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MG ZS 120
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Discussion Starter #1
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
 

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MG ZS 120
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Discussion Starter #2
Just to add I have done CHG on K-series before but it's been a while.
 

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MG ZS 120
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Discussion Starter #4
Thanks NigelOBB. The current cap is definitely holding pressure. Do you think it could be holding pressure too high?
 

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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.
 

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Discussion Starter #7
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.
 

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rover_75
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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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Discussion Starter #9
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?
 

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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.
 

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Discussion Starter #11
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.
 

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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.
 

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Discussion Starter #13
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
 

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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.

 

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Discussion Starter #15
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.
 

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rover_400_95_99
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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!!!
 

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Discussion Starter #17
Thanks Netor. I have ordered a new cap. It is very possible that it is failing to vent when it should.
 

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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.
 

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rover_75
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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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