Tempers...and My 3.5 Litre Saloon Boiling Over

Did a short run of ~30 miles over 2 hours yesterday. Details to follow. Got home, backed into driveway, and car dieseled a bit before shutting off. I could hear the coolant boiling, and it dumped about a quart of ‘green gold’ out the overflow. Next week (10-14 July) I’ll be participating in the CCCA central Ohio mini-CARavan (tour), and Grand Concours (show) putting up to ~100 miles per day on the car. Appears there will be 8-9 Duesenbergs, over 70 Packards, a handful of Pierce-Arrows, a couple of Rolls, several Bentleys, and an assortment of other pre-war cars…including one lone Jag…mine.

8:30 yesterday a.m., it was 85-degrees F (29.5 C), with 85% humidity. Sweat was rolling down my arms and dripping off my elbows. By the time we got home a little before 10:30, the temp was 93-degrees (34 C). During the trip, I drove ~6 miles in city stop-and-go traffic, at 35-40 mph. Then I accelerated to 55-60 mph for ~7 miles before a 30-minute breakfast stop. Another couple of miles and we arrived at out photo op. The car was not boiling or overflowing when we arrived at the photo op. After sitting there for ~30 minutes, we reversed the course to home, non-stop.

Approximately 2 years ago, I did a 24-hour hot-flush of the engine and entire cooling system with Evaporust. After that, the radiator top tank-to-core seam developed a leak, so I had the tanks removed, core inspected and confirmed to be in excellent condition, and tanks reattached. The water pump seems to have good flow. The car does not have a thermostat, but otherwise is in original condition. Temp gauge is inop, which concerns me, but with 3 days to go before leaving for Ohio, there isn’t a lot of time to do much with it.

I’m running a 50-50 mix of ethylene glycol. I changed the oil about a month ago and only drove it once since then for about 40 miles. It briefly dieseled when I shut it off then, but wasn’t boiling and didn’t overflow the radiator… After re-marking my dipstick prior to the oil change, it took ~14 quarts of Castrol 20W-50. I don’t think my timing is too far advanced, as the engine always starts easily and has never pinged/knocked.

Granted, yesterday was a hot day, but I was surprised and disappointed the car could not handle it. With Ohio temps expected to top 88-degrees F next week, I’m extremely concerned it’s going to embarrass me and itself on the tour, running with perhaps 100 very high dollar Full Classic cars. Thoughts?

The 3.5 litre is a bit marginal for cooling how about trying one these coolants that have better properties than water.
e.g. Red Line - SuperCool Coolant with WaterWetter 2 US Quarts (1.89 litres)



If the car does not have a thermostat, is the bypass to the water pump blocked? If the thermostat is out and the bypass is not blocked then much of the hot coolant circulates immediately back into the water pump without going through the radiator. I have seen this circumstance before where people thought removing the thermostat will improve the cooling, much to their dismay.

Another scenario is where the coolant level is too full when cold. After warming up the excess coolant will be ejected until the correct level is achieved. You indicate boiling in your first description, which should not be happening if coolant flow to radiator is adequate.

A third scenario is the timing is way off and the engine is running way too hot.

I run a standard 3 1/2 litre engine in Southern California summertime without overheating issues.


Couple of things to check, re the running on -
ignition timing too far advanced,
low octane fuel self igniting in the combustion chamber when the ignition switched of, this may be caused by excess carbon deposits in the combustion space, try higher octane fuel.
As Roger mentioned, be sure radiator by-pass blocked off.

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Thanks Peter. I bought a bottle of Water Wetter yesterday morning but haven’t added it yet. I just ordered a half-gal of the Red Line -Supercoolant with WaterWetter. It’s supposed to be delivered sometime Monday. Hitting the road in the wee hours Tuesday.

Thanks Roger & David. The car has never had a thermostat since I’ve owned it. I haven’t been able to find the correct housing for it. No, I don’t think the bypass is blocked. I’ll do something with it today. What’s a good block? Hardwood plug?

It’s not just pushing excess out the overflow. With the engine off, it’s easy to hear it boiling. And as mentioned, if the timing were way off, I’m sure I would get pinging or knocking at one time or another.

Not a lot of choice for fuel, unless I put corn-oil in it, which I don’t want to do. I’ve been careful to only use ethanol-free, with stabilizer.

My bypass blocked by stainless bolt and washer through the original thermostat mounting. Others have put dowel rod in the actual bypass hose. My car also has the ‘Aussie sock’ as an inline filter as my radiator kept clogging up after my rust treatment

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Air flow: upgrade fan or add auxiliary electrical fan

Water flow: test pump output and radiator restriction (less toxic at 100% water because I’m picturing doing this outside and making a mess)

Maybe more storm than brain?

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Thanks Tom, but I show my car as original, so don’t want any obvious modifications, like an add-on fan. I had the radiator tanks removed and the core inspected not too long ago by a guy older than the car, who’s worked on radiators all his life. He said my core is in excellent condition. I did an open flow test of the pump when I hot flushed the system, and although I didn’t get into specific flow-per-minute specifics, it appeared to be moving the coolant adequately.

I test-fit a hardwood dowel into the line this morning, then decided not to use it. Wood will decay over time, and I don’t want to risk bits and pieces of rotten wood breaking free and causing more problems. I found a tight-fitting plastic bottle cap, and forced it as far into the line as I could. I’ll wait to refill the system until I get the Red Line Supercool WaterWetter on Monday.

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Hi George, check the Pre-XK forum thread Replacement Thermostat from Moss for more details on various solutions to the missing thermostat. This thread is from May of 2024, there are other older ones.

The wood dowel insert works well. I’ve never seen a problem from decay when I have taken one out to inspect in other people’s cars. My own car has a functioning thermostat so I have only limited decay-inspection experience. I suppose a plastic piece could work as a dowel also. The key point is to assure the dowel cannot come free in either direction in the pipe while blocking all, or most, of the bypass flow path.

If using a metal block as described in the other thread, take care to assure the assembly cannot come free over time, otherwise the overheating issue will arise again.

As for fuel in the car, I’ve been using gasoline with ethanol, lowest octane is cheapest, for 20 years on my car. No issues that I know about have arisen. The car was my daily driver for maybe the first ten years and is occasional drives during the week now. The car will run fine on nearly any gasoline-type fuel since the compression ratio is pretty low.

As for water wetter, there are those who praise such solutions. I am an experimental physicist and would like to see proof of their benefit. I doubt there is harm in using such additives but my car runs fine in hot Southern California summer weather with the standard setup as it came from the factory. And my car has 70-thousandths over pistons which means I am running more heat through the engine than original design.



I had the exact same problems with my 3 1/2 Litre Mark V DHC even in Finnish summer, especially in city traffic before I did two things:

  1. Got rid of all air leaks that would lean the mixture and raise the idle when warm (or hot!) Once I rebuilt the cylinder head idle was a nice 450 rpm.

  2. Modified the thermostat housing to be able to use a Jaguar 4.2L thermostat, aka ”Ed Nantes” thermostat housing mod.

After that absolutely no boiling nor dieseling. But of course after a hot run I always run the motor for 1 min at idle after if has been stopped for a few mins, so the coolant gets some circulation, in the MKV engine bay the heat has nowhere to escape once the car has been stopped and all the extra heat will be in the cylinder head and the exhaust manifolds. YMMV.


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On my '38 SS with no thermostat, I have a large stainless steel flat fender washer with a 1/4" stainless steel bolt and lock nut through the bypass port inside the thermostat housing. This effectively blocks off the bypass function.

I have no difficulty with overheating.

An alternative would be a brass or other metal dowel rod inside the bypass hose. But you would want to remove the thermostat.

Thanks Rob. I forced a plastic bottle cap into the bypass hose. Haven’t test-driven it yet. Was supposed to be on the CCCA tour and show in central OH right now, but several days ago my wife caught COVID, so I didn’t go and forfeited my $1400 registration fee. :frowning: