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Discussion Starter #1
'98 Civic 1.6 4-dr MT sends coolant to the reservoir - the one WITHOUT an overflow port - but the coolant stays full in the reservoir and does not return to the radiator. The owner takes the reservoir and pours it into the radiator - which fully accepts the coolant. Then, the owner refills the reservoir. ~ 2 pints gets added each week. Car driven high miles each week and high speeds. 310k.

No leaks appear to exist.
No puddles.
2 new radiator caps have been tried.

Is the reservoir cap at fault? The cheap-looking white circular foam directly under the cap? Should it be replaced?

Do these overflow-portLESS reservoirs have cap evaporation issues? Vacuum-return issues?

What causes these '98s to not return the coolant from the reservoir to the radiator?

What causes these '98s to go through 2 pints like this? No smoke out tailpipe.

Thanks.
 

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Hopefully it's not a cracked block. Coolant would leak through and evaporate from the heat during the combustion cycle.

Btw, since this thread doesn't involve a 9th gen, it will be moved to http://www.9thgencivic.com/forum/other-cars/.
 

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Discussion Starter #4
Scotty, thanks. Exactly the content insight sought.

Pressure testing showed solid 16 PSI. No drop. No observable leaks, in engine bay nor cabin (and not from WP seep hole). Hoses all firmly pressurized. Car was cold and system filled.

But, after relieving pressure (accidentally, as it happens) and re-pressurizing, finally spotted teeny slow drips at radiator's bottom corner on the driver's side inside, originating from upper plastic-to-aluminum mating along the radiator's front top. Dried the areas. Leak flowed again. Same pathway. Appears as though the coolant was leaking when pressurized (not cold) - which for the owner was predominantly (96% of mileage) on interstate at high-speed driving - and was evaporating due to proximal heat sources. This does explain the coolant push to the reservoir (when hot) and the subsequent inability to re-suction back (to the radiator which was emptying).

Also, noteworthy, the 2nd new (just installed) radiator cap was leaking off pressure, slowly. New part failure. And, the first new radiator cap was solid, and should have been retained/trusted.

Oh, and the tailpipe looked good.

If the block theory bears out, will double back with you. Thanks again for it.

(Sorry for posting in the wrong part of the forum.)
 

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Cap and radiator are verified good, and no leaks above the water line? No (suction) leakage from the overflow nipple on the radiator to the reservoir? Hose under cap on reservoir has not fallen off? (it has to be submerged in the liquid)

Those 6th gen engines blow head gaskets just like the 7th gens do.
The breach will leak from the combustion chamber into the water jacket.
The telltale is pushing liquid out of the radiator but not pulling it back in.
It can overheat with less than a quart missing from the radiator.
You will not find any other "evidence" in most cases until the problem is major.

Even though you posted about a 6th gen, this info about 7th gens applies to the car (LINK to another forum)
overheating-mysterious-coolant-loss-common-causes-reference.html

 

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Discussion Starter #6 (Edited)
<Cap and radiator are verified good, and no leaks above the water line? No (suction) leakage from the overflow nipple on the radiator to the reservoir? Hose under cap on reservoir has not fallen off? (it has to be submerged in the liquid)

Those 6th gen engines blow head gaskets just like the 7th gens do.
The breach will leak from the combustion chamber into the water jacket.
The telltale is pushing liquid out of the radiator but not pulling it back in.
It can overheat with less than a quart missing from the radiator.
You will not find any other "evidence" in most cases until the problem is major.
>

Thanks EZ. Good to have your experience with the 6th gens and 7ths. Was hoping you would see the post.

Hose under reservoir cap was fine. Hose was submerged. Reservoir always had coolant.
Radiator overflow port was fine too.
New radiator cap the owner installed was in fact losing pressure and not good, fresh out of the box, but unbeknownst to the owner. Previous new rad cap was fine, but replaced, when it didn't it to be.
Radiator was not good and was leaking. When cooling, engine would have been pulling through the radiator's leak, hence, not from the reservoir.
Concur on the overheating with less than a quart too. It did overheat when 1.5 to 1.75 pints low.
And, as you say, it was tough finding evidence. (But) The radiator leak was at the top where the radiator tucks up under the top frame obscuring visibility (on the pass's side). There was run down from there (which was observable), but initially this run was associated with the (spillover) backfilling of the reservoir (from the owner having poured the reservoir into the radiator to fill "it", and then re-filled the reservoir spilling some).

On your breach from chamber into jacket, does performance loss manifest or not?
And, is one cylinder (dis)favored over another - on these '98s?

Oh, and the CTS went high resistance (-40) shortly before things took the downturn. And, this was during the 96% highway high-speed driving. (100 of 104 miles daily are interstate.) Fortunately, this was isolated and repaired the same day. And, the thermostat shortly after that was stuck, and was repaired the same day or so.

Multiple events.

Don't have access to a coolant gas tester, so, we'll see how things run tomorrow, now that the new radiator is in. I could see it fixed, and I could see it being block (or gasket). If the latter, will do the bubbles test.
 

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(But) The radiator leak was at the top where

When cooling, engine would have been pulling through the radiator's leak, hence, not from the reservoir.
Yay, you understood what I was thinking when I wrote 'above the water line' LOL
When a leak is up high, this can happen.
When a minor leak is located low in the system, it should create a slight vacuum and draw fluid from the reservoir.

If there is a significant air space in the cooling system, there won't be enough vacuum created during cooldown to draw liquid out of the reservoir.
When the (head gasket leakage) is pressurizing the system, coupled with liquid displacement, it will not draw liquid from the reservoir. This is probably what you are dealing with.
On your breach from chamber into jacket, does performance loss manifest or not?
And, is one cylinder (dis)favored over another - on these '98s?
No and no. It's gotta be a HUGE leak before it drops compression and affects performance, and that usually doesn't happen unless the head is warped from a severe overheat.

Most of these MLS head gasket failures can and will pass every last check most people think of.....while the leakage is slow and minor. Simply refilling the radiator may stave off symptoms for another month or 1000 miles or however long it takes to displace another quart out of the radiator.

Just search that other forum for things like overheating, no heat, head gasket, and you will see people have thrown every last part they can think of at the problem and absolutely refuse to believe.....and it's almost always a blown head gasket.
Many don't discover a problem until they go on an extended highway trip.
Oh, and the CTS went high resistance (-40) shortly before things took the downturn. And, this was during the 96% highway high-speed driving. (100 of 104 miles daily are interstate.) Fortunately, this was isolated and repaired the same day. And, the thermostat shortly after that was stuck, and was repaired the same day or so.

Multiple events.
Older car, things happen. And they seem to happen in threes LOL
And, as you say, it was tough finding evidence.

Don't have access to a coolant gas tester,
I've never needed that check to prove one bad, and many will pass that check even with the blown head gasket.

There are other ways.
Check the forum link above, see first post, read it thoroughly for info and ideas, watch both of the videos in that post.

Feel free to make an account and post in the forum too. :icon_wave:
so, we'll see how things run tomorrow, now that the new radiator is in. I could see it fixed, and I could see it being block (or gasket).
Simply refilling the cooling system may stave off overheating for however long it takes to displace that same significant amount of fluid out of the radiator (days/weeks/1000 miles, whatever).

If it's been overheated, the head could be warped. That's a given, and a warped head can't make a good seal.
Without looking, I think .002" (0.05mm) is the max warp spec.
MLS gaskets demand absolute flatness, and almost glass smooth machined finish that many machine shops can't achieve.
(Our local machine shop has botched a couple heads for me, now I send heads to someone an hour away simply because they can do the job correctly)

I only use factory head gaskets.

I've not been witness to a block problem from overheating (yet) on an engine that still ran.
If it overheated and someone kept driving it until it quit on its own and never ran again, I wouldn't even bother trying to save it. I'd just replace it, it's faster and usually cheaper than trying to save the old one.
 

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Discussion Starter #9
Well, so far so good. Coolant returned to (new) radiator. Dash gauge didn't rise above 3/8ths.

The article you attached Ez was worth every bit of the read. Twice.
 

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Good to hear...Let's hope it was all caused by the radiator and cap, and not anything more major.

The head gasket can fail without outside influences such as overheating, so have the owner continue checking the levels regularly.
 

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Discussion Starter #11
We're in sync. Already was hoping (planning) to see it directly everyday this week - or get daily status - and then regular status (directly or indirectly) going forward.

Oh, and on new parts failing (the owner's 2nd new rad cap), new OEM Hyundai purge control valve leaked vacuum, fresh out of the box. It held vacuum for several seconds, then ever so slight bleed off, then sudden drop. Oih.
 

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Discussion Starter #12
Update for those who helped and those it may benefit.

Been several weeks; have had a chance to check the owner's car various times; all remains well in the reservoir and the radiator; and, the gauge continues to stay at 3/8ths, including when sitting idling, despite temps in 90s.

Just wanted to status.
 

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Discussion Starter #13
For those this may help.

Update, now 5 months later:

I had opportunities to continue monitoring the gentleman's car.

@Ez1 was right.

Problems returned.

Head gasket was leaking at middle 2 cylinders.

Coolant (again) began being pushed into reservoir and not pulled back, so radiator upper header was (again) low.

New gasket; back on the road.
 
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