Mercruiser 5.7

[Johnm227]

I could use some help troubleshooting my engine.

The engine cranks but won’t fire up. I’ve already replaced the starter, the 90-amp fuse, and the ignition coil, but it’s still not turning over completely or sometimes it starts intermittently.

Also, I have a ground wire that I am not sure if it is a spare or where it belongs. It is coming out of the wire harness. I can’t find it in any drawings. I saw some people recommend putting it on the coil negative on other posts.

Any ideas on what to check next or how to identify that wire would be greatly appreciated.

 

[Fun Times]

That black wire would typically be for either the MerCruiser Mercathode corrosion fighting system that your engine model may or may not have installed or an unused/unneeded ground wire should the carburetor utilized an electric choke…Your TKS doesn’t..,

Do not install it on the coil… The engine was running fine until it wasn’t, it’s not going to be that ground wire.. that said you could connect it to the engine block if desired as it wouldn’t hurt anything.

Sounds like you should check for continuous spark and make sure you have continuous fuel pressure.

 

[cyclops222]

Battery voltage when cranking the engine ?

 

[Johnm227]

That black wire would typically be for either the MerCruiser Mercathode corrosion fighting system that your engine model may or may not have installed or an unused/unneeded ground wire should the carburetor utilized an electric choke…Your TKS doesn’t..,

Do not install it on the coil… The engine was running fine until it wasn’t, it’s not going to be that ground wire.. that said you could connect it to the engine block if desired as it wouldn’t hurt anything.

Sounds like you should check for continuous spark and make sure you have continuous fuel pressure.

That makes a lot of sense since this engine does have the TKS. I’ll avoid connecting it to the coil and just ground it to the engine block.

I’ve already replaced the coil, starter, and 90A fuse, and fuel flow seems solid. I’ll move on to verifying continuous spark next and double-check fuel pressure while cranking.

 

[Scott06]

I could use some help troubleshooting my engine.

The engine cranks but won’t fire up. I’ve already replaced the starter, the 90-amp fuse, and the ignition coil, but it’s still not turning over completely or sometimes it starts intermittently.

Also, I have a ground wire that I am not sure if it is a spare or where it belongs. It is coming out of the wire harness. I can’t find it in any drawings. I saw some people recommend putting it on the coil negative on other posts.

Any ideas on what to check next or how to identify that wire would be greatly appreciated.

As mentioned above the ground wire is a red herring ignore it. I would bet based on intermittently cranking/starting or not you have a loose or corroded wire connection- start with battery cables (both positive and nagative and both ends) also check the cannon plug for loose connection.

When it doesn't crank if you jump the starter slave solenoid (relay) across the two big terminals does it crank ? If it does this would make me think you have a bad connection between engine and helm or bad key or neutral safety switch

 

[Johnm227]

@Scott06 @cyclops222 @Fun Times do you think I need to shim my new starter?
Edit: Adding new starter and spark plugs
18-5913 Permanent Magnet Starter
18-8804-1 Spark Plug Wire Set for Mercruiser Stern Drives

 

[Scott Danforth]

what troubleshooting have you done?
randomly replacing parts without troubleshooting may or may not fix the problem

what lead you to replace the 90 amp circuit breaker, starter and coil?

do you have compression on all cylinders?
do you have spark?
do you have fuel?

see post #21 for the ignition troubleshooting. https://forums.iboats.com/threads/must-have-technical-information-for-diyers.335378/

you are also confusing turning over with firing in your post

does the motor rotate when you turn the starter? yes or no? if the motor turns, is it spinning at least 200 RPM when cranking? (use a photo tach on the crank)