[PUSB]

Hi

So last week my MGF overheated on a fast moving road. This happened after driving for a bit, stopped at services, then carried on with my journey when the temp gauge shot up and it started boiling over.
RAC man come out and topped up the coolant, and it all seemed fine. He couldn't spot any leaks etc. and the car seemed to be running ok. I was able to carry on driving without it overheating again.
I suspected the expansion tank cap was faulty so I bought a new one.

Had to go out this morning, and I was travelling at about 50mph, and the engine management light started flashing. It stopped when I slowed down. Is this likely to be connected to the overheating?

Also after I got back I wanted to check the coolant level. After letting the car cool down (about 3 hours), when I took the cap off the expansion tank it seemed like it was pressurised. I'd never really known that before on a cold engine.

 

[Julian Foulger]

I expect the worse but you need to get a Pscan or a T4 on to read any error codes. Guessing is not good but the over-heating will be most likely a symptom, not the cause.

 

[nocturneblue]

Sooo...

MIL lights can be turned off but the question is why did it overheat? Looks like you lost coolant if it overheated and the RAC man could top up the tank. Don't want to mention the 'Head Gasket' words but...if it was just a slow coolant leak elsewhere (like the usual underbelly steel coolant pipe corrosion problem) why would the expansion tank remain pressurised?

The sudden overheat seems symptomatic of an air lock. In my experience an airlock moves around - all can be fine but then it moves to the wrong place (the pump?) and the temp gauge shoots up. I found when I had an airlock (caused by a failed head gasket) the car was OK on the motorway - the high engine speed presumably kept the airlock moving - but when I pulled off onto a slip road and slowed down the airlock could settle somewhere inconvenient and the temp gauge shot up.

The presence of pressure in the coolant tank long after the car has cooled down is worrying - if you have a failed head gasket, combustion gasses can escape across it into the cooling galleries and back pressure the coolant system. You can get a carbon monoxide test kit for the coolant to determine if combustion gasses have been bubbling into it. The other way to see if head gasket blowby has occurred is to look for bubbles coming into the expansion tank and sniff it for exhaust fumes. You could look for an oily sheen on the coolant surface too (there shouldn't be one). I'm not 100% sure of the details on how to check for bubbling since if the engine is up to temperature the coolant will boil explosively if the expansion tank cap is unscrewed and you don't want to get scalded, but if the thermostat is still closed due to a cool engine the gas won't get past it (or possibly not). Maybe start the car with the expansion tank cap off and just wait and watch for bubbles as it gets to temperature.

If you find out there are exhaust gasses in the coolant tank, one way to localise the problem is to run the car on three cylinders, by taking a spark plug out. Go through each spark plug like this, one at a time. If the plug from the affected cylinder is removed, the consequent lack of pressure on a failed head gasket from that cylinder will stop any gas getting back into the coolant tank and bubbling will stop (if visible before).

Also, check for a white exhaust smoke (after the car has warmed up). White smoke indicates the presence of water vapour and therefore a coolant leak. Finally, check your oil dipstick - is there any 'mayonnaise' on it? There may be some on the inside of the oil filler cap too. A little on the inside of the cap is not unusual if you do short journeys where the car doesn't warm up properly, but a lot would be suspicious.

Original head gaskets usually fail (very approximately) in the 50k miles region. Of course, some fail very early and some go on forever, but from what I hear that's the most common sort of mileage for HGF. I'd have thought an F would be far past that mileage by now though.

 

[PUSB]

Hi, thanks for the reply.

I've checked for bubbles this morning. I started the car up from cold without the cap on. There are no actual bubbles coming up from the bottom of the tank, but where it fills up from the side (is that a bypass hose?) it was not filling consistently. That certainly seemed like there was air within the system.

The dipstick looks clean, but there is a little bit of mayo on the oil cap (you can't really see it but if you rub some tissue inside the cap you can see the discolouration on the tissue)

Engine has done 96k miles

 

[YasmineGotham]

tbh, if the engine has overheated, then its basically scrap

 

[PUSB]

Even if its still running fine with no power loss?

 

[nocturneblue]

Mine overheated when I had head gasket failure. More than once actually as after the coolant cap vented steam everywhere whilst I was stationary in traffic (much to my surprise) I waited for it to cool down, topped the radiator with water and drove it to a garage in what turned out to necessitate a short journey of a couple of stages. Not wise I know, but after the head gasket was replaced with a MLI? gasket and the head skimmed it was fine (I didn't trust the garage, I'm not sure the head needed skimming but that's a long story) and it's done 50k since, barely sips any oil (even 5W30) and can get up to 44mpg.

I think the theory behind overheating turning the engine to scrap is that the cylinder head metal goes soft (it's alloy and is presumably heat treated to some hardness). Then there are problems with the head gasket being able to retain a gas seal against the head. However, I think there's overheating, then there's 'overheating'.

When the car overheats (well, in my experience) there's a chance you spot the needle going up towards the red and pull over and let it cool down. I think you're OK there. Nothing to worry about. Then there's the type of overheating where you suddenly notice in the rear view mirror there is steam coming from the coolant overflow tank and seconds later suddenly it blows! I immediately pulled over and stopped the engine when that happened to me.

Then there's the type of overheating where you keep driving it regardless of the coolant tank venting steam, the needle being in the red and you hear the engine start to make pinking noises (pre-ignition) even when not under load because the combustion chambers are too hot. Maybe that's where things start to get a bit worrying...

Personally, although I don't know the exact circumstances of your overheat, my experience tells me overheating the engine until steam vents from the coolant cap (if followed by a rapid pull-over, engine off response) is not going to result in a scrap engine (and I believe it'd only be the head anyway that might suffer).

Disclaimer: I'm not the world's foremost expert on heat damage to engines. I just know what happened to mine and it works just fine nowadays.

Anyway, if it's the head gasket they'll have a look for any problems when the head is off I for a new gasket I presume.

 

[PUSB]

Garage did a compression test and pressure tested the cooling system and it come back ok. Annoyingly they didn't give me the figures of the compression test (do you ever get the feeling that a garage hasn't been bothered to do the job properly?)

Anyway, its still holding pressure in the coolant tank. I recorded a little video here. As you can hear, it sounds a bit like a bottle of pop being opened. This was with the car being sat for over 24 hours

 

[Man in the Car]

Garage should have done a 'sniff' test on the expansion tank using an exhaust gas analyser.

You almost certainly have a head gasket failure at one or more of the fire rings (which are the hard rings within the head gasket which seal around the head to cylinder liner interface).

It is a fairly common mode of failure, and quite often in the F/TF is due to localised overheating due to insufficient coolant flow through the exhaust side of the cylinder head, which in turn causes the alloy of the head to anneal (soften) on the exhaust side. This results in the fire rings indenting the head, thus relaxing the clamping force and allowing exhaust gases to be pushed past the fire ring and into the coolant passageway. It is this exhaust gas in the coolant system which leads to the system remaining pressurised even when it has cooled down (and that release of pressure when undoing the expansion tank cap when the engine is cold is a pretty surefire indication of fire ring failure on its own).

The head will probably need a skim and hardness testing, although a softened head can be salvaged by machining sufficient material from it and fitting a Gosnay's type head saver shim. The most beneficial modification that can be done to the head is to take a mini drill and a burr, and grind out the entrances to the coolant passageways (they were not opened out in the factory, and are usually quite restricted by excess casting material. Opening them out (taking care not to overdo it) will significantly improve coolant flow through the head, and help alleviate the localised overheating which is likely to be behind most instances of HGF (and being localised, you will not see any evidence on the dash temperature gauge).

 

[PUSB]

I think your right.

Its amazing though isn't it? That something that serious can be wrong with a car and it still on the surface run ok

 

[mhack]

i had similiar issue with my mgf, was losing about 1 inch of water a month, occasionally overheat or get close, it already had had a mls gasket fitted less than 1000 miles ago by previous owner, did a sniff test with those kits of ebay, didnt show anything, put the sniff test fluid in front of exhaust it changed colour so i know the kit works.

in the end it was head gasket gone, now looking back the main signs were over pressuring of the coolant, it got so bad that the coolant in the header tank would disappear and only re-appearing once you released the cap, the other thing i noticed now since i putting payen gasket on is it takes a good 20 mins of driving before it warms up, before it was 500 yards and it was normal temp.

check my previous threads i put a photo of the head gasket i removed which was shot to pieces but i could still use the car around town obviously didnt trusted in on anything more than a 30 min drive.