Petrol engines suck – and if they are running well they should suck a lot. Before engines developed the sophistication of today’s models the vacuum gauge was an essential part of an engine tuner’s equipment in routine use. Now it is only used when all else fails. Good engine vacuum, though, is still as important as it ever was, since at its heart every engine is essentially a suck/blow machine whose efficiency is best when working with optimal suck/blow values. Indeed with engines equipped with a Map sensor for calculating the air flow, the engine vacuum is even more important since a lower than normal vacuum causes the electronic control unit (ECU) to over fuel the injectors.
Most vacuum problems are apparent at idle speed, where a steady value of between 18 to 21 inches is considered normal, though some engines - particularly those with high overlap camshafts - can cause much lower and less steady readings. When the possible reasons for a poor vacuum reading are investigated it becomes apparent why the vacuum gauge has fallen into disfavor. The list is not short. It’s easier if the reasons for low idle vacuum are split into areas – engine breathing, excessive throttle plate opening, and vacuum leaks.
Engine breathing faults involve anything that restricts the ability of the engine to flow gas in through the inlet and out of the exhaust pipe. These faults are: Low engine compression, wrong valve timing, worn camshaft, incorrect valve clearances, blockage in the exhaust, or broken valve spring.
Biggest offender for low compression’s, we find, is the Rover 16v engine – normally we expect about 175 Psi. Below 150 Psi and problems occur with the Map sensor making the ECU over-fuel. Wrong valve timing is easily checked – unless, as we experienced recently, a Rover 16v was found to have the inlet and exhaust cam wheels interchanged. All the timing marks lined up, the engine ran at idle sort of, but wouldn’t run at speed. The fault was only found when the cam cover was removed and the cam lobe positions inspected at Tdc.
Worn camshafts with hydraulic tappets often don’t make a noise, as most of us have experienced with Astras, etc. The exhaust has to be almost completely blocked before it will cause a low vacuum reading at idle. It’s more worthwhile measuring the vacuum under full load at speed where the manifold vacuum should be almost zero. Treat any reading over 1 inch vacuum with suspicion. Better, though, to measure the pressure in the exhaust manifold down-pipe if a blocked exhaust is suspected. If the catalyst is blocked then the silencers downstream from it may have collected its debris and be blocked also.
Broken valve spring diagnosis by vacuum gauge, although listed in engine testing books, is not really an option. If suspected, then visually inspect them. If this is difficult then use a vacuum transducer to display the vacuum pulse waveform on an oscilloscope. The broken spring fault will show up as a missing or poor vacuum pulse for the offending cylinder. This is an excellent method for detecting the sticky valve syndrome associated with cramming lots of valves into multi-valve heads as well.
Excessive throttle plate opening is caused by anything that makes the engine run inefficiently, thereby requiring a greater throttle plate opening to run at the same speed than its efficient throttle plate settings. Faults are too rich or lean a mixture, ignition timing which is too retarded, and an engine with excessive mechanical drag.
Vacuum leaks can be tricky to find. A favored method is to spray carburetor cleaner onto the suspected leak and see if the engine speed changes. The problem with this method is that unless the emissions are too lean before spraying, then the engine speed may stay unchanged. Even if the emissions are too lean, a positive response is often due to some of the carburetor cleaner vapor being sucked into the normal air inlet route, and of course the fire risk. The method that we use as a screening check for vacuum leaks is an ultrasonic leak detector. This detects the ultrasonic waves emitted from air when its velocity goes sonic – as it does across a high vacuum or pressure leak. (But not across low vacuum leaks – so it can’t be used on the air filter side of the throttle plate). It’s brilliant at discounting external vacuum leaks, but unfortunately electronic injectors , distributors, and exhaust leaks also produce ultrasonic, so care is needed in diagnosis.
Another method is the stethoscope – good on moderate plus vacuum leaks, no good on small leaks, though. In practice, because all methods have their shortfalls, we use a combination of all three. Sometimes the vacuum leak can be made to ‘speak up’ to reveal itself – always worth a try. With the engine running, block the air inlet off. As the engine dies, the leak may squeak or whistle under the higher induced vacuum.
Long inlet manifolds like the Jaguar XJ series are difficult to diagnose because they can have many small hard-to-detect leaks along the length that add up to one large leak
All of these methods are for external vacuum leaks – useless for internal leaks. Most internal leaks can be found by clamping off the vacuum hose to whatever vacuum component is suspect. The most memorable example of this was a Lotus Elite which was brought to us running on only 3 cylinders at idle. When the owner picked it up and asked what was found to be the problem, I replied “Your headlights”. His face was a sight to behold. The headlights are vacuum operated, and the controller that was leaking took its vacuum supply from Number 1 cylinder inlet leg, reducing the vacuum enough at that point to cut that cylinder out at idle.
Some internal leaks, though, cannot be found by clamping off hoses – for instance the Ford V6 and the Rover V8 engines can both split their inlet manifold gaskets and then they leak vacuum through the crankcase. To diagnose this fault, disconnect the Pcv system from the relevant rocker cover, blank off the rocker cover hole that it came from, remove the oil cap and place the palm of the hand over its hole. There should be slight pressure - if it sucks at all then the inlet gasket needs renewal.
Breather systems are often responsible for air leaks – people are ingenious at removing restrictors in vacuum lines and rerouting breather pipes to strange places. Always check these when the engine is running weak because someone may have managed to reroute inlet air to by-pass the air flow meter or the carburetor.