Thermostat or water pump?

[Rising]

My diesel was running fine yesterday. This morning I went out for a quick ride (I'm sort of obsessed right now ) and I noticed my temp gauge starting to rise maybe 5 minutes in (around 2 clicks from the red clicks but it usually doesn't run that hot). I pulled over and turned the car off and went around to check things. Plenty of coolant and still have an accessory belt. So I waited a few minutes till the gauge dropped a bit and drove it home ( I was less than a half a mile).

So anyway at home I let it cool for a couple hours while I worked on my brakes and then checked some more stuff. It appears that in the coolant bottle there is a stream of air coming out but no water... so I'm guessing the water pump isn't functioning correctly or maybe its an air bubble or the thermostat is stuck. Any ideas?

I really don't want to go through any more problems... I think my wife will kill me if this car takes anymore of our money. . . Haha.

[8v-of-fury]

Well if it were absolutely fine yesterday, but today presents these problems..

I just replaced the water pump on my 88 1.6 TD and the flow in to the reservoir via the small tube from the upper radiator is very weak, but I have put nearly 3500 km since the water pump with no coolant issues. So I am not sure it is entirely related to a weak water pump.

An air bubble seems unlikely as well. Are both the top and bottom hoses attached to the radiator getting hot to the touch?

[Rising]

The top one definitely was. I don't recall if the bottom one was 100%. I could go test it. . . So what does cause the flow back into the bottle?

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[8v-of-fury]

Well it would be circulating coolant that would allow the coolant to flow to the bottle from the upper radiator.

The coolant flows from the pump up through the engine and out the two head ports. From there it freely and always flows through the heater core (unless it is an mk1 and it has the coolant valve under the hood in the heater core supply line). The front port goes out to the rad, but only flows through the rad if the thermo is open. Meaning until it opens the bottom hose should remain nearly stone cold.

There is a bypass that hugs the block nearly straight down from the front port and this should always have a free flowing path down through the pump and back in to the engine.

[libbydiesel]

There is a restrictor with a very small orifice in the hose to the coolant bottle.  I once had a small piece of crud plug that hole and had all sorts of weird coolant temperature behavior before figuring that one out (that was back before internet forums...).  I actually added a clear fuel filter into that coolant line prior to that orifice and was surprised at how many little pieces of crud that filter caught. 

[Rising]

The hose that squirts water in at the top or the hose that runs to the block from the bottom of the coolant bottle?

I'll check the bottom rad hose for warmth tomorrow. If it is warm that means water is circulating through the engine and water pump so what could be the issue then?

I guess after that a coolant flush might be good even though its all brand new. . . And its a mk1 so it has that little bypass doohickey.

Another note is the fan doesn't seem to be kicking on anymore even though I caught it running the other day. (My car never seems to warm up that much ) but I feel like it should turn on before 2 clicks off hot but that should not effect my cooling problems when moving... 

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[R.O.R-2.0]

There is a restrictor with a very small orifice in the hose to the coolant bottle.  I once had a small piece of crud plug that hole and had all sorts of weird coolant temperature behavior before figuring that one out (that was back before internet forums...).  I actually added a clear fuel filter into that coolant line prior to that orifice and was surprised at how many little pieces of crud that filter caught. 

99% of VWs have 1/4" fuel line for the coolant return hose, no restrictor in the line..

[libbydiesel]

I probably should have said that the stock hose has a restrictor in it.  If someone changed the hose and didn't move the restrictor to the new hose then it wouldn't...    IMO, it should have the restrictor because if it doesn't, then there is a decent stream there at higher rpms that always bypasses the radiator.

[TylerDurden]

I chased this question around a fair bit earlier this fall. 

Eventually I decided that the only way to be certain the WP impeller is securely attached to the shaft is to hold the impeller with a screwdriver while trying to turn the hub. Even if there is flow in the bleed line or heater hoses, it is hard to know if the impeller won't slip at high rpm and temperature, without checking it by hand.

Of course, if you test the impeller, you might as well test the t-stat too.

If both parts pass, the picture gets more complicated.

[ToddA1]

I had the above issue 2 summers ago with a fairly new pump. 

At low RPMs, the impeller would be fine and cool sufficiently.  On the highway, the impeller would slip, and run hot.  This was on a Bosch pump with a plastic impeller.

-Todd

[Rising]

Pulled the thermostat and checked the pump today. . . Thermostat wasn't opening till like 200 degrees and only partially. Tested another I had around an it was fully opened by 178 ish. That's probably my problem. The water pump passed the screwdriver stopping the pulley test. Still can't get the thermostat housing to seal properly though: /

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[745 turbogreasel]

If an old gasket is still up in the water outlet, it will leak

[Rising]

Yeah I tried to use the new gasket I just put in my old engine like a few months ago. Stupid. Ill be picking up a new one of those and a couple replacement bolts for these old rusty ones tomorrow. Thanks for the input everyone!

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[TylerDurden]

The plastic t-stat housings warp in very short time. A new o-ring will still leak if the housing is warped.

[Rising]

I have an early metal thermostat housing... should I put that on when I replace it?

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