Overheating under load! Chasing for 2+ years

Hi guys, im almost giving up, i have been chasing this problem for 2+ years, since i biught the car. it was overheating quite badly and would go to H in just 20 mins of normal driving, then it took me 2 years to rebuild whole cooling system literally twice
coolant hoses, filters, all rails, engine flashed, every single seal is new, thermostats are measured, boiled, 2 different pairs, checked. radiator recored twice, 30A efan controlled by thermoswitch, everything cleaned. apart from that i did replace all sensors, ecu, fuel line, injectors, harness, checked compression, checked for head gasket, installed thebra filters to see coolant flow. Every possible advise from forums, books taken, refilled the car 4 times, slowly, fast, nose up, down, left, right.
Literally everything…

Now, this winter i thought i got it, i was taking it for 1h normal runs and it was okeish, it would go slowly to N, then just a hinge to the right of N, and would go back and forth there as efan kicks in.

But today i took it for italian tune up, and wanted to make sure that overheating is not a problem, as i became psyho about it.

And, as i would slowly drive it, all seems good, then i started to floor it , highway, then mountain roads, avg speed is around 50mps , but i did floor it after every turn or curve, and temp slowly went pass N, then hinge to the right, efans kicked in, temp slowly kept going right, as long as i was giving her what she deserves she didnt like it. I stopped when it was halfay between N and 3/4 (i just understood that thermostats are open, both efans working, im driving fast enough and temp would not go down, so there is no point to continue that experiment) on the way back i was driving normally and temp slowly went back to N, as i got back home it was N, under Park it wen bellow N untill efans switched off.

I dont know what do anymore

P.S XJS 1992, marelli 5.3 v12

Just verifying: when filling with nose up, the bleed was open (plug was out), correct?

What year is your car and what engine does it have?


My initial gut feeling …and I think Jerry’s as well…is that you still have air in the system.

But, also, what are the ambient driving conditions? And have you done anything to check the accuracy and correct operation of the temp gauge?


@Paul_M_Novak being that he said “thermostats are measured, boiled, 2 different pairs” I’m thinking V12 for the engine, but whether 5.3 or 6.0 … ?

Your symptoms tell me that you have a coolant flow problem.
Either the thermostats don’t close the bypass completely, either the rad is too small and or restricted, or you have an internal blockage somewhere.
Did you ever check the metal coolant lines at the side of the engine? You could inspect that way the condition of the internal passages and maybe also do the Lutz mode.

It could be also head gasket, but I guess you would have other signs as wel.

yes, and i think i did it several different ways, last thing i didnt try is to add air bleed valve near heater tap

sorry, 1992, 5.3 v12

but i really dont know how else should i fill it up, i literally did it like 5 different ways.
weather is normal +25c, temp gauge is correct because it corresponds with thermoswitch on/off temperature

could be, but how to check that?
thermostats checked in the boiling water, so they do close/open up to specs
rad is original, but was recored twice by questionable shop, that could be the problem, but then why it works under normal driving condition and failing under load? if its restricted, its restricted, no?

metal coolant lines were replaced, engine flushed, housing resealed. But thats true, since i cant see inside engine passages i cant say if they are clean.

head gasket is probably no, as i dont have a single sign of that.

It’s not air in the system after two years. Any air in system eventually finds its way out.

I’d agree its a flow problem. Have you removed your water rails at side of engine? When i did mine, i was surprised to find gunk lining the passages from the block. Not enough to limit flow. But for you they could be almost blocked.

On an engine this old, I think it’s a must fix. The rubber top hat seals definitely should be replaced. If the steel rail is rusted, replace. Mine wasn’t bad, but I took the opportunity to buy stainless steel pipes.

all 4 housings were off, cleaned , resealed, new brass pipes, new pipe seals. As i said, if you take all coolant system apart, this is how i did mine, the only part of the coolant system i didnt replace/clean/check is engine block itself (but i flashed it)

Ok, let me drop some photos of the engine block, mb im missing an elephant in a room

Also, the only thing im not certain about is - radiator, could bad recoring lead to that behaviour? (overheating under load)

Same here, but you are thinking water flow, I’m thinking maybe air flow?

He’s been chasing the issue for 2 years.

Water/coolant level has been drained/filled/tinkered with many times

If one had these issues right after a coolant change, I’d agree air. But after a few days, and definitely weeks, that air will find it’s way out after a few runs. The danger of not getting air out during coolant refill is that first or second drive right afterwards.

I tilt my car up on front Left, and even have a bleed valve for heater hose. I put every last drop of coolant I can fit in there. I take it for a simple drive just to warm it up and let it sit overnight. I always have to top off by maybe half a cup. Maybe two runs. But after that, I find it’s full.

4 housings? I assume you mean the two water rails? OK, that’s good.

My temp always goes down to the thermostats (190F) when I’m moving, as long as it’s not too hot outside. It creeps up while idling, but electric fan kicks on and keeps it from moving above 200F. This means my radiator is working fine, and I haven’t had it recored yet. Hopefully yours is similar?

If your cooling system is totally redone, the last piece of the puzzle that you haven’t mentioned is water pump? And dumb question, how is the belt to the water pump? Nice and tight?

Alright, i think you gave a good food for thought, even when my car doesnt overheat under normal driving conditions, it still passes the thermostat opening temperature and only getting stopped by efan, even while driving medium speed. I think thats not right.

now whether its an air or coolant flow, i cant say.

Regarding air, last year i hoped its an air, as it would be an easy fix, so i drained, filled coolant 4/5 times using different methods.

BUT, there is one small sign that something is maybe not right with air trapped. i have transparent hose from metal to plastic tank as per book. HOWEVER even after 4/5 refills, several drives, this hose is never full of coolant neither while its hot, neither on cold. Only half way full, im not sure if its normal or not.

Radiator on another hand, lets think together, if its a bottle neck, would it cause worse cooling (not enough) under load, and sufficient without any load??

P.S water pump is new, belts new, tight.

I (and others on Jag-Lovers) have been mislead many times before about what car and engine the post was about. In order to avoid wasting time and effort I always ask to be sure what the model year and engine is at issue. When that very basic information is missing, I ask. :wink:


Have you checked your ignition timing? Does your engine have a vacuum advance and is it working?


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As a test, I would remove both thermostats and see what it does. I have previously owned an engine with similar frustrations in that it absolutely refused to open its thermostat. Mystery air bubble in the wrong place or some blocked passage in the head. IDK. Never solved it, but it would run (too) cool with no thermostat.

i have 2 transparent coolant filters installed, so i can clearly see when thermostats are open. Ok, i cant say if they partially open or full, but they do create flow that i can see.

Also, with removed thermostats half of the water flow would go back to engine no? since its 2 ways thermostats. they should close passage to the waterpump?

sorry, its 92 marelli 5.3. I have edited 1st post

I completely agree …

Well, except that if the poster shows one Jaguar listed in their profile, it should be … should be … a safe presumption that is the vehicle in question.

Other than that, as a poster, always state year, model, and engine.

As a reader contemplating the question, absolutely always the best thing to do is to verify that information.

I, and I presume many other J-L posters (based on the lack of frequency of such question being asked first and foremost) answer based on the other available information.

Most of the time it seems like we get it right … or close enough to being right that the answers fit.

That said, maybe an initial ‘pre-post’ like text box could be added added at the beginning of the first post in a thread which requests year, model, engine information as a reminder to the poster?

Hmmm … anyone with a 3D printer and sttong, clear, print media suitable for these temperatures like to experiment printing out, and trying, clear thermostat housings.

If successful, there may be a small market for them to be installed and used for test purposes?