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Author Topic:   Merc 135 Optimax Power Loss
Timothy Sullivan posted 07-28-2001 10:55 AM ET (US)   Profile for Timothy Sullivan   Send Email to Timothy Sullivan  
When pulling my kids out of the water on skis, sometimes the engine will give me an overheat light, alarm, and power back like it is supposed to. It only happens occasionally and normally the motor works great. When it happens, I shut it down, wait a few seconds and try again. It works no problem. Has anyone seen this before?


Tim Sullivan

Clark Roberts posted 07-28-2001 09:45 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Tim, the 135 Opti is fitted with 143 degree thermostats (one on each bank of cyls) which I suppose is an attempt to reduce emissions. Most outboard thermostats are in the 110-120 degree range (I think) and the 225 comes with 120 degree thermostats.. The thermostats only work at low rpm as there is a pressure relief valve which allows max cooling water flow at rpm higher than about 1500. I have replaced the 143 thermostats with 120's in my 135 Opti and think it runs a little cooler overall. A temp gage will show you relative temp and may give you insight into when and why temp goes up to alarm point... maybe it's a false alarm!? Faulty sensor?! A temp gage will sort it out for you. The temp sensed is cyl head temp and not jacket water temp ... cyl head temp is always higher than jacket water temp.
A temp gage for each bank plus a water press gage would be the ticket.. Your symptoms also point to the possible need for a new water pump impeller. A new impeller is like chicken soup.... can't hurt... In any case you need to get to the bottom of this .. all alarms should be taken seriously... Happy Whalin'.. Clark
smo posted 07-30-2001 12:03 PM ET (US)     Profile for smo  Send Email to smo     
With the cooler thermostats, aren't you afraid of getting carbon or soot buildup in the cylinders? I thought soot formed from incomplete combustion due to low combustion temps?
Bigshot posted 07-30-2001 02:45 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Nah! Depends on water temp. If you have a T-stat you are ok. Run wthout one, especially in 60 degree water and you will do damage. i like Clark's sugestion. Reason being whe n you get on plane the motor works hard then the kid wipes and you come off plane, that is when temp goes up a bit. Repeatedly doing this when skiing could throw it off.
Clark Roberts posted 07-31-2001 02:25 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
BillS, I always get nervous when I change something from "stock" configuration... I only changed to 120 thermos after talking to merc tech reps and local Merc mechanic... seems that the engine control module dials in a fuel mixture based on several parameters one of which is cyl head temp! So if I run a little less cyl head temp the ECM will accomodate by a slightly richer mixture... I had rather have the "less lean" mixture... don't need the California emission ratings here in Florida! Also I have a temp gage on each bank of cyls! Clark
smo posted 08-01-2001 09:30 AM ET (US)     Profile for smo  Send Email to smo     
Sounds like good logic. I would rather run a bit rich than risk the consequences of being too lean. How hard are the thermostats to change? Are they located at the tops of the cylinder heads like a regular merc v-6?

I have seen the same problem you have mentioned after trolling at 1000-1400 rpm, then accelerating to get up on plane. Alarm goes off after a power reset. I have twin 135s on my conquest, and also occasionally see the same problem when I have one engine in forward and the other in reverse to pivot the boat. I'm wondering if the water pickup is just sucking in exhaust from the other motor? The temp gauges always look normal when this happens, so I'm not sure what is going on. Motors are early 1998s with all applicable updates performed .

Clark Roberts posted 08-01-2001 12:25 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
The thermos are very easy to change out and are located at top of each cyl bank. One of the two machine screws on the starboard thermo housing interferes with the injector rail tubing and you must loosen the two (5/8" hex I think) nuts holding rail in place. You simply loosen the two nuts enough to pull rail back for clearance of machine screw on thermo housing. Change out the thermostat and reposition and retighten the injector rail.. which is "O" ring sealed. really a piece of cake.. you may hear a hiss when you pull rail back but it's low press and not a problem... The above may sound tricky but it's really not... total time to change out both thermos is probably 10 mins. and can even be accomplished from inside boat by tipping motor up slightly..
smo posted 08-02-2001 01:21 PM ET (US)     Profile for smo  Send Email to smo     
I tried finding 120 degree thermostats this morning at my local dealer with no luck. Their parts computer shows nothing but 143s for the 135 and 150s, either carbed or inected. They do have 120 degree conversion kits listed in the computer for lower HP motors (below 115). Do yo by any chance have a part number for the 120s, or can you tell me which motor they are spec'd for? My parts guy can't seem to find anything that will work. Also, do gaskets come with the thermostat?
Clark Roberts posted 08-02-2001 02:32 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
BillS, go to your merc dealer and get a pair of 120 thermos for a 2001 Optimax 225... don't have the part # but that should do it.. part # is right there on the computer! It's a simple drop-in replacement, perfect fit etc... just make sure you transfer the rubber gasket (circular) from 143 thermos to the new 120 thermos... a little swipe of vaseline will "dress up" the gaskets nicely. And don't overtighten the machine screws which attach the thermo covers... nice and snug will do it... good luck.. Clark.. The Old Man
PS> pardon the "over-instruction", you know how to do it...
Clark Roberts posted 08-02-2001 03:00 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
BillS, just found the package that my thermos came in... part # is 14586 and is labeled "thermostat 120"...
smo posted 08-02-2001 04:09 PM ET (US)     Profile for smo  Send Email to smo     
Thanks Clark !!
Ed Stone posted 08-04-2001 11:13 PM ET (US)     Profile for Ed Stone  Send Email to Ed Stone     
The port motor on my Outrage still seems to
run a little hotter than the starboard side.
I just recently had the 100hr service done.
Above 1500rpms there isn't a problem.It's
slow speeds like trolling or no wake zones.

Would changing the thermostats void the
Ed Stone.

Clark Roberts posted 08-05-2001 09:47 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Ed, I had not considered the warranty aspect as I did the switch with my dealers concurrence! However, I probably should not have posted this info. and others should check out individual solution and not take my lead ! You see, I never leave well enough alone and am always "fiddling" with everything! I certainly don't want to cause anyone any problems!!!! Happy Whalin'.. Clark.. the old man and the sea
Bigshot posted 08-06-2001 10:55 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
One side usually does at slwer-mid speeds due to other motor stealing it's water.

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