Since a couple of days I have constant heat coming out of the dash vents. Not much time and desire to tackle it at the moment. Was thinking of bypassing the Mono Valve in the meantime, it's getting hot here and don't think I will use the heater anytime soon. Seems to be straight forward, from pipe to pipe in the engine compartment.

Anything I should be aware of or have to take care off?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

If I were you I would just put a manual ball valve between the engine and heater core. That way you can use heat and cold until you have time to fix it, but it wouldn't be a bad idea to keep the ball valve in place just in case anyways. I put mine in place years ago after my first heater core failure. I have the ball valve at one end and the monovalve at the other end to completely isolate the heater core incase of another failure.

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1990 190E 3.0L

I presume you mean bypassing the heater core, which has near constant flow of hot coolant due to failed monovalve.


Quick and easy: Get a heater hose clamp-off tool from flaps, clamp off one of the rubber hoses to or from the heater core.


Proper repair - replace the monovalve.


Another idea: install a ball valve in the rubber hose on the driver's side, which is the feed line into the heater core.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

It's fixed, it was the mono valve. The little metal filter inside was completely clocked by dirt and rust accumulated over the years. A good clean and all is working again as it should.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

And how difficult is it to access that valve? One of mine, a gen 1 is always hot at the windshield vent lately. All summer.

this is winter, so not that concerned at the moment.

C'mon Jorn, maintain your thread

Lol. It's a while, but I don't think it was a hard job. Recall you have to remove another part to get to it and there is one hidden screw or nut, no need to drain coolant.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

Heater is still working btw.
.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

I'll need some more dirt on this before I tackle it in spring. One of our fine brethren must have a good narrative on this.