XK 150 running hot

I have a problem with my XK150 (as usual).

When idling or at slow speed the temperature will rise and the eventually the coolant will boil. At normal road speed the temperature is ok.

Thermostat opens correctly at 78 deg centigrade. Thermostat of correct type and the “sleeve” that close the bypass moves correct way to close the bypass.

The radiator is restored (new cell package). When running with the electric fan the temperature drop was ruffly (hard to measure correctly) 30+ deg from top to bottom.

Engine running nicely, carbs set by a specialist who did not detect any problems measuring the exhaust gases.

The temperature on the exhaust side of the cylinder head varied from 109 deg (fire wall) to 102 deg front .

After running the engine to 90 deg I removed the spark plugs and cranked the engine on the starter and could not detect any steam.

Checked the compression at the same time - even and around 160 PSI.

500 revs at idling.

The water pump is a new high capacity type.

The bypass is not closed well enough – the thermostat housing is corroded and some water slips by at low revs due to a lower flow of water (symptomatic high capacity pumps?) and the resistance to flow imposed by the radiator (even if it is “new”)??

Any clue what to look for?

Hi Jonas, on my 150 I use a 74 degree thermostat particularly in Australia in summer. You said you have an electric fan, is it on the engine side or in front of the radiator and do you still use the original twelve blade fan? Like you I have also thought when changing the radiator core we restrict air flow through the radiator which at best is marginal.
I have tested air flow through the radiator when the engine is at idle and found very little if any airflow at the bottom section of the radiator, the engine fan is a long way away from the radiator at the radiator base. I have also placed cellulous foam around all openings around ,under and on top of the radiator so as to direct air through the radiator and not bypass it. Ignition timing is also very important to have as per specification. On my 3.8 150 S I am now using a Spal electric fan only, attached to the radiator on the engine side . The fan is not used when I am travelling except when I slow down or in traffic. I have the fan with a thermostat and a manual switch which I use before slowing down to maintain engine temp.I can now climb long steep gradients and not go over 80 degree. Has the block been cleaned out?

Did you change the radiator cap or get a new radiator neck with your cell package? Make sure the spring in the cap is compressed to enable your cooling system to pressurize properly.

HI Ledo
The fan is placed on the front side of the radiator its a Kenlow 150 W fan.

It’s the old neck and I have ordered a new cap just in case…

of course many forum threads on hot running XKs…is this a new problem? Cooling is marginal…in steps, from worst on 120 to a little improved by 150 via improved water pumps, radiators etc, but still marginal. Coolant temp will rise in traffic, stops, idling. A thermostat, once open…does nothing else, the open temp just tells you when it is open, runs coolant thru radiator. It is then the job of the atmospheric air to lower the temp of the coolant. The coolant has one chance: as it passes thru the radiator. Air temp matters: hot day= less cooling transfer. ALL possible air thru the radiator and not around matters, at low speed it is the fan, so a shroud helps pull air thru rather than re-circulate hot engine compartment air. Coolant matters: the anti freeze mix will raise your boil temp, altitude will lower it a little, water wetter product will help by a few degrees. The pressure cap, only 4-7 lb helps raise boil temp a little, and keeps coolant in the system. All in all…100C is not boiling in the system, but still try to avoid anything over 95C. IF…it used to run cooler in same conditions but now does not…can be water pump issue, a collapsed coolant hose, worst case head gasket–look for bubbles in coolant (dont open when hot…start cold with cap off and run engine.). More complex is how a radiator is most efficient…depends on the rows, fins, and spacing. Coolant has to pass thru at the most optimal rate, too fast is not good, nor too slow. Same with the air flow.

Also have the water passages in the head been cleaned out?

What do you folks think about the idea of putting some kind of a plug inside the bypass hose or bypass port and running with no thermostat? The warm up time would be longer, but it might be worth it to get the running temp down.

no thermostat is complex Rob…once the bypass is closed, with stat in, very little water goes thru the bypass (some does…it is not a perfect seal at all), blocked off of course none would go thru the bypass to engine, so warm up is slow. Once the stat opens to the radiator it is like having no thermostat…the flow is open as much as it can be given the design of the system and water pump design to pump. The engine will heat up with no stat…the same as with one that is fully open. It is a closed system, and the coolant has now where to go but in that system, it can’t go “more” or “faster” except in small sections. Too fast thru the radiator does not give time to be cooled by the air: it is like blowing on a spoon to cool the soup…blow for 2 seconds or 10. The system was designed with a stat in it…don’t worry about a “restriction” due to the stat. The temp would only be “down” for a longer time at warm up, and that is harmful to the engine in a few ways, one is water vapors…those white puffs you see at cold start up out the exhaust…more of those equals more water past the rings and in the oil, colder for longer requires the choke, colder for longer increases wear. Same applies for thinking a lower temp stat will help…nope. The stat will open at a colder temp…but the normal engine running will bring the coolant temp soon to whatever it would have been. The options to improve cooling, if the system is clean and functional are as I described above…Race cars will have oil changed after one event, and engines rebuilt as needed, and run at WOT at speed…apples and oranges as to street cars. Nick

Yes I know all that. I just wondered if anybody had tried it in an XK.

Hi, Some time ago I did a air flow test of the radiator under various conditions on my 150. The chart gives three different situations and states air flows on a grid pattern on the face of the radiator. The 150 cooling system did not change across the production range despite a large increase in HP output, 265HP in my situation with lots of under bonnet area taken up with triple carburetters. Note that the fan tips are above the top of the radiator and under the cowling which sits above the radiator. The bottom of the radiator sits below the top of the front crossmember and the steering rack sits on top of the crossmember. With the fan further away again from the radiator I recorder zero air flow from that sector.with the engine fan only in use.

These are my findings and I am not an Engineer just an interested observer, not sure if anyone else can replicate my results.

Road and Track magazine had a letter published, Jan 1952 issue, “Cooling Modifications for the XK120”, by L.P. Saunders, Chief Engineer, Harrison Radiator Company…reprinted in the R&T on Jaguar collection. While water pump design and flow is discussed, the do-able things are fan shroud 4 Deg, radiator core and fins 8 Deg, closing the side and top-bottom space 3 Deg, more efficient fan thru speed increase (or fan blades) 4 deg. We have discussed that anti freeze raised the boil point: with a 4 lb cap, plain water it is 225F at sea level, 4 lb with 50 50 anti freeze 236 F (113 C) , with 60 40 it is 241 F. Heat expansion will drive some coolant out of the overflow. A Water Wetter type product when in anti freeze mix gives a few degrees of cooling, does not change the boil point. Since boil is 113C with 50 50, IMHO we can be ok in the low to mid 90s C for short periods of time. The engine won’t like above 95C. Nick

Thank you all for your helpful input. My conclusion the car has a flow problem in the cooling system - either water or air. Most probably too low air flow at low speed but I have a concern with the rapid increase in water temperature when shutting down the engine. But I suppose there is a heat build up in the engine which heats the water? I am looking for a suitable replacement fan both in size and power consumption. I’m not sure about the power of alternator but I think it’s 350 W. In my mind maximum power of the fan is roughly 200 W? But depending on the duty cycle of the fan.

Try a burp of the system…google it, but start cold, rad cap off, and can also loosen whatever coolant hose is at the highest point slightly…let run and warm up until thermostat opens…be cautious of hot water at that point. 80C is 176F. Note there are some precautions on after market fan blades that can contact other parts. Heat soak rise in temp after shutoff is normal.

Hi Ledo…interested on your electric fan set up…have you removed the main engine fan…also are you useing alternator or is the dynamo capable of powering the fan…your figures mention standard shroud…is this a shroud that spal supply…thanks…Steve

Hi Steve, I have an alternator installed to run the fan and have replaced the amp gauge with a volt gauge. The shroud is the original 150 set up. The Spal fan is the only fan having removed the original 12 blade fan. The electric fan is rarely used whilst driving,when approaching hills I would manually switch the fan on. The fan is used at low speeds and more importantly at idle.I have now fitted an aluminiun radiator. The set up works for me and I am happy with these reversible changes, although I can’t see me changing them in my time with this car which I have owned for 52 years.

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