Mercruiser Madness - I have a ghost

Olson30

Recruit
Joined
Feb 12, 2013
Messages
3
Thought I’d reach out to see if there are any ideas for a persistent problem with my 1994 Mercruiser ski mag 350.

When she runs, she purrs like a kitten. When she won’t, she won’t start. Until she feels like it again. Along the way, she may start perfectly, rev to 1,200 RPM exactly, and then die hard. You can watch fuel pressure fade until there’s no fuel left and it dies. It’s like the start sequence is stable and the run sequence is not. Unless it wants to be...

Comprehensive tune up complete. Compression test normal.
New distributor and ICM and ignition coil. NOID lights positive. TBI rebuilt.
VST disassembled and fully cleaned.
New fuel filter.
Main relays checked.
Alternator bench tested, checked out fine.
All connections throughout ignition and charging systems were removed, cleaned, resistance tested and reinstalled.

As you can see, it’s been a mixed bag of block and tackle diagnostics with throwing parts at it out of desperation.

Help? It’s been going on for years, I’ve been through 3 different mechanics, two of whom fired me as a customer, not for being unpleasant, but for lack of anything else to try. The boat is old enough there aren’t that many diagnostic codes that are available from connecting to a computer

As I say, when she runs, she runs great. I’m the original owner. The wife is reluctant to get on board, as am I, for fear of getting stranded yet again. I’d really like to fix the problem for good, get the boat back to predictable behavior. She’s served as a dream boat until ~2015. Never a blip, challenge or expense beyond normal maintenance.

The sporadic nature of the problem makes it insanely difficult to diagnose. You can’t force her to misbehave. As much as I’d love to pay a mechanic worth his salt to come along for every excursion in the event she decides to misbehave, it just isn’t practical...

Any and all ideas welcomed.
 

tank1949

Lieutenant Commander
Joined
Apr 4, 2013
Messages
1,911
Thought I’d reach out to see if there are any ideas for a persistent problem with my 1994 Mercruiser ski mag 350.

When she runs, she purrs like a kitten. When she won’t, she won’t start. Until she feels like it again. Along the way, she may start perfectly, rev to 1,200 RPM exactly, and then die hard. You can watch fuel pressure fade until there’s no fuel left and it dies. It’s like the start sequence is stable and the run sequence is not. Unless it wants to be...

Comprehensive tune up complete. Compression test normal.
New distributor and ICM and ignition coil. NOID lights positive. TBI rebuilt.
VST disassembled and fully cleaned.
New fuel filter.
Main relays checked.
Alternator bench tested, checked out fine.
All connections throughout ignition and charging systems were removed, cleaned, resistance tested and reinstalled.

As you can see, it’s been a mixed bag of block and tackle diagnostics with throwing parts at it out of desperation.

Help? It’s been going on for years, I’ve been through 3 different mechanics, two of whom fired me as a customer, not for being unpleasant, but for lack of anything else to try. The boat is old enough there aren’t that many diagnostic codes that are available from connecting to a computer

As I say, when she runs, she runs great. I’m the original owner. The wife is reluctant to get on board, as am I, for fear of getting stranded yet again. I’d really like to fix the problem for good, get the boat back to predictable behavior. She’s served as a dream boat until ~2015. Never a blip, challenge or expense beyond normal maintenance.

The sporadic nature of the problem makes it insanely difficult to diagnose. You can’t force her to misbehave. As much as I’d love to pay a mechanic worth his salt to come along for every excursion in the event she decides to misbehave, it just isn’t practical...

Any and all ideas welcomed.

If you can, pull or remove fuel pickup tubes from tanks. Some have small screen inserts inside of them that do get clogged. Trust me!!!!!!!!!!!!! Most people don't know that they exist. Not to be confused with anti-syphon devices... Some boats don't have them. I had to separate my NPT fuel tank fittings from its NPT aluminum tube just to examine. They are not easily noticed and small. My get feeling is that after removal and pitching them into trash, you will also have to completely cycle existing fuel through your main fuel filter. Todays fuel sucks and breaks down into varnish and collects in the bottom of gas tanks because boats are not used like vehicles and most boats are stored for months at a time. I'd also examine all electrical connectors (at night) with a friend. Wiggle them and see if boat dies or you notice spark. I am not kidding! Have fire extinguisher ready and a friend just in case. Examine all grounds very carefully. You have an older boat. If it is a salt water used boat, copper and salt water do not play well together. Look for green corrosion. Someone may have also added non-marine grade wiring to your boat (not tinned), which over time corrodes more so than marine grade wiring. I am restoring an 86 Sea Ray and ran into wiring issues, as well. Good luck!
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,317
You can watch fuel pressure fade until there’s no fuel left and it dies. It’s like the start sequence is stable and the run sequence is not. Unless it wants to be...

Inboard, VST and TBI can probably figure out the serial number but post it to make sure?

Does the fuel pump continue to run when its dying, or is power lost?
 

Cheech75

Cadet
Joined
Mar 28, 2020
Messages
9
Listen to tank1949,

Im a old timer years ago automotive mechanic, NOT a boat mechanic but as I was reading your post it was screaming to go to the sourse FUEL SUPPLY, collapsing fuel line, inside tank filter if it has , loose, cracked fuel lines fitting may be sucking air things like that, even the supply lines can internally collapse from the suction of the fuel pump and restrict the flow of fuel , just like running out of gas even with a full tank, ironically sometimes after a undetermined amount of time a old fuel line may expand after back to letting fuel flow for awhile and run great until the fuel gets restricted again. that's when the head scratching takes place.
Get a 3 gal gas tank and hook it up to the suction side of fuel pump if you can, that eliminates your gas tank. or by pass your fuel pump and hook up a inexpensive electric pump a gas line hose right to your carburetor inlet line and see if the problem is gone. yes danger in doing the above , so have fire extinguishers ready. good luck
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
51,348
if the fuel pressure is dropping, you have a restriction in the supply.
 

Olson30

Recruit
Joined
Feb 12, 2013
Messages
3
Inboard, VST and TBI can probably figure out the serial number but post it to make sure?

Does the fuel pump continue to run when its dying, or is power lost?

OF349510 is the engine serial number, thanks.
 

Olson30

Recruit
Joined
Feb 12, 2013
Messages
3
Listen to tank1949,

Im a old timer years ago automotive mechanic, NOT a boat mechanic but as I was reading your post it was screaming to go to the sourse FUEL SUPPLY, collapsing fuel line, inside tank filter if it has , loose, cracked fuel lines fitting may be sucking air things like that, even the supply lines can internally collapse from the suction of the fuel pump and restrict the flow of fuel , just like running out of gas even with a full tank, ironically sometimes after a undetermined amount of time a old fuel line may expand after back to letting fuel flow for awhile and run great until the fuel gets restricted again. that's when the head scratching takes place.
Get a 3 gal gas tank and hook it up to the suction side of fuel pump if you can, that eliminates your gas tank. or by pass your fuel pump and hook up a inexpensive electric pump a gas line hose right to your carburetor inlet line and see if the problem is gone. yes danger in doing the above , so have fire extinguishers ready. good luck

I've done the remote tank test, to no effect. Part of the problem is the sporadic nature of the symptoms. Because the problem set appears only when it feels like it, it makes it very difficult to do any diagnosis. The failure drops in as an uninvited guest.

I'll have to replace those soft fuel lines I haven't yet replaced and see where things go. Thanks.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,317
Fuel pressure depends on motor
43 psi MPI
30psi TBI
 
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