Mystery coolant leak from above the Transmistion

[Hodgimus-maximus]

Hi all,

I bought a 1975 280Z in January and haven't felt like working on it till now mostly cause it is in good running order. A few months ago after taking it out and coming back i notice a growing pool of coolant underneath the transmission. So the search began, I read the service manual I searched the forums and low and behold I found nothing that was useful. Well I finally used my camera and took a picture of the mystery hose that is pissing away all of the coolant in my car. Can anyone tell me what it is where its coming from and what it should be connected to?

Thanks

[Zed Head]

I think that's the drain tube for condensation from the AC system (AC means heating and cooling in Nissan-speak). If you're lucky, it's just one of the hoses to the heater core, not the core itself. Take the AC control panel off (the panel with the control levers) and you should be able to see where the leak is.

[Arne]

I'm not totally familiar with the 280Zs, but from the picture I suspect that's a drain from the heater box. So you may have a leaking heater core.

[Kurbycar32]

Thats definitely the heater box drain tube, but that's clearly green coolant on the outside of that tube. I suspect something is leaking within the heater box, or at least into it.

[Kurbycar32]

By the way your lucky that its not leaking all over your carpet

[Reverend]

I dont know about 280z's but i did have leaking watercock inside my 240z, coolant was everywhere. Easy to replace and not that expensive.

[ZCurves]

I agree with the others. Heater Core replacement time. For now, you could by-pass the heater core by looping the lines at the firewall in the engine compartment. This would give you time to acquire replacement parts (e.g. Core, new hoses, water ****). When you are ready, come back to Forum for instructions to replace all of this w/o removing the dash.

[LeonV]

If the plan is to bypass the core, do not loop the heater lines. Plug them.

[olzed]

If the plan is to bypass the core, do not loop the heater lines. Plug them.

Just curious .. why can't he loop the hoses.?

[draztik280]

Definitely a leaking heater core. Same problem on my 280. I looped the heater hose in the engine bay, work my way to get the heater core out, send them to the radiator repair shop. Also replace those old hoses inside as well.

[LeonV]

Just curious .. why can't he loop the hoses.?

Think about the way the cooling system works. Cool water comes from the radiator into the bottom of the block where the water pump sends it through the water jackets, into the head, and out through the thermostat back into the radiator. The heater gets it's hot water from a fitting at the back of the cylinder head and sends the return water back to the water pump inlet. With the hoses connected, the control valve regulates flow.

When the heater lines are looped, you are connecting the back of the cylinder head with the water pump inlet. This means you are now pumping the hottest coolant back into the water pump inlet which is supposed to be receiving the lowest temp coolant! The loop bypasses the radiator by giving the hot coolant a path of least resistance back into the water pump.

TonyD has done instrumented testing and included the results in the "Head cooling on cylinder #5" thread on HybridZ showing that at low load, and mid/high speed cruise rpm the temperature difference between looped and non-looped coolant temps is 50 degF! Yes, 50 degrees!

At long last more to report. The 'loop the hose when the heater is bypassed' was finally tested today.

Long and short of the testing was that with the back of the head looped straight to the pump inlet (like some do when bypassing the heater core---instead of blocking it off)the following results were obtained using the Nissan CHT as read through the Nissan ECU.

With a 25hp load on the engine running 4500rpms and the hose looped and open, CHT=220F

With a 25hp load on the engine running 4500rpms and the hose BLOCKED CLOSED, CHT=170F

Results obtained on a Superflow 900 series Dyno, with the thermostatic water valve set to maintain 160F water.

The tests were stopped at 4500rpms as the suction hose on the dyno would start collapsing above that point.

This would be similar to stock gearing at 80+ mph or thereabouts. What we wanted to see was how high we could go before the 'open hose' started to rise in temperature, but even with a collapsed hose above 5000rpms we could not get CHT above 170, whereas with the hose open and flowing to the pump inlet (which you think would help prevent the hose collapsing...) the temperature started to rise even more above 4500, so we figured stopping the test there was the best comparison. Results at 3000rpms were almost identical when the engine was 'stabilized' --- that is run in steady-state load for several minutes.

Yes, we were running the engine for several minutes at a time checking load points, etc. Let temperatures stabilize then run to a higher number.

Source: LINK

[ZCurves]

If the plan is to bypass the core, do not loop the heater lines. Plug them.

I guess that does make more sense, now doesn't it.