Air in Cooling System

Hi all,

I don't post here often, but the observant of you will remember that I posted on here a year ago after my MG had overheated on the motorway.

I never completely found out why it did that, but the only explanation that I've been given by my mechanic is it was likely to have been a dodgy expansion tank cap. I replaced it with the genuine black cap.

Anyway, since then I've been keeping a very close eye on the cooling system. One thing that kept happening was a build up of air in the radiator.

I had to have the Cambelt changed over the winter so at the time I asked my mechanic to check the cooling system over. I asked him if he thought it was the HG (thinking that if it is I could save a bit by having it done at the same time as the cambelt), and he said no. Unfortunately his in depth investigation was to take a look at the oil and coolant and to conclude that the HG was fine because both of them looked clean. His words been that the HG usually doesn't fail in any other way but between oil and coolant.

Anyway, I've thought that I can't be scared to drive the car, so I have been using it during the nice weather. Around town even in the heat its been fine. Once a week I would open the bleed screws and a little air would come out.
However last week I needed to do a 240 mile round journey. A risk I know but I thought this would be a good test. So out leg of the journey (120 miles) car drove fine, got to my destination and I checked the expansion tank. The coolant had raised a little and when cool I opened the bleed screw and a fair bit of air come out but not a massive amount.
Did the return journey, again the car drove fine on the 120 miles back. However when I got home I checked the expansion tank and the coolant level had risen from half the tank to literally filling the whole tank. I waited until the next day and checked again but sure enough it was still full. It was so full that when I opened the cap the coolant shot out of the tank with some force (keep in mind it was stone cold).

I opened the bleed valves and it was like letting down a tyre with the amount of air in there.

So for that much air pressure to be in the system after just 120 mile journey, can it be anything else other than HGF?

As a matter of fact, my engine has never mixed oil and water when it failed the head gasket, but failed between coolant and the cylinder causing a build up of combustion gases in the coolant.

I believe this is quite common with our engines.

David Aiketgate wrote:

As a matter of fact, my engine has never mixed oil and water when it failed the head gasket, but failed between coolant and the cylinder causing a build up of combustion gases in the coolant.

I believe this is quite common with our engines.

Sounds like the probable cause then?

The way to be sure is to get a garage to do a test for combustion gases in the coolant.:yesnod:

I had a similar thing about 12 months ago, system was pressuring and filling the expansion tank.
On investigation it was found that the head was cracked by number 3 inlet valve which was causing the pressuring.

I agree with David that the HGF scenario is as he describes for an F or a TF , interestingly the gaps between HGF failure's on early MGF,s were about 20000miles .On early TF,s the period was longer around 50000 miles, t he good news is that the problem is getting rarer as newer style components are fitted. If the car is driven after a cooling system event and all the coolant has been lost there is probably no other outcome other than HGF. The coolant tank tops do let go on there own and clouds of steam exit and if it happens when stationary then the situation can be recovered , but driving on only ends in tears.
Regarding repairs to K series HGF or even just cam belt changes I only trust the ex ROVER mobile guys , they have a vested interest in getting the job done properly.
The above comments are purely mine
regrds

Mike

I'm afraid it sounds very much like HGF speaking from experience those symptoms were exactly the same as mine, gradually getting worse until on the cool down cycle small amounts of coolant was making its way into no4 cylinder cause mis fire on starting.

Sent from my SM-G981B using Tapatalk

and my advice would be to mount the modified stronger HG (I believe also found on landrovers) and to mount a modified oil rail in the sump.

Just an update. MG garage has confirmed that its early HGF so its currently getting it replaced.

Its getting the N Series gasket.

I'll do another update once I get the car back