Gen IV Mercruiser 454 drops on its face above 3000 RPMs

Sorry for the long post, but I want to provide all the details here.

Not sure where else to look on this one...

Mercruiser 7.4L 4bbl carb Thunderbolt IV Ignition system.

Went on a cruise the other day. Got to our overnight anchorage and had a great day/night. The motor starts up the following morning to head back home with no sign of concern. She planed off as normal and we're on our way. About 45 minutes into the cruise home running between 3000-3150 the motor seemed to stumble. I slowed down and the re-accelerated all seemed fine. About 15-20 minutes later the motor all but shut down while at a steady 3000 RPM. I closed the throttle, the motor was dieseling and when I hit the key to shut it off it continued to "run". I pumped the throttle and it shut down. After an inspection, I restarted the motor it fired right up; we went on our way. Again after 15-20 minutes the same thing happened again. Temperature was right at 160 and all other vitals were fine. I kept the RPMs low on the way back to the dock with the exception of trying to plane very slowly one last time. I very slowly applied throttle to see how the motor would respond with increased load. Right around 2500, the motor popped through the carb and I pulled back. Got her in the slip and scratch my head...

All connections are clean on the distributor, ignition module and ground.

My guess is and ignition/advance issue...

Any ideas on where I should look?

 

Hi 7228Sedan,

The issues you're describing could be caused by a few different items.
The very first thing I would start with would be to pull the spark plugs and inspect them if you haven't done so already. How many hours do they have on them?

 

Plugs were new this year, probably about 25 hours or so on them. I didn't suspect them based on the symptoms but I won't rule them out without an inspection.

 

They're the cheapest place to start and even if they aren't the culprit, their condition may give you an idea of what is causing your problem. Was anything changed on the boat before you took the boat out and had the problem? Could even be a load of bad fuel?

 

Pulling the plugs provides lots of information about what the engine is doing, not if they might be the actual problem. Buy the manual associated with your engine/drive combo and run through the diagnostic sections. We can list the typical procedures for sorting out this sort of thing, but the first part you should buy is the manual. It sounds like fuel delivery issues and pulling the plugs will tell you some of what's necessary (rich, lean, fouled, etc.). These standard diagnostic procedures are pretty much the same on any engine, so if this is new to you, the best advice is bring it to a boat mechanic.

 

Didn't do a thing since my last run... Ran the boat a week earlier and she ran like a fine Swiss watch... I just remembered that when I started the boat up the morning after the overnight I did smell a strange "electrical odor". Upon inspection, all seemed fine. The batteries were a bit run down from running the inverter overnight and the alternator was likely at full load. I attributed the odor to that. It's possible that the odor I smelled was something in the ignition system "frying"...

 

Hi Par, I have the SELOC manual... it doesn't really speak to the specifics I am seeing. This isn't my first time around the motor (I built it 5 years ago). I've never had a motor drop on it's face like the key was turned as soon as it hits 3000 revs. Not a a rev limiter fuel/spark cut out, not a stumble or stutter, complete cut out. Only then to immediately restart and run like nothing happened again for a bit. I'll run through the ignition module & distributor sensor diagnostics as you recommend. At this point all I did was check the connections for tightness and lack of corrosion.

 

Picture a chunk of something (usually bits of deteriorated fuel line) floating up and wedging into the seat in the bowl or maybe covering the entrance or passage to the high speed jet. Though by your description, it does sound like a broken wire might make it just stop. I favor the fuel delivery side, simply because shorts or broken wires tend to keep the circuit interrupted, while a block jet or delivery seat can drop the debris, after the engine dies, so it would restart easily.

 

I could see a loss of power due to fuel, but the "switch" like shut down has me thinking ignition. I am going to trace all of the hot wires in the harness as well as the components in the ignition system. That being said, 99% of all ignition problems are fuel related

 

From what you described as the engine just shutting down like flipping a switch, along with the backfire and run-on when shut off (suggesting that the timing is moving around), I'm tending to lean towards an ignition module issue.

Just my 2¢.....

 

DSR that is exactly where I was headed as well. The run-on has me puzzled the most. As I hit the key to shut it down, I'd assume that the ignition would then be killed... The motor was not running hot at all whatsoever to cause it. I'll have to track all the wires to the module to rule out a short or multiples. Unless the short is in the module itself? Under 2500 rpm it ran fine limping back to the slip. Started and shut down fine with the key.

 

Pre-ignition (run-on, dieseling, etc.) can be caused by fuel mixture and/or hot spots in the chamber or on the valves too. The first test I would do is to wiggle all the wires, to see if you can get it to stumble or cut out. Since it starts and runs at reduced RPM's, I'd still discount a timing or module issue, though who knows.

 

Very true Par, I agree completely and I definitely wouldn't rule a fuel issue out of the situation.

Though I don't have first hand experience with the Thunderbolt system specifically, I am pretty familiar with HEI-style ignition systems and I'm leaning towards the module because I have run across very similar issues to what 7228Sedan is describing, with the module being the culprit.

Thanks,
Dave

 

Have you tested the shift interrupt switch? Disconnect it and see if the problem disappears.

 

It's a velvet drive inboard Gonzo with only the solenoid interupt. I certainly could disconnect it and try. Other than interupt the voltage to the solenoid, what could that be causing?