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Old 30th August 2023, 09:44   #1
Matt_75
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Default Rover 75 2.5 KV6 - Puzzling Cooling Issue

Hi all,

My wife's recently acquired 75 has a cooling problem that I'm struggling to diagnose. The previous owner said he thought that the coolant system was becoming over pressurised and was expelling coolant through the overflow. This is evidenced by the dried coolant around the expansion tank and on the NSF underside of the vehicle (see photos). He suspected that combustion gases were getting in the cooling system due to head gasket failure, cracked head etc. That said, he never had any overheating problems with the temperature reading 90-95 on the IPK read out, and just had to keep topping up the coolant.

I drove the car about 30 miles home, mixture of through town and motorway. I kept a close eye on the temperature in the IPK and it never strayed above 90. One thing I did observe though was that it seemed to take quite a bit longer than my own 2.5 V6 to warm up.

After getting home I let the engine fully cool down and opened the expansion tank cap, which let out a noticeable hissing noise as it was loosened. There was no obvious loss of coolant, in fact the system seemed to be slightly overfilled.

Suspecting HGF I bought a block tester / sniff tester, followed the instructions and allowed the engine to fully warm up before conducting the test (instructions said thermostat has to be open which on the V6 starts at 88 +/-2°C I believe). I had to siphon out some coolant to do the test. I did the test 3 times and each time the liquid stayed blue and the aspirator bulb was able to fully remove all the air from the expansion tank. If combustion gases were getting in, the bulb would keep sucking up gases but this didn't happen.

I also did a test with a different sniff tester kit I have, one where you allow the air/gasses to pass through the test fluid on their own. I ran this test from cold up to operating temp which took 10 minutes and again, the fluid stayed blue.

I'm struggling to think what could be pressuring the system, assuming this is the cause of the hissing noise and coolant stains everywhere. Any ideas?









Sorry the images are sideways, the uploader doesn't seem to like portrait photos.

Thanks for any help.
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Old 30th August 2023, 11:02   #2
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One more thing I'd like to add. After doing the test and letting the system cool down, I observed a build up in the expansion tank. The build up is like a soft crystaline jelly if that makes sense. I know the previous owner was topping up with tap water so it might just be due to a very weak concentration of anti-freeze, but I'm not sure. Possibly the result of mixing coolant types, delayed coolant changes or, hopefully not, some sort of head gasket in a bottle treatment.







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Old 30th August 2023, 12:13   #3
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One quick and easy check you can do is to make sure the system is self bleeding poroperly. Just pop the small pipe (the one with clip) off of the header tank neck, or you can remove it from the top corner of the radiator. If you get any air out it's likely a blockage in the moulded outlet pipe on the tank.
I had this problem on my 190 and replacing the tank helped keep air out of the system and hence reduced excessive pressure which was showing up the themostat as the weak point in my system.
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Old 31st August 2023, 08:07   #4
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Quote:
Originally Posted by Matt_75 View Post
I drove the car about 30 miles home, mixture of through town and motorway. I kept a close eye on the temperature in the IPK and it never strayed above 90. One thing I did observe though was that it seemed to take quite a bit longer than my own 2.5 V6 to warm up.
Hi Matt,

A maximum of 90 degrees is too cool. There are many possible reasons for this but as you've reported loss of coolant I would start with the simplest cause which is flattened 'O' rings on the expansion tank cap. You don't need to buy the whole cap, the rings are available separately as advised by Arctic.

Bear in mind that your cooling system should be pressurised, that's not a fault in itself.
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I observed a build up in the expansion tank ... like a soft crystalline jelly ...
I'd recommend fully draining the cooling system (including the cylinder block plug), refilling and bleeding strictly following the MGR procedure. You might even consider intermediate flushing using a product for that purpose. Be careful not to fill over the 'MAX' marker inside the tank.

Simon
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Old 31st August 2023, 10:52   #5
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Quote:
Originally Posted by Randyjim View Post
One quick and easy check you can do is to make sure the system is self bleeding poroperly. Just pop the small pipe (the one with clip) off of the header tank neck, or you can remove it from the top corner of the radiator. If you get any air out it's likely a blockage in the moulded outlet pipe on the tank.
I had this problem on my 190 and replacing the tank helped keep air out of the system and hence reduced excessive pressure which was showing up the themostat as the weak point in my system.
Thanks for your help. I have a steady flow of coolant from the bleed hose so I think I'm okay on that front.
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Old 31st August 2023, 11:00   #6
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Quote:
Originally Posted by SD1too View Post
Hi Matt,

A maximum of 90 degrees is too cool. There are many possible reasons for this but as you've reported loss of coolant I would start with the simplest cause which is flattened 'O' rings on the expansion tank cap. You don't need to buy the whole cap, the rings are available separately as advised by Arctic.

Bear in mind that your cooling system should be pressurised, that's not a fault in itself.

I'd recommend fully draining the cooling system (including the cylinder block plug), refilling and bleeding strictly following the MGR procedure. You might even consider intermediate flushing using a product for that purpose. Be careful not to fill over the 'MAX' marker inside the tank.

Simon
Thanks, Simon. My own 2.5 V6 usually sits at 93-96 on a motorway run, straying up to 100ish in traffic, so I thought 90 seemed to be unusually cool. As I said before, it took a lot longer to reach operating temperature too. I wondered if the thermostat might be stuck partially open. It had a new genuine one about 6 years and 10000 miles ago.

I have a set of replacement cap seals in the garage ready to go. I've also bought several bottles of Speedflush and 25 litres of deionised water (very hard water where I live and I don't want to use tap water for the flush) with the intention to do a very thorough flush of the system.

I've found a similar-ish thread and there's a post from yourself mentioning a flushing procedure (for the removal of K-Seal) devised by user MGJohn. It looks as if his posts have since been deleted. I don't suppose you remember what the procedure entailed? https://www.the75andztclub.co.uk/for...=301472&page=4
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Old 31st August 2023, 12:37   #7
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Thanks, Simon. My own 2.5 V6 usually sits at 93-96 on a motorway run, straying up to 100ish in traffic ...
Yes, from my experience that's what you should be seeing with an original 88 degree thermostat.
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... it took a lot longer to reach operating temperature too. I wondered if the thermostat might be stuck partially open. It had a new genuine one about 6 years and 10000 miles ago.
I agree with your conclusion based on that evidence, but KV6 thermostats are usually reliable in operation unless contaminated with K-Seal.
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... there's a post from yourself mentioning a flushing procedure (for the removal of K-Seal) devised by user MGJohn. It looks as if his posts have since been deleted. I don't suppose you remember what the procedure entailed?
Yes I do. The instructions for Speedflush are to run the engine at idle speed for a period, then drain it. MGJohn left it in and drove the car for a few days before draining. He said that this made the process more effective. He also used several separate applications.

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Old 3rd January 2024, 16:45   #8
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Time for a long overdue update to this thread (combination of constant poor weather and work/life commitments slowed down progress).

Unfortunately this problem is still unresolved. To recap, the cooling system is getting over-pressurised and will eject coolant if ran long enough. I suspected head gasket failure so did multiple "sniff tests" with the blue fluid that turns yellow if exhaust gasses are present, all were negative (fluid stayed blue).

The cooling system has been flushed multiple times with speedflush and deionised water. The expansion tank has been removed and throughly cleaned. The expansion tank cap has been replaced. The cooling system has been refilled with OAT coolant and deionised water.

We drove the car for about an hour a couple of months ago. This wasn't enough to eject any coolant and the temperature didn't exceed 93, most of the time sitting at or just below 90. Slightly low for a KV6. I let the car cool down for a couple of hours, coolant temp was about 45, opened the cap and the pressure blew it out of my hand.

I pressure tested the cooling system today. I pumped it up to 21PSI (the approximate rating of the 140 cap) and it did slowly loose pressure, about 1PSI every one or two minutes. After doing the pressure test, I noticed the coolant level dropped from Max to Min. Started up the car and the exhaust ran clear for the first 20 seconds or so, then had the usual amount of 'steam' I'd expect to see on a cold day. Ran the car for only 2 or 3 mins. Checked the coolant level, got a tiny hiss from the cap as it opened, coolant level was back on Max. Did the pressure test again, this time at 15PSI per the manual that came with the kit. Observed the same pressure loss.

I've stuck my nose in the expansion tank and must admit, it smells odd. "Chemical-ly", if that makes sense.

I'm at a bit of a loss with this one. Can anyone help with any suggestions on what to test next?

I'm thinking a compression test and possibly a cylinder leak down test, but I don't have the kit for the latter.

Edit: cooling issue aside, the car runs beautifully. If anything, it feels a little smoother and more powerful than my own KV6 75.

Thanks!
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Old 3rd January 2024, 17:59   #9
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1. Do you have hot water on each side of the heater matrix quickly from cold . . ? 2. Have you observed that you have circulation from a cold start , through the small pipe into the expansion tank. ? You may be aware that the very small hole can get blocked. I used some "s" shaped thin wire to unblock mine . This then turned into a nice free flow from being a dribble. If you start from cold , with the cap removed , then you should have circulation through the hole and into the tank pretty much straight the way. I dont know if this also applies to the 1.8 models , but i can assure you that it does on the KV6,without a doubt. Some members disagree , but i think they have the black expansion tank which may or may not do this. Something for you to check out above. ... nothing lost !!!
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Old 3rd January 2024, 18:03   #10
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I think there may be a post on this somewhere regarding unblocking the vent bypass hole. Worth reading up on.
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