VP 5.7 GSi PEFS - temp 160 at idle, 110 at speed

[YYZ-RC]

Hello,

First post here, hoping for some advice.

I have a 2000 Four Winns with the 5.7 Volvo Penta GSi PEFS. At idle after initial warm-up, the motor will run at around 160-170 degrees. As soon as the boat is underway at over 1000rpm, the temperature gradually drops to around 110 after a few minutes of cruising and will not get back up to 160 until the rpms drop below 1000 again. I have used an IR thermometer to verify that the gauge is accurate.

I recently replaced the thermostat and installed the 160 degree thermostat part # 3856961 (OEM VP). This did not fix the issue.

I searched iBoats and found a thread referencing the bypass valve being obstructed with chunks of impeller; I checked the thermostat housing including the 90 degree bypass elbow for obstructions and did not find anything. I have also recently replaced the impeller and there were no missing vanes.

What should I check next?!

Thank you.

 

[alldodge]

Pull the thermostat and verify it is installed correctly. See that when cold and closed it does seal off closed. Look on the bottom of the thermostat and verify it has 160 stamped on it.

If all checks out, place thermostat in a pan of water with a thermometer and heat the water up. See that it does not open until it gets close to 160

 

[YYZ-RC]

Thank you for the reply. I just installed the thermostat last week; brand new original Volvo Penta part and installed correctly. Stamped 160. The old thermostat (140) behaved the exact same way except with the lower temp threshold so I am relatively confident that the issue is not the thermostat itself.

 

[alldodge]

See that when cold and closed it does seal off closed.

Or with a bit better grammar, When the thermostat is installed in the thermostat housing, does it close off the water coming from the block?

 

[YYZ-RC]

I disconnected the cooling hose to the port riser and started the engine. Water was immediately flowing - I assume that when the thermostat is closed there should be no cooling to the risers?

I also pulled the thermostat and tested it, it works fine.

 

[alldodge]

There should always be water going to the risers, the thermostat restricts flow to the block until it reaches temp

This is not yours and only look at the Stat and the top exhaust connections of how water flows.

The left shows the Stat closed and only a small amount of water flows out of the block (thru small hole) and out the exhaust. The small hole is there so air can escape it it gets trapped.

The right shows the Stat open and water flows out of the block and into the exhaust. The block water is replaced by incoming cooler water

So if your Stat is not closin off the flow out of the block, it will stay cool

 

[YYZ-RC]

The part that doesn't make sense to me is that the thermostat housing is essentially two chambers. The lower chamber has four inlets/outlets: #8 (hose to power steering cooler), #23 (hose to circulating pump), #24 (bypass to cool fuel system) as well as the access port to the block. The upper chamber is just the two outlets to the manifolds. The thermostat separates the two chambers.

My expectation is that when the thermostat is closed, the engine isn't pulling in much raw water since it has nowhere to go (except whatever leaks around the thermostat), and therefore the upper chamber is relatively sealed. When the thermostat opens, the raw water would flow more readily since the raw water is now flowing through the block and up through the thermostat and thus out the manifolds.

Obviously I'm wrong so what am I missing? Is it possible to have air trapped in the system that might cause this?

Thanks!

 

[Scott Danforth]

The thermostat is being used as a mixing valve. Water is always flowing to the exhaust, however water thru the block and heads is regulated at temperature, opening and closing as needed (adding water) to maintain block temp

 

[alldodge]

Recheck hose connections to see if one is in the wrong spot

 

[YYZ-RC]

Recheck hose connections to see if one is in the wrong spot

I did; they are all correct (3 of the 5 hoses are the same diameter and it would be obvious if they were switched).

I found another post where someone had the same issue and they solved it by cleaning the mating surface between the thermostat and the housing; I did this when I was working on the boat yesterday but I didn't get a chance to run it. If that doesn't solve the issue, I will try replacing the entire housing. I suspect it was once a salt water boat and the housing is quite rusty.