Bad Thermostat Symptoms

[Shackleton]

I'm back! Been away and now a new [to me] sports car has been keeping me busy.

An unusual one for me on the Disco Td5. Twice on numerous routine checks in the last 6 months I've noticed the tiniest of splashes of coolant on the airbox beside where the top coolant hose meets the rad. Checked for pipe leaks and for a weep from the [new] coolant pump. Nothing. But I could see the fibres in the pipe end were stained coolant pink, so good evidence that it was coming from that pipe.

I could never catch it happening. I'd wipe off the splash and it wouldn't come back. The pipe was never unduly pressurised to the touch, engine and temp performing as normal, temp gauge rising normally, no discernible coolant loss from the expansion tank and no gurgles. The splash was there again on a check this morning so I decided to run some tests. I re-sat the pipe clip to make sure it was seated properly and after a run to bring the engine up to temp I felt the top hose. It was properly hard. There was also a tiny amount of coolant clinging to the end of the pipe where it had been pushed out. Oh sh#t, exhaust gases in the cooling system.

I ignored best practice and opened the expansion cap and whereas there was a hiss, that's all there was. Coolant level normal and interestingly no change in level before/after opening cap. Pipe back to normal pressure too of course. I left the car a while and then went on another run. This time with the engine up to temp the pipe had normal pressure, not almost rock hard as before. I left the car cool for an hour and went back to check again and the pressure was dropping back as you'd expect.

So just once in a while an unusual amount of pressure builds up in the system, but it's random.

Could this just be a bad thermostat, or perhaps a bad expansion cap?

I'll change them both, I suppose i'm looking for any experience with what a bad thermostat or cap can do symptoms wise.

Cheers!

George

[bear]

To make sure it's not your head gasket giving up I've seen coments on doing a "sniff test" to check for combustion gases in the coolant. I'm sure the wise ones will be along soon.

Griff

[Bowie69]

Excessive pressure is very unlikely to be a thermostat.

You can do a sniff test, but the DIY stuff isn't so successful on diesels, and besides, some cracks/gasket holes only show when engine is REALLY hot/working hard.

Sorry to say it sounds like something a lot more than £10s-worth of thermostat, if indeed it is a problem.

[sierrafery]

A faulty cap can cause what you described a thermostat doesnt...so better rule out the cap, be aware to be completely tightened cos it's supposed to keep steady 1.4 bar in the system, if it's regulator valve blocked closed it will not release the excess and if it's open the coolant will start emitting microbubbles at above 90*C which will create overpressure. It's calbrated to 1.4 bar cos that's a decent pressure for the hoses and the water's boiling point is increased to 123*C at that pressure and it starts emitting microbubbles only above 115.... only buy genuine

[Shackleton]

Thanks gents, I'll do a little more investigating today.

[Shackleton]

So after said investigating the rad is cold on the offside bottom as is the pipe leading to the thermostat, but the pipes on the otherside of the stat are hot. There actually is a change in coolant height too - It rises by maybe 10mm. Also the top hose doesn't pressurise until the engine is coming up to temp. Wouldn't combustion gases leaking in start to show almost straight away?

Dare I have a little hope that it might be a minor problem? I don't think I smell anything untoward in the expansion tank, not that that'd be conclusive

[Lewis]

When people refer to combustion gas sniffer they don't mean to smell the header tank. You can buy a chemical device which "sniffs" the header tank for traces of combustion gases

http://www.amazon.co.uk/Draper-Expert-23257-Combustion-Detector/dp/B009X95X38

[Shackleton]

I hope this is of some help to someone sometime.

My issue turns out to be a rad clogged with mayonnaise. Sonething I would never have guessed because of the following: After a head gasket replacement I flushed the cooling system 8 times. After that though the amount of oil/mayonnaise in the expansion tankwas greatly reduced so I removed the tank only and cleaned it out, maybe another 4 times. For the initial flushes I opted for adding crushed dishwasher tablets to the system during the flushes and did anywhere between 10 and 50 miles between each flush. As I say the oil stopped accumulating in the expansion tank so I assumed I'd gotten it all. Obviously not the case.

There's another thing I figured out after investigation of my coolant system. I've read on a few occasions in the past, people talking about there being little to no flow when they try and back flush the coolant system. The heater matrix is nearly always blamed as it can in fairness act as a pre filter for the rad. The natural place to stick the hose is in either of the two small pipes on the left hand side of the bottom of the rad, and more specifically the one that runs up to the fuel cooler, as the other will just direct the water flow out some of the other disconnected pipes. I did this and found nearly no flow at all so I assumed my heater matrix was almost completely blocked. I pulled the return pipe directly from the matrix to attach the hose and found it was flowing perfectly. Turns out the fuel cooler doesn't like flowing in reverse and it's this that offers resistance if you're trying to back flush from the pipe where it meets the rad.

[Shackleton]

Apologies for the repetitive posts but after further investigation it seems there's a thermostat of sorts in the aft end of the fuel cooler, which can go wrong. It's not available as a service item so it's a case of either replace the whole cooler like for like or modify another suitable heat exchanger to fit.

I'm assuming mine's fine but will know better after I've rebuilt and tested the system. Cheers