Merc 5.7L Keeps Dying

[sjohnson44]

Fresh rebuilt Merc 5.7L Thunderbolt V wont stay running longer than 5 seconds or so. Tried feeding it gas through carb and still dies.

New mechanical fuel pump, no gas in sight tube
New fuel filter
Siphon valve cleaned
Fresh Gas
New coil
Carburetor rebuilt (Mercarb) Venturies were real dirty and had a lot gunk under the venturi cluster. Bowl had a lot of junk too. Stem works up and down smoothly, new power valve, needle and seat (spring type), cleaned metering jets, new acc. pump, new check ball and spring for the acc. pump "check valve". Used carb cleaner and compressed air. All passages clear.
Reused old float and set level to 11/32" and float drop to 15/16". New float they sent did not have the measuring dots on it so I didnt even attempt to mess with it. New idle mix screw, screwed all the way in and backed out 1 1/2 turns.
New distributor cap
New alternator
New battery, 12V everywhere its supposed to be.
Choke operates normally
Timing set to 10 degrees BTC.



Im at a loss and dont know where to start now. Since carb rebuild it does idle better but still shuts off. No matter if throttle closed or open.

 

[Fishermark]

Re: Merc 5.7L Keeps Dying

Doesn't the Thunderbolt V operate through an oil pressure switch? (I'm not sure if it does or not to be honest). If it does, try bypassing it and see if it stays running.

 

[sjohnson44]

Re: Merc 5.7L Keeps Dying

I believe that is only used for the audio warning system and is seperate from the ignition circuit. I havnt seen anything that runs back to the motor from the audio warning circuit. When the switch detects 10psi or more it opens shutting the buzzer off unless there is another problem. The buzzer operates on my boat and shuts off once started. Engine has 30psi oil pressure.

 

[Bondo]

Re: Merc 5.7L Keeps Dying

I believe that is only used for the audio warning system and is seperate from the ignition circuit. I havnt seen anything that runs back to the motor from the audio warning circuit. When the switch detects 10psi or more it opens shutting the buzzer off unless there is another problem. The buzzer operates on my boat and shuts off once started. Engine has 30psi oil pressure.

Ayuh,.... Ya need to Identify just what yer motor is dressed as,...
It's NOT a 1996 vintage Mercruiser....

If it were, it wouldn't have a Mechanical fuel pump...

It sounds like a cob job built with This, 'n That....

When it dies,... LOOK, 'n See if it's lackin' Fuel, or Fire....

 

[sjohnson44]

Re: Merc 5.7L Keeps Dying

Serial number 0F770506, mercurypartsexpress.com shows a mechanical fuel pump for that engine. Other sites identify that number for a 1996 5.7L.

Im going to recheck fuel pickup in the meantime.

 

[Bondo]

Re: Merc 5.7L Keeps Dying

Serial number 0F770506, mercurypartsexpress.com shows a mechanical fuel pump for that engine. Other sites identify that number for a 1996 5.7L.

Ayuh,... I donno how that's possible,... The SBC had lost it's mechanical fuel pump boss drillings by then...

 

[Fishermark]

Re: Merc 5.7L Keeps Dying

Serial numbers are a great source... but sometimes you just have to look and see what you have. Easy enough to see if you have a mechanical pump or an electrical pump. It sounds like an electrical pump that is not getting power once the engine starts.

 

[Bondo]

Re: Merc 5.7L Keeps Dying

Serial numbers are a great source... but sometimes you just have to look and see what you have. Easy enough to see if you have a mechanical pump or an electrical pump. It sounds like an electrical pump that is not getting power once the engine starts.

New mechanical fuel pump, no gas in sight tube

Ayuh,.... I'm confused....

 

[Don S]

Re: Merc 5.7L Keeps Dying

.

 

[Don S]

Re: Merc 5.7L Keeps Dying

Do you have spark immediately after it dies? How long after it dies can you start it again.

Hook a jumper wire from the positive battery terminal to the + terminal of the coil and see if that keeps it running.

 

[sjohnson44]

Re: Merc 5.7L Keeps Dying

The yellow tube that runs from the fuel pump to the carburetor. I have a mechanical fuel pump. I installed it.

 

[sjohnson44]

Re: Merc 5.7L Keeps Dying

 

[sjohnson44]

Re: Merc 5.7L Keeps Dying

Do you have spark immediately after it dies? How long after it dies can you start it again.

Hook a jumper wire from the positive battery terminal to the + terminal of the coil and see if that keeps it running.

Sometimes takes a little while to crank again. Will try the jumper.

 

[Don S]

Re: Merc 5.7L Keeps Dying

Cut those push on/spade terminals off those coil wires and put on some ring terminals and use the nut to hold them on securely. :facepalm:

And disconnect the gray wire from the tachometer if it dies with the jumper wire.

 

[sjohnson44]

Re: Merc 5.7L Keeps Dying

Cut those push on/spade terminals off those coil wires and put on some ring terminals and use the nut to hold them on securely. :facepalm:

And disconnect the gray wire from the tachometer if it dies with the jumper wire.

There are no studs. I wish it did as that would be best practice.

 

[Don S]

Re: Merc 5.7L Keeps Dying

Gee. I wonder if you would still have this problem with the correct coil installed? Just thinking out loud.

 

[sjohnson44]

Re: Merc 5.7L Keeps Dying

Because it doesnt have studs or because its not rated to fire 8 spark plugs? Is there really that much difference between the one I got at NAPA and the one mercury says I should have? Does the NAPA coil only output 10kV and my motor needs 30kV to run correctly?

I dont mind buying a mercury coil, if its actually something worth trying. To me its all apples to apples, but Im probably wrong.

 

[rpaldrich]

Re: Merc 5.7L Keeps Dying

FWIW--

Were the NAPA coils marine coils? And were they designed for Thunderbolt ignition? I just put new coils in my Thunderbolt IV, and on the side of the coils I purchased they said something about using an external resistor (or not using it I don't recall)....

But it seems that regular auto coils and non thunderbolt coils are different from Thunderbolt WRT this resistor, That could be your problem. I read a post from a while back talking about having issues w/ non thunderbolt coils.
http://forums.iboats.com/showthread.php?t=484204&highlight=quadrajet
EDIT: Post #8 specifically.

IDK if the coil is your problem, but Occam's Razor says the simplest solution is usually the correct one. Eliminate it as a source of problems and then you can narrow it down better. A new coil is only $35.00

Let us know.

 

[John_S]

Re: Merc 5.7L Keeps Dying

Serial number 0F770506, mercurypartsexpress.com shows a mechanical fuel pump for that engine. Other sites identify that number for a 1996 5.7L.

Im going to recheck fuel pickup in the meantime.

Bondo, I think merc had allot of blocks in storage when GM stoped making the mech fuel pump boss. I have a K serial # 5.7L that has mechanical pump. According to merc, it was made Sept '96.

 

[sjohnson44]

Re: Merc 5.7L Keeps Dying

FWIW--

Were the NAPA coils marine coils? And were they designed for Thunderbolt ignition? I just put new coils in my Thunderbolt IV, and on the side of the coils I purchased they said something about using an external resistor (or not using it I don't recall)....

But it seems that regular auto coils and non thunderbolt coils are different from Thunderbolt WRT this resistor, That could be your problem. I read a post from a while back talking about having issues w/ non thunderbolt coils.
http://forums.iboats.com/showthread.php?t=484204&highlight=quadrajet
EDIT: Post #8 specifically.

IDK if the coil is your problem, but Occam's Razor says the simplest solution is usually the correct one. Eliminate it as a source of problems and then you can narrow it down better. A new coil is only $35.00

Let us know.

New coil was $72, will be here next week. We will know soon enough. Thermal breakdown would make sense with this issue, as the engine runs the improper coil builds up heat and cuts out, once cool it works again.

Appreciate the link rpald.