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Old 04-27-2016, 02:44 AM   #11
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btw... what part is that exactly with the prongs for the two wires on the oil pressure sender. Seems the switch is that round piece with the one wire on the tip of it. What is the part that the two wires actually hook to called?
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Old 04-27-2016, 08:38 AM   #12
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A miss is when a cylinder does not fire and a backfire is due to a timing issue that results in fuel igniting when the intake valve is open and resultant gases blow back up through the carb. A backfire can also be due to an improperly operating carb that dumps to much fuel.
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Old 04-27-2016, 08:42 AM   #13
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The smaller one with two wires is the oil pressure switch that provides power to the fuel pump, the other is the oil pressure sendor that is connected to the oil pressure gauge at the helm. It's resistance changes with OP (oil pressure) which varies the current through the gauge and the needle deflection is the result.
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Old 04-27-2016, 09:42 PM   #14
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Help again! On the water now... Everything ran great for about mins. Then yhe backfires started anywhere in between idle and 3/4 full thottle... Now i cant even get to full throttle, backfiring like crazy... Help Mike!!!
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Old 04-27-2016, 10:20 PM   #15
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Help again! On the water now... Everything ran great for about mins. Then yhe backfires started anywhere in between idle and 3/4 full thottle... Now i cant even get to full throttle, backfiring like crazy... Help Mike!!!

Have you checked the ignition timing? Thunderbolt V requires a wire to be grounded to set base timing. Google for how to set mercruiser Thunderbolt V timing.

When on the water use the timing light to verify the timing is advancing correctly.

Fuel can be an issue, temporary use a portable gas can in place of the built in fuel tank to see if that makes a difference.
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Old 04-27-2016, 10:25 PM   #16
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Havent messed with the timing. Dont have a light handy... Would it be a timing issue co sidering my idle and wot are fine? Could my icm be going out?
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Old 04-27-2016, 10:30 PM   #17
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Havent messed with the timing. Dont have a light handy... Would it be a timing issue co sidering my idle and wot are fine? Could my icm be going out?

If you can hit WOT the timing is probably ok. For electrical I would suspect either the coil or ignition sensor in the distributor before the icm.
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Old 04-27-2016, 11:23 PM   #18
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If im not mistaken, i can test both those parts seperately reading some guides on here... Correct?

I just know that nobody around here knowz how to fix it apparently, its feels like a hopeless situation...
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Old 04-28-2016, 12:02 AM   #19
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Maybe i should add the fact that my tach has never worked since i bought the thing. Im assuming im reaching wot based on the sound of my rpms as compared to before... Im just stumped
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Old 04-28-2016, 01:47 AM   #20
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So where in the world are you?

Got a Harbor Freight near you? Go buy a cheap timing light and tach/ dewell meter.

The tach gets its signal fron the negative terminal on the coil, gray wire. Check the connections at both ends.

Do you have a Merc manual and wiring diagrams? If not get them from the Documents tab of this site.
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1900sr, 1999, 5.0 mercruiser, thunderbolt v

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