Won't idle below 1500 RPM, Strange Ignition Problem

Seb171

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Aug 18, 2017
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Hi all, first time poster here. The forum has been tons of help in the past, but I've finally run into a problem that I can't figure out on my own. I'm handy with a wrench and a manual, but am no expert mechanic by any means :)

I just bought this boat. Engine is a 1996 4.3L Alpha with Thunderbolt IV Ignition. Engine Serial Number: 0F770091.

I took it out day one and it ran all day, although it wasn't happy at idle. It would sometimes idle fine, other times I had to continually give it a little gas to keep it from stalling. I figured I'd start by replacing some maintenance items and adjust the idle. I replaced the distributor cap and rotor, ignition wires (including the coil to distributor wire), spark plugs, and fuel filter.

Now while on muffs, I have to keep pumping the throttle to keep it the engine from stalling, I can't drop the RPMs below 1500 or so. I start checking connections, and hooking up my timing light, and here is what I find. 4 of the ignition wires are flashing like normal, and 2 are flashing at strange intervals. Even more
strange is the fact that the spark intervals on cylinders 2 and 4 are consistently wacky!

[FONT=arial, sans-serif]4 normal Cylinders: ta..ta..ta..ta..ta..ta..ta[/FONT]
[FONT=arial, sans-serif]Cylinder #2 spark: ta........ta........ta.......ta[/FONT]
[FONT=arial, sans-serif]Cylinder #4 spark: ta..ta.......ta........ta..ta[/FONT]

[FONT=arial, sans-serif]Upon first discovering this, I pulled off the distributor cap and found that the arm of the rotor broke and the metal part hanging on an an angle. It must have hit one of the points a little too hard. Defective part or maybe I didn't have it seated properly? I run to my nearest marine parts store and buy a new cap/rotor combo and install them. Same cylinder spark results as before. I've re-checked the rotor several times since, and it appears to be fine. I've tried replacing the wires and spark plugs with some of the old ones I just took out and I'm getting the exact same results. I also remove the spark plugs on #2 and 4 and put them to ground, crank the engine and I've confirmed I'm getting spark from them.[/FONT]

[FONT=arial, sans-serif]I've double and triple checked that the distributor wires are in the correct order and have gone though the TB IV Ignition troubleshooting guide. It's currently guiding me to replace the ignition sensor in the distributor. I have a hard time believing it's the ignition sensor though. From what I've been reading, the sensor either craps out or it doesn't, I find it awfully strange that the ignition sensor would function fine for 4/6 wires, and intermittently work for the other 2. Before I start spending money on parts and waiting for shipping, I thought I'd reach out to the community. Is there something I'm missing here? Did something get damaged when the distributor rotor hit point and broke? Might it be a control module issues? Has anyone seen a similar ignition problem before?

Any thoughts or advice would be greatly appreciated! Thank you.
[/FONT]
 

alldodge

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Howdy, and thanks for the engine serial number

Test procedure says its the module, so if this is one of the modules which only have 2 wires coming from it, then I would replace the module. The new modules use 3 wires and one is a ground wire which helps from intermittent firings. Now if it has the 3 wires then might need to rethink it
 

Seb171

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Aug 18, 2017
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Thanks for the reply. It's a 2 wire ignition sensor. I've ordered a new one. I'll follow up once I have it installed.
 

Seb171

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Aug 18, 2017
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UPDATE: I've just installed the new ignition sensor. The new one is a 3 wire. No difference! Wires 2 and 4 are still not firing regularly.

What could be my problem? I've followed the troubleshooting manual exactly several times and it tells me to replace the ignition sensor. Does anyone have any advice for me? I'm toying with the idea of buying another Thunderbolt IV Ignition Module and testing that. I can't think of anything else.

Thanks
 

alldodge

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Messages
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The TB troubleshooting guide starts with testing for spark. To go any further you have to have NO spark. So what is telling you that you need an ignition sensor?

My thought is, if it does not fire on the same two plugs, then first verify the rotor and cap will make contact. If they make contact, then see if there is play in the distributer shaft (side play, not up down). Will need to remove distributer to check
 
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