Mark V over heating

(Grant St Quentin) #1

Hi All,

Car is finally registered but I am experiencing some over heating issues which I am at a bit of a loss, checked the water pump while there was wear the impeller vanes where about 11mm tapering to about 8mm. the alloy plate had some corrosion about 2mm deep.

The engine galleries are clear and radiator has been refurbished. I have flushed the system but when at idle the water does not appear to move and then suddenly expands to overflow which indicates a vapour shield??on the head???

I have removed the thermostat and blocked off the by pass as a temporary measure until the modifications are done for the thermostat…? could this be the issue??

Any hints would be appreciated



(Ed Nantes) #2

Worn distributor shaft or worn carb shafts can cause over heating
Try wobbling the cam on the dist shaft { it shouldn’t] and likewise with the carb shafts [ also shouldn’t ] If they do , it can let in air m, weaken the mixture and the engine runs lean… and hot.

(Roger McWilliams) #3

Hello Grant, your overheating symptoms are consistent with bypass line not being closed. Blocked bypass should give a couple of hints as to overheating cause. If bypass is successfully blocked, then coolant flow should be slightly observable on top coolant surface with radiator cap off. Next, with bypass blocked, time from cold to warm coolant to full operating temperature should be longer than before the bypass was blocked. Finally, with bypass blocked the typical Mark V will not overheat in regular use, temperature should be somewhere around 75-80 degrees C with slow rising above that when at idle or very low speeds when radiator airflow is low (even with fan).

With a well-operating coolant system, the fluid level may rise 1-2 cm from cold to 80 degrees C. I keep my Mark V with cold coolant level just at the bottom of the circular hole where radiator fill and overflow pipe come up. Sometimes owners fill the cold coolant too high and get overflow conditions which are misinterpreted as overheating (though overheating also may give that problem).

(Grant St Quentin) #4

Thanks for the info, I have adjusted the fuel mixture as the car was running very lean, just taken her for a run the temp sat on 75-80 C as soon as I stop the car boils I think this indicates a defective water pump as there is not enough flow at low speeds? It seems to flow at RPM but not at idle or less that 1500RPM?? The timing appears OK as it runs with plenty of power up hill with no pinking.

Any other suggestions before I pull the water pump apart?



(Roger McWilliams) #5

Symptoms could be as you mention if bypass next to thermostat is not blocked well. Possibly verify block effectiveness and describe bypass blockage method in this thread.

Another possibility is non-functioning, or low water flow rate, water pump. Possibly verify flow rate proportional to engine rpm by observing flow rate out of disconnected top hose while providing fresh water to radiator. Be sure to keep electrical items dry (generator, wiring, distributor, …).

Another possibility is improper distributor advance. Possibly measure distributor advance versus engine rpm with vacuum disconnected and plugged on carb side. Compare advance curve with specification. I’ve seen overheating due to way off distributor advance. Recommend leaving vacuum advance disconnected and blocked on carb side until overall problem is solved.

Please verify that your observation of “boiling” is indeed boiling and not just rising top level of radiator coolant level. Boiling will have associated sounds immediately after engine shutoff, temperature gauge will read 100 degrees C. More description of what you associate with “boiling” may guide thoughts about head gasket issues or some of the simpler possibilities described above. And are you using straight water in these moments or is there a mix of antifreeze or other?

(Grant St Quentin) #6

Hi Roger,

The temp gauge is at 100C and the boiling in the engine can be heard rumbling, I have checked the timing and it appears OK generally if excessive pinking then over heating is prevalent. I have experienced this in the past.

I have retuned the carbies as the mix was very lean and now they work as per the description in the manual.

I have decided to pull the water pump off again and go over the impeller and the corroded backing plate and re machine this to give a better surface hopefully to improve the water flow.

I am also going to put a product from liquid intelligence that improves the tendency of engines to boil once the car is at rest as it runs perfectly on the road.

I will let you know how I fare…fingers crossed!!!

Thanks Again Grant

(Ed Nantes) #7

if you just machine the alloy backing plate , this will move the water pump rearwards and the pulley will be out off line with the crank pulley.

(Grant St Quentin) #8

Thanks Ed,

I was thinking of taking off 3-4mm and making a SS plate to ensure they line up.



(Grant St Quentin) #9

Just a quick question regarding the water flow, I have set up the car with hoses to watch the flow which is does not appear very fast then very suddenly a large volume comes out which is the sort of flow I would have expected does this indicate an air lock, cavitation in the pump or that the pump is not working as it should?


(Graham Jordan) #10

Hi Saintly1.
If you end up removing the water pump check closely the gasket between the pump and the block.
and check it isn’t fully or partially blocking the hole into the block.
I had this once with a gasket I purchased and fitted. Result was car boiled.
Also check fan is on the right way. Easy to get it wrong.
Regards, Graham.

(Grant St Quentin) #11

Thanks all seems in order the fan is the correct I think ie flow is from front to back and plenty of it, does not appear to be any blockages but the water flow is erratic. Thinking that the backing plate may be an issue so will try to sort that out and renew the seals etc. Although no leaks. Does any one know the space that should be from the backing plate to the impeller, I have about 2mm would this be an issue?

(Grant St Quentin) #12

Hi All,

Just a quick update, it was suggested by a mechanic friend that it may be worth while putting a thermostat in the system to slow the flow, I had an older original one I found in the boxes of stuff that came with the car. I checked and it opened at 80C so it has been installed.

My test run today saw the temp sit on 75-80C but did increase significantly once I stopped the engine at least 95C.

However once starting the temp dropped down to about 80-85C.

The reasoning behind the mechanics suggestion is that the flow may have needed to slowed…not sure but I will take the car through some traffic next to see how we go.

Is it normal for the temp to raise significantly when the engine is stopped?



(Ed Nantes) #13

Yes it is normal the hot water rises and the temp sensor is at the top of the coolng system. In itself nothing to worry about.

(Roger McWilliams) #14

Congratulations on possibly solving your heating problem. It is normal for the temperature to rise on the gauge when the engine is shut down. As Ed says, the water at the gauge bulb rises in temperature after engine shutdown. 95 C after shutdown is common.

Extra flow to the radiator is not a worry except possibly in coldest climate operation, it just takes longer to heat on cold startup. The original thermostat blocks the bypass when it opens. Your earlier symptoms still appear consistent with incomplete block of bypass, solved now by thermostat blocking bypass when open.

You may be done fettling the coolant flow. Still, on the electric side, making sure ignition timing with study of advance versus engine rpm with vacuum disconnected and plugged will help assure proper engine running temperature.

(Grant St Quentin) #15

Thanks Roger,

The tuning of the car seems to be a dark art, the book talks about slight pinking under load at 1550-2000RPM.

Is there a better process than this as all else seems in order. I can only assume that slight adjustment on the micrometer on the Dizzy is the process?

Any clues would be appreciated.

(Ed Nantes) #16

Firstly the dist needs to be checked for wear. Commonly the shaft gets wear in the bush and the cam starts to wear on the shaft. This is fairly easy to check. Remove the cap and see if you can wobble the cam from side to side. Wear here can affect the timing. here the usual fix is to replace the shaft and cam with one from a Bosch Dist. The dist specialists tell me the Bosch has a better cam profile anyway. The carbs should also be checked for wear See if the shaft can move sideways in the carb body. Movement will allow air to be sucked in , weakening the mixture and resulting in the car running hot.
Ideally the car should be put on a rolling road dyno where ll these things can be determined and read out.

(Roger McWilliams) #17

Hello Grant, on checking distributor for wear and advance curve, you are right there are dark art elements sometimes quickly helpful and other times too obscure or incorrect. The pinking rule of thumb works, here are some further some steps with measurable evaluations.

Service Manual page B.45 describes static ignition timing of 5 degrees before top dead centre (TDC) which is a good starting place for most gasoline engines.

Page B.61 gives further distributor specifications and conditions to check.

Page B.62 includes ignition timing remarks, including the pinking condition method.

Page P.4 has distributor specifications, including some timing advance curve numbers for mechanical and vacuum advance.

Checking distributor for wear and wobbles along with measured distributor advance curve versus engine rpm with and without vacuum advance is the next step I recommend if engine overheating persists. Worn out parts; distributor springs and mechanical advance gummed up or incorrect; or out-of-spec rotor, cap, condensor, points are worth eliminating.

On the other hand, you may be home free with decent engine temperatures now already. In that case, the measure or the static spark advance, spark advance at idle around 450-600 engine rpm, and spark advance and timing stability at around 2,500 or 3,000 rpm is a partial check of distributor function.

(Grant St Quentin) #18

Thanks Again,

Checking the fizz and carries all seems OK, I have advanced the Dizzy about 2 degrees and seems OK I will take her for a long run to see what happens

Then re tweak using Harry Potter???

(Grant St Quentin) #19

I meant Dizzy and Carbies…predictive writing!!!

(- 1950 MkV, 1959 XK150,) #20

I thought you meant you were going to use magic to tune :slight_smile: