Only 2 spark plugs firing on 3.0 mercruiser

Provi1983

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1990 mercruiser 3.0 serial# 0D442882
I was running the boat the other day and all of a sudden felt a drop in power and rpm. I turned around and started heading for the dock and felt another sudden drop in power. Got back to the and started trying to figure out what happened. Now the boat will only start if the throttle is wide open and for approximately 20 seconds the engine struggles to keep running. Eventually the engine revs up and I can bring the throttle down to 1200-1500 rpm - any less and the engine dies and I have to repeat the same process again. I replaced the spark plugs, distributor cap, and rotor and saw no difference. While the engine is running, I can unplug number 3 & 4 wire from distributor and nothing happens. If I unplug either of the other two, it dies. Checked to see if plugs were firing and they're not. I then swapped with plugs and wire that were firing and plugs from number 3&4 cylinder still aren't firing. Any ideas on what is causing this?
 
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alldodge

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Re: Only 2 spark plugs firing on 3.0 mercruiser

:welcome: to iboats

Need to do a compression test. It's sounding like you may have blown a head gasket.
 

bonzoscott

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Re: Only 2 spark plugs firing on 3.0 mercruiser

Compression test is always good, but if you're sure 3 & 4 aren't firing, it's more likely ignition. You replaced the whole distributor? You didn't mention replacing the cap. Is it Okay? Were these all OEM parts?
 

Provi1983

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Re: Only 2 spark plugs firing on 3.0 mercruiser

Sorry, I meant to say that I replaced the distributor cap - not the distributor. How would compression affect whether or not the spark plug fires?
 
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jackd1023

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Re: Only 2 spark plugs firing on 3.0 mercruiser

Do you have a spark tester? What they are saying is that if there is no difference in running condition when you pull the plugs then it may be loss of compression, If you are saying there is no spark when you pull the wires then that is ignition,
 

Provi1983

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Re: Only 2 spark plugs firing on 3.0 mercruiser

Do you have a spark tester? What they are saying is that if there is no difference in running condition when you pull the plugs then it may be loss of compression, If you are saying there is no spark when you pull the wires then that is ignition,

Please look at the attached photo and advise if the part circled is the ignition.
 

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Maclin

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Re: Only 2 spark plugs firing on 3.0 mercruiser

When changing plug wires and cap you may have introduced another problem....Triple check the firing order of the wires on the cap.
 

stonyloam

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Re: Only 2 spark plugs firing on 3.0 mercruiser

What you have circled is the coil, which is part of the ignition system. Do you have a points ignition? Normally your symptoms would say head gasket, but if you are getting no spark on two plugs, and your plugs are OK and you checked the wires, changed the cap. There is no reason to believe the coil is bad. That leaves the points or the pickup on an electronic ignition.
 

Provi1983

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Re: Only 2 spark plugs firing on 3.0 mercruiser

What you have circled is the coil, which is part of the ignition system. Do you have a points ignition? Normally your symptoms would say head gasket, but if you are getting no spark on two plugs, and your plugs are OK and you checked the wires, changed the cap. There is no reason to believe the coil is bad. That leaves the points or the pickup on an electronic ignition.

My Dad was helping me with it earlier and I remember him pointing to something while the distributor cap was removing and saying, "here are the points." To answer your question, I assume that it is a points ignition. Is there any way to test if it's the points, or the pickup?
 

thumpar

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Re: Only 2 spark plugs firing on 3.0 mercruiser

It would either be points or electronic with pickup, not both.
 

alldodge

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Re: Only 2 spark plugs firing on 3.0 mercruiser

You probably have points but a consistent misfire on two cylinders would not come from your points or condenser. Looking at your pic shows you have the plug wires installed correctly (firing order 1-3-4-2). Need to find a compression gauge at test.

Remove the coil wire from the coil. Remove plugs one at a time and install a compression gauge. Crank the engine over a few times and write down the gauge reading. Remove gauge, install plug and wire then remove next plug, install gauge and repeat.

Amazon.com: INNOVA 3612 Compression Tester - 4 Piece Kit: Automotive
 

Provi1983

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Re: Only 2 spark plugs firing on 3.0 mercruiser

You probably have points but a consistent misfire on two cylinders would not come from your points or condenser. Looking at your pic shows you have the plug wires installed correctly (firing order 1-3-4-2). Need to find a compression gauge at test.

Remove the coil wire from the coil. Remove plugs one at a time and install a compression gauge. Crank the engine over a few times and write down the gauge reading. Remove gauge, install plug and wire then remove next plug, install gauge and repeat.

Amazon.com: INNOVA 3612 Compression Tester - 4 Piece Kit: Automotive

I understand that performing a compression test is a good idea, however that is completely unrelated to the issue I'm having. When the two plugs are removed the cylinders, they do not fire when I turn the engine over.
 

stonyloam

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Re: Only 2 spark plugs firing on 3.0 mercruiser

Change the points, and make sure you have a good ground on the distributor. I suspect that the cam follower on the points is worn to the point that it is throwing your dwell way off. So change the points and make sure that the gap is correct and give it a try. If the cam on the distributor is worn unevenly it could cause two of the lobes to not be opening the points. If you do not have points handy, just try resetting the gap on the ones in there. Touch them up with a points file (not sandpaper). Another thought, are you getting a nice blue spark? If the spark is yellow, it would indicate a weak coil, or the coil not fully charging (poor points again). To check the coil do this: Pull the center wire from the distributor cap and place it near bare engine metal. Put a piece of insulating material between the points. With the ignition on, use a piece of wire to briefly jumper across the points. If you get a nice blue spark (when you remove the wire) your coil is OK.
 
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Walt T

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Re: Only 2 spark plugs firing on 3.0 mercruiser

If this were brought to me first thing I would do is a compression test. Worry about spark diagnosis after that. It is not unrelated. Trust us.
Or go ahead and change plugs, points rotor and cap instead of playing on the computer. You are confusing yourself with all this coil stuff and jumping around. Then after all that, do a compression test. My experience is coils don't go bad all that often.
It's funny how a person with limited knowledge wont listen to those who have decades of experience. I think there's someone here with the signature "why is there never time to fix it right the first time but always time to do it again"

Don't get me wrong I'm not slamming or flaming you or anyone, just pointing out you're choosing to ignore advice given to you by old grouchy mechanics who been doing this stuff for decades. We could certainly be wrong, wouldn't be the first time. In fact I hope I am so you don't have to pull a head. No matter what though let us know what it is so we can all learn and so can others.
 

stonyloam

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Re: Only 2 spark plugs firing on 3.0 mercruiser

If this were brought to me first thing I would do is a compression test. Worry about spark diagnosis after that. It is not unrelated. Trust us.

The OP stated that he has spark on the plugs #1 and #2, and no spark on #3 and #4. The plugs and wires were changed around and the diagnosis was confirmed. The plugs and wires were tested out of the head with the same results. While I would agree that normally if you loose two cylinders a compression test is in order. In this case according to what the OP has described it is a ignition problem not a head gasket problem.
 

jackd1023

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Re: Only 2 spark plugs firing on 3.0 mercruiser

may have a cracked or bad cap. The rotor acts as a trigger coil is always charged and as rotor contacts diode in cap it sparks...... I would put a inline spark tester on and verify or replace cap and rotor....
 

stonyloam

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Re: Only 2 spark plugs firing on 3.0 mercruiser

may have a cracked or bad cap. The rotor acts as a trigger coil is always charged and as rotor contacts diode in cap it sparks...... I would put a inline spark tester on and verify or replace cap and rotor....

The cap has been replaced. That is not how it works in a points ignition. When the points open the coil generates a high voltage charge that travels through the center wire through the center contact of the rotor and the spark jumps the air gap between the end of the rotor and the contact on the cap, to the plug wire and the plug. A spark tester should confirm the problem.
 

bonzoscott

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Re: Only 2 spark plugs firing on 3.0 mercruiser

It's likely something gone wrong in the distributor. The ignition system should fire no matter what the compression (no pistons even). My first thought was the distributor cam lobes as Stoney mentioned. Have someone crank the motor and look carefully to see if the points are opening on all FOUR lobes of the cam. It may be difficult to see and would be better if you could crank slowly with a breaker bar on the front of the crank, but can't get one on a 3.0. If nothing else you can pull the distributor and spin it by hand to see if the points are opening on all four. It's strange - your sure it's not firing on 3 & 4 consistently? The part where you stated "I then swapped with plugs and wire that were firing and plugs from number 3&4 cylinder still aren't firing. Any ideas on what is causing this?" - you needed to swap the whole wire, not just move the plug end to 3 & 4. You would have missed the firing order if you just moved the plug end. Try and rule out bad wires.
 

airshot

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Re: Only 2 spark plugs firing on 3.0 mercruiser

Just a note from an old timer that has been down this road a number of times....just because the parts are new does not mean they are good....in todays market many bad products that are new are sold everyday. Re-examine your "new" parts very closly, especially if they are not "mercruiser" specific parts. Just a suggestion...........
 

Scott Danforth

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Re: Only 2 spark plugs firing on 3.0 mercruiser

I understand that performing a compression test is a good idea, however that is completely unrelated to the issue I'm having. When the two plugs are removed the cylinders, they do not fire when I turn the engine over.

How are you confirming the cylinders are not firing?
 
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