My 2.5 is acting up - AGAIN- dropping the left cylinder bank, Ideas?

[Steve2ManyBoats]

Had a problem with my '91 200 hp 2.5 and thought I fixed it, but it's back...

I swapped in a known good stator and narrowed the probem down to a bad switch box that would drop an entire bank when hot, but run fine otherwise. Put in a new Mallory switch box and the motor has run fine for a dozen fishing trips.

Yesterday on the way home from a long run, same problem. Same bank. Drops all three cylinders on the left bank. As before, all grounds are good, wires are clean and where they are supposed to be and the charging system and tach work fine.

Didn't have a DVA before, but I do now. Was going use it to go through the CDI tests, but it may be hard since the problem is heat related and I can't easily duplicate it at the dock.

Kinda getting tired of doing diagnostics and not fishing. I'm considering replacing both switch boxes and the stator with new and be done with it.

Any opinions on:

1. Brands - CDI, Sierra, Mallory, Genuine Merc - any quality issues?
2. Best source of parts - price and service?
3. Can I assume the trigger is good since they rarely go bad and don't seem to kill a bank when they do?

As before appreciate the help.

Steve2ManyBoats.

[Dave Strong]

Always replace switch box's in pairs never just one.

Dave

[Go Time]

I think something is causing your stator to wig out. Noramlly all three cicuits in a switch box won't crap out at once. That's not to say something in a switch box can't load the stator and cause the problem. I would bring the DVA and measure stator input to the switch box when the engine starts to act up and go from there... Maybe your "known good" stator anit no good no more. Does this engine have a water cooler rectifier?

[Steve2ManyBoats]

Thanks for the reply. No, it's got the two yellow potted square regulators. The difficulty I'm having diagnosing this particular problem is that I need to really run the motor under load for a while before it builds up enough heat - or whatever else is the cause - before it acts up. I don't want it to totally fail out on the water far from the dock now that it's getting cold. I guess I could just run in circles for an hour or two close to home until it acts up.

This is a new problem for me as well. Most times I've had a switch box fail I get a double fire problem or an intermittent fire problem on one cylinder. Never had a whole bank of cylinders fail before.

The bothersome part is that Merc electronics seem to be really sketchy. This is not the first Merc motor I've had to chase ignition problems. Wish I could rig up an old Merc distributor and be done with electrical problems. Ran those for years in inlines and never had a single problem with any of them.

Thanks...Steve2ManyBoats

[XstreamVking]

I would think that the charging system is possibly the thing causing the problem. The yellow/clear potted charging systems were bad news from the start I have heard. Loose/crusty batt connections will kill these parts too. Is the motor charging the batts? Have you checked the voltage sitting and then running to see if it is charging? I changed these out on my 2.5 when I heard of others problems with them. The new ones are black potted.....

[Dave Strong]

I would think that the charging system is possibly the thing causing the problem. The yellow/clear potted charging systems were bad news from the start I have heard. Loose/crusty batt connections will kill these parts too. Is the motor charging the batts? Have you checked the voltage sitting and then running to see if it is charging? I changed these out on my 2.5 when I heard of others problems with them. The new ones are black potted.....

And have a actual groung wire instead of using the case to ground.

Dave

[pyro]

A bad stator doesn't need to be "open" to be bad. It can pass an ohm test, then when it gets hot, a high-voltage "leak" can cause arcs across the windings, killing supply to the pack. Hot air is more conductive than cold air-- Think of how the jacob's ladder arc chases the heat to the top.

This is why a DVA tester is mandatory for checking ignition components.

[Steve2ManyBoats]

Both batteries are Group 27s and new. The battery cables are recent. Have a voltage gauge on the boat. Volts are 12+ at rest and 14 to 14.5 when running. All grounds are whistle clean, bright metal.

I recently bought a DVA for my VOM and I was planning on running the CDI diagnostics. However, I'm thinking that it might be wise to replace the stator and both switch boxes at the same time. Don't like wasting money but don't like to be solving the same problem twice.

Dave - When you said not to use the case as a ground, not sure what you're getting at. The neg batt cable goes to the block and the voltage regulators are grounded to the block as well. Are you thinking I should run a dedicated wire/cable to the regulators from the neg batt cable?

XStream - Can the regulators correctly charge the batteries at 14 volts and still hurt the stator and/or switch boxes?

As always, I greatly appreciate advice.

Steve2ManyBoats.

[XstreamVking]

14v is ok with out any load. the charging sys looks to be ok. These parts need a good contact ground on the coil plate on the backside of them. Back to testing, or just slap leather and put all new junk on there.

[myron]

check your trigger I just had one do the same thing

[Dave Strong]

The new black potted regulators have an extra wire vs the yellow potted ones. It is ground wire for the regulator, yellow potted ones did not have this and relied on the aluminum case of the regulator as a ground.

Dave