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"No-Compression" Primer

new 7/21/04

I thought I might put together a few musings on “No-Compression” since a few list members had questions. Much of what I know goes back to the Stone Age days of carburetors & distributors but is still applicable.

As noted any diesel, 4-cycle or even 2 cycle engine has a compression phase where the valves are closed and a piston compresses a fuel/air mixture. A general “no compression” diagnosis indicates that a serious internal problem (as opposed to an external fuel or ignition problem exists)

For about $25 Sears, (and everyone else) used to sell a compression gage with a rubber fitting a mechanic could jam into a spark plug hole. With the carb butterflies jammed open and the ignition disabled the mechanic would turn over the engine using the starter and the gage would retain its highest reading.

A mechanic would write down the reading for each cylinder. Tune up books gave target readings for new engines on the spec pages. As I remember very low compression engines (8:1 CR) may have readings as low as 160 PSI, while very high compression (12:1CR) engines might read about 200 PSI. The general rule is the lowest cylinder should have a reading within 10% of the highest, which is an important rule as any engine wears. No engine at cranking speed achieves its theoretical compression ratio because all engines have some valve over-lap when both valves are open.

It is possible, but rare, to have “better than new” readings, which indicates carbon build-up or perhaps either the spec is wrong or the pistons have been replaced.

Far more often problem: a few cylinders may read 20-30% low. The trick then was to repeat the procedure after squirting a teaspoon of heavy oil in the sparkplug hole. If that fixed the piston ring problems are suspect since heave oil will temporally fix ring blow by but has no effect on bad valves.

In the days of old (before catalytic converters) engine scopes had the ability to do an electronic compression tests. Essentially with the engine running at normal temperature a mechanic using the engine scope could selectively kill the spark to each cylinder in turn and note the change in idle rpm. The change should be about the same for each cylinder, about 10% or less. In this case if the RPM doesn’t change, that is a bad thing, indicating that one cylinder is not doing its share of work. This procedure sends wads of unburned fuel out the exhaust which is bad for the cats, and is no longer recommended.

My favorite procedure is a “cylinder leak down test” which requires a $100 cylinder leak down tester. It is a gage with a nipple that seals a cylinder near TDC and fills it with air from an air compressor (sold separately). Then a mechanic checks the gage every few minutes to see how well each cylinder holds air.

Some of the same rules apply; heavy oil will fix ring blow-by but not valve problems. With a mechanics stethoscope one can listen for air leaking from air escaping into the intake manifold, exhaust manifold or crankcase.

Trusting only in my long term memory, since short term memory is shot, you may find these random thoughts also helpful:

One might also have a blown head gasket, a clue would be compressed air (or exhaust gas) showing up in the coolant. I think their there is a “morning after” test for exhaust gas in the coolant. Two adjacent cylinders also indicates probable head-gasket problems.

Valves can be bent, cracked or even have their head crack clean off. Valves seats could be just worn out and have slow leaks. That is good news; you just need your heads rebuilt. As a rule, if you need to rebuild one head rebuild both of them.

Piston rings seal a piston to the cylinder walls and they need to stay springy and free. If they gum or carbon up the piston will lose its seal and the piston may or may not be reusable.

I have seen a small nut drop down an intake and bounce around and damage pistons, it could crack a piston. In our case I fear carbon build up from the stem of the shrouded intake valve falling off and having the same effect.

In the old days one could easily remove the valve covers on a V8 engine and inspect valve and spring operation for normal operation. It is a lot more work in our case to inspect the valve operation.

One last possibility: lubrication failure. If a bearing or wrist pin starts to fail the supply of oil will not be adequate to both lubricate and cool. In an engine almost every metal part never rubs directly against another metal part, everything actually rubs against a microscopically thin film of oil which is quickly replaced with fresh oil. Any time that thin film breaks down and metal rubs on metal the local temperature skyrockets. So if an oil galley becomes plugged, or a bearing works dry something will quickly over heat and as an indirect consequence one may have a loss of compression.


Compression is a good thing. One of the by products of the four strokes of an this style internal combustion engine (intake, Compression, power, exhaust or if you prefer as I learned it in high school from a less then conventional shop teacher: suck, squeeze, pop, toohey). The purpose of the compression is to take the intake charge and squeezing it into a highly volatile charge that the spark plug ignites creating the power pushing the piston to the bottom of the stroke rotating the crankshaft and forcing the other cylinders into their respective strokes. Without a sufficient amount of compression from tightly sealing rings, valves (intake and exhaust), gakets and of course the cylinder/head material, the charge is greatly reduced in it's power producing capability and could result in washing the oil from the cylinder walls as it did in my SHO motor when it last the seal around the rings as a result of the piston burning.

Grand Ams with the Quad Four have a nasty habit of blowing the head gasket, the standard single overhead cam warps heads (I have had a couple of both flavors).

Carter Fuji

IIRC 8:1 is closer to 120psi, not that gauge readings tell you true static compression anyway, only cranking compression. What's important is that they are all even within 5-10psi




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