5.7 has spark question

[Mercruiser420]

Greetings,

IME, these symptoms can happen when the fuel/water separator gets old and starts passing water into the fuel pump and carb. I would also charge up and test the battery(s). I wouldn't entirely overlook compression..especially that it was a bit on the low side. I would do a leak down test just to confirm a healthy motor. Good luck

 

[Fun Times]

The port side engine dose turn over slightly slower when cranking starter. Could it be that?

A weak under voltage/charged and/or cranking amps of the battery would slow the cranking speed enough to keep it from starting plus cause weak spark like you've seen or an internally failing starter could cause similar issues... That is assuming all the 12v power and ground wires, cables and terminal ends are not internally or externally corroded or loose fitting at all the battery, engine and under the helm/dash area too.

Do you have a deep cycle or stating battery? How many? May want to have them tested for cracking amps. Bigger the better for CCA's.

Engine, CID (L) = Minimum Required Cranking Battery Size
4Cyl, 153 (2.5), 4Cyl, 181 (3.0), V6 229 (3.8) 6 Cyl, 250 (4.0), V8 302 (5.0), V8 305 (5.0), V8 Carb 350 (5.7), V8 351 (5.8), = 375 CCA or 475 MCA or 90 Ah

Look at the tachometer gauge, and compare cranking RPM from the good engine to the bad. You should be seeing right around 350ish + RPM at/for normal cranking speed for easier starts. High 400 RPM would almost be running while 500ish range would be considered running but pretty rough. 600+ smooth's it all out.

And find/buy/borrow/rent your/a timing light.

Try separating the engine wiring from the helm wiring by jumping the main wiring harness on the engine side such as he did with this video and talked about in second link. https://www.youtube.com/watch?v=NUoXFIo2Ab8
http://forums.iboats.com/forum/engi...99-starting-mercruiser-on-engine-stand-update

 

[Mild 85]

Got timing light. 12.7 volts on 4 batteries.

 

[JustJason]

You need to check the voltage at the coil when the engine is actually cranking over. Don't forget to disconnect your tach lead as well.

 

[Mild 85]

One thing I did notice is port side engine has only one wire going to ground side of coil. Starboard side has two

 

[Mild 85]

And as far as deep cycle or starting batteries I've never put anything other than deep cycle in my boats

 

[Fun Times]

One thing I did notice is port side engine has only one wire going to ground side of coil. Starboard side has two

One of the gray wires is for the tachometer that goes to the main cannon plug like seen in the starting video (terminal 2) and the other gray wire runs to the ignition module box. Follow them the best you can on both engine harnesses so you get an idea of what and where they head off to on the engine side of the wire harness to be sure one isn't laying about somewhere possibly causing issues to the electrical system. The one from the module needs to be connected to the coil plus be sure there's a good clean tight wire connection at the module connector by both pulling the plug connector off to inspect the wiring connections and to ensure the wire pins haven't back out/off of the connector to the module causing intermittent problems.

Are both tack gauges showing RPM at crank right now?

Also try shifting/rocking the gear handle a few times to try and help ensure that the mercruiser shift interrupter switch at the shift bracket assembly mounted on the engine is seated in the correct position to avoid any possible intermittent spark issues.

Having a starting battery onboard is considered the best option to avoid starting troubles as deep cycle batteries aren't the best option for starting an engine specially when there's trouble such as this.
Bear in mind that a battery can show good voltage but be weak on cranking amps which will affect starting and give you a false sense of the "over all" condition. It might be worth your time to use one of the other known batteries from the good engine to try on the non starting engine...And/or have it tested as well.

 

[Mild 85]

When cranking the tach on port side reads 2-3k rpm

 

[Mercruiser420]

You mentioned doing a compression test when you got it and when it was running. Did you do a leak down or compression test after it stopped running?

 

[Fun Times]

When cranking the tach on port side reads 2-3k rpm

Since you're seeing an (incorrect) RPM movement, Possibly the gray wire between the ignition module and coil is not connected to the - side of the coil like it sounds like the gray wire is connected correctly on the starboard engine. You'll need to find and confirm this possibility.

 

[Mild 85]

You mentioned doing a compression test when you got it and when it was running. Did you do a leak down or compression test after it stopped running?

I did a compression test with it cold and fuel disconnected

 

[Mild 85]

Since you're seeing an (incorrect) RPM movement, Possibly the gray wire between the ignition module and coil is not connected to the - side of the coil like it sounds like the gray wire is connected correctly on the starboard engine. You'll need to find and confirm this possibility.

Could that cause a weak spark??

 

[thumpar]

Could that cause a weak spark??

Yes. Disconnect the tach on that engine and see what happens.

 

[Mild 85]

I don't see any other wire but the one going to ground side of coil. Where else might tach be hooked up

 

[thumpar]

I don't see any other wire but the one going to ground side of coil. Where else might tach be hooked up

For a test run a lead from the + and - of the battery to the coil without the other wires on it. Something is not right with your wiring.

 

[Mild 85]

Well the starter wires were lose. And in tighten them broke corroded stud from starter off. Can just the selinoid be changed on these starters? I've never had anything but fords

 

[Mild 85]

Also what is small breaker on throttle bracket with small selinoid beside it

 

[thumpar]

Also what is small breaker on throttle bracket with small selinoid beside it

If it is an Alpha it is probably the shift interrupt you are talking about.

 

[Scott Danforth]

the main circuit breaker for the engine is sometimes mouted to the throttle bracket. the small solenoid may be the pilot relay.

you will need to pull the starter to work on it. yes, the solenoid can be replaced, however make sure you replace it with a J1171 compliant starter solenoid. you may want to simply replace the whole starter.

I personally would go thru each system.

at a minimum:
repair broken starter
clean all battery lead connections
replace the fuel/water filter and inspect its contents
replace cap, rotor, and plug wires

there is something simple that is being overlooked.

 

[Mild 85]

If it is an Alpha it is probably the shift interrupt you are talking about.

It velvet drives.

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