Intermittant Electrical/Ignition Problems 85 Mercruiser 7.4

[BermudaBryce]

Good day gentlemen, thank you for reading and helping. I use a 1985 Sea Ray Sundancer with twin Mercruiser 454 340 hp big block gas engines. (I will have serial and other model # tomororw) I was out the other day and when I went to come home the engine would crank but would not fire. I first suspected ethanol problems with the fuel and possible flooding so I limped home. Changed all fuel filters, added extra ethanol treatment (just to be sure) and still no fire in the holes. Came back the next day and changed the distributor cap and rotor and spark plugs (all which needed doing because the real owner of the boat does no maintenance) Engine was backfiring after I stopped cranking it. Checked over everything and changed the coil. Engine fired right up and ran great. Loaded up the boat and went for a cruise and within half an hour the engine died all the sudden, oil press and H20 temp were matching with other engine. I pulled it out of gear and cranked it and it fired right up, ran fine for another 10 minutes and died again, never to refire. Limped home with my tail between my legs and left it alone until today. Went down and cleaned up all the electrical connections between the square ignition amplifier pack and the distributor and the coil. Checked all my plugs wires and everything. Cranked the engine and it fired right up....idled fine for 5 minutes and shutdown. Cranked it 3 or 4 times and then fired it back up and idled for 30 seconds and shutdown. I crank it now and nothing. I'm starting to think the original thurderbolt ignition amplifier pack might be burning up my coil....but I'm hoping someone might be able to steer me on a clear course. Thanks guys.

[dubs283]

there is a troubleshooting guide in the OEM engine manual for the t-bolt ignition, the module/ampilifier will not burn up a coil, in fact coils rarely fail, they are just what people always replace for some reason

when the engine does not run have you checked for spark?? - spark tester between coil tower and ground

nine times out of ten, the reason for no spark on an older t-bolt ignition is the sensor in the distributor, does the one in there look old/crusty??

[BermudaBryce]

Yes the sensor in the distributor is orignial and there is a little bit of corrosion on the posts where the white/green and the white/red wires go to the amplifier pack. That is the one thing I haven't replaced so far. This would explain the intermittantcy of the problem. Thank you.

[cimy3]

I just went through your exact problem....followed the flowchart referenced above and it pointed me to the ignition sensor in distributor. Changed it and now it runs excellent once again.

Bought mine right here from iboats and within a few days I had it.