Sorry for the long post but.. well ARGHH. Not been here for a while, the E36 328 has gone as it isn't big enough, I replaced it with a E61 535d on a 10 plate. It came with a few issues, most have been fixed such as swirl flaps, glow plugs and controller, new main and EGR stat, oh and the tailgate wiring issue.. I had an issue after changing the stats, temps were still well below spec, it should get to and stay at 88 minimum, it is winter and it sat around 65-75. It was soon pointed out to me that the auto boxes have a gearbox heat exchanger ( referred to as HE from now on) and that contains a thermostat, so I do the most simple of tests, clamp off one of the coolant lines and go for a drive, bang on the money, temps go up to 88 and quickly as well. 2nd test doesn't take much hard work either, cold engine, clamp off both coolant lines to the heat exchanger, pull one off the thermostat side of the HE and let it drain then unclamp the connected line, sure enough coolant poured out so the thermostat had failed open. I do my homework BMW use Behr, Hella also use Behr (Hella by Behr) so I buy one from ECP, it arrives, has half a sticker cut off and a new one under it sporting Hella premium line, lots of dremel work on the flip side to remove all the BMW markings. I fit it and everything seemed fine, temps were up etc, engine got to 88+ gearbox tended to sit around 70. After a few weeks I noticed the economy had taken a dive again so I check the temps and they had dropped to mid 70's again. I call Euro's and they tell me it has to go off for testing due to it being 6 weeks ago, I end up buying a new one rather than wait a month. New ones fitted, lasts 2 days and the same again, an exchange is arranged at my local branch so I pop down to buy the new and later after fitting it return the duff one. This one lasted 2 hours, same thing, failed open as the test confirm. I had already phoned Hella by this stage and got through to a guy called Steve hudson (Head of Business Development - Behr Hella Service) so high up the ladder, he goes through a few things, bit wrong at 1st by trying to say the gearbox has an electric thermostat until I point out the HE contains the stat, he reads up on it and agrees, then asks what I expected. what coolant, I reply Comma G48 as it is approved my BMW and that was changed in November 2017 so basically brand new, he says yes it is the right coolant. I then mention I had the gearbox serviced in December by a ZF specialist using genuine parts and it is working fine, I also mention it has new genuine BMW main and EGR stats fitted as well in November and due to the car staying at the correct temp with the HE clamped off they are working fine, he agrees. He says a 3rd probably won't work as he thinks my car is the cause, at this stage I am rather pissed as I had already explained the gearbox and cooling system are 100% except for the HE playing up, we leave it at that as far as the phone call is concerned. I email after that to say the 3rd is also screwed and had failed after a few hours again, it seemed fine until I did some full gearbox load that obviously opened the stat that then couldn't hold itself against the failsafe springs and promptly failed. His attitude changed after that and he tells me to take it to BMW to let them inspect it. anyway Euro's are now out of stock so my last option for a quick parts my local main dealer who price matched Cotswold and got one the same day, the parts guy tells me after hearing my story that if this fails they will take the car and replace it. Needless to say the HE failed again after a matter of hours so I call, book it in with the threat of a £150 diagnostic fee if it isn't that ( i had done the clamp off and watched coolant piss through the cold stat again) Anyway onto BMW day, I drop it off to leave it for the day, ask to talk to the mechanic who will be dealing with it but obviously get the guy who tells the mechanics what to do, I tell him about the part I bought last week, why it was replaced and what I had done to diagnose it with my mechanic, I also tell him I have a video I had taken to show it was cold and letting fluid flow through what was a closed stat when it was new, I also tell him Hella confirmed it should be shut when cold as a thermostat does. The guys not the most polite person and basically dismisses me, doesn't want to see the video and tells me I diagnosed it wrong, I keep my calm and leave it with them. I get a call that evening, yeah we tested it and it's fine, come fetch it we won't charge you the fee. I show up and tell the girl dealing with me to fetch the mechanic who comes out, he started saying they had tested it, I stopped him and asked how, his reply, the way BMW tell us to test it, I ask again a little louder and firmer, how, explain how? He says they run it for a while, I ask where he says the workshop and continues on his speech about they run it and once the temp levels out they clamp off one of the coolant lines to the heat exchanger as it is a common problem (YES A COMMON PROBLEM) I stop him and ask why he told me I was wrong to test it like that but ok for them to clamp off a pipe, he has no answer. He continues to tell me with it clamped the temp went up one degree, I stop him again and ask what they did to replicate what a car does in its natural environment, he looked confused so I simply said "driving it against the oncoming wind that will in turn hit the radiator thus cooling what is inside if it is hotter than the incoming air" I think by this time he could see he was out of his depth He replied that they did nothing to cool the radiator, I then ask how the hell can they call this a test if they are doing that in a warm workshop with no air to cool the radiator to replicate the fault conditions as explained by me the customer. Needless to say he again had no answer to address my question instead saying that's how BMW say to do it. So I drop the bomb and say to him he was very quick to dismiss me this morning as having no idea what I am doing along with my "mechanic" and refusing to look at the video I had taken of the fault, I told him he can watch the video now, he did and again fell silent then piped up. "well you can leave the car and we will order in a heat exchanger and compare them by taking yours off the car" I said that is fine, get me a loan car to get home, they didn't have one so I had to drive it back. I explained as simply as possible, my new one has failed open, a brand new one arrives with the coolant lines shut by the thermostat then asked that he did know how one works and what its purpose is, he said he did and was aware the heat exchanger had one and all they could do now was compare a new one against mine and if indeed mine was failed open they could warranty exchange it. I decided now was the time to say, if you had taken the time to listen to me this morning and watch the video you could have pulled the pipe and seen 1st hand the coolant went straight through, ordered in another and had a working car for me now, instead this has happened. Anyway onto today, I show up to be told they are going to swap it anyway, this they do and I fetch it back, go out tonight when the roads are quiet and all seems reasonable apart from low gearbox temps, between 40 and 44, car does get up there to 88 unless you rev it high then it drops off and recovers, never does this with it clamped though! I have been out for an hour and head home, last leg involves a hill on a dual carriageway so I floor it as I turn on to 60 then back off, only a couple of hundred yards with 500 left to get home so I back off, then it happens AGAIN, 88 drops to 80, I decide to carry on to the next 50 zone, it keeps dropping until it hits 71. I give up and come home. 5 bloody heat exchangers, all made by Behr, last 2 being BMW, last one fitted under warranty by BMW. WTFH? Behr are obviously fitting defective thermostats and have been for at least a year looking at the production dates printed on the stickers. I mean how the hell can this be my car, it doesn't overheat, this thermostat is a remote one hanging off the engine block. My cooling systems 100%, BMw stats all round, approved coolant etc. I had the laptop running ISTA and the engine was at 88, gearbox at 44 when it went wrong. I think this one was stuck open a little from the start as the gearbox should be around 70 but I went with it and again hear I am, 5 out of 5 failed.. main reason for this post is those of you with this gearbox heat exchanger need to check temps, hidden menu, diagnostics software etc. Have you changed the main and EGR start to find temps are still low, this is why! To test yours, drop the undertray, running under the fan shroud is the return pipe to the rad form the HE, you can clamp this off and secure the clamp then go for a drive, did your temps go to 88+ and stay there? If so the HE thermostats failed open. Due to our cars having no temp sensor 99+% are unaware they have a problem, BMW doesn't force cool the radiator when testing for overcooling, instead they use a test designed for overheating, it doesn't cool the rad so of course it all looks fine in a warm workshop and the customers made to look an idiot, except for me, i would rather not be the only one trying to prove Behr have a bad product on the market and trying to get taken seriously by BMW is not easy! Hella/Behr have now decided it's best to ignore the problem as they have not had many returns, why would they if BMW don't test properly, and at the end of the day a replacement works for an hour, well beyond any test period the garage will do! Sorry for the long post, you can probably imagine after 2 1/2 months of this I am at the end of my tether.