Underheating 5.0 gxib

[BRICH1260]

Hey guys, I am continuing to chase an Underheating problem with my VP 5.0 gxi-b engine. Ever since I accquired this boat my boat never seems to get up to the 160 normal operating temp. My guage seems to stay in the 120-130 area, no matter how long or how hard the engine is ran. In fact the faster I go, the cooler the engine reads. It only slightly heats up at long periods of idle.

I have replaced the thermostat twice with a VP 160 degree thermostat, installed it in the correct position with the rubber gasket. I have replaced the water pump. I have replaced the temp sending unit. I have verified the guage by grounding it against the block. I have used an IF thermometer on the engine while running and it`s reading mimmick the guage`s readings. The engine starts quick and runs smooth with no performance issues.

Obviously, an underheating problem is better than an overheating problem, but I am at my wits end trying to figure out why it does not ever get up to normal operating temperature. Any suggestions to try based on the information above. Thanks.

 

[Don S]

Re: Underheating 5.0 gxib

There is what is called a bypass hole in the thermostat housing (Item C in the file below) that can get plugged up by pieces of old dead impellers, sea life, and or rust. When it gets plugged up or partially plugged up the pressure from the raw water pump keeps the thermostat open and the engine isn't allowed to warm up. Pull the housing and check the hole.


View attachment 7748163 Cooling Systems SM 64.pdf

 

[dypcdiver]

Re: Underheating 5.0 gxib

I had exactly the same problems with my engine some years back and it was just as has Don said.
You will need a flexible piece of wire to push through the bypass port. I could not see the bit of rubber that was stuck, but it could be detected with the wire and eventually pushed out.
Engine runs great now at 160 degrees and gets up to temperature much quicker.

 

[BRICH1260]

Re: Underheating 5.0 gxib

Thanks guys, I will pull the housing tonight and check that passage.

 

[The_Other_Dave]

Re: Underheating 5.0 gxib

There is what is called a bypass hole in the thermostat housing (Item C in the file below) that can get plugged up by pieces of old dead impellers, sea life, and or rust. When it gets plugged up or partially plugged up the pressure from the raw water pump keeps the thermostat open and the engine isn't allowed to warm up. Pull the housing and check the hole.


View attachment 202594

Don,

Based on that drawing I am assuming that most VP engines of that vintage (not just the 5.0) have this bypass.

That looks exactly like the thermostat housing on my 4.3GL (vintage and info in my sig) which I am wondering
if it is having a little of this same problem. It will get up to op temp but whoa it takes a long time and has to
be flogged to do it. At normal cruse 135 deg is all it will get to.....also changed my thermostat this spring (old one was
broken) and have verified gauge operation.

Thanks,

Dave

 

[BRICH1260]

Re: Underheating 5.0 gxib

You guys were right. I took the thermostat housing off the block last night and poked around in it. I found four separate piecies of rubber water impeller inside plugging up the by-pass hole. I have not yet run the engine, will take to lake tomorrow, but I have the feeling that this is what has been causing my underheating problem. Guys thanks for your help.