had a recent problem - overheat

[Mcdiver]

I have been running the ol girl hard recently.  During trolling speeds (5-6 mph) i am getting close to overheating.  Well I replaced the water pump today.  The old one still looked okay, but changed it anyway.  

The last trip I made out, it was really hard to start at the dock.  She finally started and began the run to the fishing grounds.  about half way there, at full throttle, the rpm's began to lower until she stopped and shut down.  This took about 30 seconds to complete.  She would never start back up.  Thank god for Sea Tow!

I have ordered a new set of power packs because this seems like the most common problem, just wanted to get more opinions.

Background info, In the past 4 or 5 trips out, the idle has been rough and would sometimes cut out, hard to get started, plugs have been replaced.

And most important, this is a 1992 (i think) Mercury 200 offshore 2.5l v-6.

[dburr]

Well is a bummer... What happens when you pull a plug and crank? Do you get good white/blue fire? Was it misfiring as it slowed down or was it more like the throttle was being slowly closed?

[Mcdiver]

Have not checked fire, but when it quit, it was like throttling down.

[wingtime]

do you have a temp gauge?   Did you get a overheat alarm?   When was the last time the thermostats were changed?   When was the poppet valve last changed?    Everyone focuses on the water pump for cooling.... but it is only one part of the cooking system.    You may have lost one of the switch boxes.  don't just toss $$$ at it...  test it...  do a compression test and get a spark tester and see how the spark is...  Mercs have a VERY hot spark normally.....  then check y our fuel system...  make sure it's not running lean.

[dburr]

Mike the last big Merc I ran was an 85 Marriner 150.. I had a bunch of water work it's way through that motor and it was all over the place (duh!?!..). Start fine, crank hard, then run great at higher throttle position, then die slowly, or not.  Idle great, stall, run fine then not.. Drove me nuts because it wasn't consistent.

So I started pulling filters and they were CLEAN!?!.. Only when I got desperate and started to siphon out the tank using the fuel hose to a bucket while the boat was on the trailer and jacked up a bit did the water show up.. Time I was done there was almost 5 gallons of water out of a 50 gallon tank..

Be a good thing to know what kind of fire you have and is the fuel really clean.. Easy stuff first.. That motor have CDI ignition? If so, right back to fire, like being pregnant, it works or it doesn't.. Other then that, I feel your pain and I'll cross the chicken bones for you...

An oh by the way:
A buddies 90 yam had a kill switch caused the engine to die unexpectedly and intermittently when it got warm and humid.. Complete red herring but with out boards it's a crap shoot sometimes.

Good luck!

[Mcdiver]

No water in tank and fuel is clean.  The idle problem has been happening for a while, never got hot enough for an over temp alarm.  I keep an eye on that but also understand that the water pump needs to be replaced annually.  Not sure when jd replaced t-stats last, but should probably get a set for stock.  By the way, I found the spark plugs at auto zone for $1.99 each.

[fitz73222]

OK, here we go...

First thing I would do is compression test on this engine and make sure it hasn't been damaged from overheating. When the engine shuts down after running, does it turn over slowly, like the battery is weak?  If so, we could be looking at a bearing or tight piston issue.

Next, an understanding of this ignition system could point you in the right direction.

1) Do a spark test, you can remove all six plugs, reattach to the leads and ground them to the block with cable ties or anything that will hold them tight enough to the block for a good ground. You can also do this in preparation for the compression test.

2) If you are by yourself, turn the ignition key on and then back at the engine jump the starter solenoid and see if you have any fire.

3) If there is no fire, disconnect the black/yellow stripe wire on each switchbox, (this is the kill wire coming from the keyswitch) isolate them and retest. If fire comes back, you have a bad keyswitch; which can be an issue because of water getting in a panel mounted switch and it is common.
4) If you still have no fire on all six cylinders, your problem is a stator. If your engine has the original stator these were problematic and redesigned in 1996 to a more reliable unit.
5) Your engine has two 3 cylinder switchboxes that control ignition for three cylinders each. Very unlikely that both are bad so it may have been premature to drop $500 to replace them.
6) Coils are cylinder specific so they are not the cause for total no fire.
7) The trigger which tells the capacitors in the switchbox when to discharge usually shows up as a 2-3 cylinder spark loss, but not always. Usually, the triggers themselves do go bad but because they rotate as you advance and retard the throttle, they can have a broken wire.
8) My money is on the stator or keyswitch.

Have you ever replaced the under cowl fuel filter? Sometimes people forget there is one there. Although they are a 70 micron filter and your water separator is 10 micron which should catch anything that would pass through the under cowl filter, they can plug up if not changed for years and cause a lean condition and fuel starvation. Does you engine surge when it is trying to shutdown at high speed?

Do the compression/ spark check and let me know your findings.

[GoneFission]

Mike - what Fitz said is on target (like always - he's the Mercury Man, you know?), but I weigh in on the stator.  If one switchbox goes, usually the other will run 3 cylinders and you can start the motor and it will run, just not so good.   :x   When the stator goes, you sometime first lose low end or sometimes first lose high speed, cuz the stator kinda has two outputs.  But your problem seems heat related, and I would guess the winding insulation has broken down and you are getting a short when the stator gets hot.  Look on eBay and you can find a new stator for about $150.  You can test it by checking resistance off the wires, but that does not work so well if you have a temperature related problem.  

Mercury stators have constantly been revised for improvement, but still seem to cause occasional problems.  When I changed mine out, I went from the original Revision 4 to the current Revision 35.  So that's 31 small changes Mercury has made.  Maybe the new one will last forever!    

Good luck!