87 PCM Chevy inboard temp / water pump issue

steelers7

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1987 supra comp ts6m with pcm 350 chevy

tstat usually kicks about 170, but it started getting hot (north of 200), so I got towed back in. before I shut it down, I noticed that getting on it a little or really just changing rpms would quickly drop me back to normal running temp or cooler. then it'd climb back up.

changed impeller (looked bad), tried again. same results. got hot, some throttle would bring it back down, hot again, change rpm, back down. I babied it back in that day making sure it never got much past 200.

so I ordered a new thermostat next. I had an auto one in, found out this one is special, so figured that had to be it.

I get the new tstat in and fire it in my driveway with a hose in a bucket. after watching the temp gauge for a minute or 2 at idle, I notice the bucket starts to overflow. I give it some throttle, then bring it back to idle, bucket gradually drains. for maybe 20 seconds, the bucket stays empty, engine is pulling water as fast or faster than the hose (which is usually the case). then, bucket starts filling back up and overflows. throttle again, back to idle, empties. repeat.

so its like the engine stops pumping water after some time, then a rev or change in rpm gets it pumping again.

any idea what is malfunctioning here?
 

Scott Danforth

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Replace your raw water impeller or the whole pump

It's not your thermostat

Make sure you find all the pieces of your old impeller
 

steelers7

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Already replaced impeller. Was missing a chunk or 2 but didn't tear anything apart to find them

On this engine the impeller is separate from the water pump. I don't know the system well enough to justify what could be failing to produce these symptoms

Was thinking water pump next but id like to prove it first because that sucker is just short of $1000... and not even sure I can justify that on this boat. Have a small block auto water pump in shed I could test the theory with I suppose
 

Scott Danforth

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Find the missing pieces

Raw water pump and circulation pump are two different things
 

Scott Danforth

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Already replaced impeller. Was missing a chunk or 2 but didn't tear anything apart to find them

On this engine the impeller is separate from the water pump. I don't know the system well enough to justify what could be failing to produce these symptoms

Was thinking water pump next but id like to prove it first because that sucker is just short of $1000... and not even sure I can justify that on this boat. Have a small block auto water pump in shed I could test the theory with I suppose
Not sure where you are looking, however getting the pump direct from Jabsco/Johnson/Shorewood would not be anywhere near $1000
 

steelers7

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Which ones which
Raw water pump = analogous to car water pump and circulation pump is the other with impeller?

How deep should I go for those missing chunks? Impeller housing, water pump, tstat housing? I can't see anything getting stuck in hose and if they made it into engine I'd assume its a lost cause
 

Scott Danforth

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Which ones which
Raw water pump = analogous to car water pump and circulation pump is the other with impeller?

How deep should I go for those missing chunks? Impeller housing, water pump, tstat housing? I can't see anything getting stuck in hose and if they made it into engine I'd assume its a lost cause
No no

Circulation pump is a brass impeller version of an automotive water pump. These are GM or Mehling and cost about $80 for an SBC. These almost NEVER go bad

Raw water pump has a rubber impeller and is what pushes water from the pickup to the thermostat housing. These are available direct from Jabsco, Johnson, or Shorewood for about $150. The impeller lasts at most 5 years with normal use and maintenance, or 20 seconds without water

The missing chunks of impeller will get stuck in the system and block flow

You pull things apart until you find the missing pieces

All overheating is a lack of water flow
 

steelers7

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I'll have to research what that circulation pump does. Circulates, I guess. But with only 1 hose off of it I am curious about the flow

Still also curious what mechanism is failing here to cause this intermittent pumping

From looking at it with advised eyes now, seems like if the circulation pump failed the raw water pump would still be sucking water out of that bucket, no? Which would indicate the problem lies with that raw water pump still? Maybe i fried the new rubber impeller already, maybe the bearing is bad. Maybe belt is too loose is stopping spinning. Too late to fire now, would wake the neighbors, but some things to look for tomorrow
 

Scott Danforth

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I'll have to research what that circulation pump does. Circulates, I guess. But with only 1 hose off of it I am curious about the flow

Still also curious what mechanism is failing here to cause this intermittent pumping

From looking at it with advised eyes now, seems like if the circulation pump failed the raw water pump would still be sucking water out of that bucket, no?
Raw water pumps do not "suck". They only push.

They are a flooded inlet rubber vane pump

Unless your bucket is big enough to put the whole boat in it, you toasted your new impeller

Always use muffs or fake-lake if on land

Your circulation pump circulllates water within the block and heads
 

steelers7

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I take the hose that goes to the raw water pump, put that end in a bucket, then put the garden hose in the bucket, hose fills bucket and engine draws water from bucket.

If circulation pump failed completely, would the raw water pump still push water?
 

Scott Danforth

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The bucket would need to be 8" above the impeller and have a bottom fitting to be able to flood it to prime

Again, raw water pump can not suck/draw water any distance

You toasted your impeller

Your circulation pump could be completely removed and the raw water pump would still pump assuming you have a flooded inlet to prime and you didn't burn your impeller
 

Scott Danforth

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Only a positive displacement ball & check pump or a piston and check pump can lift water
 

steelers7

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I'm not understanding
I'm no where near a boat expert, but this is the method many use for starting an inboard on land.
Something draws that water from the bucket, if not the raw water pump, then what?
And in the water, the raw water inlet is on the bottom of the boat, probably feet below where the pump is mounted on the engine
 

Scott Danforth

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You must use a fake-lake on a hose with an inboard

Or a fresh-water flush fitting

You need to replace the impeller you toasted

There is no pump in the boat that can draw water. They all only push water

When the boat is in the water, the level of the pump is 8" or more below the surface of the water. This is what floods the pump. Once moving the pickup acts as a scoop to help move water to the impeller
 

steelers7

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I think I'm doing a bad job at describing the bucket method

Fake a lake placed on water pickup outside boat, garden hose keeps plunger basin filled with water for engine

I take hose off of water pickup inside boat, put it in a bucket, garden hose fills bucket with water

Other than the position of the basin the hose fills, the same thing is happening

Nevertheless, learned a lot from this conversation, thank you

Hoping you are right and the impeller is fried, will report back once im able to check

If not the impeller it's sounding like the problem is still with that raw water pump somewhere
 

Scott Danforth

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I think I'm doing a bad job at describing the bucket method

Fake a lake placed on water pickup outside boat, garden hose keeps plunger basin filled with water for engine

I take hose off of water pickup inside boat, put it in a bucket, garden hose fills bucket with water

Other than the position of the basin the hose fills, the same thing is happening

Nevertheless, learned a lot from this conversation, thank you

Hoping you are right and the impeller is fried, will report back once im able to check

If not the impeller it's sounding like the problem is still with that raw water pump somewhere
You are describing toasting your impeller perfectly

Take the bucket, put ice and beer in it. However quit using it like you are

Install new impeller, use dish soap or ky jelly to lube

When you need to run on the trailer, connect fake lake under boat and turn hose on full. Then start the boat

Leave the hoses connected
 
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