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mercruiser 4.3 lx not starting

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ussoldier_1984

Contributing Member
Just started having an issue my mercruiser 4.3 won't start. it seems to want to run backwards a couple strokes. Timing is good new plug wires dist cap is clean getting good white spark. cant figure it out. here is a video of what its doing

 
I'd be double checking that timing and firing order on the wires.
Then compression test it. Results of comp test might point to timing chain issues or something like that more specific.
 
motor is in mechanical timing and 8 deg on ignition side like the service manual says. i have checked and rechecked plug wires. 1-6-5-4-3-2 timing order is good. I dont have a compression tester motor was running great the other week then it seemed to run out of fuel and wouldn't start after that.
 
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Are you getting fuel to carb ? Look into carb ,pump gas. do you see a squirt of fuel in throttle bores ? Fuel pump bad? Clogged filters , and the oh so obvious out of fuel in tank Bad gauge saying you have fuel but you really dont?
 
No its a complete new fuel system fuel tank checks out and fuel gauge works fine carb rebuilt 2 weeks ago. and flows great. after further investigation. It seems im only getting good spark to one and two cylinders the rest are about dead put new plug wires on it and ordered new dist cap and rotor button. (guess I should of checked every cylinder for good spark) Soon as i get the parts on Ill let you know how it works.
 
Throwing flames out of the carb means it is either bad plug wires or wires are on the wrong cylinders. Could also mean a burned valve allowing flames to ignite incoming mixture.
 
Has this been your boat since day1? If not, did you replace the plug wires going by the numbers on the cap?
The previous owner may have put the dist in any old way and #1 on the cap is NOT #1 cylinder.
 
the dist wires are on according to the mercrusier manual and Just put brand new plug wires on 3 days ago timing 1-6-5-4-3-2 checked mechanical timing to make sure number one was tdc and dist rotor pointing to number one. So I know that timing is good. Thanks for the input!
 
What year engine? What did the optic trigger module look like under the rotor? If it was heavily rusted that would contribute to the problem. Try the Thunderbolt Ignition tests to see if the optic trigger module and spark amplifier are functioning correctly.
 
boat is a 1995 and the engine is the same. Optic trigger module looks fine I dont see any rust today i put a new dist cap on and still getting hardly any spark. coil is putting out good spark, the only thing i haven't replaced is the optic trigger module every thing else on the ignition side is new, starting with new dist cap today new plugs today. yesterday new spark plug wires, last month i replaced the coil, optic trigger is working because coil is sending out a nice strong spark but yet i am still not getting good spark to the plugs wtf? engine is in correct mechanical timing, i am ready to sink this damn thing lol
 
Tests for Thunderbolt Ignition:


W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!


Check the kill switch first!


#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to the white/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts are present, remove the coil spark wire from the distributor and connect it to a spark gap tester to ground. Remove the white/green lead from the dist. terminal. Turn the ign. key to on and strike the white/green lead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if you get spark, install a new coil. NO SPARK, replace ignition amplifier.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and check it again for 12 volts. If 12 volts is present, replace the ignition sensor inside the dist. cap. If no 12 volts present w/it disconnected--ignition amplifier is shot and must be replaced.
 
in your video, it sounds like you are losing power to your ignition system while cranking. It returns to the ignition system when you release the key back to the run position. Is there a lanyard switch on your shift/throttle control?
 
run a jumper wire straight to the coil from the pos lug of the battery, see if that fixs the problem.

If yes then post and we go from there.
 
ok Ill try that today I have to pick my boat up from the dmv inspector, they were inspecting my trailer so i can get a title and tag for it. already had the boat title just no trailer title so I had to do it the hard way lol. Now if it starts fine when I run a jumper to the coil what will that tell me?
 
You tell me after you think about it a while. I'm not being a smart@ss even though you think that as you read this.

So don't post right now, think on in and then get back to me after the test, remember this is a test do not leave the jumper on, remove it after the test.

If after a day or 2 with going back and forth with posts if you decide you need a bypass we will talk about it then.

Dock gave you clues maybe you didn't read his post closely.
 
its only going to tell me when i go to start it if im losing power through the ignition system by directly bypassing the regular power source to the coil. its not telling me what is wrong with the ignition system unless when he was talking about the safty lanyard switch is bad in that case i could just jump the switch instead. that would be more specific. but going directly from the battery to the coil is not going to tell me anything besides lack of power (somewhere there could be a broken wire or bad connection the possibilities are pretty vast concedering the amount of wire and components in the ignition system.
 
your not bothering me lol i was just saying what is it going to tell me when i do the test ( havent been able to do the test cause my boat trailer is still being inspected for a title so i couldnt pick it up today :(
i am just confused about what that will tell me. I really appreciate the help. been trying to figure the problem out for about a month its just been getting to me
 
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Click on image to enlarge

What are these two wires that are disconnected? they have always been disconnected since I got the boat. boat ran great when I first got it.
 
as far as i can tell the cut out switch was working when it ran last but how do you test it when the boat wont start? someone told me disconnect the white wire with green trace to see if it is the issue is that correct?
 
ok got closer to the answer but still lost. I ran a jumper wire from the positive terminal of the battery to the coil supply and she started right up!!!!! only problem is i dont know what that tells me help please
 
That tells me that the coil is not getting 12 VDC when the key is turned to start/run position. My 87 4.3L has a purple wire going to it from the elec. choke. Check the + side coil wire and trace the wire to where it meets the 12 VDC supply for the engine.
 
ok after trying to trace where i was loosing power to the coil couldnt find it so i re ran a new wire from the ignition switch all the way back to the 10 pin connector worked great for about 5 -10 mins then lost almost all spark again tried running jumper from batt to coil still nothing thinking my new coil might have bit the dust now or could it be somthing else. this is really getting old im ready to sink this damn thing lol
 
Alright so you listened so i'm back. I had your exact problem. If i tell you i spent a solid week checking every wire, every connection, every ground. Mine is a 86'. So what i did in essence is create a new harness.

I ran the jumper with a toggle to the helm and marked it ign. When i want to start the boat pump the throttle, and flip the switch. And i crank the starter with the key. The boat starts, i run it to where i want, when i want to cut the motor i turn the key to off it will still run cause i have power to the coil. Then i flip the switch off.

I wired it like a old style race car and they still do it that way. Way old boats, model t's, and a bunch of industrial equip. are wired that way. Aircraft are wired that way also.

What i think is you have a dirty connection at the starter, main ground may be bad, or the slav solenoid if you have one or relay, could be bad or not grounded or the connection there bad.

Some solenoids have to be grounded by the base, remove and clean the base and the mounting point.

If you chose to start cleaning connections,use 100 sand paper. Make sure you disconnect the battery.

Now look at the starter see a square box on it, could the connection be bad to it.

You main plug should be held together with a wire clip, take it apart, it green and corroded ?

Now understand what i do, i do cause i have 45 years of knowledge. I want to live a long life, so i make sure everything i do i do safe.

About those 2 wires hanging, You have a trim indicator ?

With the jumper you say it runs and doesn't cut out ? If yes then it ain't your coil.

It cranks good when you turn the key ? If yes then you might chase this and never find the problem. Consider this one wire going in to the main plug bad in the plug itself. One side or the other.

My main barrel plug is history when i made my " Harness " When i pulled my motor i never undid it anyway, i disconnected everything from the motor, and marked them.

Ok to finish a bad tach wire will kill you also. People don't really understand that also.

You had it running water going to the drive ? What did the gauges do ?

Also i have no man overboard lanyard switch.
 
its early so I might of missed somthing you wrote. Now when I run the jumper from the battery to the coil it does not start anymore. I have week spark with the jumper. Prior to this though I was able to trace that the week wire is after my 10 pin connector on the wiring harness (the engine side) I checked my oil pressure switch works good (by jumping the connectors instead of it being hooked up to the oil pressure switch) still didnt start. attempted to try to check my shift interupter switch on the engine but not sure how to test it but I think I successfully jumped that switch but still didnt start so I have decided to do exactly what you said run a new wire with a toggle like you said. but im still at the problem with the jumper wire from the battery to the coil Im getting week to no spark. I do have a trim indicator that has never worked since i got the boat do you think those grounds are for the trim? what wire is the tack wire? can I disconnect it to see if it is a bad tach wire? I dont have an issue with cranking the motor starter is really strong. but im going to check to see if it is corroded on the bottom like you said. not sure if im forgetting anything (probably am lol)
 
Think I might of figured out the whole issue.... the oil pressure switch seems to have bad continuity but still enough to send 12 volts just not when im cranking I jumped it and it wants to start but extreamly low fuel and a low battery is preventing me from seeing if that is the real issue. Im pretty sure its it though Ill let you know when the battery is charged and i get some fuel
 
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