XK120 Overheating Cause?

Awesome thread! I have been running through similar issues with new-to-me FHC #680000. Initial problem was bad radiator cap, allowing coolant escape (about 1.5 gallons), followed by serious overheat. That made me nervous, and despite the fact that the new radiator cap seems to have resolved that problem, I have been sticking to short drives - and seeing the same thing, sticking around 80 while moving, but then slow steady increase when idle. From this thread, sounds relatively “normal”.

An IR thermometer can definitely help with this sort of debug. For me, I see the top tank temperature matches the gauge temperature, but then moving on down the radiator the T drops rapidly (90 at the tank, below 70 in the radiator)

Honestly I am still having a hard time picturing the coolant flow in my head (both with Thermostat closed, and open). Does anyone have a little diagram of how the coolant flows around the cooling system? If I have understood correctly, with thermostat open, there should be zero coolant flow through the bypass hose, right? If so, then a quick IR thermometer check on that hose should tell if bypassing coolant when it shouldn’t be…

Chris T.

the thermostat sleeve moves to cover the bypass flow back to engine as the thermostat opens flow to the radiator…but the covering of the bypass hole is not 100%, not because of the sleeve being too small…but it has to have some clearance…but it is a small thin opening that remains, and the coolant is not under high pressure, and it finds the path of least resistance…mostly…so flows to the radiator. The restriction of the thermostat in the housing and the size of the opening when open to the radiator allows a coolant flow thru the radiator that is like Goldilocks and the bears beds…not too slow…and not too fast. Too slow and the circulation suffers…too fast and the coolant does not have time enough in the radiator to cool enough…that is why it is not a good idea to remove a thermostat thinking it will speed up flow. (Actual real racing exception) The job of the thermostat and its design is to constantly allow cooling, then heating, then cooling , then heating…to keep coolant temp in the optimal range. As we know the air flow, and radiator size in the xk120 is not optimal under warmer conditions or idling. So often the stat will just remain open. At idle the fan just does not pull enough air thru…compared to driving 30+mph. A supplemental fan helps at slow speed and idle…but actually blocks some air flow once at speed. As I mentioned, above 30mph, 72-75 on a cool day…80-85C on a hot day…I consider normal. That with a 68 or 72C thermostat. There are some thermostats with 76, 82, 86 degree opening. It is possible a “winter” 86C could be in…then of course 86 would be the lowest you would see once warmed up.
next topic…the fender valence…I don’t think removing it will help cooling much…but who knows…(really the issue is air thru the radiator, there is much more exit room than there is intake area., but maybe a few louvers in it…or a shielded air exit scoop…air out…but not splash water in?
Nick

I’ve found that tuning is the biggest issue after the other good ideas. Check your timing advance with a good strobe light right up to 5000 rpm. Modern fuel has different burn characteristics than the early '50’s stuff. I’ve also had trouble with the vacuum advance gizmo and this can really cause overheating. Check for the accurate part number. The “usuals” just sell generics. The carb floats might be set high or low. If this fails add an electric fan like everyone else!

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Hi all,

Great thread and interesting.

I am quite amazed at the issues you have Chris and counted to 10 before replying, as I have myself not so much experience with the XK120, having owned it now for 3 Months and driven like 1800Km so far.

Overheating seems to be no issue, rather the opposite, I find the 120 really cold blooded. The highest temperature I measured was 77 (radiator coolant) and 87 (block, through the 123-ignition sensor) and this was after 45 minutes traffic at 18°C ambient.

Although plenty of suggestions have been given, just my two cents:

  • System flush cannot hurt, and there are plenty of safe products

  • I know it’s opening a can of worms on originality, but I really like the aluminium radiator

  • Electronic ignition is a simple modification, as with the radiator you can keep the original components and you know what your timing is doing, always

  • CO measurement is simple, and you know you are not too lean

  • Needless to say (has been said) thermostat is a matter-of-course service item

  • One often forgotten thing is the water pump belt and the associated pump and fan: is it tightened and turning well with the engine? In my Continental R, with a huge fan, while you would not hear it slip, it was slipping, enough to limit the the pump rpm’s.

Otherwise, I frankly cannot report that 120’s have over-heating issues, even with the original radiator (drove like 1000Km with it) it just would not overheat after highway driving and into traffic -ambient temperature being moderate-.

My two cents.

My 140 had a problem of overheating on the way to the Tennessee AGM (ambient temp in the 90’s) years ago. I plugged the bypass and put a pusher fan in front of the radiator. It is only used when I get stuck in traffic. My fan is on a manual switch but could be rigged to a temp switch.

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Thought I’d close this one out. After some experimentation, this is what seems to be working for me so far:

I’ve gone the Rob Reilly route by closing off the coolant bypass hose entirely and ditching any sort of thermostat. It hasn’t been lost on me also that my two cars came originally to me without any thermostats too. Anyway, the result now is that when driving in stop-and-go Memorial Day weekend beach city traffic, the temp reads a stable 80 C with occasional spikes as high as 83 at long signals. I do have an auxiliary electric cooling fan, but I haven’t had to toggle it on once when driving. The one thing that it seems to be useful for is that upon car shutdown, running the fan for about five minutes keeps it from spiking above 90, rapidly cools the motor down, and there’s no loss of coolant out the overflow tube. Long term, I’ll probably go with an auxiliary coolant overflow tank, but for now there seems to be no rush.

The one other modification of note is that I cut off the foot of one of Jan’s extra pair of pantyhose and it’s used to possibly catch any debris inside the upper radiator hose before entry into the radiator top. After some weeks, I haven’t found anything solid so far. Cheap insurance for the cooling tubes as I had the radiator rebuilt not so long ago.

This probably isn’t the overall answer that others enamored with bellows thermostats and bypass hoses are looking for, but it seems to be working for me so far in this part of the world.

Side note: at slow speeds, these cars are real tanks and my arms are getting a workout from manhandling that big steering wheel – as you old timers already know. Power steering has spoiled me in recent years. That’s okay though, the fun factor outweighs all else.

does the coolant temp stay above 70-75 while motoring as cruise on a normal day. If so…good, as an issue with no stat is that coolant passes too quickly thru the radiator to cool…too quickly thru the block to remove the block heat and heat up. The stat along with opening and closing to regulate, also is designed to restrict flow to the engineered gpm. As to steering…will depend some on your tire type, and tire pressure. Try 2-4 lb higher, altho the feel of road bumps may increase.
Nick

An XK120 that runs too cool. Sounds like a problem I’d like to have.

I’m running Excelsior (Coker) radials but haven’t checked their pressure in some time. Thanks for the reminder and tip Nick.

@Chris_Potempa , out of interest - how did you block off your bypass?

Chris Tann.

Chris, I removed the top of my bypass hose where it connects to the thermostat housing and plugged the hose with a brass plug that I turned on my lathe and secured with a hose clamp. On the thermostat housing, I unscrewed the bypass hose outlet fitting* and fitted in its place a plug with the correct threads I also turned on the lathe. This makes it easy to change everything back to original .

*Note: My car is an early one so it has a removeable thermostat housing bypass port fitting in contrast to later cars where the thermostat bypass port is cast integral to the housing as a whole.

I think it possible, née likely, that lack of thermostat in temps below 70F or so could result in coolant temps below 70F.

Another methond i read awhile back and used on my car is to block off the bypass port, i used an old sleeve off of a bypass style stat and used a little epoxy to hold it in place. Then drilled 3 3/16 inch holes in the rim of a regular style 70 degree thermostat. This allows a low flow for quick warm up and then normal operation once the car is warmed up. Seems to work pretty well