Question regarding initial fill vs. subsequent fill intervals.

[IveBeenRued]

Does anyone know the logic/engineering reason behind manufacturers recommending one service interval for the factory coolant and then roughly half that for subsequent intervals?

For example, my Toyota manual states: “Initial engine coolant replacement at 100,000 miles/120 months. Replace every 50,000 miles/60 months thereafter.”

My Subaru was similar. The manual states: “Subaru Super Coolant. 1st replacement interval is 11years/137,500 miles (220,000 km). 2nd replacement interval is 6 years/ 75,000 miles (120,000 km) after the 1st.”

What gives?

 

[slacktide_bitog]

I'd have to guess more corrosion potential for older cars with more miles on them

Most people never change the coolant anyway, and the cars will likely rust before a cooling system problem shows up

Just be glad your Impreza has the FB25 and not the EJ25

That first interval matches your mileage now in the sig. Do you have blue coolant on hand? How much of a problem is rust where you are?

 

[IveBeenRued]

I'd have to guess more corrosion potential for older cars with more miles on them

Most people never change the coolant anyway, and the cars will likely rust before a cooling system problem shows up

Just be glad your Impreza has the FB25 and not the EJ25

That first interval matches your mileage now in the sig. Do you have blue coolant on hand? How much of a problem is rust where you are?

I did a radiator drain and fill at 100K to freshen everything up and then a full coolant flush at 135K. Everything was good but the car started consuming coolant after the flush. I added a bottle of “Subaru Coolant Conditioner” and that stopped the consumption completely.

 

[IveBeenRued]

Before people flame me for using a coolant additive. My factory service manual has “Subaru coolant conditioner” recommended at every coolant change. Despite me confirming with the dealer that they were going to put in “Subaru coolant conditioner” they instead put in “BG coolant conditioner”. The car almost immediately started consuming coolant (3-6 fl ozs, every 500 miles or so). I added the factory recommended conditioner and went for a good long drive. Zero consumption 8K later.

Back to the interval question, I thought of a few plausible explanations.
1. Maybe the factory has some crazy additives or purity levels that allow the coolant to go double the distance/time.
2. The manufacturer doesn’t trust service centers to follow proper coolant change procedures (my Subaru dealer couldn’t even do it right!).
3. Older cars have older pipes and connections so there is higher likelihood for air to get into the system possibly causing accelerated oxidation.
4. High probability of tap water being used during a coolant change.

 

[Warstud]

Does anyone know the logic/engineering reason behind manufacturers recommending one service interval for the factory coolant and then roughly half that for subsequent intervals?

For example, my Toyota manual states: “Initial engine coolant replacement at 100,000 miles/120 months. Replace every 50,000 miles/60 months thereafter.”

My Subaru was similar. The manual states: “Subaru Super Coolant. 1st replacement interval is 11years/137,500 miles (220,000 km). 2nd replacement interval is 6 years/ 75,000 miles (120,000 km) after the 1st.”

What gives?

They probably figure some dummies are gonna dump the Factory Fill and replace it with Prestone "All Makes All Models"

 

[xgmad]

The reason why is because most of the service intervals are dump and fill, when you dump the radiator coolant, you only get so much and not the whole system, so you still have old coolant that is going to mix with the new coolant, in result you have some old coolant that will continue to degrade. That is why.

 

[2EHA]

no flushing allowed

 

[CharBaby]

IMO and probably some fact too, is that there are a number of reasons many which have been mentioned. Here are some of mine which may mimic others as well.

*On a new vehicle, every part is brand new and the coolant is the best kind/most pure for that vehicle therefore allowing for the 1st long initial D&F interval.

*Once a vehicles cooling system is drained and is introduced to air after many years and the recommended service interval drops to ~ 1/2
*The MFG doesn't know how the owner will service the vehicle or which coolant they or a shop will be using.
*As mentioned, the cooling system is now old(er) and may indeed need better maintenance, a T-Stat or water pump, head/intake gasket(s) and this also introduces air &/or contaminants into the system.
*Many MFGs don't recommend flushing due to possibly causing trapped air(bubble) into the system causing overheating. However, I do believe that there are some skilled members here who can successfully do a complete cooling system flush w/o causing that trapped air in the cooling system.

And I guess that we could go on & on with our own scenarios to show why the cooling system maintenance drops to ~1/2 the time.