[Cofty]

Hi all, first the symptoms...
MG ZS 120 heater keeps going cold, top rad hose feels like a tight balloon when hot and temp gauge rising worryingly towards hot if I idle for long on after a run. No water/oil mix and not using any water. I am assuming I have a leaking CHG that is pressurising the coolant.
1 - Your thoughts on the diagnosis please? Any ideas on how to confirm diagnosis without chemical testing?
Secondly if there is a consensus that it is CHG I am planning on taking every shortcut possible - it's my daily driver and I want to turn the job around quickly without compromising the job. The timing belt, water pump, tensioner and IMG are all quite new.
So what shortcuts are feasible?
I'm thinking about splitting the exhaust below the manifold (it's all newish and I replaced all the bolts and nuts), leaving the inlet manifold in place on the head and not removing the bottom pulley. Will take the chance to replace the thermostat while head is off as I have never changed it before. Am I right to think the engine mount will have to come off to access belt tensioner?
Any tips welcome. If I'm not wrong I hope to get away with a small parts list of head gasket, bolts and stat. I already have a cam-cover gasket here I haven't fitted yet. If head mesures flat and liners are all okay is this feasible?
Cheers, Bill

 

[Cofty]

Just to add I have done CHG on K-series before but it's been a while.

 

[NigelOBB]

Change the coolant cap


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[Cofty]

Thanks NigelOBB. The current cap is definitely holding pressure. Do you think it could be holding pressure too high?

 

[gnu]

Does your fan switch on when hot?

 

[gnu]

And if the fan is serviceable, have you bled the air out? Is the jiggle valve fitted and not seized? There are a few easier things to check before a HG.

 

[Cofty]

Thanks gnu, Yes the fan kicks on at the correct temp. I checked the jiggle valve when I replaced the inlet manifold gasket about a year ago. I've bled it a few times now but as always with k-series it's difficult to know if you have it all out. Invariably the heater works fine from a cold start. After a run it tends to go cold and that's when I have to get nervous about the temp gauge if I get stuck in traffic.

 

[NigelOBB]

Thanks NigelOBB. The current cap is definitely holding pressure. Do you think it could be holding pressure too high?

Possible they can do the reverse and not release pressure. Also check the stat and inlet manifold gasket


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[Cofty]

Thanks for your help Nigel. Should the expansion bottle cap need to vent in normal use? Would that not indicate excess pressure? My mate just scrapped a ZR I wish I had taken his cap for a spare to test it.
The IMG is less than a year old. I might change the stat since I would do it if I had to do the head gasket anyway. Thing is the stat shouldn't cause the heater to go cold - guessing that's a recurring airlock?
Maybe I will change the stat and go through the Rover recommended bleeding procedure and see if there is any difference before condemning the head gasket.
Should the coolant smell different if it is a leaking fire ring?

 

[gnu]

The question is, are you losing coolant? If no, it won’t be the head gasket.

Thermostat is difficult to do with the head on. Thermostats fail open, so usually cause a slow warm up. Worth changing if you’ve got the head off though.

worth changing the cap for a few pounds.

 

[Cofty]

Thanks gnu. The exhaust isn't steamy and water level seems stable. Is it possible to have a fault at a fire-ring that causes airlock in coolant without losing coolant? Car warm up is rapid as usual.
Would make sense to swap cap and stat.

 

[gnu]

Have you flushed through the radiator, and is it in reasonable condition? Also run a flush product through the coolant system as a whole. I’ve always seen significant coolant loss with HGF, like having to top up every week.

 

[Cofty]

My rad is only about 2 year old and I refilled with new OAT and distilled water.
The coolant smells clean ot be honest. I think a plan might be to order a cap, stat and IMG. The IMG is quite new but it's cheap and taking the inlet manifold off makes changing the stat easy and lets me recheck the jiggle valve.
I have heard talk of drilling the stat. Is that a sensible idea? Also is it worth buying the stat with a lower temp?
Thanks

 

[gnu]

My rad is only about 2 year old and I refilled with new OAT and distilled water.
The coolant smells clean ot be honest. I think a plan might be to order a cap, stat and IMG. The IMG is quite new but it's cheap and taking the inlet manifold off makes changing the stat easy and lets me recheck the jiggle valve.
I have heard talk of drilling the stat. Is that a sensible idea? Also is it worth buying the stat with a lower temp?
Thanks

My advice is to get one of the thicker black OEM IMG's as in the below thread. And yes, clean and free the jiggle valve again. The stat will be more accessible with the inlet manifold removed, I wouldn't drill it though, fit a designed.

R25 - IMG Replacement

Found another problem on my newly acquired Rover 25: the coolant level dropped and a look underneath revealed a drop of coolant under the sump. I had a good look around, including taking blind photos with the camera, and saw the IMG looked suspect with signs of leaking coolant. I...

www.mg-rover.org

 

[Cofty]

Cheers Gnu. Parts on order. I tool it for a bit of a run yesterday and it got up to 3/4 temp gauge when idling after a run and then started to drop - it was like watching the depth gauge on one of those old WWII submarine films! Top hose was tight as a drum but absolutely no water loss.

 

[netor1]

Hello, don't tight your cup too much, only until you feel good resistance on your fingers! It keeps more pressure than it should, that's why your top hose is tight as a drum!!!

 

[Cofty]

Thanks Netor. I have ordered a new cap. It is very possible that it is failing to vent when it should.

 

[gvchile]

The question is, are you losing coolant? If no, it won’t be the head gasket.

Thermostat is difficult to do with the head on. Thermostats fail open, so usually cause a slow warm up. Worth changing if you’ve got the head off though.

worth changing the cap for a few pounds.

Totally agree
For this reason, it is also convenient to check the circulation of the refrigerant through the bypass h of the heating hose.

 

[NigelOBB]

Thanks for your help Nigel. Should the expansion bottle cap need to vent in normal use? Would that not indicate excess pressure? My mate just scrapped a ZR I wish I had taken his cap for a spare to test it.
The IMG is less than a year old. I might change the stat since I would do it if I had to do the head gasket anyway. Thing is the stat shouldn't cause the heater to go cold - guessing that's a recurring airlock?
Maybe I will change the stat and go through the Rover recommended bleeding procedure and see if there is any difference before condemning the head gasket.
Should the coolant smell different if it is a leaking fire ring?

How are you getting on? The caps btw are rated at .99 they can leak over time.


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[Cofty]

Quick update. I removed inlet manifold and thermostat housing but a previous owner had over-tightened the bolts on the housing and filled the gap with black liquid gasket! The captive nuts were all starting to spin so I put it back together for now and ordered a new thermostat housing which has now arrived. So at least the jiggle valve is cleaned out (not bad but not 100%) and a new black IMG. Now I will have to change the stat housing the awkward way.
I took the car on a long trip yesterday and it coped okay. Expansion tank cap is now new as well.
I could see that the existing thermostat has been drilled.
My current theory is that the stat just doesn't cope well with long runs on hot days. It happened last summer (in heavy motorway traffic) and had been fine since. I am hopeful that when I change the stat later this week all will be well.
Thanks for the advice to leave the head gasket alone for the moment, will update the outcome.