Bad Thermostat - Mariner 15hp

[buckslayerII]

Hi - I can't find the answer to my question when searching, so I'll ask.

1997 Mariner 15hp engine that I just replaced impellor in. All tubes were flushed. Water stream out of pee hole is weak and sometimes diminshes completely. If I remove the thermostat, the stream is strong. Is it a bad thermostat, or could there be another blockage? Shouldn't the pea hole stream be strong even before the thermostat is open? Is there a bypass water circuit for the pumped water?

Thanks

 

[Texasmark]

Re: Bad Thermostat - Mariner 15hp

Depends on the plumbing but you can check your stat first. Pull your stat and look on the bottom of the pellet for a number. Also, while your stat is out, have a look at the water jacket that it monitors (engine block cooling) and ensure that it is not crudded up. If engine is like larger hp Mercs, (with which I am familiar) it will be 143F. Put it in a pan of water with a candy thermometer or other suitable device capable of accurately measuring at least 212F (temp where water boils at one atmosphere.....sea level). Heat the water slowly, giving everything time to follow the changing temp, and when the thermometer gets within approx. +/- 5 degrees of the marked temp you should see the stat start to open. Uping the temp will increase the opening. By the time you are at 160F it should be wide open.

If pee comes from the thermostat cover, the stat has to open to get it. If that's the case, water flow is dependent upon how open it is. Obviously hot working conditions mean more pee and until the engine block's water gets to 143F you won't get any pee.

If it is plumbed off the engine block, exhaust cover, or water jacket cover (where the spark plugs are located) you should get a stream as soon as you start the engine and it's strength will be dependent upon rpm's. Still, at idle, you should get a solid stream.

HTH,

Mark

 

[buckslayerII]

Re: Bad Thermostat - Mariner 15hp

Mark - thanks!! That helps. The pee tube comes off the thermostat housing, so it shouldn't open until it's warm enough. I'll test it to make sure it's working. All my other outboards aren't like this, and they pee as soon as the engine starts.

 

[buckslayerII]

Re: Bad Thermostat - Mariner 15hp

Upon further studying this system, the pee tube does come off the thermostat housing, but there is a bypass water circuit for when the thermostat isn't open. If I remove the thermostat, water comes out the pee hole very strong. The thermostat works fine according to my heated water test. If I run the engine long enough for the water in the block to become warm enough to open the thermostat, the pee stream is strong.

This all leads be to believe that the pump is fine, but the bypass circuit is plugged somewhere. Does anyone know the path of that passby circuit? What all do I need to disassemble to clean it out? Please correct me if I'm wrong on my analysis.

 

[buckslayerII]

Re: Bad Thermostat - Mariner 15hp

Guys - never did figure this out. Am tempted to just remove the thermostat and let the water run all the time, but I'm not sure if the engine would ever get to a good operating temp in this set-up.

Any more insight/thoughts this spring?

 

[carholme]

Re: Bad Thermostat - Mariner 15hp

Here is the service manual for your engine:

http://1manual.com/Mercury manuals/Service Manual PDFs/TwoStroke/827242r1/cover.pdf

You must have Adobe Acrobat reader installed to use this manual. If you scroll down to the lower part of the opening page, you will see the various engine components in red. Click on the one you want and when the first page opens, click on the "Save As" icon at the top LH corner of the page and you can save it to your computer.

Parts Listing:

http://www.crowleymarine.com/mercury-outboard/497.cfm

You will note item 42 under CYLINDER BLOCK AND COVERS that your thermostat is a 120.

If you look at the Specifications section of the service manual, page 3/7 it will show the thermostat open and closed water pressures.

Gerry

 

[buckslayerII]

Re: Bad Thermostat - Mariner 15hp

Thanks Gerry - where is that pressure measured? Out of the pee hole?

 

[carholme]

Re: Bad Thermostat - Mariner 15hp

Yes would imagine it is as it has to be somewhere downstream of the thermostat for the opening and closing pressures. Can you install a gauge where your pisser line is fitted? 1 to 1.5 psi is not much.

Gerry

 

[bbstacker1]

Re: Bad Thermostat - Mariner 15hp

I had a 2000 25 hp and currently have a 2002 15 hp, both Mercury 2 strokes. On both of these, and the dealer explained to me it was supposed to work this way, I would only get a good trickle at start up with the flow getting better as the engine warmed up. As the flow improved the t-stat was opening farther until it had a strong stream. If I was running wide open the stream would be mostly consistent, if I was idling or just trolling it would be intermittent. So, that being said what you described sounds normal to me but you should check with a dealer to make sure everything is working as designed, you should not run without a t-stat.

 

[aussieflash]

Re: Bad Thermostat - Mariner 15hp

Your Tstat should stay in.Running cold is no good for engine.Mine does the same thing...pees when up to temp.Remember to clear the pee hole(running) with piece of wire as they regularly get blocked.If your paranoid get a temp/pressure gauge.

 

[buckslayerII]

Re: Bad Thermostat - Mariner 15hp

Thanks for that info guys. Based on all my diagnostics, this is how mine should work as well. T-stat opens fine and pee hole stream is strong with t-stat removed. All passages are clear per my cleaning.

Ready to fish!!!