Check compression. I’m guessing you’ll find low compression on those back cylinders.

 

That is Usually caused by too many words in a thread title.

All kidding aside, verify that you have spark at each of the plug wires.
No spark? Fix that first.
Got spark? Check condition of the plugs.
When was the last tuneup?
Compression test like Golfman suggested is a good step too.

 

Golfman is right.

$10 says it got wet and you have weak compression caused by rusty exhaust valves. Do a compression test - easy with a $25 tool from Harbor Freight.

- Disconnect fuel pump power
- Disconnect coil primary
- Pull all the plugs
- Put the compression tester in a plug hole and crank the engine 3-4 "puffs" on the dial, record the compression you get, observe if it holds steady or drops. Write down result and cylinder #.
- Repeat for each cylinder. Release the bressure from the tool before you remove it from each plug hole.

A good result is >100psi in every cylinder, and all within ~10% of each other. Anything under 100 is suspect, any wide deviation is suspect, compression reading bleeding down while you have the tool connected is suspect.

It'll take you half an hour and you'll skin a few knuckles getting under the manifolds but it's one of the easier things you'll do.

 

Golfman is right.

$10 says it got wet and you have weak compression caused by rusty exhaust valves. Do a compression test - easy with a $25 tool from Harbor Freight.

- Disconnect fuel pump power
- Disconnect coil primary
- Pull all the plugs
- Put the compression tester in a plug hole and crank the engine 3-4 "puffs" on the dial, record the compression you get, observe if it holds steady or drops. Write down result and cylinder #.
- Repeat for each cylinder. Release the bressure from the tool before you remove it from each plug hole.

A good result is >100psi in every cylinder, and all within ~10% of each other. Anything under 100 is suspect, any wide deviation is suspect, compression reading bleeding down while you have the tool connected is suspect.

It'll take you half an hour and you'll skin a few knuckles getting under the manifolds but it's one of the easier things you'll do.

The compression gauge should not leak down what so ever as there is/should be a Schrader valve in the hose or adapter.

 

Ugh sorry - brain fart, had fuel pressure tester in my head. Thanks Mitch. Rest of what I wrote holds though.

 

I’m was hoping a bad high or low pressure fuel pump.it was running ok went on vacation used it last weekend & could barely get it on plane. Was wondering if front cylinders were at beginning of fuel rail & back ones were starving or has bad injectors. She’s fresh water Great Lakes girl with 680 hours. Tune up fresh this spring & already changed out water separator

 

The rail is pressurized and each injector sees the same pressure. Not gravity fed. You can and should test it (it needs to hold around 40psi) it's easy to do but no, low fuel pressure would affect all cylinders equally, not just a few. It could be some faulty injectors, but four of them at the same time in one half of the engine is unusual, unless somehow all their connectors got damaged by a leak over that area or something like that.

Engine needs compression (and air), fuel, and spark. A failure like yours, localized to cylinders physically close to each other on opposite banks, is really only plausibly explainable by a problem with compression. The fuel and ignition systems don't operate on groups of cylinders like that; instead ignition follows the firing order 1-8-4-3-6-5-7-2 and the injectors are generally split between two banks on Merc MPIs. Sounds like you have problems in 5, 7, 6, and 8, there's no likely failure in the ignition or fuel system which could explain that. So start with a compression test (or do a quick fuel pressure test first, just so you can put that out of your mind. It's quick to do.)

 

What you are saying makes sense but how would it have gotten wet & why would the rear half have low compression. I need to do further testing. Just did a quick rundown Monday before storms up here in Michigan. Sidelined again today by bad storms. Just speculating while I wait for Mother Nature to calm down. Really hoping I don’t need to get her rebuilt $$$$. Plus only 680 hours seems early for it to loose compression

 

The usual route is through the exhaust where water causes the valves and seats to corrode slightly, losing their seal. Hits the rear because the engine leans slightly in that direction, especially when under way. Could be years of reversion (cam overlap causes the exhaust and water to get sucked back in) or deferred maintenance on the elbows/risers (but in freshwater, that's unusual). Sure it's only 680 hours but that's middle aged for a gas motor and that's over 26 years.

Might also have blown the head gasket between two cylinders. What's the oil look like? Odd to happen on both banks, but I doubt you have four bad cylinders anyway.

Do the test, let's see what it says. You might be lucky, rule it out, and then you can start looking at fuel and spark.

 

Oil is clean no chocolate milk. Thank you for the explanation

 

Ok still haven’t had a chance to do the tests to boat yet. My further research says it could possibly be a bad ecu causing the issue. Other people with same year range have had this issue. Any thoughts?

 

Best "research" says,... do the test suggested and report back.

Note: You have one of the best boating resources at your finger tip's.

 

There's any number of things that could cause this issue. As noted above, do the tests. Spark, as well.

I'm making an assumption here so don't shoot me if I'm wrong. If you don't have the skill set to do these tests and/or make sense of some of this yourself, without help, be careful - if you do too much "internet research", you'll go down a deep rabbit hole and end up replacing things rather than diagnosing.

 

 

Just trying to gather as much knowledge as I can before doing the diagnosis. With weather & work holding me up. I probably won’t get to my boat until the weekend. I just like to know as much as I can & what to look for before jumping in.

 

OK, understood. Once you do these tests, that will start to point you in the direction of the issue.