What is the purpose of the bypass hose

fastdave · Mar 11, 2007

What is the purpose of the bypass hose around the thermostat on a 65 289? When I drive on cold mornings I have to put a piece of cardboard in front of the radiator to keep it warm. When I don't and I let it warm up enough for the thermostat to open, it will still cool down once I am on the freeway. I have had 4 different thermostats in it so I Know that isn't the problem. That is why I am thinking it has something to do with the bypass around the radiator.

D.Hearne · Mar 11, 2007

It's there to keep the water pump from cavitating and running dry. If you're having trouble keeping it warm, you need to change to a clutch fan. So it won't draw air with the fan running. You may still need to cover the radiator anyway. For sure you want it covered if you're driving while it's snowing to keep from overheating.

Pbum5 · Mar 11, 2007

It keeps a small amount of water flowing when the thermostat is closed.

What temperature is your car running at?

fastdave · Mar 11, 2007

I am running an electric fan that I have set to come on @ 200 degrees. I have a 190 thermostat. If it is cool outside the engine temp may run around 160-170. Luckly I live in Phoenix so I only worry about it 3-4 months of the year.

fastdave · May 9, 2007

I bought a Ron Davis radiator and I talked to them about having to put something in front of the radiator in the winter to keep it warm. He said I could drill a couple of holes in the thermostat and plug up the bypass port. Have you heard of doing this?

D.Hearne · May 9, 2007

What's the point in that?

zookeeper · May 9, 2007

D.Hearne said:
What's the point in that?

I second that. While I'm all for modifying anything, I really don't see where you're gaining anything at all.

j69302 · May 9, 2007

doesnt the bypass hose route water back to the inlet side of the pump bypassing the radiator altogether?

fastdave · May 9, 2007

I will have to look @ my thermostat housing when I get home. But I thought that it allowed some water to bypass the thermostat alowing some water to go into the radiator. By drilling a couple of small holes in it, that will allow a smaller amount of water to bypass but still preventing the pump from cavating. NOT positive about this. BUT.. the main reason is I have trouble maintaining temp in the winter unless I put something in front of the radiator to restrict the air flow. Kind of like a race car that puts tape on the air inlet to allow it to run @ normal wtr temp. ANY ideas or help would be greatly apricated. Also what is the best temp to run 180 or 190? I found some stock thermostat housings with a "vacuum port" hole on top of it on the downstream side of the thermostat. Any idea what this is for?

D.Hearne · May 9, 2007

Plugging the bypass passage and then drilling holes in the T-Stat housing serves the same purpose, but I'd be concerned with the pump not getting enough coolant to lubricate it, or rather keep it cool enough at operating speeds. You can also install a 205 degree T-Stat to help in winter. If you're concerned with the "cardboard look", take something like plastic sheet stock and paint it black and place it on the radiator front to keep the engine warm enough. There's nothing wrong with this method, it's been done with heavy trucks for decades. There's so many ways to cover the front other than cardboard.

ozstang65 · May 14, 2007

OK, here's how it all works.

All the hoses that attach to the water pump are on the low pressure side of the impeller. When the thermostat is closed, 2 things need to happen, water needs to circulate in the block to evenly warm up and water needs to be able to circulate in your inside heater core.

The water pump pressurises the block, a fitting on the rear(?) of your intake manifold lets water into your inside heater core which returns via the hose to the bigger of the 2 small pipes in the water pump. If your heater core is switched off, the water pump still needs a circuit to keep water moving in the block and to prevent cavitation. This is achieved by the 'bypass' pipe between the manifold and the water pump. The bypass has the added function of moving the b-heated water past the thermostat which will open if necessary.

Once the thermostat opens, the water tends to follow the path of least resistance, so although you'll still get some flow through the 'bypass' line, the majority will flow out the thermostat housing into the radiator.

If your thermostat creates a complete seal when it is closed, the bypass is very important as it lets the air bleed out of the system as you are filling it. A couple of holes in the thermostat might suffice to let some water through, but remember, the water pump is pretty big. If you give your motor a big spin while the thermostat is closed you'll run a high chance of cavitation if it can't get enough flow.

So I guess i'm trying to say that the bypass line will not be the cause of your 'underheating' problem, because even when its fully flowing, it is circulating water in the block only. The only way you can be cooling that water is if it is going through the radiator or the interior heater.

Platonic Solid · May 14, 2007

fastdave said:
... When I drive on cold mornings I have to put a piece of cardboard in front of the radiator to keep it warm. When I don't and I let it warm up enough for the thermostat to open, it will still cool down once I am on the freeway. I have had 4 different thermostats in it so I Know that isn't the problem. ...

Are you basing this on an accurate temperature gage or just by the fact that you aren't getting enough heat inside the car? Are you sure there are no problems with your heater core, control valve, vent flaps, fan motor, cowl rust, door seals, floor rust ...

mustangdave · May 14, 2007

I would replace the temp. sender first and if the gauge still read that cool, suspect a bad gauge. In all the yrs. I've been visiting this site, you are the first to complain of running too cool.

ForceFed70 · May 14, 2007

Here's my 2 bits...

- It's important to remember that the coolant flowing through the bypass hose and the heater core are recirculated through the block only... NOT the radiator.

- I second the question about how you are measuring temp.

- Your engine should have no problem staying warm. Putting a piece of cardboard treats the symptom, not the problem. If your temp readings are accurate, you have a problem elsewhere. I'd still guess thermostat or housing seal.

Jimmys66 · May 14, 2007

He's not the only one. On a 65-70 deg day mine won't get warm either, even with the electric fan off. Mine will run 150-160ish forever unless I get into town and do alot of light setting. Once it gets hot, it will stay at at 175 to 180. If I through it back out onto the highway it'll go back to 160.

I changed 3 thrermos out before I got sick of it and left it alone.

Running an electric fan,180 thermo, alum. radiator and a high volume water pump.

D.Hearne · May 14, 2007

ozstang65 said:
If your thermostat creates a complete seal when it is closed, the bypass is very important as it lets the air bleed out of the system as you are filling it.

Noooo...... it don't!! I just observed this phenomenon yesterday. Started filling the 331 with water to fire it up after a couple year slumber. Block was completely dry. Filled the radiator, cranked the engine up. Ran it for several minutes and nothing happened, water level refused to drop. I ended up loosening the temp sender retaining nut to let the air out of the system so I could add more water.

ozstang65 · May 15, 2007

Yeah, now that methinks more about it, if your thermostat is creating a seal NOTHING is getting past it! therefore no venting. Although the bypass will vent air trapped in the water pump during filling.

Apart from that I stand by everything else i said and hereby advocate a small hole in the thermostat to vent the air while filling. In fact a lot of thermostats i see these days have a small hole in them, with a tethered plug that blocks the hole somehow during use.

dennis112 · May 15, 2007

ozstang65 said:
Yeah, now that methinks more about it, if your thermostat is creating a seal NOTHING is getting past it! therefore no venting. Although the bypass will vent air trapped in the water pump during filling.

Apart from that I stand by everything else i said and hereby advocate a small hole in the thermostat to vent the air while filling. In fact a lot of thermostats i see these days have a small hole in them, with a tethered plug that blocks the hole somehow during use.

I started doing this to every thermostat I install in any vehicle. It seems to eliminate the need to burb the motor when started and less water/antifreeze is needed during the final fill when the block is warm.

fastdave · May 15, 2007

I am using an Auto meter gauge, It seems accurate,as I can watch the temp rise and actully see it drop when the thermostat opens. I completly trust it. I do burp the engine via the hole that the temp prope screws into the intake manifold. When I do put a piece of cardboard in front of the radiator to block the arirflow the temp will rise and maintain. Like D.Hearne says, trucks have been doing it for years. I have run 3 different thermostats w/ different temp settings and different manufacturers. Its not that big a deal I am just trying to understand what is going on. It has to be getting cooled by the radiator because once the cardboard is in front of it the temp will increase. I just dont like to run the engine too cool. And it is just a little pain to have to cover the rad each morning and uncover it each afternoon until the ambient temp stays above 75. Hey guys, thanks for all of your input. It has given me plenty to think about.

ozstang65 · May 15, 2007

If that's the case then I'd say that your engine is at the right temp and your gauge is reporting inaccurately. Is there any way of getting hold of a mechanical gauge or a thermometer to check what the actual temp of the coolant is?

What is the purpose of the bypass hose

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My rearend needs a stud and two nuts.

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That's why they call it "dope"

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