The Stovebolt Site Forums The Shop Area Engine Shop chevy 261 still overheating!!

hey, ive had my 49 chevy 6400 runing for awhile now,ive gotten it to idle perfectly,ive got a new temp guage and sender, a new radiator and all the stuff thats related to cooling.Ive got a problem though.Im able to drive the truck down my street no problem and maintain a temp of about 180 but on the second round down the street, the engine starts to over heat and hit 220.My fan is still running,and the water pump is flowing water.Would getting rid of the thermostat be of any help? Just give me some suggestions please so i can move to the next step and get her on the big road! Thanks a lot.

Danny

Danny,

What year 261?

What temperature thermostat (did you "bench-test" the thermostat)?

If it is a 1955-2nd series through 1963 261, are you using the original water pump or a relocated 1955-1st or earlier water pump?

If you are using a relocated earlier water pump, you might have a block that has clogged internal water passages.

Other people will chime in with other helpful suggestions.

Tim

its a 53 261 with a short shaft pump and a 160 degree thermostat

Is the top rad hose getting warm? A temperature of 220 is a way too hot since it is not producing a lot of power going around the block. If the top rad hose is cool, you may have a big pocket of air. If it is warm I would suggest trying it without the thermostat as a start for trouble shooting.

Two items that I can think of, one I plan to do on my 261, and the other helped tremendously. First is the plate adapter to raise the water pump.

The other is to check the tension of the fan belt. There could be some slippage between the pulley and the belt making the pump less efficient. I did this with my truck and cooling improved.

Switch to a five or six blade fan to move more air. Cheap and an easy first try to fix the problem.

My 54 vintage 235 w/261 head had the same problem pulled the thermostat and no more issues.

im going to try and pull the thermostat i have a 10 blade fan 1300 cfm or more so its not the fan, both hoses get hot,and the belt is tight.hopefully the removing the thermostat help! Thanks a lot
Danny

Removing the thermostat will more likely make it overhaet faster. The thermostat in addition to controlling the coolant temparature also acts as a flow regulator. Without it, the coolant will flow too fast through the radiator, and will not be able to dissapate the heat effectively. If you do want to try runing without the thermostat, use this old Flathead trick. Install a fender washer with a roughly 1/2" hole. This will help regulate the flow, and prevent overheating.

Does "short shaft pump" mean a 1954 to 1955-1st water pump? Any chance you could post a photo of the front of the engine?

How did you identify the engine as from 1953? What are the head and block casting and date codes (here are the locations)?

If the water pump is mounted high (as in 54/55), and the fan belt is good, and the thermostat is good, and the radiator is good, your engine could be clogged up internally (mine was, especially at the rear of the block).

Tim

The theory of water moving too fast through the radiator has been denied in a number of professional articles. There is no rational scientific explanation for this theory. However, according to the literature, cavitation can occur in the pump and in the motor.

The flow has to be regulated. Otherwise, the heat transfer will not occure properly. The coolant has to stay in the radiator long enough to dissapate the heat.

JMO: Given the small size of the lines in the radiator, I'd think it would be long enough... Just my thoughts after watching it slowly flow.

At idle, yes. But once the rpm comes up, the flow increases greatly. That's why it needs regulated.
I'm more inclined to go with the incorrect fan positioning or plugged passages in the water jacket.

Got to admit: I've never had the chance to look at it as the rpm comes up! :-)

A cool running engine just doesn't happen. It looks like you've done a good job on your rebuild but I see a couple things that would contribute to your low speed cooling issues. Several have been touched above. IMO....using the "short-pump" and the "pusher-fan" set off alarm bells. I tried the "short-pump" route and soon learned that you needed the mechanical fan high on the radiator to move air where the water enters at it's highest temperature. The adaptor plate and correct "early" pump gave me clearance to run a mechanical fan and combined with a 6 blade fan (Summit FLX-1615P 15") moved the fan up higher on the radiator, gave me the proper waterpump rotation speed, and my cooling issues were history. Secondly, a pusher fan is not all that efficient, especially if mounted lower in the radiator. You're pushing air through about 1/3 of your radiators surface area.

So what would I do to correct the problem? Adaptor plate, early 216-235 pump with 5/8" belt(NAPA), 15"-6 blade fixed blade (no flex), 180 thermostat (with 1/8" hole drilled in outer area), 6-7 pound cap, burp the cooling system, use a 25/75 coolant mix with distilled water and fill the radiator only until you cover up the core.

With this setup if you are still having cooling issues it's not because your cooling system is mismatched.

Dave

Drill a small hole (3/32") in the thermostat which will allow any air to pass through the hole and allow water to contact the thermostat. The hole needs to be pretty close to the 'seat' of the thermostat. If you put the thermostat in boiling water you will be able to see how it opens and where the hole should be placed.

Also, if the head came off of a 235, you might need to add steam holes in the head. In a 261, the cylinders are siamesed and there are 1/4" holes near where the cylinders meet in the block. Use the head gasket as a pattern and drill steam holes in the head and you should be good to go.

heres my water pump set up.it was actually a long shaft that we shortened to a short shaft to clear the radiator.
http://i612.photobucket.com/albums/tt202/dantheman176/DSC04340.jpg
and heres the radiator fan set up
http://i612.photobucket.com/albums/tt202/dantheman176/DSC04460.jpg

i removed the thermostat and im going to testdrive down the street again and see what happens.hope i stay around 180.

danny

Good luck and let us know how it works out.

I tend to agree with Longbox on thermostat removal. The coolant can be moving to fast to properly absorb heat. I had this happen on a DPS car we were working on aeveral years ago. Also removing the t-stat will cause the engine not to warm up to running temp untill driven a long distance. If it were me I would find a lazor thermomature and locate the hot spot in the cooling system. These thermomatures can be a little pricey so if you have a professional mechanic buddy you might ask him if he or one of his co-workers have one.

Just so you know what you are (and are not) working on: based on the location of the water pump, the engine in your photos is not a 54/55 261. The earlier 261s (you said you are a 1953) would not need a water-pump adapter plate, because the water pump would be higher on earlier 261s. The 261 came into production in 1954 (but some might have been cast in 1953?).

If you post the casting codes (and/or engine serial number), you will find out the year of your engine (if you are interested).

As suggested above, the thermostat is an easy thing to check. The condition of your radiator is the next thing to consider. The adapter plate is the next thing to deal with. And finally, a clogged block is just about the last possibility.

Ya know you may have a faulty thermostat. I have found as many bad thermostats out the box as I have found in the engine. And as a side note the same go's for condensers.

Something I noticed in the pic of the pump, the pulley looks awfull small compared to other late 235/261 engines I've seen. Are you running the 216 pulley? That will overdrive the pump, causing cavitation/exsesively fast coolant flow.