KV6 overheating.Coolant cap.
 

My MGZTKV6 2.5 has run very hot of late and after investigation and testing showed no problems with the engine we have found the problem to be the coolant tank cap. Seemed unlikely so unlikely it wasn't considered initially and was discovered by process of elimination. A cap has been fitted supplied by Xpart and the temperatures are now spot on. One to consider if anyone has an overheating/hot running KV6. Chris.S.

Thanks for posting this Chris. It's always useful to remind members of this (as well as correct operation of the cooling fan) before the subject of head gasket failure rears its ugly head. ;)

Sometimes however, it's just the two 'O' ring seals mounted on the expansion tank cap which flatten. They're not available separately from X-Part but something suitable can be obtained cheaply.

Simon

Viton O rings from Jack - https://www.ebay.co.uk/itm/133016723...m=133016723782

I fitted a brand new LR cap and the rings went flat straight away (these were standard rings, not Jack's Viton rings). Over-tightening the cap can almost certainly flatten the rings but I'd made sure not turn it past the first 'bite point' - and they still got flattened.

The owners manual page 145 says this: Ensure the cap is tightened fully after top-up is completed. It doesn't say what fully tight is and it doesn't mention over-tightening.

So it's difficult to know how much to tighten the cap on this car. Every other car I've had you just screwed the cap on until it wouldn't turn any further (i.e. it came up against the stops). The 75's cap seems to keep turning for much longer before anything mechanical stops it. Why?

Because it's spinning/tightening against the top washer, compressing it too far. I always apply vaseline to the rings and tighten just enough, and I have a 1.8:eek::D

In my 1.8t i swop out the cap every 12 months - ;)

But how do you know what 'just enough' is?

Until you feel resistance, then about another 1/4 turn:shrug: Doesn't have to be really really tight, it's not going to leak:D Any expansion vents out the side pipe:cool:

Ok, I'll put Jack's Viton rings on the new cap and go no further than 1/4 turn past first resistance. :}

Simon, "how tight is tight enough"? that was one of the questions asked of me, my answer was "not too tight" I have just checked my cap for the first time since getting the car back and it is tighter than I had it! It is on until it stopped! Regarding rings I have three old caps and none of them have flattened "o" rings? the last two fitted before the present Xpart were on the car when it ran very hot, so where does that take us " not too tight"? The first cap was on the car when I purchased it and at that time it had been tested because it was "spitting out white stuff from the headertank?cap? I had a long conversation with either T Cut, SD1too,or suzublu about this problem and read and absorbed a very erudite post on Coolant caps, SG1too - ? I am now going to examine the Xpart cap to see what it has on/or not on it comparable to the previous caps. I have already done this but forgot what was on it!!!! Chris S.

Im back, bit of a walk downstairs through the garden and garage to the drive(you feeling sorry for me)? No, ok The cap looks like any other, on the top embossed safety advice and in the centre 140, turn it over and the following round the bottom rim- PA 66 GF 30.now the orange part has a rectangle with No, repeat No number 140, but round the periphery REUTTER. N None of the other had this name on them but did have 140 in the rectangle. I think apart from possible faulty caps my problem has been "not tight enough" never been told that before but however Im inclined to favour Page 145 " ensure the cap is tightened fully" What do you think? I will run as I am and report back as soon as there is anything to report. Chris .S.