UPDATE onMecruiser 5.7 Thunderbolt no idle then no spark

bomar76

Lieutenant Commander
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Jun 27, 2002
Messages
1,963
1994 Mercruiser 5.7 L Thunderbolt (Quadrajunk) SN 0f067387 Alpha 1 Gen 2 w/279 hours...New plugs, wires, rotor and cap @ 269 hours.<br />Boat would run fine at all speeds most of the time BUT sometimes after coming off a WOT run engine would not run below 1200 rpm. Stumbles, etc as if it was starved for fuel. Makes it interesting trying to dock. Had to baby Mercotrol to get it to stay running at that speed. Any speed above that runs fine.<br />First time it happened I suspected bad fuel (boat had set for sometime with 1/4 tank). Added 6 cans of B12 (50 gal tank) 6 cans dry gas, sprayed carb down with carb cleaner (lots of gunk) and changed the Merc fuel filter water seperator. Opened primary mixture screws to 2 turns (was @ 1 1/2)and all seemed better.<br />Boat ran fine for next 5 hours, but did have a wandering idle and has always been hard to start from cold.<br />Now, on last trip, same thing happened again.<br />Drug it home, put muffs on, fires right up. No problem.<br />Let it warm up and try to restart. No start.<br />Check fuel- yep, accel pump squirts it out. Check for 12v at both sides of coil. Got that. Pull distributor cap and check rotor and inside of cap...looks like new. No spark though, verified with timing light. Put timing light on coil high tension lead...nothing. Check for loose wires, blown fuses, etc...none found.<br />Kill swith lanyard IS plugged in.<br />Now....do you think the 2 events are related?<br />I would think that if the Thunderbolt went south it would be an all or nothing event, not a random loss of low speed.<br />Ideas, men, ideas.
 

magster65

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Sep 1, 2002
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Re: UPDATE onMecruiser 5.7 Thunderbolt no idle then no spark

That's a nice little diagnostic chart :)
 

bomar76

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Jun 27, 2002
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Re: UPDATE onMecruiser 5.7 Thunderbolt no idle then no spark

Thanks akriverrat!!! :) <br />I followed the flow chart (actually earlier I substituted a known good coil from the old Baja as I suspected the coil).<br />Coil was ok, so that leaves the ignition module.<br />Anybody know a site with a better price for item #805361T4 than $169.05 (list 198.20) with free shipping?<br />Thanks tons!<br />Joe
 

swampfx914

Cadet
Joined
Jul 22, 2003
Messages
14
Re: UPDATE onMecruiser 5.7 Thunderbolt no idle then no spark

just out of couriosity does it ever backfire through the carb? I have the same year engine with the backfire problem.
 

bomar76

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Jun 27, 2002
Messages
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Re: UPDATE onMecruiser 5.7 Thunderbolt no idle then no spark

Just a very few times while trying to restart....never while running.
 

bomar76

Lieutenant Commander
Joined
Jun 27, 2002
Messages
1,963
Re: UPDATE onMecruiser 5.7 Thunderbolt no idle then no spark

Ignition module arrived yesterday, put it in as soon as I got home.<br />Replaced cap (pulling it gave more access), ran fan, turned key.......<br /><br />VAROOM!!!! :cool: :) :p <br /><br />Easiest it ever started, purred like a kitten.<br /><br />BTW, I checked local dealers for this part-none had it and 2 told me they never go bad-it was some other problem -and bring it to them and they would diagnose problem for $120.<br /><br />Thanks for the flowchart!
 
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