Consistent heat soak

[Peach1303]

1B506554
2008 Mercury Pro XS 225

I had an issue of the overheat alarm sounding with the motor this year. It would never get too high while running down the lake, nor when idling. It would happen when I shut the motor off prior to the thermostats opening up. I replaced the thermostats in it with the correct 130* tstats. Now, it seems to be doing that more often. Could this be more so of a poppet valve issue (blind guess on this)? Or do I just need to run the motor up until the point that the thermostats open up and the temperature is on the decline before I shut down the motor? Thanks

[Savage]

The boat should not start moving on plane, until the thermostats open!

[Peach1303]

Sorry, I may have misworded that. I let it warm up and allow the thermostats to open before I put the boat on plane. This is occurring when idling or coming off plane and the thermostats are then closed (or closing), causing the temperature to be rising. If I shut the motor down when the temperature is increasing, this is causing the alarm to go off upon start up again. I’ve never had this issue until this year. I cleaned the strainer over winter, had the water pump replaced last month, and I’m at around 22 psi WOT.

[Savage]

I'm skeptical that "heat soak" is causing the alarm. You should have the faults read and post the printouts

[gmbwes]

This was occurring on my buddies 2004 225 Optimax, he went down the same road changed the thermostats and cleaned the strainer. It end up being the base gasket the one between the powerhead and the exhaust adapter plate. Not saying this is what is occurring on yours just wanted to give you the info as it took him a couple mechanics and several months to figure it out.

[EuropeanAM]

Sticky poppet valve (does not slide smoothly in and out of grommet when COLD), Powerhead Gasket, or Adapter Plate Gasket are the most common causes.

[Peach1303]

Thank you guys, I appreciate the input

[Peach1303]

Have to ask...powerhead gasket sounds like a bigger job. Looking at around $1000 for that? If so, what’s a sign that it’s leaking?

[EuropeanAM]

Powerhead gasket shouldn't be anywhere near that. External leaks are easy to see with the lower cowlings removed- but INTERIOR leaks are a bit tricky (telltale signs of water in the exhaust tuner "exhaust runners", gasket obviously broken or missing when checked with a borescope, or the most fun: Scuba mask used to observe water flow while system blocked/bypassed and you note water spraying into exhaust runners).

The last one is not something I recommend, except as a last resort.

Adapter plate gasket is a bit more labor intensive... that one might approach your guessed amount by the time you factor in gaskets, labor, clamps, etc.

[Peach1303]

Thanks Don. Ran it on the lake. Definitely has something going on with it, water temp gets high towards 150* ONLY after starting up following shutdown. Took the cowl off after putting the boat in the trailer, had some water inside the cowl. Couldn’t find any drip marks so I’m guessing it’s coming from underneath somewhere.

[silvertalon]

Check the torque on your powerhead bolts. Not all gaskets "blow out". They just leak. I just fixed a 2.5, 175 pro xs leaking a lot of water into the pan (port rear), and the back 4 nuts were Way loose. Merc's torque spec is quite lite IMO. I use a Mac long reach 9/16 box end wrench and really crank 'em down tight in pattern. Could save you some dough- if air bubbles are entering, would explain higher temps. I've also found adaptor plate gasket leaks via the following; remove gearcase, attach a water feed hose to the copper pickup pipe that feeds the powerhead. Turn on the water flow gently and look up into the DS housing with a flashlight. there's a water dump hole that will expel water but if you see water leaking out anywhere else that could indicate leaks in other area's of the gasket, Hence air getting pumped up with the water flow. This trick is essential to use with Opti's that won't get the boat on plane or, very slow holeshot. Hope this helps.

[EuropeanAM]

If you try the above hose-to-water-tube trick, I would recommend you put on a diving mask or goggles.

Be careful about "over-torquing" powerhead retaining nuts. The listed torque is very specific- and over-torquing can distort the block causing sleeve or cylinder problems/failures.

This is ESPECIALLY important on 3L models (most of the above info is for a 2.5L model).

[Peach1303]

Got a call from the mechanic. He is really perplexed at this. The starboard side is hitting 180* causing the alarm per the codes he read (all 6 alarms were due to temperature hitting 180*). So far, he can not replicate the problem. Waiting to hear if the gaskets may end up being the issue). I’m lost with this one.

[Peach1303]

Spoke to mechanic. He still can’t replicate the problem. He told me he spoke to Mercury, and they asked if the thermostats were black with carbon, which to my recollection when I changed them out, they weren’t. If they were black, they would suspect the powerhead gasket for sure. He quoted me at 8 hours labor for the gasket to be replaced...but also said he didn’t want to remove it and then that not solve the issue. He also said the poppet valve was black when he inspected it. Thoughts other than gaskets?

[JOE54]

carbon on poppet spring is very good indication of base gasket leakage replace both and clean idle reliefs in tuner while its in there, scrape cuss scrape cuss

[Peach1303]

Backflush seems to have done it. I can’t replicate the issue either anymore and is doing great.