for determining engine compression loss, vacuum leaks and loss, needed spark timing advance, the engine's condition under load, leaking valves and more!
Hooking a vacuum gauge to your engine's intake manifold source (below the carburetor or EFI throttle body) can be very revealing. To begin, the vacuum should read steady. Depending upon the altitude at the shop, manifold vacuum should be in the 18-22 in/hg range at an engine idle under no engine load—with a stock or RV camshaft.
If low, don't panic yet, the engine's spark timing may be retarded. Base spark timing has an overall effect on manifold vacuum, and advancing the timing will raise manifold vacuum. Of course, there's a limit to the amount, as the engine will begin to ping or detonate if spark timing is too far advanced for the fuel's octane rating. On modern EFI engines, timing is often fixed by the crankshaft position sensor (CPS) and the PCM/ECU/ECM software programming. The computer will instantaneously and continuously adjust spark timing. For tuning purposes, it's assumed that timing is adequately advanced unless the engine is in limp-home mode.
Vacuum gauge troubleshooting and readings can include a wavering or fluctuating gauge needle. This is the sign of a valve that is not sealing or seating properly. If you see this on the gauge at an idle with the throttle closed and no load on the engine, suspect an unseated valve.
Causes of an unseated valve(s) are burned valves, bad valve seats or valves adjusted too tightly. On an AMC/Jeep engine that has non-adjustable rocker arms, there are several causes for a fluttering vacuum gauge needle: pushrods too long, a surfaced block deck and/or cylinder head with the original pushrods, high valve stem heights, or a thin head gasket. Each can cause a valve(s) to remain open when they should be seated. Driven in this condition for any length of time will either burn a valve(s) or cause carbon buildup on the valve face and seat.
Manifold vacuum is important enough to be part of the gauge cluster on race cars and other performance engine applications. The Auto Meter 2337 gauge (left) is dash mounted for continually monitoring a performance engine while operating the vehicle. At right is an inexpensive Equus 3620 Vacuum Gauge test kit. Maximum fuel efficiency depends upon the highest tolerable manifold vacuum—including spark timing advanced to just below the point of spark knock or detonation (ping)! Note the efficiency zones built into the readings for each of these gauges.
Not a sophisticated tool but surely an important tool in your tuning equipment, the vacuum gauge says a lot about an engine's running compression and cylinder pressures. The gauge can help identify poor valve lift from worn camshaft lobes and lifters, unseated valves, low compression, retarded valve timing from a worn timing chain, retarded spark timing, vacuum leaks and losses, plus the overall engine condition and tune. Unlike both a compression gauge test and a cylinder leakdown test, the vacuum gauge is a real time, running engine test!
EFI sensors that parallel a simple vacuum gauge test would be the MAP and idle air control signals. MAP factors for barometric pressure and altitude changes, important for onboard PCM/ECU/ECM tuning of an EFI fuel and spark management system.