How complete of a coolant flush is needed?

[jslt]

Hey everyone,

I've been lurking here for years to help keep my 2000 Expedition running and have always appreciated all the insight, but never had to cause to create an account. I've finally got a question that I can't answer with searching so here I am.

TLDR: Lots of coolant flushing, still getting rusty flakes. Worried about killing new heater cores if I call it good.

I've had this car since 2016. As far as I can remember, the heater has never actually worked. In 2018 I had the coolant flushed by a mechanic and they told me that the heater core was probably clogged and would eventually need to be replaced. I held off on the job because of the price. Finally, this year, I got tired of being cold and decided to try and fix it myself.

So here is where I am at. I dropped the dash and pulled the front heater core. It was definitely full of gunk. In the process, I realized that a) the coolant was pretty rusty and b) this car also has a rear heater core. I got the rear core out too and it was full of rust flakes and also totally shot.

I set up a bypass for each heater core and have been proceeding to flush system. I pulled the thermostat out and got the driver's side block drain out as well. I have done two separate chemical flushes and spent literal days of doing garden hose flushes, using the radiator drain, the block drain, and the rear heater hoses in various combinations.

My question/problem is this: while most of the crap is out of the system and the water runs clear, if I flush the water into a 5 gallon bucket and let it settle, I can still see small flakes of rust. Less, but they are still there. I pulled the old heater hoses off and looked inside the engine outlet and I can see that there is still some rust caked on the walls. I think I will forever be getting some flaking each time I do a flush, no matter how many times I do it. I've probably used 100 gallons of water at this point. The photo of the heater old hose is after I've done all this flushing (I scraped the sides with the screwdriver to show how much crap is still caked on).

At this point, I can only think of a few courses of action.
1) Just install the new heater cores, put it all back together, fill with coolant and drive it. Do more frequent coolant flushes for the next several years. Maybe try and install a coolant filter?
2) Leave the heater core bypasses, put it all back together, fill with distilled water and drive it around for a bit. Dump the water and repeat driving/flushing until no flakes/rust water is coming out, then install the new heater cores, etc.
3) Leave the heater core bypasses and try a citric acid flush(es), followed by water flushes, repeat until no flakes/rust water is coming out, then install the new heater cores, etc.

Any advice or insight would be appreicated. After all this work, my main fear is killing the brand new heater cores I went to all this trouble to install. The car has been running great and does not over heat, so the radiator and water pump seem fine.

Sorry for the long post!

 

[jr1under]

Wow, helluva first post! I got nothing re: rust but I'm responding just to encourage knowlegeable lurkers like you to become active in the forum. There's genuine expertise here but there's also a lot of people like me at the other end of the experience spectrum who would benefit.

 

[jslt]

Thanks! I've always really valued the information in forums like this (compared to black hole of facebook) but I'm usually on the non-expert side of the equation so I don't have much to add.

Random anecdote: I'm pretty sure it was from this form, years ago, that I learned I could wrap the Expy door lock actuator thermistors in aluminum foil to revive them and they have been going strong for almost 10 years like that!

FWIW, I am leaning towards giving the citric acid flush a try. I just can't see how leaving the rust in the block and lines isn't going to quickly re-clog the heater cores.

 

[jr1under]

FWIW, I am leaning towards giving the citric acid flush a try. I just can't see how leaving the rust in the block and lines isn't going to quickly re-clog the heater cores.

So, the problem is that you've eliminated the source of the rust but can't flush the residue out of the system?

I did a way overdue flush on my 99 a couple months ago. The old coolant looked like coffee w/ cream and reeked but fortunately no evidence of rust.

 

[JamaicaJoe]

I wonder if attaching a big strong magnet to the heater inlet lines would trap all that stuff over the driving cycle. Then flush that line after removing the magnet. The engine block and head being iron, you are never going to get it all out. Does anyone make a coolant filter? Maybe for marine use?

 

[Petesexpy]

My front heater core got plugged 5 years ago. That's when I learned that A) the rear heater core can be replaced in 2 easy hours, B) The rear heater core is sufficient to keep the car warm down to -10F outside temp (the only time i'm in trouble is during a heavy snowfall or freezing rain), and C) the system is ALWAYS full of rust no matter what I do. So, I replaced the rear heater core and I flush JUST THE HEATER CORE once a year. No problems so far. So I would install the heater cores and just flush both heater cores and then the system once a year. BTW, I flush the heater core against the normal flow direction.

 

[jslt]

So, the problem is that you've eliminated the source of the rust but can't flush the residue out of the system?

I did a way overdue flush on my 99 a couple months ago. The old coolant looked like coffee w/ cream and reeked but fortunately no evidence of rust.

I don’t think I can say I’ve eliminated the rust. It must be coming from the engine block. I’m assuming that some amount of previous neglect (wrong coolant or too old?) caused the issue and it has accumulated on the walls of the pipes and hoses like plaque on an artery. That’s why I’m thinking of trying one more flush with something strong, like citric acids. Maybe it will dissolve the caked on rust. Or maybe it will cause a leak

 

[jslt]

I wonder if attaching a big strong magnet to the heater inlet lines would trap all that stuff over the driving cycle. Then flush that line after removing the magnet. The engine block and head being iron, you are never going to get it all out. Does anyone make a coolant filter? Maybe for marine use?

I’ve actually wondered about this. It does seem like some people put inline coolant filters in their trucks. I’ll do some more research on this.

 

[jslt]

My front heater core got plugged 5 years ago. That's when I learned that A) the rear heater core can be replaced in 2 easy hours, B) The rear heater core is sufficient to keep the car warm down to -10F outside temp (the only time i'm in trouble is during a heavy snowfall or freezing rain), and C) the system is ALWAYS full of rust no matter what I do. So, I replaced the rear heater core and I flush JUST THE HEATER CORE once a year. No problems so far. So I would install the heater cores and just flush both heater cores and then the system once a year. BTW, I flush the heater core against the normal flow direction.

Thanks. This is helpful. It sounds like some rust in the system isn’t enough the clog up the core.

I’m think going to give it one more flush out with the citric acid and call it good.

Do you actually directly flush the rear heater core by disconnecting the hoses?

 

[Petesexpy]

Thanks. This is helpful. It sounds like some rust in the system isn’t enough the clog up the core.

I’m think going to give it one more flush out with the citric acid and call it good.

Do you actually directly flush the rear heater core by disconnecting the hoses?

Yes, that's exactly what I do and what I would suggest doing for your front heater core as well. disconnect hoses and flush it out. I think that this year I will flush it both ways, and maybe replace the coolant as well.