Hello there
i'm the owner of a 1986 C4 with cast iron L98 engine. I notice there's rust in the coolant, so this summer i cleaned the all the pipes by running the car with uncontaminated liquid until it was transparent, then refilled with yellow coolant. Unfortunately now the rust has come back again. I didn't removed the knock sensors to clean the engine block but i believe it had been cleaned by running the car (thermostat opened).
The car has 79k miles and is sitting in the garage for most of its time due to minor repairs.
I would like to kindly ask you if there's any additive or specific coolant you could recommend to reduce this effect.

Thank you

 

I think that is where you missed an opportunity.

The block holds a lot of water. I know that you were running clean water through it, but you missed some, somewhere and that is a likely place.

I am lazy and I do this **** for a living, so when I get home, I'd rather do other things than flush fluid. Therefor, I do it the fastest/easiest way that I can find and here that is....
Drain coolant from radiator
Pull block plugs/Knock sensor from blocks and drain.
Remove upper radiator and lower radiator hoses, from radiator.
Flush clean water through radiator w/a garden hose until it runs clean/clear out the other end.

Unhook the heater core hose from the water pump, connect that to your garden hose, then run the hose, until clean, clear water flows from the block drains. What you're doing here, is running fresh water through your heater core (often overlooked during flushing), then into the intake manifold, into the heads, down through the block, and out through the block drains.

Finish by blowing air through the heater core hose to push out a slug of the clean water (you can do this w/your mouth on the heater core hose or with shop air, judiciously).
Install all plugs and heater core hose back on water pump, fill with appropriate water/antifreeze mix.
Go burn some rubber.

 

Thank you for the information! Any suggestion on the antifreeze i should use to prevent the rust from coming back?
Also when refilling is it ok to also use the expansion tank to add the antifreeze when the radiator is full? Thank you

 

im not saying u should do this....but i built a contraption to plug into the ks hole, down with a garden hose, to a splitter, and i used this to both fill and then drain my ironhead, while it was running!

id allow the hose to fill the block, , wait for the tstat to open, then fill the rad, let it circulate, then id open the other splitter and it would drain the car while running. then id refill it again and i just kept doing this until it was 100% clean. it did take a while.

then drained it all and used 50/50 green coolant mix.

it was quite a contraption. the ks’s are the really only place to drain the block. and thats on on the passenger side

 

Everyone has a suggestion about antifreeze...just like oil.

My suggestion is again, motivated by experience in the maintenance industry and....laziness. I like extended life coolant b/c once again, I'd rather do other things w/my time than flush coolant annually on 5 cars. Also, experience at work has shown me that ELC WORKS and works incredibly well.

I buy 50/50 premix at home and at work; at home, so I don't add unknown minerals into the cooling system....at work so mechanics can't mess up the ratio. Thus, that solves the "when/where to add antifreeze" question. However, if you want to mix your own, I'd mix it before or as you add it, so you have a good mix and the right ratio.











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Have you have to add antifreeze since you did the flush? Reason im asking is a friend had a problem with his antifreeze turning rust colored and he constantly was having to top it off. Turned out he had leaking head.gasket.

 

I think everyone has their method of flushing cooling systems, and there's a lot of ways to do it. I've got a lot of different vehicles and have pieced together a 'flusher' that I can use in the shop over the winter. I have a well with ridiculously hard water so I never use that even for flushing which makes it that much more interesting. I took two big storage tubs and a submersible pump with some heater hoses and a Y connector to put mine together. I haven't done my Vette as yet and it will be a bit different than doing my Jeep and GM OBS trucks. On all of those I disconnect the radiator and flush it by itself, then block off the thermostat and hook my hoses up to the heater hoses. I put ~5 gallons of distilled water in one tub and run the lower rad hose from the block to the other tub...turn the pump on and run the water through until the drain tub is full. The Y connector lets me reverse the flow as I'm doing it which can make a difference.

One of my trucks hadn't been flushed in about 20 years and I think I pumped close to 40 gal through it before it all cleared up, but has stayed crystal clear for a bit over 2 years so far.

When I finish the flush I hook the rad back up and pour a gal of concentrate in, then fill the rest with 50/50 mix. The concentrate offsets the distilled water in the block and heater core and seems to bring the overall level out almost perfect.

 

Definitely pull your knock sensors... I bet when you pull them, you wont even get water out of the bores... theyll be plugged up. I had to poke a screwdriver in the bores in my 86 before any water poured out. There was a good amount of sediment blocking them up.

 

It's the only way to ensure (and SEE) what you've got going on in the bottom of the block and also, eliminate this issue:

 

thats what i use to go into the ks hole. i can send garden hose hose water in one side and then drain out the other. repeat repeat until its clean. worked well on my 85. then filled with 50/50.

 

That doesn't get the heater core, though.

 

fair, though i did this over the period of about 1hr idling so im guessing the heatercore did a couple drain cycles.

but im guessing you are think it would best to induce a flow rate through the heatercore as opposed to simply draining it?

come to think of it, i think i did a couple fill cycles through the radiator with pure white vinegar

 

A rule to go by is never use tap water for anything mechanical. Radiators, batteries, clothes iron, etc. Only use distilled water for mechanical including flushing. Its about a dollar for a gallon so its not that big of an expense.

 

Eh, yeah, I agree that pushing clean water through it is better then just draining it (to the extent that you CAN drain it)....but mostly, I'm thinking about TIME. I think your method or running/flushing it for an hour is probably more than sufficient....I just don't want to spent that much time on the job. By pushing water into the heater core, into the heads, down through the block and out the block drain(s), I can get clean water out the bottom in a minute. Maybe two. Then I can blow out the core w/air, put it back together, fill it in short order w/premixed....and move onto other things that I'd rather be doing.

 

Thank you guys, so many interesting suggestions here! I'll look on the service manual the details on how to drain, refill and bleed the cooling circuit as i understood i have little knowledge on this specific vehicle.

I have a couple of questions more i'd like to ask you:

1. does removing the heater core involves replacing gaskets or similar?
2. wrt coolant fluids, i saw the original manual says GM1825M compliant fluids have to be used. I found a couple (links below) and i wonder if being compliant with GM1825M specs is enough to consider the fluid ok for the engine or i should also avoid organic/hybrid coolants and go for inorganics (OAT, HOAT, IAT) Please consider I'm having a hard time finding valvoline or prestone here in the EU so i have to look for compatible different brands and here's what i found:

• https://partsfinder.bilsteingroup.co...cle/febi/19402 ethilene glycole + potassium violet
• https://www.mannol.de/products/opera...olant-g14#spec HOAT, green
• https://www.vierol-shop.de/en/product/V60-0070 violet

 

1. No gaskets for the heater core.
2. Any coolant will work fine, and any ELC will work better than the original coolant....IF, you fully flush out all the old coolant completely and use distilled water with the new coolant (or buy it pre-mixed). The car has no special requirements for coolant.