P38A 4.6 V8 MY2000 overheating. Intermittent

Hi All,
I would welcome some intelligent comment on the problem I am having with my RR.
It all started 2 weeks ago when I decided to replace the coolant in my 2000 P38 4.6 V8 petrol/lpg Rangie. Everything appeared OK at first but then after a couple of miles the temp needle shot into the red, steam came from under the bonnet, the lpg switched off and the cabin got cold. Stopped and popped the bonnet. Noticed the expansion tank bubbling like crazy. Waited 5 minutes then restarted the car. Everything worked normally for a couple of miles then the temp shot up again. Same symptoms. Also noticed the top radiator hose was rock hard and the bottom of the radiator was cool. In fact the radiator itself was only lukewarm throughout.
The mechanic from the local BMW garage came over and after a few minutes (when it had cooled down) he did a pressure test on the cooling system. This was OK. He then did a sniff test on the coolant by putting tubes with coloured fluid on top of the expansion tank. This was OK at first but when I revved the engine the liquid turned green indicating CO2 in the coolant. Here's is where it gets silly. Tooks the heads off and the gaskets were fine. Took the heads to our local guy Leeds Exchange Engines and he said that he pressure tested them and they were fine. There was a very slight imperfection on the LH head so he skimmed them both anyway. Put it all back together and I have the same fault but this time no CO2 in the coolant. May have been a false reading on the initial test?
I read all the relevant posts on your excellent forum and decided to go for the easy stuff first. Thinking I might have air in the system I removed all the pipes at the uppermost point and carefully filled the whole system manually. Same fault. Changed the thermostat. Same fault. Flushed the radiator out - same fault. Nothing nasty came out of the rad. I checked the return flow to the expansion tank through the small pipe and this is normal. Disconnected the heater matrix hoses to check for a good flow. No flow at idle but it gushes out when I raise the revs. Is this normal?? Took the water pump off and this all looks brand new. Fitted a new gasket. Checked the viscous fan and this seems to work OK i.e. you can't stop it when the motor is hot (in the fault condition) but it can be stopped when it is cold. Took the car out this morning with the heater matrix bypassed. Worked ok for 2 miles then the needle shot up and steam came from under the bonnet. Left it to cool for 5 mins then it drove OK. This is the nature of the fault. I can do 20 miles mixed traffic and highway with no problem and the next day do 2 miles and it overheats. I am just about to order a new radiator but I can't understand why it works perfectly one day and then overheats almost immediately the nest day.
Any suggestions would be gratefully received. I don't really want to scrap it but that might be an option. As I said, it was working perfectly until I decided to change the coolant. BTW I use the pink stuff from my local LR specialist.

Does the engine sound OK? Not really tappy like a liner is bouncing about?

What did the cylinders look like when the heads were off (any look steam cleaned)?
Maybe something simple like an airlock in the system. Have you tried bleeding the system (get the OS front end as high as possible when doing this)?

you can check condition of rad by feeling it all across the core when engine is up to temp it should be hot all over its surface if its is its fine, a head gasket leak with your symptoms would just be staining across the fire ring areas, a cracked block behind liner isnt allways apparent to see and will only show when engines hot to start with, you can only prove it by pressurising each bank preferably whilst 2/3rds filled wit hhot water

davidsmith1307 said:

Does the engine sound OK? Not really tappy like a liner is bouncing about?

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She sounds fine. Runs really sweet even when she is overheating. When I had the heads off I looked at the liners and they were perfect. I am going to take the rad off on Monday and see if there is anything loose inside that could be affecting the flow.
Like I said. It can run perfectly for 20 miles with the temp needle nailed at 12 o'clock and then the needle will go to red in 2 seconds. If I hadn't already changed the thermostat I would go straight to that. I tested the old thermostat and that seems to be fine.
P.S. The car is on 159000 miles. I have a full service history but there is nothing in there to suggest any work was done on the motor. Just the usual EAS faults and electrical gremlins.

did you check the new stat before you fitted it ? sounds like its not opening.

test the old one as well as that must be sitting around some where put it in a pan and boil it up or pour hot water from the kettle over it and see if it open

jamesmartin said:

you can check condition of rad by feeling it all across the core when engine is up to temp it should be hot all over its surface if its is its fine, a head gasket leak with your symptoms would just be staining across the fire ring areas, a cracked block behind liner isnt allways apparent to see and will only show when engines hot to start with, you can only prove it by pressurising each bank preferably whilst 2/3rds filled wit hot water

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When the car is not in it's fault condition i.e. working properly, the rad is nice and warm all over. When it is in it's faulty (overheating) condition the radiator is cold except for the very top part. The top hose will then be red hot and is sometimes rock hard and sometimes not rock hard. The bottom hose is cold when the car is in it's fault condition. It's like the car has got a dual personality. When it is working right everything is perfectly normal and then it will just flick to overheating mode in 2 seconds. Switching it off for 5 minutes is often enough to restore in to normal running. which can last as long as 20 miles.

andymalham said:

She sounds fine. Runs really sweet even when she is overheating. When I had the heads off I looked at the liners and they were perfect. I am going to take the rad off on Monday and see if there is anything loose inside that could be affecting the flow.
Like I said. It can run perfectly for 20 miles with the temp needle nailed at 12 o'clock and then the needle will go to red in 2 seconds. If I hadn't already changed the thermostat I would go straight to that. I tested the old thermostat and that seems to be fine.
P.S. The car is on 159000 miles. I have a full service history but there is nothing in there to suggest any work was done on the motor. Just the usual EAS faults and electrical gremlins.

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you cant often tell blocks cracked behind the liner, visual inspection means little it need pressure testing to prove

andymalham said:

When the car is not in it's fault condition i.e. working properly, the rad is nice and warm all over. When it is in it's faulty (overheating) condition the radiator is cold except for the very top part. The top hose will then be red hot and is sometimes rock hard and sometimes not rock hard. The bottom hose is cold when the car is in it's fault condition. It's like the car has got a dual personality. When it is working right everything is perfectly normal and then it will just flick to overheating mode in 2 seconds. Switching it off for 5 minutes is often enough to restore in to normal running. which can last as long as 20 miles.

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rad sounds fine in that case, that would sound like faulty thermostat or pressurisation

you mite just be air locked at the stat very common

biketeacherdave said:

did you check the new stat before you fitted it ? sounds like its not opening.

test the old one as well as that must be sitting around some where put it in a pan and boil it up or pour hot water from the kettle over it and see if it open

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Tested the old stat (as above) and this appears to work correctly. Just to rule out the possibility that I could have two components with exactly the same fault ( I am in the motorcycle repair business so I know that this CAN happen) I am going to devise a way of holding the stat open in order to rule this element out completely. Wish me luck.
Thanks for your help guys. It's great to be able to bounce ideas around.

martynv8 said:

What did the cylinders look like when the heads were off (any look steam cleaned)?
Maybe something simple like an airlock in the system. Have you tried bleeding the system (get the OS front end as high as possible when doing this)?

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Hi Martyn. My thoughts went straight to that. I filled each hose and component separately to avoid any air locks and ran a hosepipe through everything (at low pressure). I have parked the car "nose high" during the filling and bleeding process. Also, the small return pipe is fine and produces a good strong flow after a couple of seconds.
Cylinders looked perfect.

biketeacherdave said:

you mite just be air locked at the stat very common

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I am going to have a little poke around with my laser thermometer. Should be able to detects any anomalies in the flow.

the fact that the bottom hose is cold when it overheats indicates either a faulty stat or the stat is air locked seeing as the car was ok before you swopped out the coolant id say the stats air locked.
i chased air locks and leaks for months mine was in the end hg failure but refilling the coolant seems to be an art form in its own.
you can drill a 4 mm hole in the outer part of the stat to aid bleeding and filling

a persistent air lock is usually pressurisation

biketeacherdave said:

the fact that the bottom hose is cold when it overheats indicates either a faulty stat or the stat is air locked seeing as the car was ok before you swopped out the coolant id say the stats air locked.
i chased air locks and leaks for months mine was in the end hg failure but refilling the coolant seems to be an art form in its own.
you can drill a 4 mm hole in the outer part of the stat to aid bleeding and filling

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Just had another go at this. Ran for about 20 mins on fast idle before it went into it's "overheat mode". At no point did the radiator warm up at the bottom and the radiator side of the stat was not hot at any time. Also noticed that the heater "in and out" hoses were sometimes hot and then cold and then hot and cold again.
I had set the expansion tank level to the full cold mark after the car had stood for 5 hours. After engine start the fluid almost disappeared after less than a minute then gradually came back to about 1/4 way up. Increasing the idle speed made the fluid level drop and returning to idle let the fluid come back. Cap seems to be holding pressure as there was a hiss when I undid it to put some fluid in when the engine was cold.

Have you changed the pressure cap? When it overheats and steam appears, where is the steam coming from? The overflow from the expansion tank or elswhere? Pressure caps on these can be weird. ie if the top hose is rock hard, the cap's holding pressure, but if steam's escaping there's a leak and it's lowering the the boiling point of the coolant. The recommendation is to only use genuine caps.
Good luck
(I'm just about to pull the heads on mine - HGF (I hope) - 2 hydro-locked cyls 5 & 7)
Alastair

the fact that the bottom hose is cold when it overheats indicates either a faulty stat or the stat is air locked seeing as the car was ok before you swopped out the coolant id say the stats air locked.
i chased air locks and leaks for months mine was in the end hg failure but refilling the coolant seems to be an art form in its own.
you can drill a 4 mm hole in the outer part of the stat to aid bleeding and filling

biketeacherdave said:

the fact that the bottom hose is cold when it overheats indicates either a faulty stat or the stat is air locked seeing as the car was ok before you swopped out the coolant id say the stats air locked.
i chased air locks and leaks for months mine was in the end hg failure but refilling the coolant seems to be an art form in its own.
you can drill a 4 mm hole in the outer part of the stat to aid bleeding and filling

Click to expand...

I'm thinking that I have to explore the air lock theory before I go any further. I have printed out the section of the manual with the cooling system diagram and description and I am going to go through it hose by hose and hopefully chase out any air in the system. I will also refit the original stat which might have been OK all along as it tests OK and then chase the flow and temps around with my infra red thermometer.
Thanks again for all your help.
I spent all sunday watching videos of cooling systems and the theory and operation of same so I am all stoked up!

Update. Still getting a lot of bubbles back into the expansion tank even when cold. Coolant stinks of exhaust fumes. I have just put some K Seal in and took it out for a short drive. Seemed OK. radiator warmed up and the LPG switched over and stayed switched on. Will take it out for a longer run tomorrow. Our local LR specialist says that porous liners is a common problem and that K Seal works about 60% of the time. Fingers crossed.