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Radiator Sealant Issue?

Hi. Thanks in advance for any thoughts you might have. I was losing antifreeze fairly quickly (a couple of quarts a week), so I put Prestone Sealant (two smallish bottles) in my radiator - correction, I put it in the coolant reservoir because these new (post 1980s) cars don’t grant access directly to the radiator. I own a late 1990’s Chevy Lumina. Then I drove the car around with the heat on full, per the sealant instructions, for a half hour (the bottle only asked for 10 minutes). My coolant is no longer leaking, but now the indicator that tells me whether my engine is overheating flies from completely cold to halfway toward hot (right in the middle) and back again in seconds depending on whether I’m driving uphill or downhill. I’ve never seen anything like it. Should I be worried? Have I gummed up the works with this sealer somehow? I’ve never used radiator sealant in this car before. Thanks again.

How do you know if there’s coolant in the radiator if there’s no cap? And how do you refill the radiator? Both of my late-90s cars have standard radiator caps.

Can you tell is the sealant was drawn into the radiator?

The jumping temperature gauge could mean there’s not enough coolant in the system.

The trouble with radiator sealants is they can clog up small coolant passages that you don’t want clogged up. If the radiator leaks it needs to be repaired or replaced.

Thanks. There is a cap to release pressure but it says “do not remove” and it’s not designed to be removed. I’d have to bend the metal on the cap to take it off. I feared clogging which is why I have never used sealer before, but my brother assured me that it would take many repeated uses of sealant to cause any problem. But what you say suggests that maybe the sealant has done just that. I can’t believe they sell this stuff if it so easily can cause a problem. My employer has cut me back to two days a week, so taking the car to a garage is an ominous thought for me right now. I might opt to live without a car rather than pay a few hundred dollars to fix it. The money just isn’t there at the moment. I have a job interview on Monday, though, so fingers crossed. If I get the job, I’ll take the car in.

It has occurred to me (though I really don’t know anything about this) that an engine couldn’t heat up like that in 5 seconds and then, in another 5 seconds as I continue to drive, grow cold again. Isn’t that impossible? I think this must be a gauge problem.

Now I’m thinking that perhaps the sealant in some way has interfered with the gauge and is causing the gauge to be unreliable. Because I can’t believe that an engine in this sub freezing Boston weather, could fly from cold to the middle of the gauge in five seconds (suddenly after I’ve been driving all of 15 minutes) and then a minute later fly right back down to cold again as I continue to drive. But maybe I’m just trying to make myself feel good.

Okay I’ve learned my lesson and I’m taking it in to the garage and borrowing the cash to pay for it from a sibling. The web is just full of warnings never to use sealant. I guess I should have followed my instincts.