4.3 Mercruiser Fires but will not run

[alldodge]

The coil should have 12V any time the key is turned to RUN or START

There can be up to 2-purple wires on the + side of the coil and up to 2-gray wires on the - side of the coil

I say "up to" because the purple and the gray wires can be spliced internally so only one is there, but may not and are connected together at the coil post

 

[nola mike]

Just forget about looking for power at the fuel pump, this is not your problem

Check for 12V on the + (positive) side of the ignition coil. Not the one on the starter but the one on the coil

The reason it fires and dies when the key is released is because voltage is breaking. I'm thinking its the key switch or the kill switch

I think I'm getting confused as to what the OP is actually seeing.
1. All purple wires should have 12v at them whenever ignition is on. That's at the coil (+) and the oil pressure switch.
2. The fuel pump is powered by the pur/yel wire from the starter, through the oil pressure switch, when cranking. That wire is only hot during cranking. In the run position, the oil pressure switch closes (when adequate OP is reached) and the fuel pump is powered by the 12v from the purple wire. If the switch isn't closing, the fuel pump will only run during cranking.
3. "So when the key is on I have power to the purple wire only then when I crank the engine I have power to both purple and purple/yellow wires ...when I release key I lose power to purple/yellow wire."
This is normal.
4. So OP, you have power to both the coil and pressure switch in run position, correct? You can check the purple at the coil during cranking, or check for spark during cranking as the next step.

alldodge , from what I'm seeing, this engine doesn't power the coil through the slave when cranking, so if the kill switch is the problem then he shouldn't have 12v at purple at all, and won't catch. If the neutral safety switch is bad, the motor won't crank.

 

[alldodge]

alldodge , from what I'm seeing, this engine doesn't power the coil through the slave when cranking, so if the kill switch is the problem then he shouldn't have 12v at purple at all, and won't catch. If the neutral safety switch is bad, the motor won't crank.

If the kill switch is the issue, the coil will see 12V from the purple/yellow back feeding the purple once the oil pressure builds up. So it will start for a second and as soon as key is released, yel/pur stops 12V supply back feed and motor dies

The issue is the thread started with the OP saying motor only starts for a second and when key is released it dies. If it was the fuel pump the motor would run for some time before the carb ran out of fuel.

 

[nola mike]

If the kill switch is the issue, the coil will see 12V from the purple/yellow back feeding the purple once the oil pressure builds up. So it will start for a second and as soon as key is released, yel/pur stops 12V supply back feed and motor dies

Ah, gotcha.

The issue is the thread started with the OP saying motor only starts for a second and when key is released it dies. If it was the fuel pump the motor would run for some time before the carb ran out of fuel.

Yeah, if the bowl was full to begin with. He also said it was catching initially, and now doing nothing, so if he's been running the carb dry, and it's only getting a bit of fuel when cranking... I also thought that he had 12v on purple all the time. Guess that's the big question. Checking for 12v at the coil at run, or just checking spark would be helpful.

 

[Jgrimm1983]

Why would I have power to both the purple wire and the gray wire when connected to solenoid? When I remove gray wire from solenoid the wire does work as a ground to test purple wire.

 

[Jgrimm1983]

So just to explain better. When both purple and gray wires are connected to solenoid and put multimeter on them I get nothing. When I use battery ground I have 13+ volts on both purple and grey wires. When I remove the gray wire from solenoid I can use it as a ground when I test the purple wire (but cannot when attached to solenoid) This is all when key is turned on but not trying to start.

 

[nola mike]

What solenoid? The coil?

 

[Jgrimm1983]

Yes. Sorry

 

[alldodge]

There are 2 windings in the coil

In short, 12V on + side stays on full time, the - side gets switched ON and OFF

You want to measure voltage from the + side of coil to engine ground, not the - side of coil

 

[isaacs]

Are you getting a strong spark? If you don't have a spark tester, stick an insulated screwdriver into the wire coming off of the coil (you'll have to remove it from the center post of your distributor cap). Hold this close to a known ground, have a friend turn the key to start and you should see a spark that will jump to about 1/2 inch. If it's bright out you'll have to stick a blanket or something over your head so you can see the spark--don't let the blanket get stuck in the belt. And don't touch the shaft of the screwdriver, these things can pack a punch!

 

[harringtondav]

It won't catch at all if this was the issue

It will, and then die once the key is returned to run. I learned this the hard way when I accidentally pulled the lanyard. I freaked out. When I calmed down I saw the lanyard on the floor. Switch back up, boat started and kept running.

 

[Jgrimm1983]

OK I have 13+ volts to both sides with key in ON position.

 

[alldodge]

Run through this troubleshooting guide

 

[nola mike]

It will, and then die once the key is returned to run. I learned this the hard way when I accidentally pulled the lanyard. I freaked out. When I calmed down I saw the lanyard on the floor. Switch back up, boat started and kept running.

Yeah, alldodge corrected me above. In some engines the coil gets a feed from the starter when cranking, others are able to have voltage back fed through the purple while cranking

 

[Rick Stephens]

Yeah, alldodge corrected me above. In some engines the coil gets a feed from the starter when cranking, others are able to have voltage back fed through the purple while cranking

Mike, all mercs get power fed to the coil via purple from the ignition switch (until you get to ECM models).

The only consideration here is that IF the lanyard switch is disconnecting power through purple (whether faulty or by being pulled), will the fuel pump prime wire (Purple/Yellow) that connects to the fuel pump also supply voltage to the coil when cranking. In many electric fuel pump models the purple/yellow directly connects to the purple supplying fuel pump and choke and subsequently the coil as well.

 

[nola mike]

Mike, all mercs get power fed to the coil via purple from the ignition switch (until you get to ECM models).

The only consideration here is that IF the lanyard switch is disconnecting power through purple (whether faulty or by being pulled), will the fuel pump prime wire (Purple/Yellow) that connects to the fuel pump also supply voltage to the coil when cranking. In many electric fuel pump models the purple/yellow directly connects to the purple supplying fuel pump and choke and subsequently the coil as well.

Not sure what you're getting at... In the wiring diagram, the purple is energized during cranking by the purple/yellow across the oil pressure switch instead of via the ignition switch, even if the lanyard switch is tripped.

 

[Jgrimm1983]

Found the issue. Had a mechanic friend of mine come up with spark tester. Had spark so he tapped the carb a couple times with hammer and it works. Ended up dying again so he says the needle is getting stuck and not feeding carb with gas. He says I should clean/rebuild carb. Is there any rebuild kits you recommend or are they all the same?

 

[Rick Stephens]

Not sure what you're getting at... In the wiring diagram, the purple is energized during cranking by the purple/yellow across the oil pressure switch instead of via the ignition switch, even if the lanyard switch is tripped.

Mike, purple does not stop getting power from ignition switch when cranking. If your carburetor is already primed, the motor will start just fine without the purple/yellow fuel pump primer wire connected to the starter - just like if you have a mechanical fuel pump and there is no purple/yellow on the starter at all. The purpose of purple/yellow is to run the fuel pump during cranking. Yes, it does happen to cross connect backwards to purple once oil pressure is up, and hence to the coil, but that is not its purpose.

Jgrimm, which carburetor do you have?

 

[nola mike]

Mike, purple does not stop getting power from ignition switch when cranking. If your carburetor is already primed, the motor will start just fine without the purple/yellow fuel pump primer wire connected to the starter - just like if you have a mechanical fuel pump and there is no purple/yellow on the starter at all. The purpose of purple/yellow is to run the fuel pump during cranking. Yes, it does happen to cross connect backwards to purple once oil pressure is up, and hence to the coil, but that is not its purpose.

Jgrimm, which carburetor do you have?

The point was that if the man overboard switch is tripped, that should cut power to the coil, since purple from the ignition is interrupted. However, WHEN CRANKING, that voltage will be backfed through the switch via the purple/yellow--the coil would still get voltage, even though the ignition switch isn't powering it. With a mechanical fuel pump, this wouldn't be true--the coil wouldn't get any power (ever, unless it's a 470 and runs a separate coil wire from the slave) and the engine wouldn't catch if the MOB switch is tripped.

 

[Jgrimm1983]

Top Numbers on the carb are 807764
bottom are 5z01

Does that help?