Cylinder drop test -- when to disconnect wires

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Author Topic:   Cylinder drop test -- when to disconnect wires

Frank O posted 02-02-2009 09:46 PM ET (US)
A mechanic suggested conducting a cylinder drop test on my 1994 Mercury 3.0L Offshore 225-HP 2-stroke carbureted outboard to help diagnose rough running. In the test, spark plugs are disconnected one at a time to see what effect this has on the rpm. A substantial drop means that cylinder is probably firing well, whereas if there is little or limited effect this implies that there is a problem with the cylinder. By conducting this test in conjunction with testing spark plug wires with a timing light, it's possible to differentiate between an electrical vs. fuel-related problem.
I've heard two opinions about how to carry out the cylinder drop test:
1) Leave the engine running throughout. Disconnect each spark plug wire using a well-insulated pair of spark plug pliers.
2) Turn off the engine when disconnecting each wire. Make sure to ground out the disconnected spark plug wire. Then run the engine briefly to observe rpm. Turn off the engine, and repeat with the next spark plug wire.
I gather that the advantage of the first approach is that you get a better sense of subtle changes as each wire is disconnected. The advantages of the second approach are that it's safer, and it also avoids electrical damage to the stator or power pack that might be possible if the spark plug wire were ungrounded.
Any suggestions on which approach would be better with my 225-HP 2-stroke V-6?

seabob4 posted 02-02-2009 09:53 PM ET (US)
Why not do a compression test? A whole lot safer, and less 40,000V ready to zap you!
You're looking for a "bad" cylinder, than I assume you have already ruled out plug, coil, and wire. If you haven't, that's where you need to start. If you have, do a compression check. That'll point out the offending bore in a heartbeat.
Frank O posted 02-02-2009 09:56 PM ET (US)
Actually, yes, I'm going to do a compression test too. That in conjunction with the cylinder drop test and a check with timing light would all offer different views of what in particular is wrong with the problem cylinder, no?
A2J15Sport posted 02-02-2009 11:48 PM ET (US)
The "cylinder drop test" is a valid one. A compression test will not tell you if you have a clogged carburetor or if the ignition is malfunctioning.
Jeff posted 02-03-2009 09:13 AM ET (US)
You mean a "Leak Down" test?
Frank O posted 02-03-2009 09:42 AM ET (US)
A leak-down test is yet another test -- it involves pumping compressed air into each cylinder with dedicated test apparatus.
A2J15Sport posted 02-03-2009 10:07 AM ET (US)
The "drop test" tells you if a cylinder is contributing, or not. IF it is not, the other tests may tell you why.
A2J15Sport posted 02-03-2009 10:10 AM ET (US)
The cylinder drop test is used extensively throughout the automotive industry.
It is called other things, such as: "cylinder contribution/percentage". It is the same thing. How it is done-varies. Most often, it is accomplished via test equipment.
Frank O posted 02-03-2009 12:10 PM ET (US)
I guess the question I was originally trying to get at is:
If I disconnect each spark plug for a few seconds each while running the engine, do you think there's much risk of damaging electrical components?
Tohsgib posted 02-03-2009 12:30 PM ET (US)
Won't hurt a thing on that engine. If you don't keep it running you probably won't notice it. With 6cyls it can be a very slight drop sometimes..hence why people blow them up more often because they don't realize they are running THAT poorly.
A2J15Sport posted 02-03-2009 12:55 PM ET (US)
No issue whatsoever.
Your tach. and your ear are the best indicators for cylinder contribution.
The tach. should drop an equal amount when each plug wire is disconnected.
Frank O posted 02-03-2009 01:14 PM ET (US)
Thanks, guys. I had a hunch that with an engine this size what you say would be the case.
outragesteve posted 02-04-2009 02:46 PM ET (US)
Doing a cylinder drop test is a good test to find a "dead" cylinder. Depending on the engine, it may lead to a fuel or electrical problem. Although it may be hard to do, dropping a cylinder is more noticeable with the engine under a load. This can be done in a test tank, with a test prop or hooked to a dyno. I have done this test on a trailer, backed in the water, in gear, with some RPM. This can be dangerous, so do not attempt this if you unsure of the procedure. I also find it easier to pull the plug boots if you first secure two long tie wraps to the plug wires and "pre-pull" the wires first. You do not need to pull the wire off the plug very far. The tie wrap allows you to place the boot back on that plug and move to the next. Goog luck.
Jerry Townsend posted 02-04-2009 04:12 PM ET (US)
Dropping a cylinder and noting the change in the rpm can be very informative - loaded or unloaded. On car and truck engines - I have used an ice-pick with another aid (that I always have in each tool-box) - a 3 - 5 feet length of 10 guage wire with alligator clips at each end. Use the wire to connect the ice-pick shank to the ground - and then gentally push the ice pick point through the boot until it contacts the spark-plug contact.
This is a lot faster - and the fine penetration through the boot is immediately sealed and is essentially a no-never-mind.
The plastic handle used on ice-picks normally prevents any shocking - but all plastics are not perfect insulators - and wrapping a rag around the handle may be necessary.
The wire with alligator clips at each end is real handy for trouble-shooting electrical problems. --- Jerry/Idaho
Frank O posted 02-05-2009 09:44 AM ET (US)
Interesting approach, Jerry. I have to say, though, that it's kind of against my nature to use a test approach that damages a component (puncturing the boot) even though I'm sure it doesn't cause later problems. I agree with you about the handiness of wires with alligator clips -- I think I have three or four of various lengths in my toolbox.

seahorse posted 02-05-2009 10:00 AM ET (US)

A safer way to do a cylinder drop test is to use a double alligator clip jumper wire to ground out a single spark plug wire at a time to the engine block. Then run the boat to see if there is an rpm drop or if the "shorted" plug wire makes no difference in drivability. You have to start and stop the motor each time you short the plug wire.
Be aware that you cannot disconnect a plug wire and not ground it on most 4-strokes as they use a dual coil system that fires 2 cylinders at the same time (wasted spark system).
Running a motor with a plug wire disconnected and not grounded, stresses the ignition system and could damage the coil or even the CDI module. Sometimes a coil will burn through the insulation and arc to ground due to the high voltage induced in it, rendering it unusable.

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Author	Topic: Cylinder drop test -- when to disconnect wires
Frank O	posted 02-02-2009 09:46 PM ET (US) A mechanic suggested conducting a cylinder drop test on my 1994 Mercury 3.0L Offshore 225-HP 2-stroke carbureted outboard to help diagnose rough running. In the test, spark plugs are disconnected one at a time to see what effect this has on the rpm. A substantial drop means that cylinder is probably firing well, whereas if there is little or limited effect this implies that there is a problem with the cylinder. By conducting this test in conjunction with testing spark plug wires with a timing light, it's possible to differentiate between an electrical vs. fuel-related problem. I've heard two opinions about how to carry out the cylinder drop test: 1) Leave the engine running throughout. Disconnect each spark plug wire using a well-insulated pair of spark plug pliers. 2) Turn off the engine when disconnecting each wire. Make sure to ground out the disconnected spark plug wire. Then run the engine briefly to observe rpm. Turn off the engine, and repeat with the next spark plug wire. I gather that the advantage of the first approach is that you get a better sense of subtle changes as each wire is disconnected. The advantages of the second approach are that it's safer, and it also avoids electrical damage to the stator or power pack that might be possible if the spark plug wire were ungrounded. Any suggestions on which approach would be better with my 225-HP 2-stroke V-6?
seabob4	posted 02-02-2009 09:53 PM ET (US) Why not do a compression test? A whole lot safer, and less 40,000V ready to zap you! You're looking for a "bad" cylinder, than I assume you have already ruled out plug, coil, and wire. If you haven't, that's where you need to start. If you have, do a compression check. That'll point out the offending bore in a heartbeat.
Frank O	posted 02-02-2009 09:56 PM ET (US) Actually, yes, I'm going to do a compression test too. That in conjunction with the cylinder drop test and a check with timing light would all offer different views of what in particular is wrong with the problem cylinder, no?
A2J15Sport	posted 02-02-2009 11:48 PM ET (US) The "cylinder drop test" is a valid one. A compression test will not tell you if you have a clogged carburetor or if the ignition is malfunctioning.
Jeff	posted 02-03-2009 09:13 AM ET (US) You mean a "Leak Down" test?
Frank O	posted 02-03-2009 09:42 AM ET (US) A leak-down test is yet another test -- it involves pumping compressed air into each cylinder with dedicated test apparatus.
A2J15Sport	posted 02-03-2009 10:07 AM ET (US) The "drop test" tells you if a cylinder is contributing, or not. IF it is not, the other tests may tell you why.
A2J15Sport	posted 02-03-2009 10:10 AM ET (US) The cylinder drop test is used extensively throughout the automotive industry. It is called other things, such as: "cylinder contribution/percentage". It is the same thing. How it is done-varies. Most often, it is accomplished via test equipment.
Frank O	posted 02-03-2009 12:10 PM ET (US) I guess the question I was originally trying to get at is: If I disconnect each spark plug for a few seconds each while running the engine, do you think there's much risk of damaging electrical components?
Tohsgib	posted 02-03-2009 12:30 PM ET (US) Won't hurt a thing on that engine. If you don't keep it running you probably won't notice it. With 6cyls it can be a very slight drop sometimes..hence why people blow them up more often because they don't realize they are running THAT poorly.
A2J15Sport	posted 02-03-2009 12:55 PM ET (US) No issue whatsoever. Your tach. and your ear are the best indicators for cylinder contribution. The tach. should drop an equal amount when each plug wire is disconnected.
Frank O	posted 02-03-2009 01:14 PM ET (US) Thanks, guys. I had a hunch that with an engine this size what you say would be the case.
outragesteve	posted 02-04-2009 02:46 PM ET (US) Doing a cylinder drop test is a good test to find a "dead" cylinder. Depending on the engine, it may lead to a fuel or electrical problem. Although it may be hard to do, dropping a cylinder is more noticeable with the engine under a load. This can be done in a test tank, with a test prop or hooked to a dyno. I have done this test on a trailer, backed in the water, in gear, with some RPM. This can be dangerous, so do not attempt this if you unsure of the procedure. I also find it easier to pull the plug boots if you first secure two long tie wraps to the plug wires and "pre-pull" the wires first. You do not need to pull the wire off the plug very far. The tie wrap allows you to place the boot back on that plug and move to the next. Goog luck.
Jerry Townsend	posted 02-04-2009 04:12 PM ET (US) Dropping a cylinder and noting the change in the rpm can be very informative - loaded or unloaded. On car and truck engines - I have used an ice-pick with another aid (that I always have in each tool-box) - a 3 - 5 feet length of 10 guage wire with alligator clips at each end. Use the wire to connect the ice-pick shank to the ground - and then gentally push the ice pick point through the boot until it contacts the spark-plug contact. This is a lot faster - and the fine penetration through the boot is immediately sealed and is essentially a no-never-mind. The plastic handle used on ice-picks normally prevents any shocking - but all plastics are not perfect insulators - and wrapping a rag around the handle may be necessary. The wire with alligator clips at each end is real handy for trouble-shooting electrical problems. --- Jerry/Idaho
Frank O	posted 02-05-2009 09:44 AM ET (US) Interesting approach, Jerry. I have to say, though, that it's kind of against my nature to use a test approach that damages a component (puncturing the boot) even though I'm sure it doesn't cause later problems. I agree with you about the handiness of wires with alligator clips -- I think I have three or four of various lengths in my toolbox.
seahorse	posted 02-05-2009 10:00 AM ET (US) A safer way to do a cylinder drop test is to use a double alligator clip jumper wire to ground out a single spark plug wire at a time to the engine block. Then run the boat to see if there is an rpm drop or if the "shorted" plug wire makes no difference in drivability. You have to start and stop the motor each time you short the plug wire. Be aware that you cannot disconnect a plug wire and not ground it on most 4-strokes as they use a dual coil system that fires 2 cylinders at the same time (wasted spark system). Running a motor with a plug wire disconnected and not grounded, stresses the ignition system and could damage the coil or even the CDI module. Sometimes a coil will burn through the insulation and arc to ground due to the high voltage induced in it, rendering it unusable.