Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

New Engine, New Radiator, Sludge in coolant reserve!

Hi all. I recently had a new engine and radiator installed in my 1993 toyota pickup. I checked the coolant and oil every day since the install, then drove it 250 miles or so without checking the coolant, and found that all the coolant was gone from the reserve tank, and there was a sludge at the bottom. I don’t see any leaks, and the thermometer indicator was normal. I don’t understand where the coolant could have gone, given that it’s a totally new motor and new radiator, new hoses.

I’m worried about the competence of my mechanic, not noticing a dirty coolant reservoir and maybe messing something else up. Advice?



Refill the coolant reservoir and then keep an eye on the coolant level…

After replacing the engine/radiator, there may have been air trapped in the cooling system, and the air was replaced with the coolant in the reservoir.



If it is only the coolant in the reserve tank that is low, I’d presume that there was air in the system and once the air was expelled it made room for the extra coolant.
I would refill the reserve to the correct level and keep an eye on it daily. If it still drops in the reserve, then take it back to the mechanic that put in the engine.

Speaking of engines, was this a new engine …fresh from the crate. If not and it was a used engine…that engine may have had a bad head gasket straight from the junk yard. If so the mechanic was duped by the recycler and never knew about the head gasket. But then the recycler may have not known either.

Look at it this way; A car gets rear ended and scrapped. The owner never mentioned that there was a leaking head gasket because he wanted the most from the insurance to replace the car. The scrap yard gets the car and presumes that because it was hit from behind, the front half and engine/tranny are ok.
They sell it to your mechanic as a good used engine and the mechanic also thinks the engine is good.
You get it and now YOU have the head gasket problem.

Talk to your mechanic if it continues to use coolant. If he does a lot of business with that recycler, the recycler may pay part of the cost to do the head gaskets.


The engine is remanufactured by ATK, with 3 yr warranty…pretty sure it’s not the head gasket. Hopefully it’s just trapped air…but what worries me is that it was only after a week or so of driving it that the coolant in the reserve got consumed. Seems like the air would get expelled within just a few minutes of driving the car, no?

Some engines require that they go thru a heating up/cooling cycle before all the air is purged from the cooling system.

And sometimes that doesn’t work. And you have to manually purge the air from the cooling system.


You’ll know soon enough. Clean the sludge out of the reservoir, refill, and keep an eye on it.


If you CAN clean the sludge out of the reservoir, do it. It may be leftover from the previous engine. That stuff can be nearly impossible to get out of some reservoir types. Agree with the trapped air theory. Add more and watch it.

The people who really know might correct me but after spending the money for an engine in this case I would replace the reservoir and flush the cooling system. It couldn’t hurt to play safe.

The sludge may have come from the heater core.

Define “new engine”. Many people consider a 100k miles engine from a boneyard as a “new engine” once it’s installed in their vehicle.

Just wanting to clarify that bit…

1 Like

[quote=“tobin, post:4, topic:96368, full:true”]
The engine is remanufactured by ATK, with 3 yr warranty…pretty sure it’s not the head gasket. Hopefully it’s just trapped air…but what worries me is that it was only after a week or so of driving it that the coolant in the reserve got consumed. Seems like the air would get expelled within just a few minutes of driving the car, no?
[/quote]There’s your answer.

Sometimes, after a repair, you’ll find the coolant low because the mechanic didn’t get all the air out of the system. Also, what kind of sludge is in there? I’ve seen dirt get into the coolant reservoir, so it might not be coming from the engine.

I would remove the reservoir, clean it, reinstall it, and keep a close eye on it for a couple weeks after you top it off. Everything might return to normal in terms of coolant level and condition.

This is useful to know. I’ve heard of the trapped air hypothesis but thought it wouldn’t apply, since it took so long to start guzzling the coolant. But sounds like that’s probably it. The sludge was still green color, but very thick and opaque. The old engine had been treated badly…most likely it’s leftover from 20 years of running with the old guy?

Concur w/above, probably just some air was in the cooling system. At this point I’d be more concerned about the sludge in the bottom of the plastic overflow reservoir than the drop in coolant level. If you are certain that bottle was clean at the time of the engine replacement, suggest to do a couple of drain and refills of the entire cooling system and sieve what comes out to see if there’s further problems lurking inside the engine.

If that all turns out ok, it’s always a good idea when refilling the cooling system, afterwards to open the radiator cap (not just the plastic bottle cap) and idle the engine in the driveway for 20 minutes until the radiator cooling fan comes on a couple times. Make sure while doing this the heater is set to “max heat” so you get maximum coolant going through the heater core. At some point you should notice a big surge of flow into the radiator which confirms the thermostat is opening. That process usually rids the cooling system of all the entrapped air. Especially for trucks which sit higher off the ground so the radiator cap is the highest point in the system. You can ensure this by driving the front wheels of the truck on ramps of course.

If all that still doesn’t solve the loss of coolant, either it is going into the combustion chambers due to a faulty head gasket, it is leaking into the oil again due to a faulty head gasket or oil cooler, it is leaking into the transmission fluid (if your truck has an automatic), or you have a small external leak somewhere (hose connection, radiator, etc) that is evaporating before any drops of coolant are finding their way to the floor of your garage. For the head gasket possibilities, mechanics have ways to test for that and do it all the time; for the latter there are dyes that can be put in the coolant, then you use a special lamp to spot where the leak is occurring.

I should add I own a Corolla of similar vintage and been losing a little coolant. About a cup every 6 months. I just recently discovered it’s coming from a leak where the metal return hose connects to the water pump housing. My solution is to just top off the coolant every 6 months.

You could have the cooling system pressure tested to verify that there are no leaks.

Not exactly. What happens is that the air in the engine gets pushed into the reservoir as the coolant in the engine warms up and expands and it “burps” out of the coolant in the reservoir. Then, when the engine cools off again and contracts after the engine is shut down, it draws the coolant out of the reservoir to replace the air that was in the engine before it was started. You end up with all coolant in the engine, and none left in the reservoir.

The air moves from the engine to the reservoir and out the reservoir as the engine warms. The coolant from the reservoir then gets drawn into the engine when the engine cools.

Hopefully that’s what you’re seeing. But monitor it and let us know. We do care.

Thanks! I will.