paalw

I am about to drain the coolant before tb/wp job. Do I run the engine and set the AC to full heat, or full cold, before I drain? Would like to get as much old coolant out as possible.

Thanks.

John Speake

Set heater to hot. Remove expansion reservoir cap. Drain the rad then remove the two block drains. Use a nice large pan to catch the coolant from the block drains as it splashes all over the place !

FredR

The valve that controls flow to the heater is open by default so when you drain down it should more or less empty the heater circuit without having to do anything else other than remove the coolant reservoir cap. If you are doing the TB/WP the cooling jackets are emptied when you remove the two drain plugs. If you are doing that on jack stands trying to avoid getting a glycol shampoo is quite testing.

hacker-pschorr

Using a funnel connected to one of the heater core hoses I pour in a gallon or two of distilled water which comes out the other hose. If you are really meticulous, measure what is in the bucket and know that much distilled water is already in the system when you calculate your 50/50 mix after it's all put back together.

It's also a good idea to back / forward flush the radiator really good, most are surprised what kind of gunk comes out of there.

paalw

Thanks guys

davek9

Having the car level makes a big difference IMO, remove the Rad drain plug, then open the cap to allow air in, then remove the Block drain plugs, wear safety glasses or a face shield when doing the block
As said its a antifreeze mess as it hits everything on the way down from the block

Dave K

Rob Edwards

Disconnect the short hose from the rear of the passenger side head, and blow compressed air through the line, it'll purge everything out of the heater core and push it back to the water crossover and out the bottom of the block drains.

linderpat

Rob - should there be any pressure limitations? I just saw another thread elsewhere regarding 911s where the discussion was about blowing out coolant systems with air pressure and damaging lines and other internal bits. The consensus was no more than 8 to 10 psi, if you do it at all.

hacker-pschorr

That's why I just gravity flush with distilled water. I've also seen some nasty debris come out of heater cores that may not be pushed out with just air, better agitated with some clean water and out the other house versus having it run through the head.

Now that I think about it, best to go reverse from the water flow, better chance of knocking some of that debris loose.

This isn't a 928, but an example of what the inside of a heater core can look like:



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Rob Edwards

Good question, I don't know for sure but since the system operates at, what, 12-15 psi, I wouldn't go much higher than that. If you wanted to flush the heater core you could just disconnect the 'U' hose (928 106 321 03) from the water crossover and collect the flushing in a bucket. Might be interesting to see what comes out, I can't remember the last time I used my heater. Should probably run it.

dr bob

Hacker -- I don't remember seeing those turbulator inserts in the 928 heater core. Those are usually used on high-transfer rate applications where fluid viscosity changes a lot with temperature change. Same as oil and transmission coolers in the automotive environment. They break up the thick outer boundary layer, allowing the hotter thinner oil in the middle to contact the tube walls. Apparently they also act as crud filters, perhaps enough to think about adding an inline crud filter ahead of the heat exchanger itself.

hacker-pschorr

I noted above: "This isn't a 928"

It's from a VW / Audi, first photo I found from my collection of heater core crud. Those little spiral tubes were probably a great idea on the bar napkin....

dr bob

They're a great idea in practice too.

Most heater cores are flattened-tube arrays with some form of turbulence-generator formed on the inside of the tubes if needed. This design helps a lot in space-constrained areas like the dash. If you've looked at the inside of a new Behr 928 radiator you've seen them. The recesses in the insides of the tubes fill with scale if the system is under-maintained, so by the time most folks peek into the tubes they are scaled up and appear to be smooth inside. The scale is a good insulator, so even tubes that look to be clear on the inside are usually coated in the recesses and have diminished capacity. On bigger exchangers there's a basic mechanical cleaning, followed by some interesting water-blasting inside to clear the accumulated deposits. Avoid these needs by maintaining the system, particularly avoiding use of anything besides good coolant in distilled water.

Three other screens are currently dedicated to solving a heat transfer model. No turbulators. Back to work!

Mongo

Or when an engineer looked at a tapeworm.

vanster

I was told to flush with distilled water and a half cup of Tide . Is this valid?