3.0 Mercruiser having a funny 5 mins

[Denis O]

Firstly spec. Chaparral 180SS with 3.0 Litre Mercruiser inboard with 21 hours with carb, full tank of gas and all fluids 100%

Was out today for the first time in a couple of weeks. She fired up on the slipway no problem and off we went. Got about 6 miles and the engine spluttered a little. We were running at around 3500 revs. In hinsight it felt like it was holding back a little from the start. The splutter was minor, and it had done it before a few times in the past at higher rev levels so I didn't really worry.

It then lost all power, shuddered a little and I pulled the throttle back to neutral. It dropped to tickover and was fine. The rear passengers said they could smell petrol and the engine had a loud tick. The tick went off after about 15 seconds and we decided to head back to the slipway.

I was pretty sure this was fuel starvation of some sort so we ran at about 1500 revs for a while and no real problem apart from how slow we were going. Pushed it up to 3000 revs to get her on the plane and was moving along at about 25 mph and again we lost all power. Throttled back to neutral and she ticked over again without problems. The ticking was back, no louder, and it went off after 15 seconds. Tried a few times at 3000 rpm and she would run for a few minutes then die. If I didn't get the throttle back to neutral quick enough she would cut out completely but start easily straight away.

Next I tried a little Italian tune up and went for a full bore start and to WOT. It got up on the plane, ran for about 10 seconds at 4000+ revs and then lost all power again. We managed to get bak to the slipway at low revs and the boat is now back at home and awaiting some TLC.

Does my fuel starvation diagnosis sound feasible? If so how is the fuel system made up. I can see where the fuel comes out of the tank via a pipe in the rear left corner (looking from inside the boat). It then goes off to what looks like a glass bowl which I suspect is the filter or mechanical pump and then converts to a plastic clear tube that goes to the carb. Is there an electric pump in the tank. With the exit at the top of the tank I'm guessing it needs something to push it up there. Is the ticking relevant. Can't remember if it was there before when all was good.

If it's not fuel starvation are there any other possibilities to sort it out.

Cheers guys.

 

[dingdongs]

was it this years fuel or last?if you have a fuel filter in line change it.you could try running on an outboard tank to eliminate fuel altogether.

 

[Denis O]

Fresh fuel. The boat, although it's a 2003, was only used for the first time in April this year. All fluids and impellor changed before first trip.

Good point on the outboard tank. That way, if it's actually a fuel delivery problem, it can then only be the carb, jets etc.

 

[cadser]

Make sure there is no water in the carb. If the boat has been stood for a while condensation builds up in the tanks and low points of the fuel system.

 

[1973keith]

Could be several things mentioned already but if you can eliminate contaminated fuel it sounds like there is a restriction in the fuel getting to the lift pump supplying the carb.

Assuming it's a simple jerk diaphragm pump on the side of the engine, if that gets a high vacuum on it then it could make a loud ticking as it's not following the cam properly. Like a valve tappet that needs badly adjusting...

Check the fuel tap is wide open.
Check the filters between the tank and the lift pump.
Check the tank vent is not blocked.

good luck!

 

[spannerman]

As others have said check the system from engine back to the tank, there is one thing you could check, its a USCG requirement on american boats that they have an anti-siphon valve in the fuel system and its mounted in the fitting where the fuel exits the tank, you just unscrew the elbow/union and inside there will be a ball and spring, these can stick and cause fuel starvation, so we poke the ball, spring, and seat out and throw it away so the fuel has no obstruction.
European spec boats don't have them so it doesn't compromise safety, as long as the fuel system is properly maintained which goes without saying on all boats but especially petrol boats.

 

[Denis O]

I've now got the fuel filter out and there was a little **** in the bottom but nothing major so I suspect that's not the cause. By the way, does the filter go back with the centre hole at the top or the bottom. I would say top but you never know and I didn't notice when I took it out.

I'll try the elbow on the tank exit. I'll then try the other points mentioned.

My favourite is still to try an outboard tank which can eliminate pretty much everything apart from the carb.

Thanks for your help so far.

 

[tinkicker0]

Seems like fuel starvation, but you mentioned a loud tick too. This could indicate a sticking exhaust valve if a fuel system tune up does not sort it.

 

[Paul1962]

Had something very similar with my Mercruiser V6 4.3 set up in my 1996 Bayliner.

Changed all manner of components and with this Forums advice narrowed it down to the fuel pick up tube in the tank.

Removed it from the tank and found that the pick up tube has a small piece of mesh at its base where a large lump of debris was mostly blocking the pick up tube.

Of course, the fuel filter on engine was perfect (yes, I had changed that too), the debris was not getting through the fuel pick up tube mesh!.

Took the level sender out of the top of the tank (no inspection hatch on my tank), totally defuelled the tank and cleaned it out - there was a lot of debris and rubbish cleared out.

Hope you sort it out.

Regards

 

[alan006]

A friend of mine had a simular problem with her Mercruiser powered speedboat and all the symptoms seemed to be suggesting fuel starvation. However it turned out to be the ignition amplfier module that was starting to break down. This was confirmed by taking the module to a Mercruiser dealer with a computer diagnostic system that the ignition module was plugged into and checked.
The dealer explained that he had seen this problem before where dampness had got to the module. I hope for your sake this is not the problem as the new module was about £500.
Good luck.

 

[Denis O]

Had the local Mercruiser guy up who overhauled the fuel system from top to bottom. He then plugged in the diagnostics and ran the engine through various rev ranges and all was well. Diagnostics showed no problems so it goes back in the water tomorrow to see if it's still good under load.

Wish me luck.

 

[Denis O]

So we took the boat out yesterday and exactly the same thing happened. Anything over 2500 RPM and the stalling was back. Got back to harbour, moored up and went for beer and sardines Called the Mercruiser guy who came down to the harbour and we took the boat out. Problem was replicated and he set about the elbow at the top of the tank; that one with the anti syphon mechanism.

Once complete the boat then ran like a dream so problem solved; happy days

Thanks for all your help guys especially "spannerman" who wins a virtual pint for the correct diagnosis.