Coolant flush
Thread Starter
Advanced
Joined: Jan 2025
Posts: 63
Likes: 8
From: Texas
Coolant flush
I have a 1991 Coupe L98.
So I read on the forms how to do a flush.
this is what I did
0. TURN ON HEAT and raise temp to max (this is so it's not forgotten).
1. drain radiator (pet-**** on passenger side).
2. close pet-****.
3. Fill the overflow tank with distilled water.
4. SLOWLY fill the pressure tank with distilled water to 1/2 inch below the bottom lip.
5. start car, within 1-2 minutes, add additional distilled water, keeping level 1/2 inch below the bottom lip, then cap off.
6. bring idle speed to 2k rpms for 5 minutes, watching both temp gauges (analog and digital on dash). Both should climb somewhat evenly -- do not allow to overheat, but you want the temp to get to around 200 for at least 5 minutes.
7. turn off engine.
8. allow to cool for about 15 minutes.
9. open radiator pet-**** (do not touch radiator cap).
10. drain radiator -- remember system will have some pressure and will be hot). After 1-2 minutes, open radiator cap (pressure should be relieved so no spewing or leakage).
11. repeat steps 5-11 at least 4 times, but continue until water drained out of radiator is nice and clear (or your down to just 2 gallons of distilled water).
I did this till water came out clear^
My question/ issue is When I went to do my fainal adding the coolant and water at 50/50 mix. It will only take 1 gal of coolant and 1 gal of water. I cannot add anymore. I thought the system held a lot more than that. Any thoughts or ideas? I let it ideal with the cap off to let bubbles come out but that didn't do much. the system is defiantly holding pressure. Not sure what to do or if something is wrong.
So I read on the forms how to do a flush.
this is what I did
0. TURN ON HEAT and raise temp to max (this is so it's not forgotten).
1. drain radiator (pet-**** on passenger side).
2. close pet-****.
3. Fill the overflow tank with distilled water.
4. SLOWLY fill the pressure tank with distilled water to 1/2 inch below the bottom lip.
5. start car, within 1-2 minutes, add additional distilled water, keeping level 1/2 inch below the bottom lip, then cap off.
6. bring idle speed to 2k rpms for 5 minutes, watching both temp gauges (analog and digital on dash). Both should climb somewhat evenly -- do not allow to overheat, but you want the temp to get to around 200 for at least 5 minutes.
7. turn off engine.
8. allow to cool for about 15 minutes.
9. open radiator pet-**** (do not touch radiator cap).
10. drain radiator -- remember system will have some pressure and will be hot). After 1-2 minutes, open radiator cap (pressure should be relieved so no spewing or leakage).
11. repeat steps 5-11 at least 4 times, but continue until water drained out of radiator is nice and clear (or your down to just 2 gallons of distilled water).
I did this till water came out clear^
My question/ issue is When I went to do my fainal adding the coolant and water at 50/50 mix. It will only take 1 gal of coolant and 1 gal of water. I cannot add anymore. I thought the system held a lot more than that. Any thoughts or ideas? I let it ideal with the cap off to let bubbles come out but that didn't do much. the system is defiantly holding pressure. Not sure what to do or if something is wrong.
Instructor
Joined: Dec 2024
Posts: 216
Likes: 85
From: Phoenix, Arizona
Draining from the rad is at best 50% of your fluid (which would explain the 2 gal refill vs. near 4 gal total capacity). Engine block /heater core holds the remainder, which would drain through your knock sensor on the undercarriage. With that said and based on steps taken, you've reasonably flushed** your old coolant by the process of continuous dilution and the process you followed is certainly more effective than those that just drain the rad once, clean and refill. You've at least cycled clean water through the entire system multiple times and dumped it. During the refill it sounds like you've burped the system as well, so you should be good to go. Good time to check your rad cap for proper function too!
Side note - given you flushed with distilled water I think it is reasonable to assume that the engine block was near 100% water. If you refilled the remaining 2 gallons at a 50/50 ratio, your total system is closer to 75% water / 25% coolant...which is more ideal for the Texas summers you endure.
Side note - given you flushed with distilled water I think it is reasonable to assume that the engine block was near 100% water. If you refilled the remaining 2 gallons at a 50/50 ratio, your total system is closer to 75% water / 25% coolant...which is more ideal for the Texas summers you endure.
Thread Starter
Advanced
Joined: Jan 2025
Posts: 63
Likes: 8
From: Texas
Draining from the rad is at best 50% of your fluid (which would explain the 2 gal refill vs. near 4 gal total capacity). Engine block /heater core holds the remainder, which would drain through your knock sensor on the undercarriage. With that said and based on steps taken, you've reasonably flushed** your old coolant by the process of continuous dilution and the process you followed is certainly more effective than those that just drain the rad once, clean and refill. You've at least cycled clean water through the entire system multiple times and dumped it. During the refill it sounds like you've burped the system as well, so you should be good to go. Good time to check your rad cap for proper function too!
Side note - given you flushed with distilled water I think it is reasonable to assume that the engine block was near 100% water. If you refilled the remaining 2 gallons at a 50/50 ratio, your total system is closer to 75% water / 25% coolant...which is more ideal for the Texas summers you endure.
Side note - given you flushed with distilled water I think it is reasonable to assume that the engine block was near 100% water. If you refilled the remaining 2 gallons at a 50/50 ratio, your total system is closer to 75% water / 25% coolant...which is more ideal for the Texas summers you endure.
Thank you!
Safety Car
Joined: Mar 2017
Posts: 4,197
Likes: 1,321
From: LeClaire Iowa
Shop vac flush works! - CorvetteForum - Chevrolet Corvette Forum Discussion
Your cooling system is different than my 96 LT1 but I bet you could figure out how to use this method to flush out your block. Dan
Your cooling system is different than my 96 LT1 but I bet you could figure out how to use this method to flush out your block. Dan
