Spent about 3 hours farting around with this today squeezing all the pipe work to try and remove the suspected air lock even put front wheels on ramps, strange thing is outside temp was 3 degrees when i started the engine so the FBH fired up the heater inside at this point was blowing warm air, as soon as the FBH shut down the air went cold again. I'm thinking of buying a vacuum coolant pump and doing a vacuum fill. Top hose on the rad gets fairly hot but not to the point you cant hold it, the pipes going into the bulkhead are only mildly warm. Wondering if the thermostat is staying open ??

I even went for a blast down the bypass did a James bond when i hit 2500 rpm all cars behind properly disappeared in a massive cloud of smoke which cleared about 10 seconds later i was proper embarrassed lol (that's a job for next summer), and still no heat. Also own a 1976 Land Rover 101 3.5 V8.
Previously owned 1991 Range Rover Vogue 3.9 V8.

Landy95 - I don't have a FBH, so you would think that it would be easier to fix
Also have the same scenario as you with the hoses. I have a new thermostat fitted & checked the live values on the IID (89 - 91 degrees C).

I'm miffed at how the heater is getting fed when the control valves are shut?.
As the return flow from the bulkhead goes to the metal pipe, back towards the rad, however the other hose from the adjacent metal pipe goes to the auxiliary pump then to the control valves.
Now if these are shut how does it feed the matrix or do they only partially shut?.

But still no heat at a certain speed revs (45 mph / 2500 rpm).

I was losing coolant and found the top hose seal was the culprit. Fitted new hose and then started having problems with the coolant level. The top hose was also cavitating when it had completely cooled.
Along with fitting a new expansion cap, I let the engine get as hot as it could, switched off, and immediately removed the expansion cap. The first time I did it the expansion bottle nearly overflowed. It had to be air that forced the water level to rise.
When the engine had cooled completely I managed to add nearly another litre of coolant.
I did this a few times and now it has a stable coolant level.
So is this another way to burp it? Worth a try. Good luck.

Right, Managed to find some time today & see if there were any obvious issues. Took various hoses off & all had fresh coolant coming out of them.
Ran up to temperature & fan on Max, cabin nice & toasty. Checked all hoses with Infar red temperature sensor, showing good temperature differences on feed & returns. However found both the A & B hoses very very soft, so soft on one that I could squeeze it shut.

Therefore I will check & change the thermostat next weekend & see if the pressure increases.

My TD6 started doing this issue this morning on 200+ mile run down the M6 & M1 which was flipping annoying. Same as described earlier, motorway speeds blowing cool, slow down and temp of air being blown is warm/hot. Watching posts with interest as now even looked under the bonnet yet.
No issues with heater up to now and mine is 17 years old but something has to have changed.....

As an additional comment to the above, coming back last night (same 200+ miles) the heater seemed to be working perfectly as it had been. Haven't done anything in between, the only difference was the drive down was 3-4 def outside temp and coming back 6+ deg. 2004 Vogue SE TD6
(Also a LR 90 V8 & RRS, just because)

Ok, I've just about had it with this issue now
Changed the thermostat (although there was nothing wrong with the old new one). Also checked both & confirmed that they worked ok.

Driving around below 1500 rpm & it gets too hot, resulting in me turning the heat down. Increase the revs to above 1500 rpm & the heat stops

I've bled & bled the system, so really can't see that this is an air lock issue, just don't know now

Sorry to hear that Highroller, i finish work tomorrow for xmas so will have plenty of time to try and sort mine out. Will keep you posted. Also own a 1976 Land Rover 101 3.5 V8.
Previously owned 1991 Range Rover Vogue 3.9 V8.

Hi all, spent some time removing the heater control valve today from offside inner arch in engine bay, stripped it down and found the seals to have expanded with age, even with no voltage on the solenoid it was hard to blow through the engine side, i played around trimming the seals but they were pretty knackered so decided to buy a complete new valve assy from amazon on prime next day, i will also order a seal kit and recon the old unit and keep as a spare. Ive attached some pics of the valve in various stages of stripping for reference for anyone attempting the same.

The larger diameter that sits in the solenoid is attached to the piston shaft with a small weld i snapped this off and filed it down flush with the shaft to enable removal of this portion which then allowed full disassembly, note if you do this you will need to re-weld it or loctite it back onto the shaft. I also did a sketch and captured the sizes of the piston plunger if anyone wants the sizes i will add it also.

I will let you know how it goes once the new unit is installed.

Click image to enlarge

Dunno why only one pic is enlargeable???? Also own a 1976 Land Rover 101 3.5 V8.
Previously owned 1991 Range Rover Vogue 3.9 V8.

landy95, I take it that this has cured your issue then.

The new heater control valve arrived this morning i have just fitted it and took it around the block and i can confirm i now have heat i had to have the windows open it was that hot woohoo well chuffed.

Highroller did you change your heater control valve just wondering if this is your problem maybe restrictive flow is giving your strange speed/heat issues.

This is the link to the heater control valve on amazon they only have one left!!!

https://www.amazon.co.uk/gp/product/B07X87...&psc=1 Also own a 1976 Land Rover 101 3.5 V8.
Previously owned 1991 Range Rover Vogue 3.9 V8.

landy95, Replaced with a second new one & a second new thermostat. Still the same

Edit just seen extra posts which didn't load
If you have access to diagnostic ic equipment, you should be able to monitor heater matrix temperature answer as engine coolant temperature. Has the coolant system got good flow? Seen a few other vehicles where the impeller has come off and causes alot of issues with heating etc

Yes, IID tool gives a live reading of the coolant temperature between 88 -92 degrees the lower temperature is at an average speed (Motorway / dual carriageway). Coolant flow appears to be good.

Engine heats up as you would expect & the gauge sits bang in the middle, never goes above this. Well it did back in the summer once, when it was fully loaded with trailer & race car. It moved to the one o'clock position. Pulled over to check for leaks etc, but nothing & has never gone over the twelve o'clock position since.

It's not that the heater is not working, it just cools down at above 1500 RPM. The matrix pipes cool down. This can only be a result of the control valves closing & restricting the flow to the matrix?.

I know that the matrix is protected by these to avoid high pressure at higher engine revs, but the matrix is only fed by these two inlets & one return.

Back to this 1 again, after finding the paper gasket on my lower thermo stat mod was dripping coolant iv fitted a standard thermo housing for now I'm getting intermittent heating. Nice and hot on tick over goes cold as soon as I drive it.......I'm sure it's an air lock as it usually is but same as every one else iv bleed and bleed it, even my trusted back flush with low pressure hose didn't work. Mite be time to try out my new toy, negative coolant filler thingy.