28 OCT-DEC 2016 engine professional

What is a common thread with any type of problem? Symptoms…there will usually be symptoms. That remains true whether you are trying to diagnose a sickness, a leaking roof or a rough running engine. Often times, we are too hasty in our efforts to get a quick and inexpensive fix that we simply treat a symptom and leave the underlying problem unattended only recur a short time later. We naturally throw a bunch of things at a problem and it goes away. Sadly, when we encounter the same problem again, we don’t really know the fix.

There are many publications on the methodology of “Problem Solving” and they are very helpful in many facets of life from home, health and industry. Some processes can be exhaustive and time consuming and we know that is hard to employ in the hustle of day to day business but we can take a common sense approach and use the tools effectively. You don’t have to have tons of charts, files and a computer system just to keep track of where you are in the process. What you will need is a desire to fix the problem, not the symptoms.

So what is critical to fixing any problem that we may encounter? The key is to attack the problem with a cause and effect mentality. If you approach problems with that mindset, you will be more likely to drill down to and find “root cause”. We all know that every weed has roots and they will continue to be a problem until we address or kill the roots. Likewise, every problem has a point at which it propagated or began, hence “root cause”.

In the process of root cause analysis, you will treat symptoms. As you treat the symptoms, you will find that sometimes the problem is corrected and stays away. If you want the process to be effective, you need to keep track of your changes so that you don’t lose track of what you have done or you will be scratching your head thinking “well the problem is gone but how did I get there?” In a perfect world, you re-create the problem by

turning off the fix or treatment and see if the problem comes back. If the problem does come back and you can reapply the fix and it again goes away, you are able to use that knowledge to correct the process that causes the problem or how to correct someone else’s problems that you are dealing with, like an OE design failure that is prevalent in the service industry.

When was the last time that you went to a doctor and said, “Doc I don’t feel well” and they check your temperature to find that it was okay and they just sent you home? More than likely, they are checking blood pressure, pulse rate, weight; look at ears, nose and throat as a baseline and progress from there.

Now how is this related to engine diagnostics? As an industry, we are still servicing many engines that may not have on board diagnostics “OBD”. Therefore, it is more important that we use cause and effect principles to properly diagnose cylinder performance issues. If we have automotive machine shops, we probably know that compression is affected by proper ring seal and cylinder bore distortion, valve to seat seal, guide wear and the list goes on. Much if not all of this information has remained unchanged for many years but we often forget the basics that we were taught long ago or expect technology to have wiped out the need for the basics.

To properly diagnose cylinder performance, we are going to need some tools to help us measure what is happening? Two common tools that are used when trying to diagnose cylinder performance are the compression gauge and the leak down tester.

The compression test gauge is a good tool and can tell you if a cylinder is weak but what it cannot tell you is where EXACTLY the losses are going. A proper compression test requires that you do a wet and dry test, that the throttle plates are fully open and that the starter can turn the engine at max cranking speed. Many times, not all of these requirements are achieved. The following is an

example copied from a manual that calls out the specific procedure for performing the compression test on a particular engine.

Compression Test Procedure1. Charge the battery if the battery is

not fully charged. 2. Disable the ignition system. 3. Disable the fuel injection system. 4. Remove all the spark plugs. 5. Block the throttle plate wide open. 6. Start with the compression gauge at

zero, and crank the engine through four compression strokes (four puffs through the spark plug hole).

7. Make the compression check for each cylinder. Record the reading.

8. If a cylinder has low compression, inject approximately 15 ml (one tablespoon) of engine oil into the combustion chamber through the spark plug hole. Recheck the compression and record the reading.

9. The minimum compression in any one cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 690 kPa (100 psi). For example, if the highest pressure in any one cylinder is 1035 kPa (150 psi), the lowest allowable pressure for any other cylinder would be 725 kPa (105 psi). (I035 x 70% = 725) (150 x 70% = 105).

(continued)

Losing CompressionThe rest of the story…

BY CHUCK LYNCH

30 OCT-DEC 2016 engine professional

LOSING COMPRESSIONBY CHUCK LYNCH

• Normal – Compression builds up quickly and evenly to the specified compression for each cylinder.

• Piston Rings Leaking – Compression is low on the first stroke. Compression then builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.

• Valves Leaking – Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil.

• If two adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase the compression; the cause may be a head gasket leaking between the cylinders.

After performing the compression test and taking into account the complaint that brought the engine to you in the first place, it is a good practice to perform a cylinder leak-down test. An engine leak-down test is basically a compression test in reverse. Instead of measuring the engine’s ability to create pressure, compressed air is introduced into the cylinder through the spark plug hole. One gauge on the tester measures the pressure of the air entering the cylinder and the other measures the percentage of the air escaping (or leakage) from the cylinder. The leakage percentage will indicate the condition of the cylinder and overall condition of the engine.

Leak-Down Tester

Leak-Down Test

Before applying air pressure into the engine, the cylinder being tested must be placed at Top Dead Center (TDC). The piston must be at the top of its

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travel and the intake and exhaust valves must be closed; so it is a good practice to rock the crank tool back and forth slightly to verify TDC. When the compressed air is allowed to enter the cylinder, the leak-down tester will measure any loss of air escaping past valves and/or piston rings and possibly head gasket.

In addition to getting an overall picture of engine condition, the engine leak-down test is an excellent way to pinpoint where problems are before tearing down the engine. The nature of the air escaping through the components is going to provide an audible indicator as to where the air is escaping so that you can isolate the problem.

• Intake valve: Air and noise out of the intake, carburetor or throttle plates indicates a leak at the intake valve.

• Exhaust valve: Air and noise out of the tailpipe, turbocharger or exhaust manifold means an exhaust valve leak.

• Piston rings: Air and noise out of the PCV valve, oil filler cap hole or dipstick tube means the air is pushing past the rings. Suspect ring or cylinder wall wear.

• Head gasket: Air bubbles in engine coolant seen at the radiator filler cap could mean air escaping into the coolant past the head gasket.

• Cracked cylinder head: Bubbles in coolant or coolant being pushed up out of the radiator neck can also indicate cracks in the cylinder head or cylinder walls.

The percentage of leak-down that is allowed is going to differ based on bore diameter to some degree because larger bore diameters will allow larger ring end gaps but an industry accepted rule is 20% for a typical stock passenger type engine. If you are seeing 30% or more, engine service is recommended. The racing industry would not typically be satisfied with any leakage that would reach a double digit percentage.

So why would I even do a compression test if I can learn so much from a leak-down test? A compression test can tell you if the engine has the correct components, if a cam lobe is worn, if there a restriction to incoming air and so on. A particular engine build will have a standard compression pressure range and leak down does not provide standard pressure data.

I hope that this article is seen as more than just a compression and leak-down article. The tools of problem solving are really based around the fact that for every action there is a reaction and if you don’t understand the interrelationship of upstream and downstream factors, you can quickly have yourself headed in the wrong direction.

In review:• Ask “Why” to create an understanding of the

interrelationship of the problem and symptoms(continued)

32 OCT-DEC 2016 engine professional

LOSING COMPRESSIONBY CHUCK LYNCH

• Map where you have been and where you are headed. Documentation can be tough but I venture to say you do it when it comes to fiscal responsibility. It will be less painful when you are presented with the same problems in the future.

• Have standards and stick to them and it will be much easier to see when you are off course. We humans have a tendency to get lost without boundaries. For example, some standards that we probably have always known such as 20% percent leak-down max, .002” seat run-out maximum, .0005” cylinder taper maximum.

I know that this article is going to reach many shops that do not deal with engines that are running in the vehicle. What I do know, based on calls that we receive at AERA, you do become involved when there are problems after the machine shop work is done. Maybe you can use this knowledge to better

help the technicians diagnose a problem and salvage our paying customer’s faith in the engine shop and the installation shop if they are not the same. Whether it’s the daily commuter, fleet vehicle, or Grandma’s station wagon, we are all on the same team when it comes to getting that vehicle back on the road.n

AERA Technical Specialist Chuck Lynch spent 20 years of his career at Jasper Engines in many roles, including process engineer. He has also worked as a quality auditor, analyzed tooling needs, coordinated procurement and training for equipment and tooling, incorporated the use of super abrasives, coordinated failure analysis of components, and more. For more information, email: chuck@aera.org.

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