Mercruiser 4.3 Crank But no Start (spark issue)

[ESully94]

Hi everyone, new to boat life and new to the forum. I recently bought a 1990 (old I know, but in great shape) maxum 2100 with a mercruiser 4.3. First few days of having the boat it started right up never an issue. One day I forgot to turn off the radio when covering the boat (it was raining really hard) and it drained the battery. Was anticipating it to be a simple battery swap but that was not the case.

Battery was replaced, then bolts on starter snapped. I ordered and replaced the starter. I have also replaced the solenoid, ignition coil, distributor cap and rotor, spark plugs and spark wires with no luck. When I check the spark at the distributor, I get one initial really strong blue spark when cranking. After the initial spark, there are no more sparks until I reset and recrank.

From reading around the forums like a mad man, I came to the conclusion the problem could either be a bad ignition module, possibly a shift interrupter? Perhaps something else I am overlooking?

All 4 wires that come off the thunderbolt IV ignition module have power which go to the coil. Purple is good, grey is good, white with red stripe is good. Getting very low readings on the white with green stripe. Pretty sure this wire controls the feed being sent to the coil in order to generate sparks in the distributor.

Any help would be greatly appreciated as I am ready to bring it to a local marina and call it quits. Money is not the issue and have no problem bringing it to a professional. I just wanted to sort the issue myself so I am knowledgeable about my older boat and know how to service and maintain it myself

[mmwjr]

So does the engine crack over when trying to start? Why did the stater bolt break off? Is it possible water got into the engine? I’ve not heard of a case where spark is there but goes away after a few cranks, how are you verifying this?

[ESully94]

The main cap in the center of the coil was removed, and an insulated screw driver was used to check the spark output. From the main plug wire, there was one initial solid blue spark and is gone after legit one spark.

I couldn’t tell you how the starter bolts broke, there is no mount or brace for it, and the starter was dated from 2011 so I’d imagine corrosion on the bolts.

Engine does not crack over. Starter engages, solenoid is working and everything. Just no spark to fire it over.

Spark plugs when I remove them show no signs of firing at all. Completely clean when I pull them out.

[mmwjr]

With the spark plugs out can you spin the engine over by either hand or the starter?

[ESully94]

Yes the engine spins plenty. If I had spark it would fire right up. Just never achieving ignition.

[mmwjr]

Ok, then it’s either the ignition sensor in the distributor or the module which is no longer available. You can find used ones on eBay but it must be for the V6.

Worst case is get an aftermarket marine distributor which is a complete ignition system with coil. Mallory, Sierra are two such suppliers.

[ESully94]

I also forgot to mention that I was using a light tester on the electrical connectors, and found that all of the wires coming from thunderbolt IV ignition module all carried strong currents aside from the white and green wire that goes from ignition module to the distributor itself. Leads me to believe the coil is not getting ready power, and therefore cannot generate a spark to be transferred into the distributor.

[ESully94]

Thanks!!

Upon further inspection, I noticed the shift interrupter switch was super corroded and actually stuck!! Cleaned and lubed it and now it’s moving freely, but not sure if I need to replace the switch as the metal arm holding the wheel on it seems to have bent a good portion. Could a shift interrupter cause no spark if the kill switch on it is engaged?

[mmwjr]

Originally Posted by ESully94

Thanks!!

Upon further inspection, I noticed the shift interrupter switch was super corroded and actually stuck!! Cleaned and lubed it and now it’s moving freely, but not sure if I need to replace the switch as the metal arm holding the wheel on it seems to have bent a good portion. Could a shift interrupter cause no spark if the kill switch on it is engaged?

Yes, that’s exactly what it does when shifting out of gear. It kills the ignition for a very short time to remove engine load on the dog clutch to make it easier to shift out of gear.

[Joey9851]

Originally Posted by mmwjr

Yes, that’s exactly what it does when shifting out of gear. It kills the ignition for a very short time to remove engine load on the dog clutch to make it easier to shift out of gear.

Would this prevent it from starting altogether? Little backstory:

Christmas 2022, we took our 2011 Bayliner out with a Mercruiser 4.3 and Alpha 1, SN: 1A633081and spent about 6 hours on the water here in Hawaii. Went out the next day, put the boat in the water and it would just turn over but never fire off. My initial thought was spark, fuel, air. I started with fuel, and drained some of the tank to see how much water was in the sample, and had a very, very thin line on the top. I replaced the fuel and water separator. Still no change. Put in a new carb, distributor, plugs and wires. Rotated the engine to TDC on compression, set the new distributor in and still no change. So then I replaced oil pressure sensor, ignition coil, and fuel pump, still no change. I just need to figure it out and it’s driving me bananas. Before anyone asks, I did check the lanyard and it’s in the run position, I’m getting voltage everywhere. This is my first boat and really want to enjoy it some more before leaving Hawaii.

[mmwjr]

Originally Posted by Joey9851

Would this prevent it from starting altogether? Little backstory:

Christmas 2022, we took our 2011 Bayliner out with a Mercruiser 4.3 and Alpha 1, SN: 1A633081and spent about 6 hours on the water here in Hawaii. Went out the next day, put the boat in the water and it would just turn over but never fire off. My initial thought was spark, fuel, air. I started with fuel, and drained some of the tank to see how much water was in the sample, and had a very, very thin line on the top. I replaced the fuel and water separator. Still no change. Put in a new carb, distributor, plugs and wires. Rotated the engine to TDC on compression, set the new distributor in and still no change. So then I replaced oil pressure sensor, ignition coil, and fuel pump, still no change. I just need to figure it out and it’s driving me bananas. Before anyone asks, I did check the lanyard and it’s in the run position, I’m getting voltage everywhere. This is my first boat and really want to enjoy it some more before leaving Hawaii.

Yes a misaligned shift interrupt switch can cause the engine from starting but I would first confirm there is spark. If you have spark then the interrupt switch is not the issue. Is this a carburetor or fuel injection engine? If carburetor you can try spraying some starting fluid in the carburetor to see if the engine will run for a few seconds. Regarding your fuel water is heavier then gas so it would be on the bottom and not the top, you may be getting phase separation that can occur with Ethanol fuel in which case the tank needs to be drained and refilled with fresh fuel.

[Joey9851]

It’s carbureted. When the issue first started, I had spark, but I don’t have spark now. Last night I went and did the test on the interrupt switch where you trim down and check the alignment of it. The y-shaped piece that moves when you shift to F or R didn’t move much at all, and the flat portions of it didn’t engage the switch. I’m really not sure where to go from here

[mmwjr]

You can try temporarily disconnecting the shift interrupt switch to see if the engine will run

[yahoo82]

After the battery ran down did you replace it? I had an odd situation similar to yours in that the engine would crank no problem but did not fire up. Turns out my batteries were likely damaged over the winter and although they showed 12v at rest, and they could crank the motors, the voltage drop prevented the motor from firing. I always assumed if there was enough power to crank, there was enough for ignition but I guess that’s not always the case.


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[ESully94]

I charged this battery fully (the one that the boat came with) and bought another battery with identical specs. Both batteries when on my charger read as having all cells alive and well. Brought the boat to a local marina as it’s beyond my capabilities now. Surpringly I’m not frustrated either. This is my first boat ever. And I learned a whole bunch about how it works in the process and learned how to diagnose issues. And once the mechanic tells me the issue and how he fixed it, I will be able to recognize and fix any issues I face moving forward myself

[Phillbo]

Check the kill switch at the throttle.....

[Joey9851]

That was the first thing I checked. It’s in the run position but I’m not sure if the boat is recognizing it. I finally found a mechanic to look at it next week

[Joey9851]

Originally Posted by Phillbo

Check the kill switch at the throttle.....

So I had a mechanic come out and the ignition coil was bad. Replaced the coil, but spark is weak. Using a spark tester, it’s orange when it sparks, when I would expect to see blue. I’m getting 12v at the coil when starting and 6.8v constant with key in the run position. I’m getting fuel as well. The only thing I can think of is the shift interrupt switch is bad or the module inside the distributor is bad. I’m going to order a new module and the local Mercruiser shop has the interrupt switch. I’m truly at a loss here. Is there anything I’m possibly missing?

[mmwjr]

Couldn’t the mechanic isolate why the spark is weak? I would suspect the ignition model based on all the other things you have already changed.

[Joey9851]

He stopped working on it when it wouldn’t turn over anymore. But I’ve learned that it does that when it turns over without getting ignition. I ordered the module for the distributor last night and once I get it installed, I’m going to try again. If that doesn’t fix it, I’m getting it in the shop