XK120 Overheating Cause?

Can I get some thoughts on possible causes for why my '120 overheats (670236 – April '50) and more importantly, what I should be focusing on to effect a fix? It slowly drifts in to an overheated state after about 10-15 minutes of just idling only. Fixing this has been made all the harder as I’ve done so much work to it over the past two years leading to this point – there’s sure to be something that’s still out of adjustment.

*No water found in the oil and no oil found in the water.
*All valves found to be within 0.002" of specs.
*Compression on all cylinders found to be within 10 psi of each other. No oil pressure issues either.
*Radiator was boiled out and fitted with an extra row of cooling tubes.
*New later XK120 water pump, cooling fan, hoses and tested NOS bellows thermostat fitted (Smiths).
*Rebuilt distributor with diaphragm and copper core plug wires.
*Rebuilt carbs with new throttle shafts and acetal bushings line reamed to fit.
*NGK BP5ES plugs gapped to 0.022" and points to 0.014"

The car starts easily enough and has been static timed per the book, but if I continue to rotate the distributor clockwise, I notice the idle speed continues to noticeably increase. Also, I believe the carb mixture to be rich as my eyes tend to water in the garage even with the door open but no breeze. It also seems to run just a little “lumpy” at idle. I have both of the jet screws initially turned 2 1/2 turns out as that’s what one of my books said to start with.

Thoughts on what to focus on next in terms of priority would be appreciated. Thanks.


In what way does the overheating manifest itself? Only on the temp gauge or also cooling fluid escaping or even steam? Reason I ask is: have you calibrated the temp gauge?

Bob K.

At idle for 10-15 min will heat up any xk120. What does coolant temp do while normal driving, on mid temp day? Do you have a 4lb radiator cap of the correct type. If your car runs at 72-82 on a normal drive…I think you are just fine. It may climb at stop lights thru town. Cool down quickly once back to motoring. IMHO normal. Run 50-50 antifreeze, unless you need extreme cold weather protection, use one bottle of a “water wetter” cooling aid…(only gives you a few degrees…but why not.)

as to other set up items (not having to do with your idle temp) :
If you had your distributor rebuilt, vacuum unit is good and working, you may try 025 to 028 on your spark plugs, what is your timing? 5 or 7 or 10 BTDC? I’d suggest 7. and diizzy points at 016.
suggest a check of your spark plug wires for resistance…just pull one both ends, can leave pretty much in place, and measure…should be quite low resistance over the length of the wire.
They tend to run a bit rich…especially at idle. How is it a normal driving rpm. and do a plug check after a good run…where you can pull into garage without much slow rev time. You can also follow the manual instructions with the “lifting of the dashpot side pin” test. For your early car/carbs the lift pin test tells you about the setting of the OTHER carb. Err on slightly rich…a brown to blacker plug but not sooty.


How about gunk buildup in the block, head, intake manifold?

You could try putting a clamp on or plug in the bypass hose so you can be sure nothing is being bypassed when it is warmed up.


My first reaction to your post was: They all overheat at prolonged idle, don’t they? But seriously, as has been observed, what does it do when you drive it? I got caught up in a massive traffic stop/go scenario a week or so ago in mine (due to the closure of a major highway that routed traffic into our small town and if I had been aware of could have been avoided) after tolerating this for 10 minutes the temp gauge was up to the 90C point (ambient temperature was not excessive, we are talking Ontario in April). After leaving the worst of it behind I had to throw the clutch in and rev it a few times as it was already percolating the fuel in the intake manifold. As long as is mine getting some airflow through the radiator even on very hot days it will run at 85C or cooler, but it definitely does not like prolonged idle or traffic jams, even though I had the radiator re-cored. They certainly were not designed with today’s traffic in mind.


yup, as Chris said, a bit more…even on a hot 90+F summer day…once at speed…above 35mph…mine still runs at 80-85C. On a cooler day in the 80sF, will run at 70-80. so with 50 50 antifreeze, boil point is raised a little, still I try to stay below 90C. Because of the coolant temp climb in slow traffic and stop lights, I installed a windshield washer pump/water reservoir kit up under the front fender…up high and in back. Run the little hose to the front of the radiator, split it with two nozzles aimed at the radiator front. In traffic when the coolant temp gets to 85 or 90…push the button–sprays some water on the front of the radiator fins…increase idle a bit at stop light, and watch the temp drop by about 10D…it will work a few times, to get you thru traffic. The option is to turn off the busy street to some side street where you can get back up to 35mph or so for a while. Just parking it will cause heat soak to the carbs area and a chance for a temporary no start due to vapor lock til it cools down. Nick

That was my first thought. When I did my rebuild I was astonished by how much crud was in the block. The head and intake manifold too but especially the block.

Wow, well this is certainly eye opening. These replies read like the memories I have of my father’s '62 Oldsmobile coupe. As a kid, I’d sometimes ride with him on a Saturday to the family business in Inglewood from Palos Verdes on surface streets only (this is southern California). We’d get mostly there before he had to turn in to the big Xerox factory parking lot where a jug of water was then removed from the trunk and he’d slowly top up the radiator. I remember him saying how the car was not designed with a big enough radiator and it was something he just lived with. I think that was the start of the end of domestic cars in our family. In '73 he replaced the Olds with a new Datsun 240Z. Anyway, there’s a sense of deja vu with the big motor/ small radiator thing. Unlike my father, at least we have workarounds such as aluminum radiators and auxiliary cooling fans. Moving on…

When driving my car at speed, yes, the temperature then comes down. It only seems to be an issue with prolonged idle. Otherwise, driving the car is a non-event. Nice power and no pinging so far but I have yet to pull some steep grades.

Radiator cap is a reproduction 4psi item. Coolant is the standard orange stuff at a 50/50 ratio with distilled water.

Timing is statically set between 5-7 BTDC judging as best as I could via the teeth on the flywheel before that little arrow.

Gunk in the block, head, inlet manifold? Nope, no smoking gun there. About what you’d expected from a previously well maintained but old engine. Of course, I can smear some black goo off with a fingertip, but, there’s hardly any thickness to it.

The gauge was professionally rebuilt within the past year. No steam, there’s just the beginning sounds of percolating noises in the radiator when the needle is around 100 – then the car starts running rough at idle and wants to die.

I think I’ll continue with the fine tuning and look for any improvements. If it’s still an issue then I’ll consider an auxiliary fan. Somehow, though, that feels like admitting defeat and makes the ownership prospect of a '140 that much more appealing – in spite of the ugly bumpers :sunglasses:

1 Like

I think your car is just fine and normal…all set, other than maybe adjust of the two ignition gaps, a little fine tune of carbs. I do have the aux fan only used in traffic when I also use the water mist spray…the two together, rarely used, get me thru a few slow minutes…I will often drive 10 miles further on roundabout back roads to get to the other side of town to avoid the dozen blocks of slow down and stop lights. Why worry…and the country roads are much nicer anyway, less risk of being rearended.
and…lucky you: The XK140 cooling system is BETTER than the xk120.

Going off on a tangent with an observation here:

Would punching louvers in the interior engine bay splash shields assist with cooling by helping to disperse engine compartment hot air? As opposed to doing so with the bonnet? No, I’m not thinking of doing this. But I was thinking of this after observing a picture of Rob Reilly’s FHC and how the carbs take their air intake forward of the radiator (at cooler, ambient air temperature) in contrast to the OTS models where the air intake is aft of the radiator and at a much higher temperature. Why the distinction?


Sounds pretty normal to me particularly given your location. Kind of like the Moss box, all part of the “delights” of driving an XK120 (?!). Back following initial purchase I would take the XK up to Mosport or down to Watkins Glen without giving it a second thought, however, back then one could drive from my area past Toronto on the 401 enroute to Mosport and never have to stop. I honestly cannot recall the last time I was able to do that in recent times. Now I avoid major roads, partly to avoid the possibility of traffic jams, but also as I am terrified by those who wish to “see-what-the-funny-foreign- car-is” and close to within inches of my rear end. An hour spent out in the country driving on scouted-out low-traffic, smooth paved roads now gives me my essential fix. When my engine was rebuilt it was put in the tank and boiled/pickled (whatever they do) so no sludge clogging up the waterways. And I agree the bigger bumpers on the 140 spoiled the XK lines.


1 Like

AFAIK, the official reason for the remote air filter on FHC/DHC was for less intake noise.
However, lower intake temperature could be a factor in So Cal.

Rather than punching holes, you might try removing the lower valences and see what you get.

There have also been people who raised up the rear of their bonnets a bit to let some heat out there without resorting to louvers.

Do you have a secondary confirmation on the engine temp?

important is to get all possible air thru the radiator…making sure the closing panels on each side and bottom block off pathways for air. With exhaust manifold on the opposite side, most heat is there. I notice that after even a long drive, the top surface of the bonnet is not very warm to touch…compared to say other surfaces from solar heat. Even with the big iron engine…not far below. Cool air to carbs as we know has become important in modern cars. I noticed in Formula 1 there are rules about the fuel temp…has to be a certain minimum for the start grid…cooler fuel-more horsepower…same as cool air…more dense. I did wrap my coil in exhaust insulaton wrap. When a kid, My dad made an air duct to the fuel line in our old car, out of a few emptied peaches cans…we had to eat the peaches road side…for a cure for summer vapor lock…it worked.

Chris, search the E-Type archives for Michael Frank’s Thermostat White Paper. I provided three photo’s showing that the Smiths bypass thermostat does a poor job of sealing-off the XK120 bypass port after the thermostat has opened. I would put a restrictor in the bypass hose to assure the bypass circuit isn’t contributing to the problem.

Also, check that the cooling fan isn’t mounted backwards. This was the case with a friend’s XK120. When the fan was mounted backwards, the car needed an electric supplemental cooling fan. After that was sorted, the electric fan became unnecessary.

Chris, good points on planning my outings. The first time I had it out left me rattled by the inordinate amount of attention it was getting when all I really wanted to do was focus on traffic, monitoring the gauges and simply taking in everything the car was signaling to me. I foresee in the future drives done very early on weekend mornings, nearby outings during non rush hour times and back road trips and holidays when others aren’t.

Rob, I’ll consider taking the exhaust side lower valence off. I already keep the intake side one off as it makes it so much easier to work on and make adjustments. I don’t have secondary confirmation on the engine temp but your comment serves as a reminder for me to pick up one of those handheld IR temp meters that Harbor Freight sells.

Nick, I have those radiator side cooling valence brackets on along with the one at the bottom of the radiator. My father used to have stories about vapor lock too – mostly as an excuse to extend a date with mom when he was still courting her.

Mike, I’ll look into pinching off the bypass hose temporarily to see what effect that has. Rob mentioned it too so there must be something to it. And I’ll search the E-type forum for that paper.

Left unsaid so far – and I write this for posterity – is to check the brakes and wheels too. One of the first things I did when I got the car was to do a re-rebuild of the master cylinder (early type) since one of the rubber cups wasn’t seated right caused the brakes to drag. Also, a previous mechanic had lubed the front hubs and tightened the spindle nuts so there was NO end float making it difficult to push.

And don’t forget fuel line clothespins as heat sinks!

Or in our case for those of us with the early cars, retrofitting those thick black phenolic spacers between carbs and intake manifold along with the longer studs. Thanks.

Probably all of us with early cars should do this.


Just be careful if you remove the intake side valence that you do not get caught out in a sudden downpour. I recall being somewhat embarrassed when driving a young lady home years ago and getting caught in a shower. The lack of the side panel combined with the spray from the RF wheel quickly drenched the distributor resulting in rapid flame-out.


Good point, thanks Chris. I’m already always watching for so much as a single drip of water from the water outlet the designers conveniently placed above the distributor cap anyway.