[Scottie1961]

Originally Posted by Brian Belcher

just finished a bunch of research on yours today. Seems that your ICM is prone to filling up with water. I don't know how but there were enough out there that Mercury totally changed the replacement module. I read where folks would pull the module off and would just shake it. They could hear the water inside.

Makes you go Hmmmm. How in the world would that much water even be close enough to the module? I wonder if it's condensation that has built up in the modules over the years. I might pull mine off to see if I can hear anything. I"m still not sure if this is a IV or V module so that would confirm that as well. It's definitely mounted on the distributor but I thought there was a IV version that mounted that way. Haven't had time to do the fuel pressure check yet.

[Scottie1961]

Fuel pump worked like a champ. Cranked for 15 seconds and got just under a pint of gas. Looks like it's the ICM. I pulled it just now and it says ThunderBolt IV on the module. That means it's the older module without knock sensor? Now I'm at that question that I've seen others stress over. Should I replace the whole ignition system with a newer HEI or try to find an ICM?

[Scottie1961]

Originally Posted by Brian Belcher

Well, the coil didn't fix it?
The next thing is the ignition module. I don't think it's mechanical cause heat doesn't generally affect these components. (sans vapor lock!)
I believe you have the Thunderbolt V. Do you have a buddy you could borrow his to troubleshoot?
I'll loan you mine if you'll pay for shipping.
HudsonSteve. Will mine run his?

I don't think the V will work on my ignition if it has the IV module will it? I might try to find someone that has a working module just to confirm. Thanks Brian for the offer.

[Scottie1961]

OK, call me crazy but I think this engine was set at 0 degrees advance timing at idle. Just for fun I threw the timing light on and it showed advance at zero. Just to confirm since there are no timing marks, the deepest v is 0 and then BTDC goes 4, 8 and 10 represented by each point on the timing mark plate. I adjusted to 8 degrees BTDC and the thing started really purring. However, I want to make sure this is not false feedback. Can anyone confirm the marks on the timing plate? Not sure this explains everything I'm experiencing but missing 8 degrees of advance could be significant. It looks like original merc paint so did they paint over the markings or are they just not there?

[Brian Belcher]

Did you have to ground the purple (?) wire before looking at timing?
Wish I had the answer you're looking for but I have the '97 454.

[Hudson River Steve]

According to the manual, the Thunderbolt IV does not require you to do anything special to time the system. The Thunderbolt V system requires that you ground the PUR/WHT wire to place it in base timing. You also need to ground this wire to adjust the idle speed or mixture. https://www.google.com/url?sa=t&rct=...24jmmNvYWTFpmQ

[Brian Belcher]

Awesome link Steve ! I didn't ground while doing carb adjusts. Thanks bud!!

[Scottie1961]

Reporting back on the issue. I have the problem resolved and will review everything I've done which may help someone in the future with a similar problem.

This all started with a tune-up including spark plugs, rotor, dist. cap and fuel filter. The engine would "diesel" after a hard run so I suspected the plugs may have carbon buildup. After the tune-up all seemed well with the world until about 40 minutes in and I started experiencing engine cutting, coughing and backfires over about 2500 RPM's.

I initially thought fuel supply since I had changed the fuel/water separator filter. Turns out the fuel pump was fine and now has another brand new filter to keep him happy. After some suggestions from Hudson Steve and Brian Belcher I began looking at the ignition system. The coil tested out OK, but I replaced it since it had some pretty significant carbon tracks on the insulator. Still had the same symptoms so many of us felt the ICM was suspect so I pulled that off to just take a look. It looked OK with no signs of heat or water intrusion.

At the same time I checked the timing and was surprised to see the advance was 0 degrees at idle. I adjusted the advance to 8 degrees and it seemed like the motor was running well. I also pulled the Ignition sensor and it was corroded which probably didn't help. Replaced the sensor and alakazam it seemed like that fixed the problem. HOWEVER, when running the boat I discovered intermittent backfiring and still running rough but not as bad. Threw the timing light on and number one hole showed internittent firing. Pulled the wire from the distributor and no change in engine idle speed. I also confirmed there was plenty of fire through that distributor plug as my hand got a dose of 40000 volts. I also noticed the number 8 wire which is right next to number 1 was showing firing at a very fast rate. I noticed when I checked the timing the first time the number 1 wire was not firing regularly. I thought my timing light (very old) wasn't picking up the pulses. (Another clue that I ignored)

It all began to make sense. The number one wire must have a crack in the shielding and was jumping over to the 8 wire. All that to say I went to O-Reilly's this evening in a small town about thirty minutes from the lake. Bought a cheap set of chevy plug wires and swapped out 1 and 8.
Got back to the lake at sunset and then pure joy at hearing the small block purr like a cat in a dairy farm. Left the trailer on the ramp and had time for two good runs down the lake. Ran and sounded better than I've heard it since I bought the boat.

The plug wires on the boat looked fairly new and were Sierra Performance wires. I "assumed" they were fine and didn't bother changing them due to laziness. I still think the sensor was part of the issue but the wires should have been replaced when I did the tune-up. Thanks for all the help guys! My grandpappy said we should learn from mistakes but they don't have to be our own!

[jad050]

wow, I haven't here for a few years, early on this forum answered a lot of the questions I had when I first got my boat. The first posting I see today in forum describes symptoms of what I was researching.

Background, boat was idle for over a year, due to "life getting in the way". Took boat out, ran great for about 15-20 minutes, then chugged along at around 2700 RPM max. This has happened, water in fuel. So, I changed filter, and since The gas was old and I wasn't sure if I had stabilized it, I drained the tank. Back on water this weekend, now I am getting the exact same symptoms described here. Engine starts right up, and flies for first 10 minutes, then struggles to keep at 3K rpm. shut engine off, to check fuel line. restart engine after 20+ minutes runs great for another 10 minutes...

I have the tiny 4.3 engine, think I might have the V ignition, Are the coils the smae, i.e ohm specs... It's cheap enough I might just replace it for peace of mind. Hoping for fairly cheap solution...

Jim D
1996 Shabah 4.3LX

[Scottie1961]

Hey Jim, if you know there was water in the fuel system I would confirm that is resolved before checking other systems. When you said you checked the fuel line did you take the line off or check the filter? You might dump the fuel filter into a clear jar and see if any water settles to the bottom.

If the fuel is clear then a good place to start would be to test the coil, I think they are the same resistance specs, and pull the distributor cap. I wonder if condensation could have built up somewhere that could cause intermittent problems. I've found a timing light is invaluable for checking on whether you're getting good fire from the ignition.

Be methodical and don't move on to the next thing until you're certain albeit plug wires etc