Reading a vacuum gauge

Motor, Aug 2001 by Layne, Ken

Just to be sure that the evaporative emissions system doesn't interfere with vacuum testing, disconnect and plug the canister purge hose and its manifold port. If you're testing an OBD 11 car, check for evap-related DTCs when you finish testing to be sure none set.

Run the engine at idle, low cruise (1800 to 2200 rpm) and high cruise (2500 to 3000 rpm). Note the vacuum readings, and any fluctuations, at each speed. Next, hold engine speed steady at about 2500 rpm for 15 seconds and read the gauge. Now release the throttle and watch the gauge as the speed drops. The vacuurn reading should jump as the throttle closes, then drop back to its normal idle reading. if vacuum doesn't increase at least a couple of inches when you release the throttle, you may be looking at worn rings, cylinders or valves.

Idle vacuum for most engines is about 18 to 22 in.-Hg, but some may produce only 15 to 17 inches at idle. (Remember what we said about experience.) If vacuum is steady and within these ranges, the engine and fuel and ignition systems are operating normally.

If vacuum is steady at idle but lower than normal, the ignition or valve timing may be retarded. Low compression, an intake leak or tight valves also can cause low vacuum at idle.

If the vacuum reading fluctuates within the normal range-the gauge needle bounces around a lot-uneven compression (broken rings or leaking valves or head gasket in one or two cylinders) is a likely culprit. An uneven air/fuel mix, erratic ignition timing, a misfire, misadjusted valves or a manifold leak near one or two cylinders also are possible causes.

If vacuum drops intermittently at idle, one or more valves may be sticking open or dragging. Higher-than-- normal vacuum at idle is a common clue to overly advanced ignition timing, while low vacuum can indicate retarded timing.

Low vacuum also can be an immediate clue to a plugged exhaust. To check further, run the engine at about 2500 rpm for about 15 seconds. If vacuum drops during this period and does not increase when you close the throttle, you're almost certainly looking at a restricted exhaust.

Vacuum Fluctuations & Power Balance

Several of the guidelines in this article have distinguished between steady vacuum gauge readings and fluctuating readings, where the gauge needle bounces up and down erratically. This may seem secondary-almost inconsequential-but it's an important distinction. A steady but abnormal vacuum reading indicates a problem common to all cylinders. Things like incorrect ignition timing or an old, tired, high-mileage engine affect vacuum equally for all cylinders. A bouncing needle, however, usually indicates that the problem is localized to one or just a few cylinders. Here's where power balance testing enters the picture.

Compression testing on many late-- model engines is flatly impractical from a labor standpoint for a quick engine evaluation. That's especially true on some of the weird vans for which removing and reinstalling spark plugs is a two-hour job. It's relatively quick and easy, however, to connect a vacuum gauge to the manifold and your engine analyzer to the ignition system.