Mercruiser stern drive runs for 30 min then dies

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flyinghull

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Long sorry story, need some help folks.
1996 350 cu in V8 mercruiser stern drive, Celebrity Bow rider, I have owned it for 16 years.
Symptoms are that the boat runs fine for 30 minutes or so. Up on plane, back down to idle, back on plane, back to idle, no problems. Then dies and will not restart. Dies when on plane, or just putting along at 1200 rpm. It will not restart with the throttle in any position. Will not restart with carb cleaner injected into the carburetor. Engine just turns over. Wait some time, 20 min plus, it will restart with difficulty, run some, then die again. This has happened 15 times. Wait till the next day, runs fine again for 30 min.
Ok, so what have we done to fix this.
Changed the gas in tank multiple times. Looked in the tank by removing the level sensor. Can see a few bits of dirt floating around but looks good overall. Put various fuel stabilizers in to deal with any water in the gas. But remember I pumped it out at least 4 times. Removed the dip tube, put a new dip tube in with out a screen on it. Gas that I removed runs fine in the truck.
Changed the water separating filter several times. This was the most recent fix attempt. No change.
New spark plugs. New carburetor. New starter solenoid. New thermostat. Oil changed. Oil pressure looks ok, couple of ticks up on the gauge when 1200 rpm or more.
Boat temp shows 160 or so when hot. Tried running with out a thermostat, temperature was lower, 145 but same problem.
Taken this to 2 different mechanics. No change. Frustrating because they only run it on the trailer. This happens when under load, running the boat.
Cleaned the rotor and distributor cap. There was a little corrosion, but they look fine.
Boat coughs and sputters once or twice when it dies.

Any ideas are certainly appreciated.
 
Suggest clean out or disconnect the breather pipe as if sounds like when the air pressure in the tank gets to low the fuel pump can't suck fuel, the next day the pressure has eased and it runs fine.
 
no idea of the setup or specifics.
in the old times, when one mentioned something similar on a car (with a carb!) it was most likely fuel pump (mechanical) overheating on the engine block and stopping pumping fuel. I've no idea if you have fuel injection or a carb so following maybe pointless.
When it stops do you have fuel on the engine? Can you check? Does it smell of fuel if you try to start with WOT?

What could be heat soaked and mulfuction in the engine bay?

good luck!

V.
 
Or, just loosen the fuel cap when it stops, and see if that sorts it out....

Consider Fuel pump, and also Rotor arm replacement, as they can have internal issues. I would suggest electrics, as you tried Eezi start in the inlet manifold, which would have fired up an engine starved of fuel...

This is prehistoric engineering, so I'm going back to olde worlde ideas!!
 
The 'thingy' in the distributor! I like that.

It used to be cheap enough to replace coil, condenser and plug leads (or just the one from coil to distributor).

The first bad coil I had (ford xflow) was an easy fix after I'd replaced the carb, serviced the distributor, changed the plugs and leads, skimmed the head and ground the valves.
 
Sounds to me ,if it takes so long to restart,that something needs to cool down considerably in order to function again.Suspect electrical.
If fuel was the problem,as the blockage occurred it would remain there until physically dislodged,not just clear itself away after a fixed period of time.
Had exactly the same type of problem with a smaller Mercruiser engine and went down the fuel route to no avail.Including pulling carbs to bits several times,lift pumps,filters etc etc.
In the end replaced distributor cap,ignition coil,rotor arm and condenser in one go.
As all were available as pattern parts,not that much money involved and
problem solved.It to ages to track down the problem and as only happened under load,could not be replicated unless underway.
To this day suspect it was actually a dual problem and merely swapping one item in the chain left a faulty part still in situ,it may have been something as simple and uncomplicated as the rotor arm,whats to go wrong ?.
I would be looking at the electrics.
Never managed to stop it stalling though :)
 
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I had a merc petrol in my first speedboat, and it had an intermittent fault with a secondary oil pressure sensor. This would cause the engine to randomly die. In the end the resolution was just to rewire the sensor. Note that this isn't the same as the oil pressure gauge sensor.
 
Sounds very much like an ignition problem, have you checked to see if there is a spark when it cuts out? If not then check if there is 12v on the coil + terminal, if not then work back through the harness to the gnition switch. If there is power at the coil check the - terminal it should pulse when cranking if not then you have a problem in the distributor with the sensor which generates a pulse for the module.
If you have a spark does your engine have an electric or mechanical fuel pump, as I have seen on most US imports they use an electrical pump on carb engines which is wired via an oil pressure sensor down near the oil filter, these can play up and cut the power to the fuel pump, but it should still start on eezee start so I lean towards an electrical fault.
 
Well worth replacing cap, rotor arm, leads and coil, might be dear but once its done its done.
Repower marine not bad on prices they also sell on ebay.
My cap furs up once a year or so, who uses alloy on a boat rotor cap! dont forget the spring loaded centre pin in the cap
Not saying this is the correct one but gives you an idea price wise.
http://www.ebay.co.uk/itm/Ignition-...929550?hash=item4197ef7f4e:g:qNQAAOSwpDdVfI0a
 
Had exactly the same problem with an efi 5.0 V8 merc...chased it and chased it in a similar fashion to the OP........fitted a new distributor....sorted. Thought it was fuel but definitely wasnt.
 
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