470 cooling question.

[bently]

What has been dones:

upgrade frome 3" to 4" heat exchanger, last year
new exhaust flappers (should not affect cooling)
removed and cleaned riser - no restrictions
new new rubber boot when replacing flappers
new water pump impeller kit with new housing
removed and inspected upper housing and no heat damage
cleaned all water passages so no parts of old impeller left stuck anywhere from PO, I have not replaced an impeller yet with fins missing

Issue

Engine starts runs up to temp 160 ish on gauge, but on idle it will slowly creep up to 190ish. If I increase throtle up off idle it will slowly drop down to 170ish on the temp gauge. I have a clear tube that runns from the transom to the inlet to the heat exchanger and can see that it is full of water and it is moving through the tube. I have used a lazer heat gun on the engine and can get readings of up to 168 to 174 but I know those are just surface temps and the internal water is hotter. My gauge could be off by 10 to 15 degrees but that does not explain the rise in temp on idle. The old impeller was new end of last year and looked good when I replaced it earlier this week and ran to find the same running conditions.

What could I be missing?

 

[stonyloam]

Does it hold temperature after dropping down from 190? If so I would suspect the thermostat is opening slowly for some reason. Might want to check it.

 

[bently]

Once the temp drops and I put the throtle back to idle she will slowly start to climb again. I have not done anything with the thermostat and it could be the weak link as it could be weak and openning slow like you are suggesting.

 

[alldodge]

What has been dones:

upgrade frome 3" to 4" heat exchanger, last year
new exhaust flappers (should not affect cooling)
removed and cleaned riser - no restrictions
new new rubber boot when replacing flappers
new water pump impeller kit with new housing
removed and inspected upper housing and no heat damage
cleaned all water passages so no parts of old impeller left stuck anywhere from PO, I have not replaced an impeller yet with fins missing

Issue

Engine starts runs up to temp 160 ish on gauge, but on idle it will slowly creep up to 190ish. If I increase throtle up off idle it will slowly drop down to 170ish on the temp gauge. I have a clear tube that runns from the transom to the inlet to the heat exchanger and can see that it is full of water and it is moving through the tube. I have used a lazer heat gun on the engine and can get readings of up to 168 to 174 but I know those are just surface temps and the internal water is hotter. My gauge could be off by 10 to 15 degrees but that does not explain the rise in temp on idle. The old impeller was new end of last year and looked good when I replaced it earlier this week and ran to find the same running conditions.

What could I be missing?

Just throwing something out, wonder if in this case you don't have enough restriction do to increasing the size of the heat exchanger (HEX). I'm guessing there was no previous issue until old HEX was replaced. Now there is to much heat, maybe you have been unable to move enough water to fill the larger HEX. If this is the case maybe reroute the exiting hose higher to make sure the HEX is full. If this does work there may be an issue with the exhaust getting hotter, don't know just spit balling

 

[Scott Danforth]

step 1 - verify nothing is plugged between the raw water pump and heat exchanger (like the power steering cooler)
step 2 - with the motor fairly cool, disconnect your clear line, and with the motor running just above 1000 RPM, you should fill a 5 gallon bucket in about 16 seconds.

FYI - the raw water pump behind the 470 pushes about 20 GPM at 20 PSI. more than enough for either the 3" or 4" HX. unless there is a restriction.

 

[dccordell]

step 2 - with the motor fairly cool, disconnect your clear line, and with the motor running just above 1000 RPM, you should fill a 5 gallon bucket in about 16 seconds.

Is it really this much? 5 gallons in 16 seconds?

Mine only does about 3 quarts in 15 seconds, which I thought was fairly accurate according to this document from Mercruiser: http://www.boatfix.com/merc/Bullet/82/82_04.pdf

Although to be honest, I did this test using muffs in the driveway, not on the water.

 

[dccordell]

Doh.. double post..

 

[stonyloam]

Is it really this much? 5 gallons in 16 seconds?

Mine only does about 3 quarts in 15 seconds, which I thought was fairly accurate according to this document from Mercruiser: http://www.boatfix.com/merc/Bullet/82/82_04.pdf

Although to be honest, I did this test using muffs in the driveway, not on the water.

Yeah at 1000 PM 3 qt, vs probably 5X that at WOT seems reasonable. The raw water just flows through the tubes and removes heat from the coolant, the thermostat controls engine temperature, as long as enough heat is removed by the raw water.

 

[bently]

Ok, from what I have just tested my impeller pumps the proper amount of water at 1000 rpm's. If I had a bad/weak thermostat, what would be the signs? this weekend the engine would run at the 190 to 200 range in 80 degree lake water.