I noticed the low coolant light coming on every other week in my 1991 Vert.

The overflow tank has plenty of coolant in it, and seemed that it was not returning to the pressure tank.

I then noticed that the pressure cap's seal was completely deformed, solid and brittle.


It seems that the rubber seal has been getting sucked back into the pressure tank, as you can see in the photo its no longer flat, but covers the whole spring now.

I just bought a new GM cap and installed it last night.
This morning when I opened the cap (engine completely cold), there was still a vacuum in the pressure tank (I could feel and hear air rushing in as i pulled the cap off.
The rubber seal of the new cap was also bent inwards, luckily its soft still and not brittle.

Has anyone had any experience with this sort of issue, would you think its another defective cap thats not opening the return valve, or is there a blockage in the overflow tank/line. I would think not, as coolant can easily flow to the overflow tank.

Thanks!

 

I would check for a collapsing hose, esp. lower or a bad cap. Radiator hoses commonly have a reinforcing spring inside that helps prevent radiator hoses from collapsing. If this reinforcing spring is faulty or missing, the hose may collapse when there is a vacuum in the cooling system. Another cause could be a faulty radiator cap. A radiator cap works to maintain a constant pressure in the cooling system. As the antifreeze mixture begins to cool after engine shutdown, a vacuum is created. If the pressure cap doesn't equalize this pressure, a radiator hose can collapse. The small vacuum valve in the radiator cap may not be working properly.

 

I just replaced the radiator cap with a new one, and it is doing the exact same thing, the large rubber seal at the top is getting sucked into the expansion tank.

I checked the hose that goes into the overflow/recovery tank and blew through it no problem with flow back and forth in that hose.

The lower radiator hose does not collapse at all. The upper hose only collapses when you accelerate the engine and the thermostat is closed, otherwise its fine after.

The small vacuum valve in the cap is working fine.
There is still a vacuum left over in the expansion tank after the car is cooled. It just seems like something is not letting the overflow/recovery coolant back into the expansion tank, even though the lines are clear.

 

It seems a stretch to say you have had 3 bad caps. Are you losing any coolant? I would be looking at the overflow tank and hose. That tank has a vent to allow air in the tank to enter and leave as the coolant level in the tank rises and falls in response to coolant temperature. If the vent is clogged, or the hose collapses under vacuum, I would expect to see the symptoms you report.

There should never be residual vacuum or pressure in the overflow tank. When the car cools off after being at operating temperature, remove the cap on the overflow tank to see if there is any suction there. If so, you have a blocked vent, or a collapsing overflow hose.

 

There is a check valve in the cap that allows the coolant to flow, so blowing thru the hose will not tell you if the check valve is good. If the vacuum valve in the radiator cap fails, the resulting vacuum in the cooling system can cause a radiator hose to collapse. But you said you just replaced it so maybe not.

A collapsed radiator hose does not necessarily indicate a bad hose. Buildup of chemical deposits or debris in the cooling system will gradually constrict coolant flow. This and other coolant blockages can cause system overheating and a vacuum sufficient to collapse a weakened hose. My top hose has a reinforcing spring in it to keep it from collapsing. I would replace that hose and do a system flush and then a heater core flush.

cross post w/ above, just sayin'

 

I am only losing coolant every other week or so, the expansion tank will report low coolant through the MIL when I apply the brakes hard enough that the level drops that low.

It just seems that the coolant is happy to expand into the overflow tank, but not return.

I pulled the hose of the overflow tank again, and blew through it and also sucked on it and the coolant flows through really easily.

The cap of the overflow tank also works venting air no problem.

The large rubber seal on the new cap is showing signs of being sucked into the neck of the expansion tank as the old one did, although no where near as bad.

I also checked the operation of the return vacuum port on the cap, and it opens as it should to allow return flow.

Something is not adding up here.

On another note, when burping the system, which I have done many cycles the past few days... is it correct to warm up the engine until the thermostat is open, raise the rpms to 2000, bubbles come out (im thinking these bubbles are from the expansion tank itself, since it is higher than the filler neck) keep the rpms up and put the cap on, or reduce rpms so the level drops, fill up, then put the cap on?

I noticed if I drop the rpms after the bubbles come out, the level drops significantly, almost 1 litre is gone. So not quite sure here which is the correct way to do it.

Cheers guys and thanks for the responses!

 

Poof!..........says the head gasket or cracked cylinder head.

 

You could confirm this with a compression test- but would these two conditions result in sustained negative pressure in the cooling system after cooldown?

 

Compression is good on all cylinders, and cant be a head gasket as you pointed out, there is still a vacuum on cool down.

Something is just not allowing the overflow coolant back into the expansion tank.

Driving me nuts, maybe it is the metal hose that goes to and from the overflow tank.

 

Coolant does not just disappear. If it's not going into the overflow tank, it's not dripping externally, and it's not in the oil...it's going out the tailpipe.

It is not at all unusual for an aluminum cylinder head to crack and only lose coolant when it's at operating temp due to thermal expansion. During cool down is also when there would be a vacuum as the liquid condenses in the system.

Find a leak and you'll find your problem. Don't find a leak, then prepare to pull the heads.

 

Sorry if I didnt make it clear, coolant is going to the overflow tank, just not being drawn back in properly in cool down.

There is a vacuum to pull the coolant back as the engine cools.

The only reason the coolant level drops is because of the expansion to the overflow.

 

You could try an experiment- rig a temporary hose in place of the overflow hose. If you remove this element from the system, you can tell by the results if it is the issue.

 

Great idea thanks!

 

Also, consider that the vented cap on the overflow tank may not be letting air back in after engine cooldown. Not likely, but possible. You could confirm this by removing the cap when you shut the car down and park it, then see if the coolant in the tank is drawn back into the system when it cools off.

 

[I think the last couple of posts were on target. It's possible for the vent on the overflow to work one direction allowing pressure flow into the overflow tank but an obstruction in the vent may be acting like a check valve and seals off the vent under vacuum. Same for the overflow tube from radiator to overflow tank. If it's too soft it will still allow pressure flow but can be collapsing under vacuum and preventing reverse flow.

 

This might sound silly but do you have the correct cap there are probably 100 different caps..

 

One last suggestion; You might have 20 years worth of crud floating around in the surge tank. This wont affect pressure flow to the tank but could obstruct reverse flow from the tank. Might try really flushing out the overflow. When I first got my 86, I couldn't believe all the crud I flushed out of this tank.

 

Have the correct cap, its from Mid America, and is 15lbs.

The line to the overflow tank is made of metal i believe?

Will try that with the cap off and see if there is any difference.

Cleaned out the overflow a few years ago, and you are right there was a bit of sandy like substance.

Another thing that is strange, every time after driving the car, there is always a tiny stream of coolant on top of the overflow tank leaking out of the cap somewhere, maybe from the bumpy roads, who knows...

I tried pulling the cap on the expansion tank while it was still a tad bit warm during cool down, the instant i let up on the tank, I could hear all the water in the overflow line bubble back into the overflow tank.

Maybe there is a blockage somewhere in the return line under vacuum as you said. Perhaps a blast with an air gun will do the trick.

Either way will report back with what I find out, and thank you all again!

 

Doesn't this tell you the line is not blocked and the cap is blocking the coolant return.

 

The cap works fine, the return valve opens as it should, that is why there is a vacuum in the upper level of the cap, and causing the upper rubber seal to be sucked down.