Dies Suddenly while Cruising

[JoDo]

89 2000sc with a 5.0 Mercuiser and Quadrajet. Cruising along about 3000 rpm then quits suddenly. No spit or sputter just bow dropping deadness. I had the carb completely gone through by a reputable shop before I launched for the first time this year. They told me I had water in the tank as the carb had a lot of water. I completely drained the tank, new separator, added fuel and took her out. Cruising, cruising, no problems then 5-10 min in it just quits. I found the coil wire was broken so I nursed it to a friends dock and returned with a new wire set. Same problem. In my own uneducated reasoning, I don't think it's fuel. Just for grins, Immediately after dying I operated the accelerator manually and it squirted plenty of fuel. When pulling the coil wire and lying it close to the coil there appears to be plenty of spark. But just smells of ignition to me, not fuel.

Thoughts?

[mmwjr]

Sunder dieing would be an electrical issue. Bad coils or ignition sensor in the distributor are know for this type of fault. You'll need to troubleshoot these using the merc guide next time it occurs.

Thunderbolt IV?

[JoDo]

Thanks Mike, yes there's an ignition module bolted on the port side that says Thunderbolt. Whether it said "IV" I do not remember.

[mmwjr]

Originally Posted by JoDo

Thanks Mike, yes there's an ignition module bolted on the port side that says Thunderbolt. Whether it said "IV" I do not remember.

If your engine still has the plastic cowl over the flame arrestor it should say it on there.

Is the module a square block about 4x4x2 inches? If so old style TB IV.

[JoDo]

No it is somewhat triangular mounted with three bolts to the port cooling manifold.

[mmwjr]

Ok that is the newer TB IV.

[JoDo]

How is this tested? Is it a likely culprit? Looks like a spendy part.

[mmwjr]

Originally Posted by JoDo

How is this tested? Is it a likely culprit? Looks like a spendy part.

See post 2 for likely causes. Go to the documents tab to get your merc manual which has the troubleshooting guide.

[JoDo]

If it only happens when I'm cruising along that makes it intermittent doesn't it? Potentially all could test fine?

[mmwjr]

You will need to test while the fault exists to find the issue.

Electric parts get hotter while running at higher rpm than sitting at idle.

Have you tried letting it idle at the dock for 30 minutes to see if it dies?

[JoDo]

Well I've ruled out coil because I put a new one on. I figured for $40 replacing a 27 year old coil probably wasn't a bad idea anyway. It did still die however. Then it would start but wouldn't stay running unless I kept the throttle opened. Then it would stay running. Then it died. Then it wouldn't start at all but did after a squirt of starter fluid into the secondaries. Then as I was putting to the dock (1500 rpm) it died. It started up and I was able to putt all the way back. Man, this is sure a puzzling issue. I'm starting to wonder again if it is fuel related.

[mmwjr]

A 1989 5.0 would have a mechanical fuel pump, I would perform a pressure test on it. It should be 6 - 7 psi, you'll need an adaptor that inserts between the fuel line and carb input that has a nipple to connect the pressure gauge.

[JoDo]

Thanks again Mike for the help you give so many people here.

Let me ask you about vacuum. I now notice There is a vac port on the carb that is not connected to anything This photo is not of my actual carb but its a google photo of the port I'm talking about

Also there's a black plastic port on the distributor cap that is not connected to anything. These are vac advance and should connect to eachother correct?

[JoDo]

The fitting on the dist is likely a vent I discovered. But on the carb regardless that should not be open should it?

[speedysprocket]

Sounds like a carb rebuild is in order. Kits can be had for $30-$40.
While the carb is off, you can tell if that's an idle vacuum line or a throttle vacuum. Regardless though, it probably should be plugged.

[JoDo]

Just rebuilt by reputable carb shop .

[speedysprocket]

Might be worth it to take the top cover off and see what's in the bowl. Even though you rebuilt it and pumped the tank, still very possible some junk was still in the very bottom of the tank and has made it into your carb now.

[JoDo]

Good point. I went up to try a pressure test but the lake was so choppy the boat was all over the place then two pontoons full of partiers showed up at my friend dock so I just gave up for the day. Won't go out for the 4th but I can live with that. I'll get back at it Tue.

[mmwjr]

Yes on the distributor its a vent.

There is no vacuum advance on a boat as it operates at constant high rpms.

A marine carb should not have a vacuum port. Try putting you finger over it when the engine is running to see if you feel a vacuum. Either way plug it.

[ss3964spd]

That particular vacuum port in the photo is above the throttle blades and therefore won't likely have any vac at idle, or very little, but will when the throttle is open. Regardless, as Mike said, plug it.