The Stovebolt Site Forums The Shop Area Engine Shop Cooling issue on a 261

I started my truck today and let it run for a while and I noticed it was starting to get hot. Tempature gauge went up to 212 degres. I shut it off. I noticed at this point, the top cooling hose was not hot and the radiator wasn't hot either. I twisted the radiator cap and it was under pressure but not hot. help?

This was the first time I had run it for an extended period of time and it was mostly so I could torque down the manifold bolts. Any idea as to why the top hose wasn't hot? It didn't seem as if the water was flowing into the radiator. BTW: It didn't have a thermostat in it when I got it but it has a 160 in it now.

VA ,
next time ya start it up leave off the cap an top off the water level , after it warms up an the thermostat opens ya should see a drop in level an water movin .
Sounds like ya a simple air lock at the water pump

Do you have the original 261 thermostat housing for the special by-pass thermostat with additional outlet?
If so, how are your hose connections routed?

On the road again: What? It has a big housing but it I don't know about special... I don't have any extra hoses...

Have you tested the thermostat outside of its housing in a cup/bowl/tin of hot water, to see if it is opening?

This has nothing to do with your problem, but the 261 thermostat came with a hole in the flat "edge" to let a little fluid leak through at all time. Most modern thermostats do not have this hole and people drill a 1/8" hole in the edge (outer ring) of the thermostat.

No I didn't test it I just put it in the truck... I did notice that it was a difficult fit as I couldn't get it to fit but one way. Is there a special 261 type thermostat?

OK I've searched the archives and see the extra holes and hoses. I don't have any of this hooked up. Mine is just like a regular thermostat. Do I have to have all the extra hoses? Where does that fit with the intake and exhaust manifold?

Virginiafiretruck,

You probably already know these facts. The thermostat is directional. If has to go in with the spring or pellet facing the 'hot side' (I think).

Coolant flow is from radiator through bottom hose and engine, out through top hose into tank.

If the system is blocked...thermostat not open, coolant cannot circulate through upper hose into the tank.

If the thermostat is stuck, in backwards, or sitting in a air pocket, it can't open. I can see no reason a brand new thermostat wouldn't function properly. Lots of folks toss them in a pan of hot water on the stove just to verify that they are opening..ye of little faith!

Check things out and give it another try...'good on you' for watching the temp gauge!!

Stuart

Ive gotten brand new thermostats before that didnt work. Maybe you got a crappy one? Sure sounds like a thermostat issue though...

Thanks. What happens if I just remove it like it was when I got it? If I remember it correctly, there was some type of mount that the thermostat fit into when I put it in the truck. It seems to me "if" I'm remembering it correctly, the spring was facing up away from the engine block as that was the only way I could get it to fit. And I remember saying to myself, "that doesn't look right" but it was the only way it would fit. So do I need to drill a small hole in it also? I thought I was past this part of the restoration and onto the fun stuff... Nope but I'll get it right even if I've got to back up a few steps. BTW: I have an extra thermostat in my garage so maybe I'll put it a pan of water and see if it works.

Sounds like you installed a standard thermostat for a 235 engine and upside-down.

The original larger 261 bypass housing requires a special by-pass thermostat, if you want to connect it as stock.
With a regular thermostat (and stock hose connections), if it even fits and is installed with the right direction of flow, you would still have a large portion of hot coolant bypass the radiator.
With a regular thermostat, you should remove the stock bypass hose that connects directly to the waterpump and plug both ends.

Virginiafiretruck,

It will not make the world stop if you just pull it, figure it out down the road and move on to some task that screams "FUN"!

Stuart

I want to get it right even if it isn't screaming fun... There are metal plugs on the "special 261 housing" and on the water pump. Is that what you mean? What is the outcome of using the regular 235 thermostat? Where do I find a 261 special thermostat? The 261 in the firetruck was added later in the sixty's so it has been a pain to address the issues so far. I'm happy with it but I wish I knew this stuff when I put the thermostat in. I just figured it didn't have one so if I put one in a 235 would work just fine like in all my other cars... Wrong answer. Thanks for taking the time with me on this one.

Good - the plugs mean somebody already set it up for a standard 235 thermostat.
Just check if the thermostat is installed correctly.
An original working 261 bypass thermostat is practically impossible to find.

Ok. Thanks. So does the spring part of the thermostat go toward the head or away toward the top hose? I just picked up the thermostat in my garage (180 degree version for a 235 motor) and I'm got a hot pot of water to see if it will open. Ok that worked as I see how it opens. Do I need to drill hole in it? I can se the thing is labeled so the spring goes toward the head and not the hose.

I'm going to get this fixed as I really enjoyed sitting in the truck listenng to it run. Might as well do it now before I get the hood and all the fenders back on. Again thanks.

Yahoooo, it's marked for direction, so no mistaking that. Drilling the hole in the flange is a 'shade tree trick' to allow it to pass any trapped air and a trickle of coolant to insure smooth operation.

Tim Bloom recommended leaving the radiator cap loose until the patient stabilizes, this is a great idea. Try it again and it will work.

Stuart

I'll give it all a try. I can't believe I screwed this simple part up. I've put these things in my old cars without a problem...

All that matters is that it works and doesn't over heat in a parade. Which degree thermostat is better with a heater in the truck: 180 or the 160? I've got both...

If the cooling system is top notch, the thermostat should be able regulate coolant temperature pretty close to the chosen value.

The 261 is a newer engine and 'probably' designed to run at the higher temp...180! Parade duty can be tough on the rigs cooling system unless it is clean and working well.

The fact that the truck has a heater should have no bearing on thermostat selection.

If the tubes in the radiator are partially plugged and the block is cruddy from rust and such, be prepared to 'steam' your way through the parade route.

Stuart

I had it all cleaned out at the local radiator shop and it is under warranty. Motor all cleaned and boiled out too. New water pump too.

Just a question: I turned on the heater for the first time allowing water to flow thru it. Could I have got air from the heater lines/core to the thermostat? It might just be a connection...

Virginiafiretruck,

I feel like I'm on a chat line. That is a possibility. When a fresh engine is filled for the first time, the system will invariably have air pockets and bubbles in it. This is part of the mentality behind drilling the little hole in the thermostat flange.

The hole allows trapped air and small amounts of coolant to sneak by and purge the system. Your heater would have to have a shutoff valve of some sort in one of the lines, or it would be in the loop at all times.

What happened to you with the over temp is one of the most common ailments of a new start-up. It is VERY minor, you were sharp enough to catch it, and it is probably cured as I type.

Stuart

It does seem like a chatline tonight. Anyway Thanks for the info. I'm going to take a look at it again and make sure it is working properly. Soon I'm going to pull the thermosat just to make sure it is in correctly and replace the 261 special housing with a 235 housing. I's just like to get a better fit before I put it all back together.

Honestly, I really appreciate the help. Thanks again and take care.

VAfiretruck.

I had a similar issue with my 261 at first firing, My issue was the fan belt was not tight enough to spin the waterpump fast enough at idle.

It is worth checking out...

Thanks. I'll check on that too. First thing is remove radiator cap and let her run. I'd like to see if the thermostat is opening or not.

VFT, I think you ought to stick with the original 261 setup. It was designed to eliminate a very common 235 problem of hot spots from steam when warming up.
This is from an article in the tech tips section of inliners international: 'In its pure big truck form the 261 had a larger thermostat housing holding a double acting thermostat: This is designed to circulate water through the block and head before the thermostat opens to allow hot water into the radiator. Thus, no internal steam hot spots during warm ups, expecially in winter. This is particularly important with the very cold coolant. Vital engine spots can become very hot before the total coolant becomes hot enough to open a normal thermostat on the front of the block.' You can tell its a thermo housing from a 261 if it has two holes drilled for the smaller hose fittings in addition to the large one that feeds from the radiator. It is slightly larger than the 235 housing and it contains a hard-to-find by-pass thermostat."
Jerry

I understand that part but I can't sem to locate the double acting thermostat nor do I have the hoses to connect it all as it originally was built. It came back to me without a thermostat installed so I figured it was an improvement just to put one in.

I'm going down to the garage this morning to fire it up again and watch to see if the thermostat is opening. Now that I have a better idea of how it all works (or should work) I can anaylize the situation and figure out what to do next.

I ran it this morning but it would not open the thermostat. So I cut it off and I'll work on it next week. Thanks for all the help!

I found the bypass thermostat at Parts Plus. The parts guy had the sense to look in the book for big trucks or maybe it was commercial trucks. Hard to remember, it was 2 years ago. I'll try to find a part # tonight. You can get fittings and hose to make the bypass hose at any parts store.

I couldn't find my old receipts so I called the store. Stant #13926. I've ordered a back-up. I'll know for sure if this is correct when I pick up the part. It is listed in their commercial truck application for a 60 series truck.

My 1960 shop manual shows part # DV50 and is listed as a dole pellet bypass thermostat.

Thanks for the info.

Today I had to pull the radiator due to leaking in the top. I took it to the radiator shop for repair but it still leaked after it was repaired. I ordered a replacement radiator this evening...

Also I pulled the "special" 261 thermostat housing and put in a 235 single action housing and thermostat. I hope this will make the cooling system reliable on the road. Picture link below: 1951 firetruck picture

New radiator (Classic Parts ie: Old Chevy Duty)arrived today so I go to install it... Doesn't fit... Too big... Back to the drawing board.

Sometimes it gets so frustrating that I just want to quit. Thanks for letting me complain.

Update: I had the new radiator mounting brackets moved to provide clearance for the fan blade and it fits. It is much longer than the old radiator and it covers the handcrank starter hole but I guess that larger radiator has a benefit. It actually keeps the truck operating tempature cooler than the originial radiator had before. Anyway, it ran for several hours today without over heating. I drove it around most of the afternoon and it was nice to finally get it back on the road. Thanks for all your assistance.

Bill

The way to tell it the therm not working(((install with spring or bellows toward engine head or block or heat source..If there is no bypass hose or other source for coolant to move past therm drill therm at outer area...Remove therm and run engine to see if you have coolant movement thru radiator if none look at water pump,,lower hose,,belts....doc

It's working but the truck isn't running hot at all.