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Mercruiser 5.0L losing power

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wr18962

New member
Hi,
I am new to this forum and an inexperienced mechanic. Unfortunately the nearest dealer is 100+ miles away.
The engine is ~2003 with low hours but was laid up for 3+ years before I got it 2 years ago.

The Mercruiser 5.0L had been running fine but recently has developed a series of what initially seemed unrelated problems. Intitially it sporadically lost power but was abbe to run fine at low revs. I believed this to be a dirty fuel filter and after replacement it then ran fine for an hour or so when it began to lose power again and subsequently died altogether and would not restart.

There was no spark and this was fixed by replacing the ignition coil.

After about another 10 hours running it began to lose power again. It is intermittant as it sometimes gets to full power and then loses it again. I am suspecting the fuel pump. I don't have any way of measuring the fuel pump pressure.

Is replacing the fuel pump the next logical step ?

Cheers

-Lawrence
 
Ayuh,... I'd start by checkin' the contents of the fuel filter for Water....

Ya might also have a restriction in the fuel tank, run it on a remote tank to test that theory...
 
Let me be the first to welcome you aboard !

The boat sat for 3 years ?

1- i would pump the gunk and junk off the bottom of the tank or pump the tank empty.

Clean the filter in the fitting that enters the carb. You have a water sep. dump it into a clear container and let sit. Look for water on the bottom.

Me i would pump the bottom of my gas tank. One small soda bottle full. When you see clear fuel entering the bottle stop pumping.

Check your anti-syphon valve ( one way valve ) it is the fitting that the fuel hose connects to that draws fuel from the tank.

Also when was the last tune up ?

Check your vent.
 
guyjg said:
Is it EFI or carbed?
Click to expand...

Thanks for the responses; The engine is a 5.0L carbed with Thunderbolt V ignition s/n 0M641732 fitted to Alpha 1 Gen 2 Stern drive.

I will do a complete clean of the fuel system as this seems to be the most likely cause; I had been concentrating on the electrical system because of the coil failure last summer but this seems to be a bit of a red herring. I will probably replace the rotor, cap and HT leads anyway.

Last tune up was prior to the boat being laid up so now 5 years ago.
 
I don't want to scare you but i feel compelled to inform you about the t-bolt system. Nice when new, but when they start to go it ain't cheap for the parts.

If push comes to shove and your looking at a 400 dollar+ module don't do it.

I have installed one piece dist. with a coil built in for around 300 dollars. By passes the t-bolt system completly, it is one wire in one wire out and one wire for the tach.

For now go after the fuel system get that running good.

Good luck !
 
I'm not a fan of Merc's TB EST system either.
If given a choice between replacing any of these expensive OEM parts -vs- a replacement, I'll take the replacement system.

wr18962 said:
There was no spark and this was fixed by replacing the ignition coil.

I am suspecting the fuel pump. I don't have any way of measuring the fuel pump pressure.

Is replacing the fuel pump the next logical step ?
Click to expand...
Lawrence, the TB system requires Merc's OEM ignition coil.

Here's my suggestion.

Use the age old and tried and proven P of E (process of elimination).
For this to be most effective, we test/check one item, and only one item at a time.

Perhaps begin with the fuel system.
Pull the fuel tank pick up tube. If this is screened, check the screen
If the filter is suspect, simpy replace it.
Since this engine is carbureted, these fuel pumps are not all that expensive.
Look into the switch controlling side of the pump system. There will be a L/O pressure switch, and perhaps a relay within with the start-by-pass system. Merc's trouble-shooting section for this is fairly thorough.



Most any issue can be solved by throwing enough new parts at it..... but we don't offen learn what the actual cause was.
When used methodically and systematically, the P of E will never fail us!
The P of E more so pin-points the issue leaving us better prepared for a similar issue in the future.


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