Mercruiser 4.3 runs great, then wouldn't start. HELP!

[S.S Aquaholic]

Hey All,
I picked up a nice used 2001 Caravelle with the MerCruiser 4.3l EFI this year and it's giving me some trouble.
I'll start by saying the boat runs fantastic. Issue I'm having is it will not start after running.

The first time I offloaded the boat at the dock, fired it up, backed it around a tied it up to load up gear. 10 minutes later when we all came back to go out it would not fire. It cranked strong, battery and gauges were all operational, but it would not turn over. I messed with it for about an hour before putting it in neutral and advancing the throttle to WOH and got it to start.

Second time were out all day, tubing, tying up to shore...the typical things you do during the day. I probably started and stopped it 20 times that day. As we were about to leave, I went to fire it up and it did the same thing as it did at the dock the week before. so naturally, I tried the ol' WOH trick and nothing. just cranked and cranked but would not fire. The only thing I can think of is that we pulled close to shore so this kids could check out a rope swing. I fired it up once as we were floating closer to shore (don't have an anchor), pupped it in reverse and then killed it after a few seconds to coast backwards. When I went to fire it back up 5 minutes later, she was not having it. so...we had a friend tow us back to the dock (about 25 minutes), and once we got back, I gave it a crank and wouldn't ya know...it fired up. Super frustrating...and my wife was giving me that "and you wanted a boat look" Anyone have any suggestions? I'm at a loss.

 

[alldodge]

Howdy

Your using two terms which have me conflicted which are, cranked and turn over.

Is the starter turning over the motor "its cranking" or is it NOT cranking and not turning over the motor?

Cranking and turning over mean the same for me

 

[S.S Aquaholic]

Cranking, meaning the motor is moving but not firing. So the starter engages, motor turns...does not fire up. Hope that clarifies.

 

[alldodge]

Thanks
Check you kill switch (also called man overboard). If it doesn't make a good connection then there is no power to the gauges or motor, but it does crank

If it doesn't make connection there will be no beeps when key is turned ON.

 

[S.S Aquaholic]

Plenty of beeps happening...

 

[alldodge]

ok, thanks

The key applies power to the ECM, which in turn supplies power to the Main Power Relay (MPR). The MPR applies power to the rest of the motor.

The MPR and the fuel pump relay are the same and can be swapped (86-865202T). The relays are lso can be found at about any auto parts store (not marine specific, just different part number). When the motor is cranking and the ECM gets distributor pulses then the ECM turns the fuel pump relay ON which then supplies fuel.

Do you hear the fuel pump come ON when hearing Beeps?

Going to need a voltmeter and measure the pink wire on the ignition coil to see if you have 12V present

Being a 2001 and a MPI it will have the HVS distributor. The HVS uses a flat style cap which has had many failures. The cap can look great and still not work, or cause misfiring. The cap may not be the issue but its recommend to replace and if it does not fix the issue keep it as a spare, because they fail without warning and still look good.

 

[S.S Aquaholic]

The fuel pump operates as it should. I hooked up the muffs in my driveway this morning and it fired right up. Killed it a minute later, and then fired it back up 5 minutes later with no issue. So, the issue is intermittent at the moment. Could firing up the motor and turning it off within 10-20 seconds flood the engine? That doesn't seem likely, but I'm stumped. Do these motors have an OBD hookup? This is really bugging me...hate to get up to the lake and have it happen again. Grrrr.

 

[alldodge]

Start and stop will do nothing to a EFI motor in the way of flooding, or much else

It doesn't have a OBD but does have a similar connector to to connect the Rinda Techmatepro or Diacom software. Both will read codes and data of the motor while running. The Diacom can also record data for playback later

https://www.rinda.com/marine/marine.htm

 

[alldodge]

Hey GA_Boater just realized this is in the OB section, please do the honors Thanks

 

[GA_Boater]

Hey GA_Boater just realized this is in the OB section, please do the honors Thanks

Did a rare thing, tossed an outboard and installed a Mercruiser. All in less than 3 hours including cutting the keyhole. :lol: Thanks AD.

 

[Furbird]

First of all, I'm not a boat mechanic, but I do work on cars. I know enough about boats to be dangerous. So YMMV. Hey, just being honest! But what you are describing is a failure only when hot.

I do know that on the carbed versions with a fuel pump the pump only activates on oil pressure. So if your oil pressure switch is failing (which is separate from the sender going to the gauge on the carbed versions) that could make the fuel pump not activate. At colder temps the oil is thicker, so the switch is activating, but at warmer temps when the oil is thinner it won't. Might be worth checking with a meter or just replacing since it's only acting up when hot. Obviously this only applies if the EFI version is set up like the carbed ones.

On my Firebird, I had a situation where the fuel pump was going out. Cold it would start fine, but when hot it took forever to get it to start if it would start at all. Fuel pressure was dropping to zero and it took continuous cranking to get it to build up to fire off. Turns out I had two failures; one being the fuel pressure regulator on the intake manifold, and the other was an internal check valve in the pump that maintained pressure on the lines when the pump was off. The regulator failed first, so you had to build up enough vacuum in the regulator to get the pressure to the injectors. Unfortunately the check valve failed right after that, leaving me stranded and had to get towed. They unload me from the tow truck, and the car fires up because the fuel pump had cooled down (check valve is in the pump and got stuck open when hot. Eventually they just get stuck open permanently.) Gotta love it.

So that's a very long winded way to say it sounds to me like something is going on with your fuel pump. It's either not getting voltage, or it's failing internally. A fuel pressure gauge would be the tell-tale tool to have.

 

[alldodge]

I do know that on the carbed versions with a fuel pump the pump only activates on oil pressure.

No, not on a boat

 

[Furbird]

No, not on a boat

Did I need to specify "electric" fuel pump? There are literally two active threads on the first page or two talking about how the 4.3 fuel pump doesn't activate unless there is oil pressure, and how to bypass it.

 

[Rick Stephens]

Did I need to specify "electric" fuel pump? There are literally two active threads on the first page or two talking about how the 4.3 fuel pump doesn't activate unless there is oil pressure, and how to bypass it.

You are correct. An oil pressure switch is the main power supply to the pump. It also gets power from the outside lug on the starter to prime during cranking.