View Full Version : Is Every Plug Firing

04-12-2006, 11:24 PM
How do I make sure each plug is firing. I put the timing light plug wire sensor on each wire and it trigger the light. Does this mean each plug is firing (maybe not correctly) or just that a current is getting to the plug each time the rotor triggers a spark to each wire?

04-13-2006, 07:46 PM
"Pull 'em out and look at them. Even if yu have no idea what they are supposed to look like, you can spot any marked difference. TL sensor is just telling you spark is in wire not that plug is actually firing. Another riskier way is to pull each wire (with special insulated plug wire pliers) and listen for difference in engine sound/rpm drop,,done wrong it'll light you up like a christmas tree! Make damn sure you're not in contact with metal or water.

Do you have some sort of problem, or just curious?


04-13-2006, 10:12 PM
"Thanks for the feedback. I have several posts here noting the fact that my boat will rev up at the dock, but underway it won't go about about 2200rpm, it reaches their about 1/2 throttle, then more throttle just makes is shiver a little. Long story, you will see my posts relative to not going above 2200rpm and bad gas posts. Here is what I have done: I think gas from last fall was a little of the problem. Bayliner Ciera 2680, 5.7L. 2BBL merc carb. Thunderbolt V Ignition. New plugs, gapped correct, new wires, new cap, new wires, new gas filter (will change again now that I have new gas), adjusted valves. Have not replaced Coil. Was running rough, smelly, smoke. With what I have done all that has gone away, running better, but still that high speed lack of power. I will put a coil on anyway. I will put a new fuel filter on. I am now leaning towards rebuilding the carb. It is a 1999. I think my only problem now is now power past a certain throttle position, but only when under way. Thoughts?"

04-13-2006, 11:25 PM

Coils almost don't go bad! Really sounds like fuel delivery. Have you, checked secondary fuel filter/screen in carb (just inside carb large nut that fuel line screws into), check fuel tank vent for restriction, if plugged up tank creates vacum that pump can't overcome, but engine will run fine at idle. If I couldn't pinpoint problem I'd take chance on fuel pump long before replacing coil.


04-14-2006, 10:51 AM
"Hey, I just thought of something: You might simply have crossed ignition wires. A mechanic did that to me last year after doing a compression check--drove me nuts!

Unlike a car/ truck installation, a rigidly mounted marine engine doesn't shake when a cylinder isn't firing right--even two of them.

Try this: Warm the motor up thoroughly and let it idle slowly. Next, put on some heavy gloves and, using pliers with insulated handles, remove one spark plug wire at a time. Hold it a couple of inches away from the cap, listen to how the engine sounds, then plug it back in. You should hear a definite change in rpms as you do this, and for every plug wire. Pick a logical starting point and go around the cap, noting how much the engine slows down for each plug wire pulled. They should be identical.

Now, if you see NO change at all at a plug wire, note it's location. With a bit of luck at least one will be a dud, menaing a bad wire/ plug/ cap (or something really bad: hole in piston, burnt valve, etc.) But if you see two wires that don't do a thing, SWITCH them and see what happens.

Good luck!


Mike Green
04-14-2006, 07:17 PM
Have you looked into a restricted exhaust?

04-15-2006, 06:45 PM
Thanks for the advice. Exhaust is fine. Am checking the tank vent. have new fuel filters coming. Last resort is to rebuild carb. I will see if any one plug is not firing following the advice above. The intake manifold was off when I replace the head gasket. I wonder if a leak in its gasket could cause this issue.

04-15-2006, 07:39 PM
Intake leaks usually cause the RPM to raise and you wouldn't be able to lower the idle speed to a normal level. Think of it as not being able to control the throttle completely.

04-16-2006, 04:49 PM
"Today I used insulated pliers to pull wires off the distributor while running. The first wire I pulled off did change the pitch of the engine. Two more did not but I noticed something curious. When any plug wire was off and away from the distibutor cap, there was significant arcing between the positive terminal on the coil to the coil wire to the distributor. I can't imagine this being normal, it is like a short. Last summer when the boat had problems, it was an overheating problem. The top of the coil looks a little funny. I am planning to replace it this week."

04-16-2006, 11:58 PM
"When you changed the plug wires, did you change the coil wire? I have seen more bad coil wires on Fords but it happens on GM motors, too."

04-17-2006, 06:38 AM
"yep, brand new set. Marine wires for that engine."

04-17-2006, 06:40 AM
"yep, brand new set. Marine wires for that engine."

04-17-2006, 07:51 AM
"At low RPMs and no load it will take a relative low voltage to fire a plug, 10,000 volts or less. when you pull a wire the voltage will build up to the max rated for the coil 30,000 to 50,000 volts. that is why the arc occurred when the wire was lifted. Also the arc should be a big fat blue arc with lots of noise. I have seen where a motor will rev fine at the dock but not under load due to a weak coil."

Eddie from Oregon
04-17-2006, 10:41 AM
"Michael, you could have easily damaged a plug wire or gotten a defective one in your new set. Happens A LOT!! About the only other thing it could be is a weak coil, which is the only ignition item that you did not change. Be sure you get the right type of replacement coil. Some require external ballast and some do not. You DO NOT NEED a high output coil but an epoxy filled coil would be a good upgrade. Coils rarely are a problem and often just quit entirely when they do go bad but when they are weak, high speed miss is a possible symtom.
I know it seems frustrating but if you did some simple ignition trouble shooting you probably would have fixed the problem long ago. Guess how I know that."

04-17-2006, 09:41 PM
"Thanks. At least I will have all new ignition components which is probably a good thing since engine is 7 yrs old and I do not know its total history. The coil looks a little abused on top. Plus I noticed some substance where the wire plugs in, almost a gray think grease, but more like clay."