Mercury outboard overheating

[99 Sierra]

I have a 1987 mercury175 that is overheating I have replaced the water pump,t staats,poppet valve and it is still overheating when its running the stream is very weak and and the water pressure gauge reads 15psi. If it has something blocking a water passage where would it be block. Or any ideas on what it could be would be greatly appreciated thanks

[flabum1017]

Could be plugged up in the exhaust chest or in the adpter plate ..... are both cylinder heads getting hot?

[99 Sierra]

At what temp do t stats open mine don't open till 180 what is goin on

[Fish]

i think they should open around 140 or 145

[flabum1017]

how are you measuring the temps?

[99 Sierra]

Measuring with a digital temp probe in water one opened at 160 and the other was 180

[99 Sierra]

Yes both cylinders are getting hot. Just put a temp gauge in this year what should this thing run at the sending unit does not read the water ,reads the head temp (metal on metal)

[rock]

If your stats are opening at 160 and 180 that is the problem. To my limited knowledge Merc makes 2 stats. Ones that opens at approx. 120 and the other approx 140.
Rock

[Jay Smith]

Yep sounds like the T stat target opening points are WAY to hot...

Just for my curiosity whats your water pressure at WOT ?
If its over 25 PSI (with an accurate gauge) you have an obstruction of spent water leaving the block and that will make one red hot !

Jay

[Dave Strong]

You could "cook" the t-stats in water to check, stick a thin tag wire in between t-stat valve and hsg opening and suspend it in pot of water and heat they should begin to open at 140deg, drop off wire but will not fully open till higher temp, make sure that they fully open. Sometimes they only open part way. Or swap T-stats side to side and see if temps follow stats, it is normal for one side to open before the other but they should open at close to same temp.

Dave

[luckybuff]

Just had a similar problem on my 86' 175. Found my quicksilver relief valve block gasket leaking, preventing it from opening. So I made my own gasket!!! Runs fine.

[99 Sierra]

@jay smith my lower unit has the side scoop on it and at wide open (when it was running right) the PSI was at at 25 the water pressure gauge is a mercury one and I'm not to sure what you mean (obstruction of spent water).

[rock]

There are small holes in the adapter plate that allow water to exit the engine. If these plug they will not allow spent cooling water to flow thereby increasing heat and they will increase water pressure considerably. At 25 psi wot I would not think that was an issue.
Rock

[Ron V]

Careful about trusting thermostats even if they test OK in a pan of water on the stove. I had a brand new pair of stainless ones (the best) for my V-4 that tested perfectly on the stove, and would overheat the motor on initial startup every time. Wound up drilling the tops to get them to work. Just because they are new and pass a test doesn't mean they will work in the motor. Don't know why, but it happened to me.

I don't know about the older Mercs but my '05 runs at about 160-170 and has since new. Only time it runs in the 140s is when the water is cold. The stats may open at 140ish but the motor temp is actually higher.

Sounds to me like you could have a compound problem though.

[Jay Smith]

That 25 lbs at WOT ( if your gauge is accurate ) is getting to a point where there may be a restriction of spent water leaving the block causing the heating issue.

High water pressure signal to a known healthy accurate gauge normally points to a restriction in spent water leaving the system.

Several good places to look for obstructions ( if the water pump assembly is healthy and you've confirmed an accurate gauge )

1. Divider plate passages
2. In the block at the point where the divider plate openings are
3. Poppet valve assy.
4. Tuner adapter where spent water passes leaving the block
5. T stat areas

Good luck,
Jay