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Flushing the engine block and radaitor


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OK, so I want to flush and drain the radiator and engine block.  I know there is a petcock at the lower bottom of the radiator. And this is used to drain the rad. On the engine block on the driver side is another petcock to drain the engine block.


I want to put into some Prestone radiator flush and cleaner and then run the engine up to temp. Then drain the rad. Then want to drain the engine block via the petcocks. And then refil with water to flush out the entire system prior to installing the new green antifreeze.


Question I have what is the best way to perform this maintenance:


1. Put in the flush cleaner and run the engine up to 160-180 to circulate the cleaner.

2. Dump the radiator and then open the petcock on the engine block.

3. Now when I refill the system, I assume that I fill the radiator to the top and then start the engine and the water will then start to flow back into the engine block OR do I need to remove the thermostat housing and the thermostat and then fill the system with water and or antifreeze.


Not real sure of the best way to perform this maintenance and not damage the engine.


Any and All advise is welcome.


Rich Hartung



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I pretty much do what you do, though there is a T fitting you can get that you splice into the heater hose.  This allows you to hook up a garden hose to "power flush" the block after the cleaner has had time to work.  Prestone flush and fill is the name. 


As for a refill, You don't need to pull the thermostat.  I use a device that pulls a vacuum in the cooling system and let's you refill the system under vacuum.  No air then. 

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sniper thanks for th einfo.  Since I have on the head a hole for the heater value to thread into the head I thought that I could also use this hole with some adapters to perform the back flush of the engine block.  I have the bypass external metal connector on my engine and I also have a manual shutoff valve and the rubber line runs back to the heater. This line would be the return line back to the water pump. 


If I used the hole in the head for the heater then I could use the garden hose to back flush the engine and open the petcock on the driver side of the block to flush out the block. Would this work?


Would that be an easier way to do the flush? Yes also have to drain the rad first and also flush that out with clean water. Then put inthe flushing and cleaning chemical and run the engine upto temp. Let the engine cool down then empty the radiator of water and then do the backflow flushing as described above. 


Rich Hartung

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18 minutes ago, desoto1939 said:

If I used the hole in the head for the heater then I could use the garden hose to back flush the engine and open the petcock on the driver side of the block to flush out the block. Would this work?


You could do that.  Sometimes the heater core needs flushed too, my 72 Dart had no heat till I flushed the core, several times in each direction with nothing more than a garden hose.  Heater worked great after that, hmm now that I think about the it the heater in my 51 is weak. 

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Remember to put the radiator cap back on on the initial start. There will likely be air in the system and this can cause a radiator eruption without the cap. Been there and done that. This was the first fire-up on my totally rebuilt ‘51 Dodge. It was immaculate under the hood before said eruption and took a lot of cleaning up. 

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I worry about modern flush products .... they may be too strong for a brass/copper radiator or heater core. .... I only feel this way, I have no proof they are.

Modern day products are made for modern vehicles.  ..... Photo is only a example of what modern products are designed for ... Aluminum & plastic.





I know nothing about prestone flush, I'm sure it is a fine product. I know Bar's leak was a excellent product 30 years ago. It was a brown liquid with little pellets in it that looked like rabbit ********. ..... Today it is changed & spouts it is liquid aluminum.  ....... My only point is, Prestone flush has been around just as long & I'm certain it has changed to work on modern cars.


I just felt more comfortable flushing my engine out with vinegar. Is a mild acidic rust remover, will not hurt my original radiator or heater core.

I just felt more comfortable using something slow & mild. I added 2 gallons and finished with water. I would start & run the engine through several heat cycles. I left it in there for a few weeks.

When I used my flushing T & flushed it out, it still came out pretty brown. So I did the same process again, next time I flushed it came out pretty clear.


This is not a 15 minute flush. If your antifreeze is still green with no brown streaks .... You are golden. A 15 minute flush is fine.

If you are seeing brown in your green antifreeze, probably have some rust going on in the block & water distribution tube.

Will a modern product remove rust from a iron block & copper water distribution tube?

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I recently flushed my radiator, newly rebuilt engine and heater core. I wasn't very scientific about it....

I dumped in a jug of the prestone flush, drove it 150 miles over a 3 week period.

When the time came to dump I opened both petcocks and stuck the garden hose in the top of the radiator. I let the water flow until it ran clear from the radiator neck, radiator petcock and engine petcock. Then I started and ran the engine a while with the hose still running and petcocks open. I closed the petcocks and turned off the hose, ran the car till it was warm (had to cover the front of the radiator so it got warm enough for the thermostat to open). Opened the petcocks and drained as much as possible, refilled with distilled water, ran a few minutes after the thermostat opened and drained again. Final fill was 50/50 distilled water and green prestone. Took it for a drive to get it hot and topped it off. 


So far it seems ok.


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I have been reading everyone comments, thanks. But here isa question for the experienced  guys. My cooling system holds 19 quarts, but how many qts are in the engine block only?


Here is my plan: Put in the cooling system flush and run the car for a run and get it up to temp to open the thermostat.


Then dump the radiator. Then pour in clean water 1 qt at a time to determine the number of qts needed in my radiator and then subtract that from 19. That should provide me with an approx number of qts in the engine block.


After flushing the rad and letting the engine cool then at the rear of the head is the connection for my heater supply hose.  Take off the hose and install a coupling so I can attach my water hose .  Open the block petcock and then flush the block.  May be do this several time to get a clear stream of water.


Then close the block petcock.

ia the heater hose opening inthe top of the block via a funnel pout in straight AF number of qts based on my calculations, a pint of radiator Water pump and anti rust agent and then some distilled water until the water level is near the top of the block.


Install the shutoff valve and reconnect the hose to the heater.


Then pour in the approriate amt of AF in the rad and distilled water and antirust agent.


Run the engine up to temp, check for any leaks. Then check the levels again and also check the level of protection.


Sounds like alot of work but this my way of flushing the engine and rad. What are your thoughts.


Rich Hartung


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