Hello Everyone,

I recently acquired a used boat with a pair of 1999 Mercuiser 525SC carbureted engines, and just fired them up for the first time yesterday. I quickly noticed that the starboard engine was surging at idle, and adjusted the carb to smooth it out, which worked out well. I also noticed that the engine is blowing quite a bit of steam from the exhaust, when compared to the port side engine. I took a short video and re-watched it a few times to confirm.

In chatting with a former Mercuiser mechanic today, I'm concerned that the engine is not getting enough total volume of water, i.e., the inadequate water volume is super-heating in the headers, mostly evaporating, and turning into steam. Visually, it appears that there's an equal amount of water exiting via the exhaust though. He said that Mercruiser utilized a nylon fitting insert through the transom back then, that eventually collapses, thus starving the engine for water. Remedy involved pulling the engine away from the transom, removing the drive and gimble to access the fitting, replacing the fitting etc., and then re-assembly.

My plan was for a very limited sea trial this weekend, but now I'm leery of hurting one of these expensive, premium fuel-thirsty noise-makers. Any thoughts?

Thank you,

Were you running this in the water or on the hose? If on the hose I would put it in the water and see what it does before worrying too much.

If water is spitting out the exhaust you can put your hand in the water and not be uncomfortably hot you should be fine. Neither motor should go over 160 on the thermostat. Also you should be able to put your hands on the exhaust manifolds without being uncomfortable for several seconds.

Thanks for your quick reply Donzi Matt. To answer your question, I was running it on the hose.

Ok, thanks for the heads up Rex. I'll give that a go tomorrow and see what I find. I'm hoping all is fine. Really just wanting to enjoy the boat a bit.

You don't list location but in cold air it will steam more.And my starboard steams more than port as well.

Ahh, good point MichiMike. The boat is in FL and it's warm here. Thanks for the input. Much appreciated.

Does it have the stock Merc/Gil manifolds???? They tend to crack internally just under the riser, especially with salt use. I had the same issue with steam that developed.

Also, the 525SC intake manifolds do not like salt. They corrode badly around the internal water ports.

Been there, done that. I had to replace the 8 year old Gil manifolds at 250ish total hours with 200 or so being salt prior to me owning it.
Also did a top end refresh around 350 hrs and had to have all intake manifold water ports welded on.

What part of Florida are you in Donzi?

run in the water, hose iffy on water flow