Mercury 225 3.0L not working all of a sudden

[AdamBorz]

1996 Mercury 225 Seapro Offshore Serial OG384600 carbed version: Been lurking, first post as I am getting stumped.

Here is the story, Thing ran like a top until it didn't. Late last year, was screaming along on the lake like usual, mid run, suddenly lost RPMS (had her propped to run right around 5500 rpms) and dropped off plane to around 2500 rpms... didn't sound like she was missing so WTF and no alarms. Wouldn't get back on plane no matter what I did. Since then, I have read numerous websites, replaced all coil packs (it was old just in case), spark plugs, fuel lines/ filters all the way to tank, I premix fuel 50:1 but the oil tank still in outboard so unhooked its wiring so as not to get fault, no warning lights, I checked TPS which I thought was reading backwards until I read somewhere on this site to not use the ground wire, it is good, checked compression all around 100 psi/ cylinder, didn't see any fuel spitting out front of carbs so don't suspect a reed issue, but am pulling carbs off soon. I just received all new head gaskets which I'm going to change just in case.. along with rebuilding the carbs just in case...

I read on this site about the exhaust honeycombs being clogged so that got me excited. The last run out, I noticed the Exhaust boot from exhaust plate to cover came loose and it was blowing exhaust and oil into the cover. I found on here that the exhaust holes can get clogged so I felt that was likely my problem as well. Everything else checks out that I check. Prior to the last run, I removed oil tank in the outboard, just attached the hoses to each other to plug it up. Unhooked the wiring so not shorted to ground. The tach is working fine (crank sensor seems good), unless the ECM is screwing up, and I haven't checked timing but using a car timing light I verified all coil packs are firing.

Anyhow, engine needs air, fuel, compression, spark.. I rebuilt the fuel pump if I didn't mention but I don't have a pressure gauge to check it. This is the carbed version so doesn't need much. I pulled the powerhead and I guess I was expecting to see these honeybomb holes I read about but I guess they are below another plate?? That is my question for now, if I remove this exhaust plate I'll see these honeybomb holes? Hopefully I'm on the right path, don't want to ditch this outboard she ran awesome... until she didn't. So now I'll rebuild the carbs, new headgaskets etc. I ordered all new exhaust plate gaskets they are coming but what I see so far doesn't look bad but I really don't know what I am looking at. Not a mechanic but am not afraid to get my hands dirty.

Hope the pics show up...

Any suggestions?

[JOE54]

that early 3.0L engine is gonna have a different tuner than the the one your reading about but carbon can still be a problem there and it will not cause the sudden problem you described ,blown base gaskets can as well as a leaking exh boot , would not mess with heads or carbs just yet

[AdamBorz]

There was a lot of oil inside the cover when I noticed the exhaust boot was torn. I know I premix to 50:1. I was wondering if some banks or cylinders weren’t firing and the gas evaporated leaving the oil?? The exhaust story peaked my interest as I had a flapper break on an I/o once and jam itself in the exhaust shutting down that half of motor. Just in case, I just bought a used known to be good ECU on eBay. I’ll check that when it comes in. Also going to recheck the spark under load in the boat when I put back together.

im already this far so I’m going to remove that lower exhaust plate and check out the tuner. Gotta wait for new gaskets to arrive.

I had replaced all the carb bleed hoses and the thing idles like a champ. It was sneezing out the port side and I noticed blown tubes; replaced them all. Can’t believe I can’t figure this issue out..,

[AdamBorz]

Also, when I unhook the temp sender and the TPS I get the warning light flashing. I unhooked shift interrupt and didn’t change anything. In fact when I manually activate the shift interrupt I can hear the engine almost stall so I’m guessing that is working! I unplugged the TPS when idling and the engine sped up some.

I’m wondering if I didn’t blow a carb gasket??? Or something got past filter that last time WOT and clogged the high speed part of carbs? I did unscrew the access ports to high speed needle and run a needle brush in all 6 jets… no change.

[FeFanatic]

Have you verified timing is correct and advance arms are working?

[AdamBorz]

Have you verified timing is correct and advance arms are working?

No, don't have a marine timing light, do have a automotive one, can I use that?

[AdamBorz]

So I pulled the plugs tonight to see how all 6 look, not a 2 stroke outboard expert (this is the first I've ever worked on), so here is how all 6 plugs looked, pretty clean around the whole plug except right at the ground electrode which was black and oily. I would expect them to be black completely around the threads... or is this normal?

I have a online manual BTW but some of these questions/ answers aren't in there.

[JOE54]

im gonna quit assuming folks have the correct plugs in their engines , wrong plugs QL77CC is the only plug for your engine

[AdamBorz]

im gonna quit assuming folks have the correct plugs in their engines , wrong plugs QL77CC is the only plug for your engine

im using what my manual recommends and what I’ve always used in this engine. Not that I’m opposed to switching to the champions

[AdamBorz]

Page from my digital manual.... if I should be using other plugs I'll switch but I've used these for years...

[AdamBorz]

On a 4 stroke I would say the plugs were being steam cleaned... getting water or coolant in the cyclinders to clean the plugs like that. I'm not sure how these plugs should look.... maybe I'm ingesting water?

[bpicinc]

Replace the plugs

[lpugh]

Too much low rpm use, running to cold, very rich run, too much oil or low voltage from coils
Restore the oil injection to OE and ql77cc plugs, check/replace thermostats would be first on my list for a fishin boat.

Oil injection is very reliable, much more so than the boat owners in getting the right oil mix, helps tremendously to avoid plugs looking like those do.

You cannot vari the oil ratio like the oil injection can do as needed!

[AdamBorz]

Too much low rpm use, running to cold, very rich run, too much oil or low voltage from coils
Restore the oil injection to OE and ql77cc plugs, check/replace thermostats would be first on my list for a fishin boat.

Oil injection is very reliable, much more so than the boat owners in getting the right oil mix, helps tremendously to avoid plugs looking like those do.
You cannot vari the oil ratio like the oil injection can do as needed!

I would hook up the oil injection but the previous owner unhooked it. It is all there except for the linkage to the pump. How do I know if the pump works? And I add exactly 50:1 ratio, but this last trip I was definitely trolling along for miles.... and there was a mess of oil in the engine compartment, the exhaust idle boot ripped and spewed oil everywhere which is weird that there was that much oil and not gas (unless evaporated) For guiel, lets just say I put in 40 gallons of gas, I multiple by 128, then divide by 50 giving me the exact oil amount to add (102.4 ounces in this case). However, I realize that gas will evaporate and the mixture does get rich. I did change the thermostats when this problem occurred, didn't fix the problem and the temp sensor works and when I unhook I get the blinking red light.

I have the powerhead off at the moment, but I'm gonna reassemble when the new gaskets get here and do a spark check under load in the water. I'll recheck the TPS voltage, I have new carb gaskets and head gaskets/ O-rings, will probably do that next. Did I mention the carb bleed lines were sneezing on the port side, maybe a lean sysmpton so maybe one of the carb to intake took a shit??

I'm pretty much at a loss, but the thing has compression but granted the seals are 27 years old !!!

[AdamBorz]

New Champion plugs on the way. So I have new/used ecm coming, exhaust boot, exhaust plate gaskets, head gaskets, carb gaskets, replaced thermostats, all fuel lines. The thing idles like a champ but won’t rev past 2500 rpm’s in gear. Still gonna inspect the exhaust tuner for blockage (tomorrow)

[JOE54]

dont pull those heads yet

[AdamBorz]

dont pull those heads yet

so you think when I clean up the gasket surfaces on the exhaust plate and change that exhaust idle boot and change the plugs that’ll fix my issue? I hope you’re right but I’m at a loss as to how that little boot can affect the power of the outboard. Wouldn’t all the exhaust still be getting out? I haven’t pulled the exhaust plate yet, hope to look at it tonight. Looks like that is the top mounting point of the outboard right?

Manual says to permabond the exhaust boot, ok to use 5200 or is there something special for exhaust?

Not a mechanic so sorry for the dumb questions but this 2 stroke is pretty simple engine, no cam, timing belt, valves etc it’s really perplexing why she just quit “going”!! Not many electronics either unless the ecm just quit advancing timing. I’ll have to watch the timing under load for sure.

Thanks for the responses so far, I’d rather tear into the heads and intake this winter and just use the boat now if I can get her running. I got the gaskets so I figure won’t hurt to open up the heads and check cooking passages etc since she gets used in salt.

I’ve had this outboard about 4 years, changed impeller and lower oil but otherwise she has run like a rocket; until she just quit on the lake that day end of last year. I really figured I would find clogged fuel line when I changed it all but nope!

[JOE54]

those head bolt are one time use only , that engine is a giant vacumcleaner under that cowling when running where would you think all that oil /gas and water from boot went hmmmm , 5200 for boot , saltwater better order those headbolts then

[AdamBorz]

those head bolt are one time use only , that engine is a giant vacumcleaner under that cowling when running where would you think all that oil /gas and water from boot went hmmmm , 5200 for boot , saltwater better order those headbolts then

So I pulled the thermstat hoses off (to replace since I have powerhead off had to unhook from exhaust plate) and I noticed oil was in the line, kinda had me thinking about water intrusion in the heads, then you mentioned the engine being a giant vacuum cleaner; I guess the engine would suck the exhaust water right back into the carbs (from exhaust boot) especially since the intake goes around to the side of the engine. That would explain clean spark plugs too I guess.

After my exhaust plate gaskets arrive, I'll put the engine back together and do some more testing on the lake so I can put her under load. Will read up on the checking timing, but I know all 6 cylinders are firing, each plug looked identical plus I previously checked for spark with the timing light on each cylinder. Bought 6 new coil packs, new thermostats and put new plugs in (albeit the NGK kind that were in there already just put new ones in), replaced bleed lines, gonna have new exhaust boot, all new fuel lines, went to marina today and bought the correct zipties that fit around the hoses instead of generic ones I usually use, so will redo all those... will leave carbs and heads alone for now and will put in Champion plugs. Will report back next week when all my parts arrive and I get them installed, oh was going to rebuild the fuel pump again just in case, but since I had a buddy pumping it while I tried to rev up and it stayed hard, the carb bowls are filling up so that isn't the issue...

Sorry if my though patterns jump around and seem random, but I've been dealing with this issue since end of last season and have done a bunch of work already and occasionally something I did pops into mind and I post it.

[JOE54]

bud those plugs are not water washed they are fuel and oil soaked , they would be sparkling white if water washed