Hi and thanks for taking the time to read this. I really hope someone can help me figure out whats wrong with my defender as I'm meant to be driving it across Scotland next Saturday for a weeks holiday!

Firstly its a 1996 300 tdi 90 that I've had for nearly two years now. Its been great all this time (with the exception of the usual maintenance issues and parts failing due to age) always pulling like a train and never missing a beat but since replacing my radiator (for a metal bearmach one) last week I seem to have constant problems with airlocks forming in the thermostat and radiator (where the top hose connects).

I've tried all the methods I can find on here to bleed it, from undoing both plugs and overfilling the expansion tank till first the rad overflows, resealing the plug and then doing the same for the thermostat & I've also tried replacing the thermostat plug for one from B&Q that allows me to hot bleed the system but either of these only seem to work for a short distance (5-10 miles at 50+ mph) before my heater blows cold and the temp starts to creep up towards the red & I have to do it all again?

I'm beginning to panic that its a head gasket failure but its still driving as well as ever, not smoking and the oils clear of water and the waters clear of oil so I'm hoping theres just something stupid I've missed to get rid of the airlock or theres air getting drawn into the system somewhere? I've also checked the hoses and the top hose gets hot and the bottom hose stays reasonably cool + the new rad feels cool all over
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To give you a bit more info, in my list of jobs I've done recently was that the p-gasket had gone, which with Lerks how to I managed to replace it along with my water pump (waterpump was still ok but figured while I had it off). This unfortunately resulted in the original rad splitting the metal at the top hose connection due to it being rusty and not liking having the hose removed though hence the new radiator.

Sorry for the long post but just trying to give you all the info I can in advance + any help & advice would be much appreciated! - Kev

head gasket would be my first thought

I was hoping you werent going to say that... Is there any way of checking for sure as I've read on other forums that sniff/pressure tests might not show up anything on a 300 tdi because of how the gasket usually goes or because the head warps...

most alli heads warp ,but with 300tdi been a simple block with no liners often have slight gas leak to water way as first sign ,will show up as dark stain across fire ring on gasket ,providing you havent a leak

Damn this isnt sounding good for my holiday! Is the only way to tell by taking the head off then to see what state the gaskets in?
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Thanks James, like I said your advice is much appreciated & if your ever up in my neck of the woods give me a shout and i'll buy you a beer or three!

to be 100% unless you leave it a while ,if head gasket is good you will have a clean shiny ring on block ,head and gasket around the cylinders,quite a simple job on a tdi,are all pipes getting hot when up to temp including little bleed pipes from rad and thermostat housing to expansion bottle

I havnt checked all the pipes from the expansion tank but when I've been bleeding the thermostat using the B&Q bleed nipple any coolant thats landed around that area has turned to steam, altho thinking back I have grabbed them and not noticed them particularily hot though (but wasnt checking to see if they were and cant remeber how hot the car was at the time).
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How hard would you say taking the head off is given my limited time and the fact I'm a willing but not massively experience diy mechanic (so far I'm still wearing my L-plates but I've done the rad, front prop, p-gasket, water pump, replaced my back door, fitted a swing away carrier, new brake pads and replaced a friends drag link on her 300 tdi disco)

quite easy but check those pipes and top hose to ensure thermostats opening,theres a brass bung on lh side of block just in front of exhaust down pipe remove to drain block first ,then just a matter of removing inlet manifold 2 x 13mm nut 2x 10 mm headed bolts ,glow plug wire on rear plug ,oil breathercyclone,injector pipes ,rocker shaft,head

Thanks againJames will do! Really hope its just my thermostat but guess I'll find out when I fire my wee beastie up tomorrow. Lol - if not looks like my learning curve has just got a whole lot steeper.

Does the fact the top hose is getting hot but isnt ever particularily hard (+ I had problems getting it to stop leaking when the new rad was fitted) mean anything?

getting hot means thermostat is opening ,as should all pipes ,rad (new rads can fail )etc proving good circulation and poorly gasket ,obviously as coolant is lost circulation will fail in parts as you noticed with heater tank cap blows at 15 psi iirc so system will allways have some pressure

Definitely not looking good then. Altho I've only checked the rad once and that was earlier today when the car was staying hot even when I'd bled the system (yet again) but it felt cool to the touch + I think the expansion tanks only blown once that I know of & that was after I'd prob left too much coolant in it after the previous refill/bleed and I noticed it dripping when I opened the bonnet to bleed it again.

Thanks again and I'll let you know how I get on with my hoses and thermostat + I might try checking to make sure all my jubilee clips and o-rings are doing their job too just in case I can avoid trying to fit any more big jobs into this week!

check little pipes and the valve ,rad should be hot as other parts ,its difficult as all problems give same symptoms ie poor rad will cause overheating so coolant to be lost through cap ,but so will hgf

Thanks again for sharing your expertise & I mean it if your ever up in Dundee (or Glencoe next week if we make it) the beers on me - if only coz I might get some sleep tonight now without sitting trawling the net trying to find some rare reason for it not to be a hgf! At least now if it is, I'm a little less apprehensive about giving it a go (of course that might just be the whisky )

no problem the quicker you start the quicker its done ,sometimes actuallt finding out practicably is only way

Well thats me taken it for a wee run to get it up to temp & all the hoses got hot + the rad felt lukewarm but no overheatng this time. The thermostat was pretty empty though when I checked it once I got back so I'm thinking it is prob a hgf :-(

I did notice more coolant had been leaking from my top hose at the rad & when I checked my bottom hose the thinner part of the 'y' had been rubbing on my water pump pulley and was hissing & leaking too, so before I give up competely & whip the head off (or coz I'm short on time get my mates garage to!) I'm away to replace those hoses tonight in the hope it might be them.

get a couple of bleed valves - yu can then bleed the air out when hot. theres a few on fleabay.

Hi Mad - thanks for that but I've already got one anyway on the thermostat that I've been using but the airlocks still keep coming back intermittently.

Its weird but they usually only appear after I've been running it for a while over 50mph (the temp usually only goes up a wee while after the heaters blow cold) and sometimes disappear for a while again once I drop below 50mph only to come back again later!

Hopefully I'm not getting ahead of myself but I think I might be the jammiest thing since jammie dodgers were invented!

Thats my two new hoses fitted, the system bled (I turned off my heater before I emptied the system to hold that water and then fitted the new hoses, filled the expansion tank till the rad overflowed, fitted the brass bung and filled again til the stat overflowed and stopped bubbling then fitted my b&q bleed screw thingy. I then started the beast with the expansion tank cap off & turned the heater back on to let any extra air work its way out before replacing the cap). I got back in the landy to be greated by instant "heat" & set off on my test drive...

I gave my wee beastie a good hard run at 50-70mph round the city bypass before powering up the biggest hill nearby (that I knew it had overheated on before) & with the exception of the heaters going cold twice (fixed with a short burst of my bleed screw each time with the engine running) it didnt overheat in the slightest!

I know its still early days but fingers crossed the Beast lives to fight another day! Thank you all for your help & advice

Mate I've got same problem with a 300 TDI. Heater cold till thermostat opens tryed 3 thermostats. Will the same. Replaced head gasket water pump starting to become a nite mare