Thermostat question

B

BillWilliams

Member
Member
Joined
Jan 1, 2000
RO Number
2904
Messages
279
I'm trying to help my dad out before he takes off on vacation. His boat was running warmer than it should and if he rev'd the engine, it would bring it down some. It's an '86 MerCruiser Alpha I.

Anyway, I pulled the thermostat and put it into a pan of water and heated the water to see if it opens. It's a 140 degree thermostat and it did begin to open around 140 but was not fully open until around 170 or so. At what point does a thermostat begin to open based on its temperature rating and when would it be fully open?

We're hoping it's not the impeller since he's short on time.

Thanks
 
It should probably be fully open by 150. For a quick check of the water pump impellor, remove one of the hoses leading to the thermostat housing. Have someone start the engine while you check the water flow. It should give you a good flow at idle (I think it was 5 gallons in 15 seconds, but I'm not sure of the exact number; just look for a good, solid flow.) The flow should increase as you bring the engine up to 1500 RPM. Keep your bilge pump running to remove the water from the bilge. If the flow's good, your impellor should be OK. I'd suggest you change the thermostat and run the engine to see how she cools. His problem might be the thermostat or it might be the manifolds and/or risers. Start with the easy and cheap things first. Hy
 
Thanks for the suggestions.

There are basically 6 hoses going to the housing. The two larger ones from the circulating pump on the engine and to the block itself. The other 4 are heater hose size, 2 attaching to the top of the housing right above the thermostat and two attaching to the fittings with the bobbit valves (whatever you call the contraption with the plastic balls and springs).

Does it matter which one to remove?

Thanks
 
quote:
Originally posted by littlebookworm

It should probably be fully open by 150. For a quick check of the water pump impellor, remove one of the hoses leading to the thermostat housing. Have someone start the engine while you check the water flow. It should give you a good flow at idle (I think it was 5 gallons in 15 seconds, but I'm not sure of the exact number; just look for a good, solid flow.) The flow should increase as you bring the engine up to 1500 RPM. Keep your bilge pump running to remove the water from the bilge. If the flow's good, your impellor should be OK. I'd suggest you change the thermostat and run the engine to see how she cools. His problem might be the thermostat or it might be the manifolds and/or risers. Start with the easy and cheap things first. Hy

Click to expand...


I think that RW pump flow spec of 5 gals in 15 seconds is at at least 4000 rpm, maybe full throttle , which is why I never actually tried it. The pump impeller vane tips need good contact suction at idle & slower rpms , but doesn't even contact the pump housing at high rpms, so it could be worthwhile to not just check at idle.
 
I'm not sure of the number of seconds to test because I haven't had an Alpha outdrive for over 10 years. I do remember having to fill a 5-gallon bucket within a certain number of seconds. Do a google search and you should get the number. Basically you need a good flow.You want to try to get the hose which feeds the water circulation pump which is at the front of the engine (in place of an auto water pump). The fact that you have the thermostat-balls setup presents another possible source of overheating problems. Check that the balls move freely and are not stuck in place. The 'balls' system is notorious for giving flow problems. Hy
 
No big deal to replace the stat while its out. You can also cook both stats together to compare but I think the problem is elsewhere.
 
If you look at Mecr manual #15 at page 6A-8, they go over the procedure. The test is done for 15 seconds at 1000 rpms. The output is between 3-4.5 quarts depending on the gear ratio if the pump is in the outdrive. For a pump that is driven off an engine belt the flow should be 7.5 quarts in 15 seconds.
 
Thanks all. We ended up changing the impeller late last night and that helped. The temp help steady around 140 or so running off our water hose. I know that is not the necessary flow for higher RPM but at least it was holding steady. Now they need to drop her in the water and see how things go.
 
Did you replace the pump housing, too. It's important to replace impeller and housing as a set.

Checking temp and waterflow on muffs is not always accurate. Should be checked in the water. Thermostat is fine.

At that age check water hose between the bell housing and transom assembly. Was there any difference in temps at different trim angles of the drive??
 
Charlie, I can't say for sure since my Dad replace the impeller while I was not available. I ended up helping button things up and then was there when he started it. He's out on the water now and has indicated everything is working good.

The water hose between the bellhousing and transom assembly was replace just a couple of years ago when we did the bellows.
 
jtybt15: Did you replace the pump housing, too. It's important to replace impeller and housing as a set.

In what reality is someone going to replace their whole T housing or pump housing with an impeller change? Can I call you or PM you for some extra boat $$$ when I change my impeller next? I suppose you have 2 or 3 on board as well as t-stats? Inspection for rust and clogged passages yes, but immediate replacement with a T-stat or impeller, give me a break.
 
profishnsea,
You gotta follow along a little better before making off the wall observations.
 
Water pump housing profishnsea - it's a $10 difference in the pump kit for a Merc Alpha Gen 1. It's made of plastic and should be replaced with impeller. On the GenII it's metal and lasts a lot longer.
 
I stand/sit humbly corrected. Not sure if I would want anything to do with heat or water to be made of plastic on my engine but they do what they do to cut corners and make you spend $$$. Peace
 
You must log in or register to reply here.
Back
Top