Mercruiser 4.3L Alpha Gen 2 cold hard to start

timjet

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I know this is common problem with this engine. As I understand it the electric fuel pump will only energize when there is oil pressure so no fuel to the carb until oil pressure is achieved during cranking. So the fix is to pump the throttle several times and position the throttle about 1/2 way in neutral. Crank the engine long enough to get oil pressure then release the key but leave it in the on position. The fuel pump is energized while there is oil pressure. Doing this a couple of times and the engine should start. OK, this seems like a very poor design leading to premature starter failure.

So has anyone bypassed the oil pressure switch to allow electrical power to the fuel pump? How about wiring a momentary press on release off switch in parallel with the oil pressure switch. Then on a cold engine only, you turn the key on, press the newly installed switch for about 15 seconds. This supplies voltage to the fuel pump. Release the switch and start the engine like every other engine in the world.

Does anyone see a problem with doing this?
 

achris

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You have missed one critical part of the circuit. On the starter motor there is a terminal that only get energised when the engine is cranking. If you have a look at the circuits you'll see it. That terminal is connected to the fuel pump, and while the engine is cranking, the pump should be getting power...

All that said, what engine is it? The 4.3 litre Gen IIs have a variety of flavours, and what's going on will depend on which flavour you have. 2bbl carb or 4bbl? If 4bbl, Weber or Rochester? Best post the serial number, or at least a year and the exact model (4.3L, 4.3LX, 4.3LXH, ect)

As for a momentary switch to power the pump... Sounds like a way around whatever problem you have, but not a fix for the problem, ,just a 'band-aid'....

Chris.....
 

wrench 3

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A lot of commercial applications have a momentary contact primer switch installed from factory. So it is an acceptable practice.
 

thumpar

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Don't bypass the safety systems that were original. Like achris posted the pump should run when you are cranking the engine. If it is not then figure out why. If it is then you have a different issue.
 

timjet

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OK, thanks guys for the replies. The problem is the engine is very hard to start when cold. I have to pump the throttle 20-30 times to get it to start. Once it's started and warmed up, it's not a problem for a day or two.
1994 Mercruiser 4.3 L alpha 1 gen 2 outdrive. 4 BBL weber carb.
The engine was just replaced with a Jasper rebuilt.

Because it's so hard to start cold, I am assuming it is a lack of fuel hence the momentary switch to start the fuel pump. However like Chris said it will not work if the engine has to be cranking, so I could bypass everything and run it direct from the battery. But I'm open to any suggestions.
 

timjet

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Chris, I just looked at the wiring diagram. There is a yellow/purple wire from the starter solenoid to the fuel pump, but there is also a purple wire from ignition key to the oil pressure switch and then to the fuel pump. As I look at the diagram it seems to me that voltage is supplied to the pump when the starter is engaged and also to the pump when the oil press switch closes. This is consistent with what you said -

"On the starter motor there is a terminal that only get energised when the engine is cranking. If you have a look at the circuits you'll see it. That terminal is connected to the fuel pump, and while the engine is cranking, the pump should be getting power..."

OK, yes but if that were the only circuit then voltage to the pump would stop when the key is released from start to run, hence the purple wire from the oil switch. If the oil pressure switch is bypassed then voltage is always supplied to the pump when the key is on, engine running or not, not a good idea unless it's just momentary on a cold engine.

Am I misreading the diagram?
 
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thumpar

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You are reading it correctly. When the starter is cranking the voltage is supplied from the starter. Once it actually starts the oil pressure switch provides the voltage.
 

bruceb58

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Before you start it next time, have some one pump the throttle while you look down the carb throat and make sure you see streams of fuel going onto the carb.

I had the identical issues when I first bought my boat and I just had to clean out the accel pump section of the carb. Now it starts fine.
 
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timjet

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Before you start it next time, have some one pump the throttle while you look down the carb throat and make sure you see streams of fuel going onto the carb.

I had the identical issues when I first bought my boat and I just had to clean out the accel pump section of the carb. Now it starts fine.

I will do that and report back.
 

timjet

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You are reading it correctly. When the starter is cranking the voltage is supplied from the starter. Once it actually starts the oil pressure switch provides the voltage.

Not saying I'm going to do this, I do want to check the carb first, but if the above is correct then bypassing the oil pressure switch will energize the fuel pump with the key in the run position, correct?
 

thumpar

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Not saying I'm going to do this, I do want to check the carb first, but if the above is correct then bypassing the oil pressure switch will energize the fuel pump with the key in the run position, correct?
Yes. Not a good idea though. If there happened to be a fire it would be fed even when the engine is off. That is the reason for the oil pressure switch.
 

timjet

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Yes. Not a good idea though. If there happened to be a fire it would be fed even when the engine is off. That is the reason for the oil pressure switch.

I will report back on my carb findings as suggested by bruceb58. However the fuel pump will only be energized with the KEY ON and the oil pressure switch bypassed, correct?
 

Fun Times

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Chris, If you have a look at the circuits you'll see it. That terminal is connected to the fuel pump, and while the engine is cranking, the pump should be getting power..."
Chris happens to be a master with Mercruisers circuitry....:becky::bolt::thumb:

OK, yes but if that were the only circuit then voltage to the pump would stop when the key is released from start to run, hence the purple wire from the oil switch.
Once the engine is running at idle speed, the carburetor's "fuel bowl" is/should be full enough to keep the engine running smoothly during the millisecond that the electrical circuit changes from the starter to the oil switch. With the system working correctly, the engine would never detect the change.

If the oil pressure switch is bypassed then voltage is always supplied to the pump when the key is on, engine running or not, not a good idea unless it's just momentary on a cold engine.
As a safety procedure, just be sure that the fuel pump turns off when the engine shuts down for any reason while the key is in the on position so the fuel will not flood the engine or bilge area.

However the fuel pump will only be energized with the KEY ON and the oil pressure switch bypassed, correct?
On the starter motor there is a terminal that only get energized when the engine is in cranking mode.....The starter terminal will not power the fuel pump with the key in the run position...Only in the crank position thus the reason for the oil switch to take over and supply 12v power with engine running.
 

thumpar

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I will report back on my carb findings as suggested by bruceb58. However the fuel pump will only be energized with the KEY ON and the oil pressure switch bypassed, correct?
That is correct but you don't want to rely on the key switch to be your safety device.
 

achris

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OK, thanks guys for the replies. The problem is the engine is very hard to start when cold. I have to pump the throttle 20-30 times to get it to start. Once it's started and warmed up, it's not a problem for a day or two.
1994 Mercruiser 4.3 L alpha 1 gen 2 outdrive. 4 BBL weber carb.
The engine was just replaced with a Jasper rebuilt.

The Weber carbs have an 'unacknowledged' (by Merc) problem... When you shut them down, they dribble fuel down through the main venturi until the bowl is empty, hence your hard starts. I also noticed that the fuel pump seems to lose prime as well. I haven't had a chance to do any proper diagnosis on that part of the problem, but I do suspect they are related.

timjet said:
Because it's so hard to start cold, I am assuming it is a lack of fuel hence the momentary switch to start the fuel pump. However like Chris said it will not work if the engine has to be cranking, so I could bypass everything and run it direct from the battery. But I'm open to any suggestions.

That's not such a good idea. The reason the oil pressure switch is in the circuit is so if the engine stalls out for any reason (without the key being switched OFF), then once oil pressure drops the power to the fuel pump is shut off. Boats are particularly nasty places to have an uncontrolled fuel leak. It stays in the boat, and has caused deaths in the past.

Chris, I just looked at the wiring diagram. There is a yellow/purple wire from the starter solenoid to the fuel pump, but there is also a purple wire from ignition key to the oil pressure switch and then to the fuel pump. As I look at the diagram it seems to me that voltage is supplied to the pump when the starter is engaged and also to the pump when the oil press switch closes. This is consistent with what you said -

"On the starter motor there is a terminal that only get energised when the engine is cranking. If you have a look at the circuits you'll see it. That terminal is connected to the fuel pump, and while the engine is cranking, the pump should be getting power..."

OK, yes but if that were the only circuit then voltage to the pump would stop when the key is released from start to run, hence the purple wire from the oil switch. If the oil pressure switch is bypassed then voltage is always supplied to the pump when the key is on, engine running or not, not a good idea unless it's just momentary on a cold engine.

Am I misreading the diagram?

No, you have it spot on.

The problem with what happens with the carb dribbling and the pump losing prime is that the pump takes forever to pick up fuel again. One way around that is to install an outboard style fuel primer bulb in the line between the tank and the engine. Just before starting the engine, give the bulb a couple of squeezes and fuel is available to the carb. That used to work really well with the old mechanical pumps as the fuel flowed right through the valves and you could fill the carb bowl up and the engine would start right up.... But the electric pumps don't valve the same way, and unless the pump is running, you can't push fuel through it... So, how to work around that? Most people simply short the oil pressure switch (as you suggested), switch the key ON, give the bulb a couple of pumps and away they go. But, as pointed out already, that bypasses the safety feature of the oil pressure switch, and leaves you open to some 'unpleasant' consequences.

Two ways around the problem as I see it would be a) a latching relay initiated by the operator pressing a button on the dash that would allow him/her to then go back to the engine to prime it, with the relay being de-energised by either a timer (30 seconds would be enough) or the action of cranking the engine...
Option b) would be easier to implement. That of putting a momentary switch inside the engine bay (obviously a flame proof one!) that the operator holds while priming the fuel bulb....

Hope this helps...

Chris..........
 
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timjet

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. But the electric pumps don't valve the same way, and unless the pump is running, you can't push fuel through it... So, how to work around that? Most people simply short the oil pressure switch (as you suggested), switch the key ON, give the bulb a couple of pumps and away they go. But, as pointed out already, that bypasses the safety feature of the oil pressure switch, and leaves you open to some 'unpleasant' consequences.

Option b) would be easier to implement. That of putting a momentary switch inside the engine bay (obviously a flame proof one!) that the operator holds while priming the fuel bulb....

Hope this helps...

Chris..........

Yes Chris that helps a lot. Thanks so much for the explanation about the fuel draining out of the Weber carb and why it's so hard to start. I've had this boat for 17 years and it's always been soooo hard to start when cold.

OK, option B sounds good and I understand it. The priming bulb would of course be located between the electric fuel pump and the fuel tank. Probably easiest to install it just before the fuel filter/water separator. Since the electric fuel pump does eventually prime itself, would it work just to eliminate the priming bulb and bypass the oil pressure switch with a momentary switch and just keep it pressed on for a little longer until the electric fuel pump is primed and supplying fuel to the carb?

Perhaps the answer to that is you would not know when the electric pump is primed and supplying fuel to the carb and by installsing the priming bulb you would "feel" the bulb harden as fuel enters the carb. Correct??
 

achris

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...Perhaps the answer to that is you would not know when the electric pump is primed and supplying fuel to the carb and by installsing the priming bulb you would "feel" the bulb harden as fuel enters the carb. Correct??

Not quite. As the fuel is being sucked through the bulb and not pushed through it, the bulb will soften as the pump fills the carb
 

timjet

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OK, I checked the electric fuel pump with a multi-tester and it received 12 volts when the engine is cranking, just like it should. However when I bypassed the oil pressure switch (not sending unit) I did not get 12 volts at the fuel pump with the key switch on. I used a jumper wire to connect the two wires on the oil pressure switch (bypassing the switch), turned the key on and there was no volts at the electric fuel pump. I then check for voltage on the purple wire connected to the oil pressure switch with the key on and there is no voltage there either.

Can someone explain why I'm not getting voltage at the oil press switch (purple wire) with the key on??
 

alldodge

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OK, I checked the electric fuel pump with a multi-tester and it received 12 volts when the engine is cranking, just like it should. However when I bypassed the oil pressure switch (not sending unit) I did not get 12 volts at the fuel pump with the key switch on. I used a jumper wire to connect the two wires on the oil pressure switch (bypassing the switch), turned the key on and there was no volts at the electric fuel pump. I then check for voltage on the purple wire connected to the oil pressure switch with the key on and there is no voltage there either.

Can someone explain why I'm not getting voltage at the oil press switch (purple wire) with the key on??

I think you have made a bad connection with your meter leads (+ or -) or you blew a fuse moving things around. If your engine will crank, the purple wire has 12V on it, or if you turn the key to ON and the alarm/buzzer sounds you have 12V on the purple wire. Below is the schematic

4_3L elect diag.jpg
 
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