Here is a tough one

[74 Carlson the II]

Late 70's early 80's mastercraft with a 351 ford, have a overheating problem. First off engine runs really good and is strong, the problem is with the thermostat installed it will over heat and the stat is a 140F degree. I tested the stat on the stove in a pan with the stat suspended in the water and useing my digital multimeter with temp capabilities. The stat starts to open at 142F. So with the stat in I can't get the temp below 180F and with it out it wont go above 165F. I am very reluctent on pulling the stat because thats not the way I roll, That egine at one point and time ran just fine with one installed so I really dont want to leave it out. The sea pump has a new impeller and gaskets, belt tension good, Circulation water pump good, back flushed from sea pump to the water pick up, flushed oil cooler. When it gets close to 220F I can rev the engine and get it down to 180F and at Idel it will get hot with in 2 minutes and with the stat out and I rev the engine it will go down to 120F.    Any Idea's. Thank you.   

[Rich_V174SS]

A few things come to mind. First recheck the impeller to make sure it's not spinning free of its hub which will reduce the amount of water it pumps. Also check the front and rear faces of the impeller housing to make sure they're smooth and flat, without grooves worn in them. If they're worn you won't get as good suction/pumping action. Next check the thermostat housing and exhaust manifolds/elbows for any rust blockage that can restrict the flow of water out of the engine. Restrictions in the exhaust water outlet passages will act similar to a thermostat because water can't exit fast enough to keep the engine cool. If they're plugged up they should be replaced. If they're getting hot when the engine is running that's an indication of not enough water flowing through them.

[74 Carlson the II]

Thank you Rich, I will try all in that order. I did inspect the pump for groves but didn't check flatness or for a spun impeller but will check again as it doesn't take that much time to pull it down. Thank you once again.

[Eric_Michael]

Stranger things have occurred.  I am not a Ford guy, so I can only speak in generalities....

I had a very similar problem with a stock Chevy 305.  Hot days it ran cool.  Cool days it ran hot.  Rev the engine and it would cool off.  Temperature would spike and then drop down.  Spike, and then drop.  Only on cool days, never in hot weather.  Supposition: sticking thermostat.

I had a row of 'stuck' thermostats lined up on the shelf.  I then went after everything else - radiator, hoses, etc.  I tried drilling 1/8" holes in the thermostats to allow for pressure equalization.  No luck.

Since everything else had already been replaced, I swapped out the water pump.  I put on a new 'Flow Kooler' brand high performance water pump.  Problem solved!

My point...  When all other possibilities have been eliminated, try something different...and use it as an opportunity to 'upgrade' parts!

HTH,
-Eric