My 2012 NA AJ133 mysteriously accumulates excessive pressure in the coolant system, and I can’t identify where it could be coming from.

I ask that before leaving a response, please read the full description and try to make sure that your reply doesn’t disregard things I’ve already done, checked or seen.

Only knowledgeable responses from those that are familiar with the system please. I list a good bit of detail that I hope can paint a useful picture of the situation to experienced wrench-turners and mechanics that know this engine reasonably well, beyond the basics.

That said, any helpful, experienced and knowledgeable ideas are gratefully welcomed. I certainly appreciate the help in this.

Symptoms are:

The problem first presented itself by a random misfire failure driving down the highway, after which I took it to the dealership to identify/fix. The dealership didn’t get to look at it for a week, and when they did, they said I needed a new engine – they said they found a tiny amount of coolant in a cylinder, and nothing more. They didn’t have enough time in the evaluation to charge me even an hour.

From there I took it to a reasonably good, experienced European indy mech that had serviced it regularly for a couple years before I owned it. He looked at it, looked into the same cylinder (boroscope) and other cylinders, and said they didn’t see anything to make a diagnosis from, possibly a slight trace of something, maybe fuel. All he saw was a slightly fouled plug. Cleaned it, ran fine.

I put a bottle of Blue Devil through it after that, flushed it out & refilled with fresh coolant, and no more misfires since, cooling system cools the engine fine.

Typically after running for 50 or more miles, excessive pressure accumulates in the cooling system and remains after cooling. Top cooling hose expands and remains so until surge tank cap is removed to relieve pressure.

Coolant level lowers by as much as 3 or 4 inches from the FILL line while under pressure, but returns to the line when pressure is relieved because hoses contract to normal size/shape then.

Pressure can get so high that slight amount of coolant might leak from the thermostat bypass port at the front of the coolant pump, but very rare and only after building more pressure than usual from driving far and in hot weather without cracking surge tank cap to relieve pressure at some point.

There is no overheating, no faults. The heater and A/C work perfect. Engine coolant temp is consistently around 185 – 190 degrees F (from OBD).



What I’ve checked:

I have replaced cooling hose from the throttle body to the rear heating pipe, the hose from the engine output pipe to the thermostat, and from the coolant pump to the thermostat. I have replaced the coolant pump and the thermostat. I replaced the FoMoCo surge tank cap (200 kPa (30 PSI)).

None of these changed anything in the mystery pressure accumulation.

Combustion test shows no exhaust/combustion gas in coolant. The coolant stays clean, no oil, and never any coolant in the oil.

I did a leak down test on all cylinders. Bank 1 had a couple cylinders that might have been about 15% loss – within acceptable limits, sound coming from oil filler cap only (no intake/exhaust/coolant leak)

Right bank solid (slight hissing at oil cap as expected). No cylinders produced even a trace of bubbles in the coolant tank (I used 80 PSI for leak down tests).

I’ve emptied/filled the coolant a few times , running the Blue Devil through it, replacing hoses, pump and thermostat, looking for clues, and each time pulled a vacuum on the cooling system, no leaks – solidly maintaining the vacuum after closing the valves for many minutes after turning off air, so there is no leak there. Solid.

Between repeated solid vacuum on the cooling system and good-to-great leak down test on all cylinders, I don’t know where the pressure can be getting into the cooling system from .

I know that coolant expands when it gets hot, but it returns to normal when it cools – in a closed system anyway.

I know that if I let the pressure accumulate without occasional relief, it will probably leak where it can.

 

I am not understanding how you conclude you have excessive pressure in your system. The overflow cap on the vehicle is what allows the sealed system to be pressurized. The system needs to be pressurized to increase the boiling point of the coolant and water mixture, without this pressure the car would boil over at a much lower temperatures, when water/coolant boils it turns into gas and expands, it needs to be in liquid form to be able to provide protection to the engine as the liquid picks up the heat from the metal, gas form it doesn’t. The cap limits excessive pressure and would overflow and vent of that pressure goes past the set PSI. Air from let’s say a blown HG would allow air in the system which lowers pressure and boiling over would happen at a much lower temperature, That will cause the engine to overheat quickly as air and gas does a terrible job at transferring heat. Are your hoses soft where it’s increasing system capacity by stealing it from the reservoir? That might be what’s it’s happening. I guess you need to run a pressure test in the system.

I also don’t understand the whole Blue Devil flush thing. You added that to prevent a leak?

 

Whilst I accept you say there is no combustion products in the coolant, what you have here is the classic symptom of headgasket failure. Is the pressure still there when the car is stone cold ? HGF has various failure modes, one being your symptoms, but other modes can result in coolant getting into the oil, or a failure between two adjacent bores.
There is no other source of excess pressure than the combustion gases. As Ricardoa1 says, it is normal for pressure to build up in the engine as it warms up, this pressure being limited by the pressure cap on the coolant header tank. Normally, with an engine being correctly cooled, the pressure never gets to the blow-off limit on the pressure cap. Then when the engine cools, the pressure disappears. If pressure remains with a cold engine, it is, as I said, a classic symptom of HGF. The only way you'll find out is to lift the cylinder heads, but before you do, I'd get a second test on combustion gas products.

 

@12jagmark, how was the cooling system bled following replacement of the various parts?

Does the coolant level ALWAYS return to the cold full mark on the expansion tank once the pressure is relieved?

An X351 I looked at some time ago exhibited the same symptoms you describe and it turned out to be a faulty reservoir cap, which you indicate you've already changed.

 

Yes, and as I said, the system remains under high pressure well after cooling.
This is evident by the top radiator hose being visually bloated, much air escaping out of the surge tank, and the coolant level rising as much as 4 inches (Range Rover expansion tank, not the Jag) when removing the cap - well after cooling down - the day after driving - not hot at all.
My wife's Jag (same year, same engine) doesn't do that.

I've tested for combustion gases several times, and had a mechanic test at his shop too. I think I need a 3rd opinion, and since I've got a lot of combustion gas test fluid left, I keep testing it myself too. It's got to be there.

I agree with you, it's got to be a slight head leak. But there are no other signs than the residual pressure.
Therein lies the mystery.

 

I fill the cooling system with a coolant system test & refill kit.
How that works is, it pulls a vacuum on the system, which I let sit for about 10 minutes or so (varies widely, I've done it a few times) to make sure it maintains a vacuum indicating no leak.
Once that's done, I lower a hose the hat comes from the kit manifold into the bottle of coolant that I have.
Then I open the valve to the coolant, which (because of the vacuum) sucks the coolant into the system until completely full.
you MUST have the heater circuits open (front & back) to avoid retaining air in there.
Then once the vacuum is gone, displaced by coolant, there are a couple of purge caps in the cooling system - one on the heater chose, one on top of the front engine outlet pipe.
The surge tank is at the correct level at that time.
Then I remove the refill kit manifold from the surge tank orifice, and put slight air pressure into the surge tank - but remember, it is filled to the correct level with coolant, so I'm only putting pressure in the system, not air.
Then at that time, with slight pressure on the coolant system, I momentarily open those two purge ports, one at a time and slightly - just enough to let a tiny bit of coolant out to ensure the air is completely bled-out. Then re-secure the purge caps.

Then I run the truck (this is the AJ133 in my 2012 Range Rover, not my wife's 2012 XJ) and monitor the temperature to make sure it doesn't exhibit the wonky temperature readings it would when there's air in the system.

That's when I know that I've properly filled the system and bled-out all the air.

YES the coolant ALWAYS returns to where it was, and where it belongs after I open the tanks cap, and relieve pressure - even a day or two or three after it's been operated.

One thing that could do something like this would be an old cap, but I did test the one that was on there, and then (since a new cap from JLR is like $14) replaced it with a new one anyway.

All I can figure is a very slight & conditional head gasket/warped head. But then I should test positive(test fluid turns from BLUE to YELLOW) for combustion gases in the coolant system.

 

Curious that whilst using the vacuum system to remove air from the cooling system that it maintains the vacuum. If the cylinder head(s) and or block are warped, I fail to understand how it can hold vacuum.

Perhaps a third opinion is a good idea.

 

Reminds me of a case of an engine with a very thin crack that was only leaking when the engine was hot.

Considering all you did and described, my first move would be to doubt the results of the gas leak tester. Are you sure the test fluid you use is ok? Maybe worth controlling it.

 

Exactly. I agree.

And if you could think of a way that I may be mistaken, please let me know.
But it's pretty simple. Once the vacuum is stable, I close the air (supply) valve, and let the vacuum remain. At that point, hoses are collapsed. Pretty simple.

Caveat - prior to replacing my coolant pump, I would get a slight vacuum leak from the bypass hose port on front of the cooling pump because the bushing around the bypass hose nipple was slightly damaged. Once I replaced the pump (including new bypass hose port flange), rock solid.
I've used my coolant system test & refill kit many times & very familiar with it.
By the way, I've rebuilt a few engines, from 50cc 2-strokes to a Chevy 454 V8, and a few in between. And I've taken care of all of our cars over the last 40 years along the way.
My daytime job is to analyze, diagnose, improve designs and fix military systems like laser, EO, pneumatic, hydraulic, mechanical. So I do have a better than average technical fault isolation capability.
So although I can't say I don't make mistakes or miss something once in a while, I'm more experienced than average.

That's why I'm glad to get the input of some who do this for a living - those that can point out something from experience or specific knowledge that I miss, obvious and simple or not.

 

A "cracked" head or block is one of the options I consider, but not real familiar with how they happen, how they present, and why it would happen after a long time and many miles of no problem - meaning no overheating event.
I don't understand how/where a block would crack, and if the PCV system is working fine, why that would build pressure.
But you may be right. I just can't find much info on a cracked block situation.

All things considered, I'm thinking if I put a lower pressure coolant tank cap on, it would relieve the excessive pressure to a reasonable amount.
The stock cap is 30 PSI, pretty high. If I put a 15 PSI cap on, it might help. And I consider this because it does not have an overheating problem, and doesn't boil over at temperature even with the cap off.
It just seems to build pressure when operating at highway speeds/load for a while.

 

You have done a great job troubleshooting and I was shocked too and checked my car to make sure about that 30 psi cap rating!!
But mine is the same and I have never seen such a high rating before.

Let us know how the 15 psi version works.
.
.
.

 

I picked up a 16 PSI cap at Autozone and put it on there. Now I'll keep an eye on it and report back after some long, hot highway trips.
Before I put the 16 PSI cap on, but after I replaced the top, bottom and other major radiator hoses the other day, I notice that the fresh hoses don't expand as much if at all.
The system still retained high pressure after fully cooling down, but I'll see if the cap reduces that pressure.
If so, all I can figure is that I have a very early case of a warped head, or possibly something cracked? But not enough to cause any real operational problems.
But again, after replacing the hoses I pulled a vacuum on the cooling system & it holds a great vacuum. Maybe it wouldn't be so great with the engine hot, but I'm not going to drain coolant from a hot engine just to check the vacuum.

 

Good luck with the new cap hopefully you get many years of service. I wonder if the TStat is not opening creating pressure from the pump as things are not circulating. You definitely seem to have a peculiar situation. I can’t imagine the system needing to reach 30PSI for any reason other than a busted radiator fan and extreme heat, engine would fail way before that pressure is reached in my opinion. You would be over 300f

 

Thanks Ricardo, the thermostat is new (and I've ran it with the thermostat removed too) as is the pump. This is a 2-stage thermostat, and when it's closed, it's in bypass - by design to recirculate.
I'll probably have to dig into it like you did, pull the heads, get them (especially bank 1/right head) and replace gaskets, and may as well replace timing chain guides, all that stuff. Pretty much a standard high-mileage maintenance action on these Jag/Land Rovers; to me an annoying but acceptable maintenance for such an otherwise nice vehicle.
If we wanted cheap, easy & reliable, we'd be driving something else.

I'm curious to see if the 16 PSI cap keeps the residual pressure reasonable.

 

Here's what you need to solve your issue:
https://spingroup.global/aj133-engin...test-facility/
May be a bit overkill, tough.

 

Impressive, something every Jaguar Training Center should have.

 

Mark,
The following remarks are based on experience with steam and hot water systems (mechanical engineering):
Reducing the system pressure is a bad idea. At 16 PSI (30.7 absolute) water will flash to steam at about 250 degrees F.
The system design 30 PSI (44.7 absolute) water will flash to steam at about 275 degrees F.
Water with antifreeze will be a bit higher but the differential will be about the same.
The local heating at the cylinder heads likely requires the ability to avoid flashing to steam to allow proper coolant flow.

You state that, since the spark plug was cleaned by the indie, the engine runs properly and all systems function properly.
The only evidence of malfunction is "over pressure" as evidenced by the bloated hoses and displaced coolant.
No mention is made of coolant loss via the pressure cap.

If the hoses are expanding significantly enough to cause the level of coolant in the reservoir to drop as much as stated, they are defective. New or not.
(Note also that local elevation can exacerbate the problem. Are you in Denver or Miami?)

Best regards,
Bill

 

Thanks for the helpful input, Bill. I appreciate the help!

Actually I didn't clean the spark plugs, rather I let the mechanic replace them - for a few reasons - 1: They were not going to charge me much for the diagnoses they found (if I remember correctly), and 2: I was ecstatic to hear that they didn't find the same dire prognosis that the dealership did.
And 3: mainly, to get a fresh set of spark plugs in there - it had over 100k miles..
Otherwise, I would have changed the plugs myself, since it's so easy & the plugs are cheaper if I get them myself.

I've had the chance to drive with the 16 PSI cap for about a week now, including 30-mile stretches of turnpike at speed between 75- 85 MPH, on a couple days up and over 95F.
All the while, I monitor my ECT by OBD, constantly between 188 - 190 F, solidly. So it seems to be keeping the targeted temp perfectly.
If this were my 2003 Range Rover 4.4L M62 that is designed to run up between 212- 220 F, I'd understand the need for a high pressure cap more. But at a thermostat that begins to open at 188, it seems to work okay SO FAR (I still need to monitor of course).

No coolant loss, including via pressure cap.

I have replaced the hoses last week, along with the trial of the 16PSI cap. Not because they're defective, but because of age.
The top hose was not bloated to the point of imminent failure, but more than a hose should be, and more than normal. You would probably not notice it if not looking specifically for a difference. I only see the difference at the connector junction, where the hose mates the plastic. With no pressure, the hose is slightly smaller diameter than the plastic connector. When hot & under pressure, slightly larger diameter than the plastic connector.
I prefer to change cooling system components prior to failure, so I replaced the set last week.
With the 16 PSI cap and new hoses, the new top rad hose does get firm as it should, as expected, but does not expand at all at the radiator connection.

While this specific application of the AJ133 is in my 2012 Range Rover, my wife's 2012 XJ is the same engine, so a cooling failure is as critical in that too as it is in all aluminium block & head engines I suppose.

 

Thanks for the helpful input, Bill. I appreciate the help!
As an ME, you surely have great memories of fluids (engineering) classes. What fun!

Actually I didn't clean the spark plugs, rather I let the mechanic replace them - for a few reasons - 1: They were not going to charge me much for the diagnoses they found (if I remember correctly), and 2: I was ecstatic to hear that they didn't find the same dire prognosis that the dealership did.
And 3: mainly, to get a fresh set of spark plugs in there - it had over 100k miles..
Otherwise, I would have changed the plugs myself, since it's so easy & the plugs are cheaper if I get them myself.

I've had the chance to drive with the 16 PSI cap for about a week now, including 30-mile stretches of turnpike at speed between 75- 85 MPH, on a couple days up and over 95F.
All the while, I monitor my ECT by OBD, constantly between 188 - 190 F, solidly. So it seems to be keeping the targeted temp perfectly.
If this were my 2003 Range Rover 4.4L M62 that is designed to run up between 212- 220 F, I'd understand the need for a high pressure cap more. But at a thermostat that begins to open at 188, it seems to work okay SO FAR (I still need to monitor of course).

No coolant loss, including via pressure cap.

I have replaced the hoses last week, along with the trial of the 16PSI cap. Not because they're defective, but because of age.
The top hose was not bloated to the point of imminent failure, but more than a hose should be, and more than normal. You would probably not notice it if not looking specifically for a difference. I only see the difference at the connector junction, where the hose mates the plastic. With no pressure, the hose is slightly smaller diameter than the plastic connector. When hot & under pressure, slightly larger diameter than the plastic connector.
I prefer to change cooling system components prior to failure, so I replaced the set last week.
With the 16 PSI cap and new hoses, the new top rad hose does get firm as it should, as expected, but does not expand at all at the radiator connection.

 

Mark,
Thank you for posting. It was an interesting problem,
Best regards,
Bill