I need help with an engine issue diagnosis please!

S

Still LauCo

New Member
Sep 20, 2007
408
Grosse Pointe Farms, MI
Boat Info
300 Sundancer 1996
Engines
Twin 5.7L EFI Alpha One Drives
What I thought was a fuel problem, is not. I need somewhere to look.

(These are fuel injected Merc 350's)

My starboard motor fires right up, runs great. If I turn it off and let it sit, it won't start back up until the engine is cold. It will crank, but that's it. (I recently replaced that battery)

Yesterday, we took a 20 min cruise, and hung out in an anchorage for about an hour. The engine would not start. Last night, after giving the engine plenty of time to cool down, it started right up.

I need to fix this obviously, but I'm puzzled where to go and what to do. When it runs, it runs great. Any assistance is MUCH appreciated.
Thanks!:smt021
 
It sounds like a vapor lock problem. I have experienced vapor lock on both of my previous boats and my symptoms were the same as yours. Mine was caused by "heat soak" at the fuel cooler. The fuel runs thru a cooler that uses fresh sea water to keep the fuel cool enough so it will not vaporize. What was happening with mine was the hot engine cooling water would backflow into the fuel cooler when the engine was turned off and cause the vapor lock. Letting the engine idle for about 3-4 minutes before turning it off generally prevented the problem. Some people also install one way valves in the water hose to prevent the back feed from the hot engine as well. One other thought is your fuel cooler could be partially plugged up and that may be all it takes to allow heat soak in the fuel cooler when you shut down your engine thereby causing the vapor lock.

The next time it happens open your engine hatch and try putting some ice on the fuel cooler itself to see if that helps or if that's not possible try running a fan pointed at the fuel cooler for several minutes and also have your bilge blower running to get as much heat out of the engine room as you can.
 
Dave's advice is probably right on, but if it doesn't work, try a new set of plugs, wires, distributor cap and rotor. My 250 did the same thing and a tune-up solved the problem.
 
Thanks for your replies. I am going to get a tune up anyways on both motors and the gen, and will also try idling/blower and see what happens.
Much appreciated!
 
It does sound like vapor lock. In a pinch you can often start by opening the throttle all the way while you crank.
 
I think great advice you have been give, my money is on plugs or plug wires, most of the time when an engine wont crank warm or hot, check the plugs first, happy boating, Mike
 
New spark plugs, distributor cap, and rotor each season takes care of boating season headaches! Bad ignition wires can cause fouled plugs as well. If the ignition wires are older than 5 years, you may want to replace them! And don't forget the dialectic grease! Annual fuel filter replacement and periodic fuel line filter replacement in the carburator is also a must. And how about fuel additive to prevent ethanol seperation and clogged fuel injectors? Star-tron works wonders during the season! And fuel stabilizer over the winter in the full tank to prevent condensation and fuel degredation.

My engine purrs like a kitten!
 
keokie said:
It does sound like vapor lock. In a pinch you can often start by opening the throttle all the way while you crank.
Click to expand...

Not with EFI engines. EFI turns off fuel flow during start when the throttles are advanced more than 3/4.

Best regards,
Frank
 
Still LauCo said:
What I thought was a fuel problem, is not. I need somewhere to look.

(These are fuel injected Merc 350's)

My starboard motor fires right up, runs great. If I turn it off and let it sit, it won't start back up until the engine is cold. It will crank, but that's it. (I recently replaced that battery)

Yesterday, we took a 20 min cruise, and hung out in an anchorage for about an hour. The engine would not start. Last night, after giving the engine plenty of time to cool down, it started right up.

I need to fix this obviously, but I'm puzzled where to go and what to do. When it runs, it runs great. Any assistance is MUCH appreciated.
Thanks!:smt021
Click to expand...



Do you mean that the engine turns over and does not start or do you mean that the engine will not crank at all? It is very common that the wiring harness on your engine has shaken itself loose and is not making complete contact at a plug in juncture. Most engines have a round plug with about 8 to 14 pins inside, so this plug is about the size of a 50 cent piece. I just helped a friend with the same symptom you describe on his 2003 8.1 volvos,and sure enough it was his harness that was the issue. Appearantly when the engine would cool off, the contraction of the harness would re-connect itself enough to restart the engine. This also happened to me on my 340 sundancer when it was brand new.

Hope this helps.
 
A good engine needs fuel, air and a spark to start-up. Then boat or car manufacturers put the engine into a computerized environment to assist with reliable and efficient operation of the engine. Over a dozen sensors work with a Computer and wiring to control the engine. USUALLY, it is a sensor or the electonics that create problems, especially where the engine is fuel injected. I would start there... but would also look to the fuel system since I have heard of problems with pumps and filters not working properly when heated beyond a certain point. Maybe a few more details would help hone down on what parts to focus on. If feasible for the 350's, consider switching the ECM between the two engines and see what happens.
 
So far some good suggestions. Not trying to jump in the middle of things but try this next time your engine will not start. Once the engine turns over and does not start, remove air cleaner cover and housing. Unplug one or both of the fuel injectors. Try and fire it up. My only $5 dollar bill says it fires right up! This will not cure your problem but will shorten your diagnosing without spending a penny. Although a tune-up never hurts and can save some gas money in the long run!!

All certified Merc shops will have a plug in dianostic reader that will read sensors and most likely will be needed to fix your problem if engine fires after pulling a injector or two.

Only reason for suggesting the above is the engine fires up and runs well when cold but not when warm.

Vapor lock is always a solution but an injected engine is not as common as a carburatored engine.

Only other little tip may be fuel seperator. But I think you have one seperator for both engines. If you have independent seperator, change it as it's just a maintenance item anyways.

later,
frank
 
Actually, vapor lock is quite common even with fuel injection.
 
I doubt it's vapor lock. When I repowered my Chris with a 6.0 MPFI the yard warned me about the potential for this, particularly because I selected the engine with the "returnless" fuel system that could overheat the fuel.
Vapor lock occurs due to high temps (it hasn't happened to me yet here in FL)- I doubt it's hot enough in Michigan to do this, although your symptoms (hard starting when hot) fit.
 
I thought I would update this thread. I had both engines and the genny tuned up, but it turns out the fuel pump on the stbd engine was going bad. It would spray fuel, but the "pulses" were really weak, causing the problem. New fuel pump goes in today.
Thanks for your suggestions!
 
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