HGF diagnosis

Maintenance: diagnosis and prevention of HGF

Problem: Steam is errupting from my car- what is the cause?
Action: stop as soon as is safe and practical. Stop engine, and open the boot. Then as the engine cools, perform the following checks:
Look at the coolant expansion tank. Is there any coolant left? Is it boiling? Is it Green?

If yes, then the steam erruption is likely to be due to a failure of the coolant cap. To confirm, perform the following checks:

Remove the Coolant cap and examine the brass valve. There is a rivet which should be central. If it is not (as in A, the cap on the right), the cap has failed. Replace.
Check the coolant. The level should be above the minimum mark, and the colour green (as pictured in 'B'). There should be no evidence of oil or brown gunge. If the latter, then the head gasket has failed.
Remove oil filer cap and dip stick (C). There should be no evidence of a milky (mayo) residue- just oil. If present, the head gasket has failed.
Check under the car. If a pool of water, remove engine inspection cover and look at the front right corner. If water is seen existing (arrowed, 'D')- the head gasket has clearly failed.

Visual checks for evidence of a HGF

hgf_checks.jpg (40585 bytes)

Thanks to Jon Fredheim for the picture of the failed head gasket (D).

In the event of a head gasket failure, do not attempt to drive the car any further. Arrange transportation to an MG/Rover dealership or specialist for replacement.

Problem: my car is overheating in slow moving traffic, but appears fine on the motorway.
Action: Investigate as soon as practical- overheating of aluminium engines is dangerous as it can lead to either premature head gasket failure or damage/warping of the engine itself.

The most likely cause of a failure of this type is the malfunction of the ECU temperature sensor or its electrical connection to the engine management system. To check, open the engine inspection lid, and examine the following parts. A replacement sensor (part # GTR206) costs less than 11 GBP including VAT, so replacement is a sensible precaustion.

Locate the ECU water temperature sensor- it is found on an elbow from the cylinder head, arrowed in A and B. The housing is brown plastic.
If replacement is planed, wait until engine is stone cold before replacing sensor with a new item (pictured C).
To remove electrical connector, carefully prise off the circlip, and withdraw loom. If you wish to test function of the sensor in situ, run engine until warm (water temp approx 80-90 celcius). Take multimeter, and measure electrical resistance across the two terminals. It should be in the range of 300-400 ohms. If significantly greater, then the sensor has failed and will require replacement. Replace when stone cold. Unscrew and replace with new sensor sparingly smeared with a gasket compound.

Replacement of ECU temp sensor

tempsensor.jpg (41140 bytes)

A: location of sensor at front left corner of engine cylinder head (arrowed)

B: close up of sensor (brown coloured, arrowed).

C: what the sensor looks like (part #GTR206)

D: removing the wiring loom requires release of a circlip- careful not to loose!

Similar faults can occur with the dashboard temperature guage making it under-read coolant temperature. Its location is immediately below the ECU temp sensor mounted in the verticle plane.

Preventative Maintanence to avoid HGF

As per hand book instruction, regularly check the level of coolant in the expansion tank, topping up as necessary. Any rapid changes in level should arouse concern.
Keep an eye on the oil and water temperature guages. If any evidence of overheating (or the engine behaving as though cold), then check the ECU temperature sensor as described above.
Check the condition of the water ways under the car- they are made of steel, and can therefore rust- it is not unheard of for coolant to slowly leak away because of corrosion of these pipes on cars of 3 years of age or more.
If a pre-1998 MGF 1.8i, it is recommended to update the inlet manifold seal with the later type. Early manifold seals are prone to premature deterioration, and can be a cause of water loss and even precipitate a head gasket failure.
Ensure that the coolant is correctly bleed, and that the auto-bleed valve is functioning correctly. Click here for more information on this.
ALWAYS avoid over-revving the engine from cold: never exceed 3000 rpm until 60 celcius is showing on the oil temp guage.

There are three bleed nipples in the cooling system of the MGF. One is located by the radiator (A), the heater (located on the front bulk head behind the bonnet shroud, B) and in the engine bay, on the header rail, pictured from two different angles (C, with filter removed observed from the luggage compartment & D from inside the engine bay with inspection cover removed. The latter provides the easier access).

To bleed the system of air, keep the resevoir topped up with coolant (50:50 water and Unipart Super3 antifreeze) and individually remove the bleed screws one by one to release any trapped air. Drive the car a short distance, and repeat again once the engine is cold.

Location of the Bleed Nipples

bleed_nipples.jpg (46856 bytes)

A potential solution to the HGF problem?

Another potential cause of the HGF problem has come to light as a result of MGF related motorsports. Basically, from what I gather, the problem is as already been highlighted on the MGF BBS, namely that due to the remote nature of the radiator causing larger than 'normal' [than that found in FWD applications] changes in coolant temperature entering the engine. This causes the standard type thermostat to occasionally be fooled and close when the engine is actually hot, thus allowing localised 'micro' overheating. This phenomena, over a period of time, may weaken the perfectly good head gasket leading to failure.

The common solution, apparently, is to drill four equi-spaced 6 mm holes in the thermostat ring. This mod allows the cooling system to continue to function in a normal fashion, but crucially also some circulation when the thermostat is shut. Thus, when the thermostat is inappropriately shut, some cooling circulation continues avoiding the excess oscillations of temperature caused by a negative-feedback system such as this with a remote effector system.

There is a draw back- the warm up time is increased. It has been suggested to me that a compromise for a road car would be one (or possibly two) 6 mm holes- which would result in a slower warm-up time, but still adequate closed-thermostat circultion to prevent a temperature peaks causing head gasket failure.

Thermostat modification: see Dieter's thermostat site.

If your MGF has ever suffered from a head gasket failure, we would very much appreciate it if you could enter as many details as possible into our HGF listing. From this we hope to learn more about the causes of the HGF, and help prevent them occuring in the future- thankyou. Enter http://www.shame.4mg.com here.