Mark v cooling system expectations

hello all, i am a new owner of a mark v 3.5 and dont know what to expect from cooling system. i have been struggling to keep it cool. what kind of temps do these cars usually run mine seems to be 90 c when idling for while. is this normal?

Welcome Brad.
Depending on if you are in Minnesota or Arizona, 90C or 194F could seem a little high, but not dangerously so. Possibly you could benefit with a radiator and engine block flushing. Investigate whether the thermostat is in place and functioning, or if the bypass hose has been blocked.

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HI Brad,

I used to have the same issue on my MKV 3 1/2 DHC, it was mostly due to a bad thermostat (I did the Ed Nantes modification and use an E-type 4.2L thermostat) and air leaks into the inlets, resulting in high and lean idle.

Also very likely there are all kinds of particles and gunk inside the water passages, running a Bar’s Nural treatment and flushing completely the block and the radiator would probably make things better. YMMV.


Dear Rob, I just joined the Forum and as I had the same problem with my Mk V I want to reply but, I live in Germany. Anyhow the car was going to extreme high temperature coming from overland streets to the city and specially with stop and go traffic. At least it figured out that there was the wrong net at the radiator. I found a special shop in Netherland who changed the net according to the ones they used in the 50th. The result I could´nt believe. In the hot summer stop and go in the city and the temperature is not above 90 degree Celcius.

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Welcome Jorg.
You use the word “net” but I suspect it is a poor translation from German kuhler kern, and perhaps you mean the core of the radiator, or possibly the thermostat. Either one would certainly help to reduce the temperature.

sorry for the delay in response, i have had head off, i flushed the cooling passage, had radiator boiled and flow tested, timing is correct, carbs are correct, i have the bypass blocked off but do not run a thermostat i have 20-degree temp drop in radiator i have removed water pump and checked impeller and looks good. one thing i found odd was i have 150 degrees coming out of radiator but 180 degrees coming into water pump. makes me suspicious of water pump it appears to flow in radiator but will do some more testing when time permits temp will climb just sitting at idle and does not seem to be effected by a fan blowing into radiator

Although I never heard this it is indeed called Kühlernetz and that (core) is what he meant.

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For your “odd thing” in temperature, there are two ideas which come to mind. First, your bypass block possibly may not be complete. A sufficiently complete bypass block can be achieved with a wood dowel inserted into the output from the thermostat towards the water pump. And I assume your thermostat has full flow towards the radiator.

A second possible contributor to your 150/180 oddness is the way in which you measure the temperature. If you are using an infrared thermometer, the material which is sampled by the thermometer can have a substantial effect on what the indicated temperature reads. Infrared thermometers need to be adjusted, calibrated, to the material at which they are pointed. Since you say 150 and 180 degrees, I imply you are speaking of Fahrenheit temperature degrees. An infrared thermometer in the engine bay easily can have a 30 degree difference on different materials at the same temperature.

Thanks David, my translator failed to find that word.

My SS came to me with no thermostat, but also nothing blocking the bypass. I have no idea how long the previous owners ran it like that.

I blocked the bypass port flange with a stainless steel bolt, nut and washer, like this:
bypass plug in thermostat
It has been running fine, though I only run it May through October.