How to check the condition of your engine

By: Paul Tuzson - Story & Photos

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engine 2 engine 2

Engine problems? Here's two quick ways to find the source...

From Unique Cars #296, March 2009

Engine testing

It would be nice to be able to see the condition of an engine without pulling it apart. Of course that’s not possible, but you can get a pretty good idea of what’s going on in a motor by means of these two excellent and affordable diagnostic tools – the vacuum gauge and the compression test gauge.

In a normally aspirated engine the descending pistons create a vacuum when the throttle is closed, or partially closed. The way in which the engine creates and maintains this vacuum under certain conditions gives a good indication of its condition. This is because many of the problems an engine can develop will affect the vacuum present within it. Similarly, the creation and maintenance of high pressures is also vital, and fortunately it’s also measurable.



1. All car enthusiasts should own and know how to use these.



2. Clearly, this engine is going to need some work. But what might its exact problems be?



3. This engine looks much better but looks can be deceiving. As it happened it was in good condition as far as vacuum is concerned, but plenty of engines dressed up to look decent are pretty sad on the inside.



4. There are people who’ll swear that the inside of the engine you’re buying looks like this...



5. When, in fact, this is closer to the truth. A bit of detective work with a vacuum gauge and compression tester can tell you a lot.



6. Some owner’s manuals specify vacuum at idle, others list compression test pressure. Some specify both along with the maximum permissible variation between cylinders.



7. Compression testers come in two basic types: screw-in and hold-in. Either works however screw-in types will never suffer from not being held in place firmly enough. On the other hand, hold-in types are quicker to use between cylinders.



8. Screw-in compression testers have either fixed fittings with double threads for both spark plug sizes, or removable fittings with single threads. Some removable fitting gauges have quick connectors that allow the fitting alone to be screwed into the plug hole before the hose is attached. These are the easiest to use.



9. Vacuum test gauges have either plain hoses that can be pushed onto vacuum fittings or cones that can be pushed into vacuum sources. A leak free fit is essential or your readings will be worthless.



10. On an older carburetted vehicle there will usually be a spare vacuum source at the base-plate of the carburettor. However, the line leading to the vacuum advance on a distributor shouldn’t be used. The line leading to the brake booster is also a good test point. If you do use this as your connection point, don’t drive the car with the booster disconnected. If you do want to drive it with the gauge still connected, use an appropriate T-piece.



11. The vacuum source should be exposed directly to manifold vacuum.



12. Things the owner didn’t know about were certainly going on in this engine.



13. Vacuum gauges measure pressure too. Although we’ve gone metric, Imperial scales are a good thing because so many of the unique cars we’re interested in are from America.




14. Normal Engine 1

This is what you want to see. If your gauge has a green zone the needle should probably be in it at idle if your engine is healthy. This gauge calls 18-23 inches of Mercury normal. You should also perform a cylinder balance test. To do so, simply unplug each cylinder one at a time. The drop in vacuum for each cylinder should be the same. If the reading doesn’t drop far then the cylinder you unplugged isn’t contributing much to the overall vacuum because it has a problem. If the reading is a few inches lower than specification, but steady, you probably have worn rings. Confirm these results with a compression test.




15. Normal Engine 2

With the engine warm and at idle, rapidly open the throttle, let the engine speed up considerably until the needle drops to zero. Then, rapidly close the throttle. This will create a sudden shortage of air in the manifold and your vacuum reading should rise to perhaps four or five inches of Mercury above the at-idle position. If this doesn’t occur, you may have worn rings. To confirm this, perform a compression test in this cylinder and see if the result is lower than the manufacturer’s specification.


gauge-3-Very Low Manifold or Carburettor leak.jpg


16. Very Low

If the reading is very low and steady, then there’s a leak somewhere in the inlet tract. It could be a manifold gasket, carburettor gasket, manifold fitting, etc. Find it and plug it.




17. Leaking Head Gasket

If your needle is indicating reasonable vacuum but regularly falls by several inches quite sharply, you probably have a leaking head gasket. If the drop is considerably more then it will have failed between two cylinders. When one of the cylinders fires, pressure passes into the neighbouring cylinder when it’s on its induction stroke. This passes out through the open valve and into the manifold causing the increase in pressure.




18. Leaking Valve

If a valve sticks and doesn’t close properly during compression, part of the contents of the cylinder will be pushed back into the manifold reducing the vacuum. This will show up as a reading that drops a few points irregularly. So will a leaky valve, due to cylinder pressure on the combustion stroke.




19. Leaking Valve Guides

When your valve guides are leaking, the needle will fluctuate a few points as shown. This is because air from the rocker cover leaks past the guides and reduces vacuum. The needle fluctuates because sometimes the leakage from a guide is pulled into its cylinder when the valve opens and doesn’t contribute to reducing vacuum in the manifold as much.

A burnt valve is a bit different to one that sticks because it never seals properly and compression pressure won’t push it shut and stop the leak. When the cylinder is fired, some of the pressure escapes back past the valve and reduces vacuum regularly by a few more points than a sticking or leaky valve. This happens every time the cylinder fires. Again, a cylinder balance test will pinpoint the problem cylinder. When it’s unplugged and doesn’t fire, the problem will be absent.




20. Weak Valve Springs

When a motor is running fairly fast, weak valve springs won’t hold the valve on the cam lobe. At maximum lift the ‘nose’ of the lobe passes under the lifter and the valve spring stays compressed for a touch longer than it should which leaves the valve open longer. This happens randomly which is why the needle fluctuates. Running the motor faster makes the problem worse and the needle fluctuations will reflect this. If the fluctuations are rapid but not constant, you may have a broken valve spring that affects the reading as it tries to close.




21. Exhaust – Good

If you open the throttle slowly until the engine is at about 2000rpm and then close it quickly the pointer should rise quickly due to the sudden lack of air. In a system without excessive back pressure the pointer should return to normal quickly and smoothly. If the return is slow and irregular, back pressure is likely the problem.




22. Exhaust – Bad

If you maintain the engine speed mentioned above, back pressure will be indicated by a gradual reduction of the reading first obtained at 2000rpm as the manifold gradually fills up. Similarly, if the reading is fairly high when the engine is first started and then drops back to zero or just a bit higher and then climbs to a low reading, it’s because the exhaust is blocked. Initially, the pipe is empty but when it fills up flow is blocked, back pressure enters the manifold causing increased pressure, then the engine settles down to the level it can run at with the exhaust restriction.




23. Late valve timing

Late valve timing means that the piston has already travelled some distance down the bore before the valve opens. This reduces the efficiency with which air is drawn into the cylinder. The reduced vacuum in the cylinders shows up as reduced vacuum in the manifold which creates a low, but steady, reading on the gauge. Mind you, this is a sign of very late timing.




24. Ignition Faults

If the reading is off by a point or two in either direction and the needle moves slowly over that range, you’ll have some sort of an ignition fault that’s causing power drops in one or more cylinders. The needle moves as it does because the fault isn’t regular and the cylinder or cylinders make power with varying efficiency. A lot of different things can cause this. There’s too many to go into here, but we’ll do another article about electrical testing in a coming edition.




25. Poor Carburettor Adjustment

A poorly adjusted carburettor will cause vacuum to read low and fluctuate slowly over a few points, because the engine won’t be running at its maximum efficiency and the fuel supply will be irregular. It can look fairly similar to ignition faults.

| Read next: How to pull down an engine


There are some other things you can do with a vacuum tester. Setting your idle mixture to give the highest vacuum reading possible with the motor running at normal operating temperature means that you are supplying just the correct amount of fuel to make it run at maximum efficiency. It’s the same with setting the ignition timing at idle. The higher the vacuum, the more efficiently the motor is running. Of course setting the timing and idle using these methods isn’t ideal. But they’ll get you close if a timing light or tachometer isn’t to hand.


Many vacuum gauges are also able to measure pressure like the one shown and you can use them to do a dry check of your fuel pump. If you disconnect the fuel lines from the pump, attach the vacuum gauge on the outlet side and then operate the lever a couple of times, you should get a reading roughly equal to the rated operating pressure for the pump. Connecting the tester to the inlet side of the pump should show a reasonable vacuum.

So, a vacuum gauge and compression tester can tell you a great deal about what’s going on in your engine. Make sure you add these instruments to your tool kit.

Mike’s Dyno Tuning in Victoria showed us how all this works. If you’d prefer someone else to do the work, you can give Mike a call on 0402 846 418.


From Unique Cars #296, March 2009

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