I just installed the engine I rebuilt and started to break in the cam. I ran initially with the radiator cap off, then put the cap on as I reached 190*. After a few minutes of running at 2500 rpm, the engine started to overheat, so I shut it off. The moment I shut it off, the radiator started violently spewing fluid out of the overflow. I shut it off, let it cool to 180*, then went back to try again after topping off the radiator. This time, I left the cap off. As soon as I started the engine, the temperature dropped to 160*, indicating that the cooler fluid that was sitting in the radiator had flowed into the engine. After a minute, the temperature climbed to 230* and I shut it off again. This time, without the cap ever having been put on, in spewed like a volcano and drenched my rafters. Why am I overheating, and why does the radiator only shoot fluid once the engine is turned off? I am stumped!

 

Well, I ain't no expert and I'd certainly wait around for some more opinions. That said though I can tell you that temperatures typically rise once you shut down a motor. The air is not passing over the radiator as it was when the motor was on so you get a brief temperature Spike when she shut down. That impart good cause the volcano.

May be a stupid question but are you running straight water or 50/50 with coolant?

Lastly, when you rebuild the motor did you have it dunked? As in did you have it boiled out to make sure all the crud was out of every nook and cranny?

 

Did you replace the thermostat? If the old one that once worked was sitting around and you re-used it, it's probably bad.

 

The thermostat is new-ish. Less than a year old. Since the thermostat attaches to the intake on a 460, I did not disconnect the thermostat at any point during the rebuild. Just pulled the intake and set it on the bench until I was ready to put it back. I have a box fan in front of the truck blowing on the radiator.

 

I am using a 50/50 mix of coolant and distilled water. The engine was hot-tanked when I took it in for machine work. I did the final engine cleaning myself after it was machined.

 

I would think that since the temperature dropped from 190 to 160 the instant I started the engine for the second time, that the thermostat was open and allowed coolant to flow into the engine. Am I thinking about that wrong?

 

Well, it might be a PITA but could you take it out and check it in a pot of hot/boiling water? I only say this because it happened to me - same hot water spouting like a geyser as you. Engine got too hot during break in. Put in a new thermostat and everything was good after that.

Just humor a grumpy old goat....if you're sure the T stat is good, please disregard....hahaha.

Did you pull the old water jacket plugs (not the freeze plugs but the square threaded ones) and clean out the block's innards?

 

No, you're right about that...do you have a fan shroud?

 

No fan shroud....there is a body lift so the fan is slightly offset from the radiator. Then again, this is the same setup I had before the rebuild and it never overheated. Is there a reason a new radiator would be the issue? I have a box fan sitting in front of the radiator, doing the break-in inside the garage.

 

I will take a stab at this check the thermostat and how tight is the engine I know it is a fresh rebuild but maybe some of the clearances are to tight, on the violent eruption here is my theory. While the engine is running the water pump is keeping the flow going throughout the engine and lets say keeps the water in the engine when you shut the engine off well the water is no longer moving and maybe being on the point of boiling anyway shoots out the path of least resistance as it boils.

 

I think I left my t-stat out when I broke my engine in, and also kept the water hose on just if it decided to overheat.

I've read where some have sprayed the radiator down while breaking in the engine with assistance of a helper.

 

My plan of action is to advance the timing, set the carb a bit richer, pull the truck into the driveway and have the wife spray down the radiator while it runs. Those seem to be the top three to try, according to the internet.

 

I'm voting that the lack of fan shroud is a likely culprit. Had a similar situation in the past during engine break in. That's the reason I only run water during a cam break in, less mess. We got around it by running a garden hose through a PVC adapter into the lower hose and just flushing the block with continuously freshwater for the break in. 8 years later the trucks still going strong.

 

What type of head gaskets did you use?
Is "front" front and only one facing 'up'?

The only time I've ever run into this is when my buddy put both gaskets facing up in his Bronco.
He went crazy with bigger radiators and high flow water pumps before he finally tore it back down.

Infrared guns are cheap now.
Makes it easy to find a hot spot.
There obviously is one, or it wouldn't be a volcano every time you shut it off.

 

Did you replace the water pump. If you did. Did you put the metal plate between the block and pump. I have seen that before and will give you some really strange heat issues. And yes. If motor is tight they will run warm. And with no shroud and the fan not lined up with radiator. Then expect heat issues. So do the water on the radiator routine until you are ready to drive it. And if the timing is late they will run hot. So check the timing get it set and do your break in. If you added a zinc supplement to the oil then the break in is not that huge an issue. I never did like starting a new engine and running at 2000 + rpms for 30 mins. I prefer starting setting the timing then go drive them vary the speed and rpms up down. The biggest issue is if it has a new cam and lifters they need the zinc either in the oil or a coating of cam lube at installation.