Sluggish out of the hold and loss of top end

[Hez]

1996 175 HP Mercury Mariner
Serial - 0G360900

Hey guys:

I'm looking for a little knowledge and support from the reliable Mercury board again.

Summary:
Motor was running good - then took her out and she didn't have as much power. Sluggish out of the hole, loss of top-end, and only running about 4800-5000 RPMs

The ol' motor was running great 3 weeks ago - had her hitting about 5800 RPMs pushing 65.3 MPH.

Took her out the next weekend and noticed that she was extremely sluggish to come out of the hole (from 2 seconds to 5-6 seconds) and had lost about 10 mph on top-end. I was planning for a trip to SC the upcoming week, so I immediately called my mechanic to see if he could check her out. He did a compression test, 120-130 across all 6 cylinders. He found some spark plugs that welded themselves to the electrodes, so he replaced all 6 spark plugs and a worn out hub.

I made the trip to Lake Murray and ran the motor all week - same performance, but ran strong - made long runs, just with about 80% of the power. She is running low on RPMs too - can only get it up to about 5000 RPMs WOT and trimmed all the way up.

I have 2 tanks - one in bilge and one under the seats...performance was the same out of both tanks.

What's been replaced in the last couple of years:
All new fuel hoses
New primer bulb
New fuel pump
New pickup hose in the rear tank
new thermostats
New head gasket
ECU Rebuilt
Power Packs (3 years ago)


If you have any suggestions or recommendations - I'm all ears.

[JOE54]

spark on all 6 ?? you got any pics of those plugs , you may have a detonation problem going on there , also may need to isolate non working cyls

[Hez]

Hey Joe,
I ran it this morning when I went fishing, and removed my plugs to take photos upon your request.

I noticed one of the plugs (I am calling it #3, left side of head, bottom plug) looks brand new still....could this be the issue?











This is the one that looks brand new











Here are all 3 plugs from the left side of the head:

[styler]

You need to find out if its sparking ... looks like no to me. And yes that'll be your problem if it isn't.

[fly by night]

As stated check the fire on each plug with a timing light to determine which one is or might not be firing, trial and error as on any engine.

[Hez]

Picked up a spark tester. Will check all plugs tomorrow. Thanks guys

[JOE54]

Shure could , bottom port /left/ passenger is #6..... 2,4,6 ..P...1,3,5.... STBD

[Hez]

ah...#6 - thanks Joe.

So...I did a spark test this morning and seem to have spark on all plugs...even #6. There's a slight chance that it's slightly weaker - but hard to tell. Also checked compression again and consistent over all cylinders.

When I pulled the plug for compression test, I inspected it and it still looks fairly new.

Video of spark test 1:
https://youtube.com/shorts/g77Y2fRc6RY


Video of spark test 2:
https://youtube.com/shorts/C3x-DMpgj3k


The video is not great...but I think you can see the spark.

[Hez]

#6 Spark Plug

[JOE54]

could be coil dropping out under load , might be looking at injector failing be careful could loose that cyl if its inj stuck and you keep running it

[Hez]

Ok, I bought a new coil. It's probably worth it to install the new coil and see if it does better under load. If not, then it's likely the injector then.

[Hez]

ok - new coil installed...confirmed spark. Gonna go water test it now

[Hez]

Ok - out on the water now. Pretty much same performance - still sluggish out of hole and loss of top end. Guess it’s time to hit up the shop

[leonreno]

Did you check gaps on the plugs? I would also pull the plug off that plug and run it and see if it changes. If it doesn’t make it worse then probably the problem. Either fuel or spark.

[Hez]

Yes, checked gap on plug - it was within spec. As mentioned above...all cylinders have spark.

I checked with my mechanic that replaced the plugs and did compression test, and he agrees it is likely a clogged or bad injector.

Gonna try to get it in the shop this week, dependent on availability.

Thanks guys

[JOE54]

be sure he looks at wiring harness

[fishing fanatic]

Gonna try to get it in the shop this week, dependent on availability.

Thanks guys

Curious if there is an update on the findings of this one?

[Hez]

Dropped it off at the shop last Tuesday - and still waiting to hear back with a final word on the diagnosis. Will be sure to update this thread when I have news.

[JOE54]

[Hez]

Ok, picked the boat up from the shop today...

They replaced 2 power-packs and all plugs and said it was running like a scalded ape. I mentioned the wiring harness, as suggested, said he checked everything along the way.

Immediately after picking up from the shop I went straight to the boat ramp.

Pretty much the same results as before...but you can tell she's running slightly better.

RPM at WOT: 5200
Top Speed: 56 mph

She's definitely idling better...when I took her in I was having to feather the throttle every few second to keep her running....now she idles smoothly and stays running. Just not running like she was a month ago...or for the last 4 years.


I took her right back to the shop just to let them know that it wasn't running 100% - but was running slightly better, and asked if there was anything else they could think of that might be an issue.

Mechanic's response was that it wasn't worth chasing down a few hundred RPM...

Not sure what to do at this point...I don't want to continue throwing money at it without good results. So far, I've spent about $1100 chasing this issue.

Gonna take her out this weekend and just go fishing while I consider my next move.