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Rapid Head Gasket Change - Tips Please

6K views 30 replies 6 participants last post by  Rover 416 Si Automatic 
#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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#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.
 
#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?
 
#10 ·
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.
 
#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.
 
#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
 
#14 ·
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.

 
#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.
 
#28 ·
I know that this is a bit of an old thread, but exactly the same thing happened to my 416 Si Automatic yesterday. The needle rose to 3/4 and then just sat there. It was hot yesterday and I had been on the motorway for about 2 hours. The thermostat was replaced only a week ago as the old one had failed. The fan never seems to turn on though, which leaves me to believe that the fan temperature sensor or even the fan are dead.
 
#20 ·
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.
 
#21 ·
A massive amount of cooling issues diagnosed as hgf are wrong, due to the bad press reports, the vast majority of cooling system issues are other more simple causes, hence so many occurrences of I have changed the head gasket and it’s gone again, when in reality it wasn’t the source of the prob in first place.


Sent from my iPhone using Tapatalk
 
#24 ·
Hi, sorry for the delay. I think last I wrote was that I took the thermostat out only to discover the thermostat housing bolts were overtightened. I ordered a new one and had it sitting around for weeks. As the weather cooled down I had no problems and continued to use no water. Anyway I finally got around to replacing the stat on Sunday - I had to replace the starter which was getting lazy so once the tools were out I decided to do both jobs. As you will know it's a bit of a fiddle but not too bad.
I did all the right things but airlocks are inevitable so it might be another few days before I'm sure but it seems good. Somebody had drilled the old stat which I read can lead to rapid fluctuations of the stat opening/closing under some situations. The new stat heats up the engine amazingly quickly. I will let you know if it is all fixed.
Two or three jobs to do asap - horn sounds continuously (relay in MFU?) and handbrake is rubbish so I'm going to rebuild rear calipers. Wish me luck battling with the internal circlips!
 
#25 ·
Sounds like a good job. (y)

I refurbed the brakes on my zs when I had it. I found the handbrake mechanism is prone to seizing, so a clean up and a grease is necessary occasionally. After a few years I replaced the rear callipers altogether as they started to drag, even if regularly stripped and cleaned.


 
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