Loss of spark under load.

[500dollar744ti]

1990 MCM 350 Magnum, not sure of serial number.

Under heavy load, like getting on plane or accelerating faster while planing, I notice a loss of spark when I toggle things like trim tabs or engine trim. It happens more often in damp or rainy weather, sometimes it doesn't do it at all. I first noticed it when my alternator belt broke and I had to get home on just the battery power, it was having a lot of trouble firing because of the lowered voltage but with the two giant deep cycle batteries, there should be enough to keep the ignition system running for hours before dropping out.

After I put another alternator belt on, it ran great. Then today I was running it and I noticed the ignition cut out when I tried to retract my trim tabs.

I am thinking there's a ground problem somewhere, is there a common place to check for this issue. I'm sure the expert mercruiser guys have seen this "engine stalls when trimming drive up" thing before.

Thanks.

 

[Bondo]

Re: Loss of spark under load.

1990 MCM 350 Magnum, not sure of serial number.

Under heavy load, like getting on plane or accelerating faster while planing, I notice a loss of spark when I toggle things like trim tabs or engine trim. It happens more often in damp or rainy weather, sometimes it doesn't do it at all. I first noticed it when my alternator belt broke and I had to get home on just the battery power, it was having a lot of trouble firing because of the lowered voltage but with the two giant deep cycle batteries, there should be enough to keep the ignition system running for hours before dropping out.

After I put another alternator belt on, it ran great. Then today I was running it and I noticed the ignition cut out when I tried to retract my trim tabs.

I am thinking there's a ground problem somewhere, is there a common place to check for this issue. I'm sure the expert mercruiser guys have seen this "engine stalls when trimming drive up" thing before.

Thanks.

Ayuh,.... Start at the (-) negative terminal of the batteries, 'n work yer way to the ignition circuit on the motor,....

Connection by connection, as any 1 of 'em could be The bad 1,...

 

[Fun Times]

Re: Loss of spark under load.

Hi there, 500dollar744ti thanks for the help you provide here on iboats, It's very much appreciated. It looks like it's your turn at needing a little advice. I hope we can pull through for you.

First the usual questions, Power and ground cables are clean and tight? No signs of corrosion on the ends or just inside battery cables ends on the copper wires under the cable insulation at all locations.

Time to see what the alternator is putting out both volts and amps. Belt slipping?

Your description of loss of spark under load seemed a bit confusing, I would take it that you drop engine power when you power up an accessory item?

One test to try would be to see if it happens on battery one, two or all.

How much power (RPM) do you lose?

 

[500dollar744ti]

Re: Loss of spark under load.

When I say loss of spark, I mean when I touch the trim button while planing, the engine stalls, like the key was turned off. I notice the stall so I immediately release the button and it starts running normal again. It doesn't happen all the time, only sometimes but it seems to be getting worse which means it's compromising my reliability, time to fix it.

 

[achris]

Re: Loss of spark under load.

Does it happen with the remote control throttle lever in all positions, or just one? That's assuming the trim button is on the remote control lever. I'd be looking for shorts somewhere in the trim wires...

EDIT:... Are we talking about the Mercruiser power trim and tilt system or a separate 'trim tabs' system?

 

[glenn property of pam]

Re: Loss of spark under load.

theres a lot of points of failure that could cause the problem you are having especially if you add in the fact that the belt broke and could have whipped and caught anything around it. I would look near the distributor for a ground wire and make sure that is still attached. By the sounds of it the problem is weak connection on either the positive/negative. Check and clean all the ground connections and look for any that could have been damaged when the belt failed

 

[Fun Times]

Re: Loss of spark under load.

When I say loss of spark, I mean when I touch the trim button while planing, the engine stalls, like the key was turned off. I notice the stall so I immediately release the button and it starts running normal again. It doesn't happen all the time, only sometimes but it seems to be getting worse which means it's compromising my reliability, time to fix it.

Thanks that helps. Do you know the model of your remote control?

 

[500dollar744ti]

Re: Loss of spark under load.

It's this one..

It happens sometimes when I trim the outdrive but it also happens when I trim the tabs as well. It also happens in all throttle positions where the engine is under heavy load. This means it won't occur revving in neutral and it won't happen at no-wake speeds.

That also leads me to think, I grounded the trim tabs directly to the battery so maybe it's as simple as a corroded connection there.

There's a voltage drop somewhere that's clearly causing this, I think I'm going to have to do as Bondo suggests and start chasing grounds.

It's getting to be the end of boating season and I may not be able to 100% confirm I have this fixed until next summer. Not sure if I'll even get another chance to go out before it gets too cold.

 

[bruceb58]

Re: Loss of spark under load.

Its not your remote control. Its a voltage drop issue. I had a similar issue and it was because the cables at the starter terminal had loosened. I would take the connections off of the starter and clean them and reinstall. There are two cables on the starter solenoid stud. One of course goes to the battery but the other one supplies your entire electrical system. You can have a voltage drop on that wire but not the one going to the battery and get the condition you are experiencing.

Its possible its a ground but I doubt it. Generally, the ignition system is grounded to the block. if the block didn't have a good ground connection to the battery, you probably would have issues cranking.

 

[bonzoscott]

Re: Loss of spark under load.

Things will run fine as long as all is in order. When your alternator belt broke, it increased load to the battery. Connections get hot and create resistance. Go through the terminals with emery cloth or such.

 

[bruceb58]

Re: Loss of spark under load.

When your alternator belt broke, it increased load to the battery.

Very slightly and not enough to cause an issue.

 

[500dollar744ti]

Re: Loss of spark under load.

I'm certain it's a voltage drop issue, I just need to find where it's happening. I didn't know there was an issue before the alternator belt broke because the charging voltage was high enough to mask the problem. Now that I'm aware of it, I can recall times in the past where trimming up caused the motor to stumble but it didn't happen again.

I guess I 'ought to start by measuring voltage at the coil while cranking and when the engine is running, then compare that to battery voltage.

I'll also check the starter terminals.

My volt gauge always reads 12.5-13.0v when running but I don't trust it. I use the readout on my GPS which is always 13.4-13.8v.

 

[500dollar744ti]

Re: Loss of spark under load.

Also, as I was heading back to the boat ramp this past Saturday I was having issues in the pouring rain with misfiring but the more I ran it, the better it got. After I had been running on plane for 10 minutes, it cleared up completely and I was able to get full power and trim the drive as needed with the rain still pouring down.

 

[500dollar744ti]

Re: Loss of spark under load.

Today I checked purple wire at the coil and found it was getting 9.5v with the key on, once the engine was running, coil voltage went up to 10.5v. I found the rest of the electrical system getting 14.3v.

I went over my battery cables and they are clean, I also found a bunch of grounds that were corroding away at the back of the block. I thought for sure this was an issue with the coil voltage so I cut them all, replaced the terminals and secured them, making sure there was good continuity throughout.

Coil voltage remains the same, are my readings within spec or is it supposed to see at least 12v at the purple wire?

 

[bruceb58]

Re: Loss of spark under load.

Make sure you take all your measurements with the ground of the meter on the battery negative post. Only use this as your ground reference so you get consistent readings.

While using a long test lead on your negative battery post, measure voltage at the input and output of your ignition switch. From those readings, it will tell you what direction to look in.

edit: One more place you should measure is in/out of the man overboard switch.

I am not a Mercruiser expert. What ignition system do you have?

 

[achris]

Re: Loss of spark under load.

...I am not a Mercruiser expert. What ignition system do you have?

1990 350 Magnum will have Thunderbolt 4....

Basically it just needs a good 12volt supply and a good ground, to both the distributor and the ignition module...

 

[glenn property of pam]

Re: Loss of spark under load.

10v at the coil sounds correct. Theres a resistor wire that drops the voltage to the coil. As long as the coil is a external resistor and has not been replaced with a coil that is marked as a internal resistor then this is not your problem.

 

[achris]

Re: Loss of spark under load.

10v at the coil sounds correct. Theres a resistor wire that drops the voltage to the coil. As long as the coil is a external resistor and has not been replaced with a coil that is marked as a internal resistor then this is not your problem.

Not on Thunderbolt 4... There should be the full battery voltage there....

 

[500dollar744ti]

Re: Loss of spark under load.

Ok, now we're talking, it's thunderbolt IV and I don't have full battery voltage at the coil. Is it fed directly from the ignition switch?

 

[achris]

Re: Loss of spark under load.

Ok, now we're talking, it's thunderbolt IV and I don't have full battery voltage at the coil. Is it fed directly from the ignition switch?

Yes....

Basic circuit is power from the battery goes through a circuit breaker, then the harness plug, then the fuse in the red wire up by the key, then through the key switch and back into the harness, then through the harness plug and into the engine harness to the '+' side of the coil...

Oh, somewhere up by the dash side of thing you might have an ignition kill switch. Should be in the remote control box, but may or may not be wired in...

HTH