Thermostat Gets Stuck, Or Is Slow... Or Is Something Else Going On?

[monkeyman]

Hi all, first, the detail:

 

1975 260Z 2+2

L28 (rebuilt a few years ago)

Radiator rebuilt (with new core) about 4 months ago.

 

I have this problem where the car overheats. I have tried a few things and am yet to get to the bottom of it. 

What seems to happen is that the water in the car will slowly heat up as per normal (by viewing the gauge in the dash), but often it will go past the mid position where it usually likes to sit. If I don’t notice this it will keep going until the radiator caps lets it all out and I get steam and water everywhere. 

If I do notice it, I pull over, scratch my head, look around, and then when I get going again it drops back down to the mid position and sits there happily. If the car hasn’t been used for a few days, it will always happen. If I have been driving it that day, it probably won’t happen (though it might). 

 

So the engine has been relatively recently rebuilt. I have had the radiator re-cored. Tried new radiator caps. But it all seems as though it is the thermostat getting stuck, or being very slow to open. I have replaced it a number of times, got a mechanic to do it once, even used an OEM themostat. One time I put the one I pulled out into a pot of water on the stove and it open when it should (from memory I think it was about 85 degrees).

 

It seems so much like it is the thermostat, I can’t think of anything else it could be, but after having replaced it a number of times (and my mechanic too), I am a bit lost

 

Has anyone else had any similar experiences?

 

[Dionysus]

Remove the thermostat and see if the problem still happens..

Things to consider...

Air pocket?

Even new radiators can be blocked?

No gunk in the water?

 

[Linton]

Remove the thermostat and see if the problem still happens..

 

do not remove the thermostat and leave it out you will get more problems, but I would suggest you replace it, while its out nip the housing back on no gasket ,remove the bottom radiator hose and stick the garden hose in it, open the heater tap and reverse flush it out the cap, cant hurt

[ZED83]

Sounds like thermostat is getting stuck. Take it out and drill a second small hole in it. Should help it open!!

[Dionysus]

do not remove the thermostat and leave it out you will get more problems, but I would suggest you replace it.

I never said to leave it out, just process of elimination.

 

Why replace it if it's the radiator at fault..

[bjszed]

I had this issue. I'm guessing you don't have the bypass lines hooked up from the thermostat housing ? Without the bypass lines hooked up the thermostat doesn't see circulated water until after it has opened which makes its initial opening very slow due to the only heat it sees as conduction.

The fix is either to re-install the bypass hose or remove the jiggle valve so a small amount of water will flow past the thermostat which heats it up properly....and also run an oem thermostat. The jiggle valve stays closed during operation from the pressure in the block, it opens from gravity once the engine is off and pressure lowers. 

[cruza]

Had an old valiant that used to do just that, drove me totally nuts for two months, turned out to be bottom radiator hose had a spring inside it to stop said hose from collapsing (sucking together). The spring died from old age and the hose got soft and the spring then partially blocked the water movement. If you dont have that problem a TEMPORARY cure for me would be the old torana XU-1 trick of cutting out the center of an old thermostat (sounds like you might have a couple lying around) and fitting that, pick one with the smallest center to cut out as the idea is to slow the water down enough for the cooling process to happen but still run warm enough that its not a pig to drive. This is a quick fix only while you sort thru the other possibilities, but it will work and take the pressure off(no pun intended).

[dat2kman]

The olden day method,,,

Pull it out, put it into a pot of water, bring it to the boil.

Opens,, good.

No open, bin it.

[adam]

I had a very similar issue a few years back. Getting to operating temperature would always involve coolant blowing out of the overflow tube. 

 

I noticed it had something to do with the thermostat opening. Turns out the jiggler may have possibly been installed the wrong way on the thermostat (I know! How stupid does that sound).

 

I pulled that little sucker (jiggler) out and all has been well since. I'm guessing it was something to do with the jiggler not releasing pressure since the flow of water had caused it to close the hole (being the wrong way).

 

Now before you say it, thermostat was actually thought to have been installed the wrong way, I flipped it over and the over pressuring/over heating had stopped but temps would never stay consistent or warm (for example driving on a freeway). Looked online and it was indeed installed wrong so I flipped it back, ripped the jiggler out and everything's now good as gold.

[bjszed]

Yeah, removing the jiggler allows some coolant circulation past the thermostat so it opens normally. This is only necessary if you have deleted the bypass hoses that were hooked up to the bottom of the thermostat housing.

[Riceburner]

+1 Adam. I'm the same, I always cut off the jiggler and drill out the hole to 2.5mm. By doing this you'll have a tiny amount of flow even when the thermostat is closed and it will sense the temp quicker.

 

Was the cylinder head pressure/crack tested at the last rebuild?

[Gtv240z]

Do you still have original style mechanical fan? the viscous clutch is probably stuffed. if your not leaking coolant and with the cap of can see good coolant circulation your stat should be open and waterpump probably working well.

 

these clutches are pretty prone to failure

[monkeyman]

Thanks to everyone for helping out. Every post gave me something to think about, or taught me something, or gave me some direction in my quest to fix my baby. 

Who’s ever hear of a ‘jiggle valve?!?!?!

So I found the problem and was gob-smacked!

I pulled out the thermostat. It had a jiggle valve (never noticed it before). Into the pot on the stove and it opened between about 82 and 87 degrees. All good. 

There was a pipe coming out the bottom of the thermostat housing: the bypass. It split in two directions, one hose to the carbies and one back around to the engine block (so this was the bypass hose). The hose to the carbies was blocked. I tried to clear it out with a screwdriver but there was a spring inside making it difficult (presumably to maintain the shape) so I just left it. 

I capped off the pipes to the carbies. The pipes through the carbies seem blocked anyway. 

I had the bypass hose off at the thermostat end and tried to blow into it. Blocked. Not a bubble!. So I pulled the hose off (it’s a long one) and tried again. The hose was blocked! I put a garden hose to it and a second later something came shooting out. I found it, and it was a lump of deformed plastic. I bit like a squashed plastic version of the rubber caps i used to seal off the pipe to the carbies. 

How did it get there!?! I was gob-smacked. I was smacked in the gob!. 

I will take it along to my mechanic next time I see him. See if he recognises it.

So I put everything back together. I left the jiggle valve untouched (I had planned to remove it as per some suggestions here). I took the car for a drive and the temperature went past its usual midpoint to about three-quarters. Too high, but I had air bubbles in the water that could be causing problems. I took it out again later tonight and she was great!!

I will keep an eye on it and if it gives trouble, out comes the jiggle valve. But I suppose that having it out could disguise other problems (like my blocked bypass hose), so I will keep it if I can. 

Thanks to everyone who posted, I was lost until I got help here.

[Riceburner]

It shouldn't get to 3/4 on the temp gauge, that's at 100 deg C!

 

The steel pipes into the SU mainfold are always blocked, the fittings in the manifold corrode unless they've been replaced as part of a refresh. The flow through that manifold will be zero, that means no flow through housing past the thermostat and poor temp control. The bypass hose off the housing is a question mark shape, you'll never get a screwdriver down that.

 

There's most of the shite temp control. That's why the 'jiggler' is removed and the hole is enlarged. Drill it to 2.5-3mm, it's very a common on the L4-L6 engines.

 

Next time when filling the cooling system, leave the car on stands/jack at full height while filling, start the car and squeeze the lower hose when running to push any air up to the radiator, leave the cap off and you will see it bubble in the neck. Then repeat.