[Scottie1961]

Originally Posted by Hudson River Steve

I assume you have a Thunderbolt IV ignition system with the standard Mercruiser coil (392-7803A4). If so then the primary resistance should be 0.60-0.80 Ohms and the secondary resistance should be 9,400-11,700 Ohms.

Tested the coil and everything checked out. Primaries were .60 and secondary was 10000 and change. I did notice the ceramic insulator around the output was dark brown on one side. Not sure if that's normal or indicates some arcing or other heat related issue. Coil wire and plug wires all look to be in great shape.

[Hudson River Steve]

If there is any doubt about the coil I would just change it out, they are fairly inexpensive. It seems to behave like a good coil when cold but it could be overheating and shorting when it warms up.

Sent from my XT1254 using Tapatalk

[Scottie1961]

Originally Posted by Hudson River Steve

If there is any doubt about the coil I would just change it out, they are fairly inexpensive. It seems to behave like a good coil when cold but it could be overheating and shorting when it warms up.

Sent from my XT1254 using Tapatalk

I had the same thought. For $75 or whatever the coil costs it's worth it to make sure. Thanks for the help Steve!

[Scottie1961]

I installed the new coil and a larger fuel filter. Took the boat and it ran like a champ for a couple of runs down the lake. After about 45 minutes started the missing symptoms again. I stopped the boat and opened up the engine compartment. Sat around for about 10 or 15 and took off with no problem but about a mile down the lake it started again.
I have two theories about what is happening. Either the fuel pump is weak and is struggling to keep the fuel bowl full or heat is causing intermittent failure in an ignition component. I've got a fuel pressure tester but there's not room to get the fitting in front of the fuel line at the carb or the pump. I may need to build a temp fuel line with rubber hose to splice in the pressure gauge. The fuel pump doesn't make sense though because it runs so well starting out. As for ignition I guess I can run through the ICM diagnostics. I hope it's the fuel pump as the distributor looks like a pain to get out.

[Mariahsteve12]

Thanks Scottie. I am dead in the water until I can get this resolved. Eagerly awaiting your reply. Thanks again.

[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?

[GaryDoug]

I'm not really that familiar with these boats, but if you have a mechanical fuel pump, I think you have a sight tube to indicate a leaking or ruptured diaphragm in the pump. As I under stand it, if you see fuel in the sight tube, the fuel pump needs service. Keep in mind that as the engine heats up, the fuel will tend to vaporize and will need more pressure to keep that from happening. What pressure works when the engine is cold may not work when it's hot.

[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?

Thank you Brian! I might take you up on that offer. The motor is a 95 so does that mean it's a V or is there another way to determine? I'm going to hook up the fuel pressure gauge tonight and see what I get. If the fuel pump looks good then I think the ICM is the next thing to tackle. I'll let you know once I get the pump tested. Thanks again!

[Scottie1961]

OK, I've been thinking through this issue and making sure I'm accounting for all the anomalies when the engine starts running rough. It seems ironic that the engine starts running rough around 2600 RPM's pretty consistently. This is the start of the range, I believe, when the ICM provides full advance to the ignition. I have also noticed a low idle speed at times which I can't directly correlate with the other issue but it happened on Saturday after I was supposedly letting the engine cool down.

Would either of these other bits of data lean toward the ICM or still possibly point to fuel delivery? Does the ICM control idle speed by altering the spark? If so then those two things seem possibly related. I suppose the Ignition Sensor could also cause some sort of issue but it seems that would be more erratic in nature. I'm still going to test the fuel pump tonight but wanted to get thoughts on these other symptoms. Or, I'll just say to heck with the whole stinking mess and replace the distributor with one of the electronic replacements and call it done

[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.
One fella got a couple oz's out of his.
Bad news is, if it's that, it's $450.
I was originally looking at the knock sensor. You may want to double check that. You'll need a descent volt meter. You can test it with a timing light and a hammer.