5.7 thunderbolt weak spark

Status
Not open for further replies.

westie612

Cadet
Joined
Jul 2, 2013
Messages
6
Have a 87 5.7L 260 hp mercruiser alpha 1 with t-bolt ignition. I have been through the Seloc manual and have followed the troubleshooting. Not alot of info avail. Ultamatly it suggested to replace the sensor inside the dist. I have also replaced the cap, rotor and coil. When I place coil wire 1/4' from ground spark is yellow not blue. even weaker at the plugs. When I crank the engine it almost starts right away. Have tried starting fluid with same results, assuming it's not fuel delivery. Any suggestions
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: 5.7 thunderbolt weak spark

Ayuh,... Welcome Aboard,... Do ya have a full 12v at the (+) at the coil,..??
 

westie612

Cadet
Joined
Jul 2, 2013
Messages
6
Re: 5.7 thunderbolt weak spark

Oops! No with the key on = 0 volts. while cranking = 3.8 v
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Re: 5.7 thunderbolt weak spark

When you say "across coil" what do you mean? The voltmeter negative lead should be on a good engine or battery ground. The positive lead of the voltmeter goes to the positive terminal on the coil, that will give you the voltage at the coil.
 

westie612

Cadet
Joined
Jul 2, 2013
Messages
6
Re: 5.7 thunderbolt weak spark

Thank you! your very helpful. Ground to the + side of coil is 12.9v with the key on. drops to 8.4 when cranking
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: 5.7 thunderbolt weak spark

drops to 8.4 when cranking

Shouldn't be dropping that far. Check clean and tighten all battery cable connections (Both positive and negative cables and each cable has 2 ends)

If they are clean and tight (With a wrench, not with a wing nut and your fingers) then get the battery load tested. It's showing too much voltage drop.

With that low of voltage while cranking I would have to bet the starter even sounds slower than normal.
 

westie612

Cadet
Joined
Jul 2, 2013
Messages
6
Re: 5.7 thunderbolt weak spark

1 new battery. other tested good with a load test. cleaned connections when I replaced the batteries 7 days ago. New starter too.
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: 5.7 thunderbolt weak spark

It's not going to start with 8V to the coil while cranking.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: 5.7 thunderbolt weak spark

1 new battery. other tested good with a load test. cleaned connections when I replaced the batteries 7 days ago. New starter too.

Ayuh,... Check the Other end of the battery's Ground(-) cable,...

You could also have a Bad cable,... or more likely, cable End,...
 

westie612

Cadet
Joined
Jul 2, 2013
Messages
6
Re: 5.7 thunderbolt weak spark

I will check all cables and the selector switch. Thank you all for your help....much appreciated....but for now, there is a prairie dog town calling my name in South Dakota
 

Ajsikora24

Recruit
Joined
Jul 3, 2019
Messages
1
I am having a very similar problem on a 1982 Mercury 5.7. Weak spark at coil.

Mine is a new engine as of last summer with new coil, distributor, slave solenoid, starter, plug wires and alternator. 12 ish on the positive and negative side of coil with ignition on and as low as 4.5 when starting.

Still trouble shooting but I’m interested in any suggestions

thanks
 
Status
Not open for further replies.
Top