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MG MGF Technical - Water boils@100 what about Oil?

I'm still working on trying to find out what is going on with my oil temp gauge. The water temperature is just below the middle all the time, but the temperature guage can reach 140 degrees!

I know all oils are different, but at what temperature do they boil and lose their lubricating properties?

I've read the recent temperature teaser thread (probably the most useful thread we've had on this board), but where is the sensor for the oil? Does anyone have a picture and part number?

Thanks, Alan

Spookily enough, I just bought some Mobil 1 for the little Red Rocket. It supposedly can keep an engine, almost in Brand Spanky new shape for a million miles operating at temperatures between -54 deg celcius and +300 degrees Celcius.
It's totally synthetic, bloody expensive, but extremely good.

Kieren Gibson

Water only boils at 100degC @ 1bara. The purpose of the pressure system is to raise the boiling temperature, as water is a very good heat conductor above 100degC. Hence the fans turning on and off at 107 etc... Which is why if the pressure cap leaks, the water boils at 100degC before any of the cooling fans or thermostat trigger. If the cooling water boils, it disappears... then HGF?

One could debate the purpose of a water temp gauge, as it will display only the temperature of the water passing the sensor. Which will never be above the boiling point of water. Perhaps pressure is a better monitor of cooling system health?

The oil system is not deliberately pressurised. Normal oil doesn't boil until well over 200 degC.

Does that make sense?

N837 OGF


The sensor for the oil temperature gauge is located in the oil filter housing, attached from below behind the oil filter. You can see it if you look under the car in front of the driver's side rear wheel (you may need to raise the rear of the car depending on your ride height- in that case don't forget to support it properly on axle stands). According to the workshop manual MKI cars up to vin 001017 have an 150C sender and from vin 001018 onwards have a 170C sender. I don't know what type of sender MKII cars have.

VVC cars have an additional oil temperature sensor mounted on top of the VVC hydraulic control unit. This is used by the ECU to detemine how quickly the VVC mechanism should respond to cam period change commands. However, I don't think you have to be concerned with this one in your case.

If you have a MKII car, I remember a few threads some time ago mentioning a problem with a faulty batch of sender units giving to high temperature readings - have a look in the archives.

Spyros Papageorghiou

Your oil shouldn't go above 125 deg C, above this some oils will burn off some of the lighter fractions or additives. Some oils will tolerate extremely high temperatures and are very stable viscosity wise, others are not so good... If in doubt, keep below 125 deg. Remember oil deteriorates with age, contaminants and shear forces on the molecules render an oil a lot less effective quite quickly.

Dave Andrews

There were a batch of new cars that were reading the oil temperature as being too high.
This was (I think) due to a wrongly installed component. eg. Older style sensor in new car.

Will not cause problems, but reads wrongly.

Have a look on the BBS archive to check this out.
I think it was Roger Parker that posted this.

Hope this helps,

ps. Before the topic crops up, unless you are Dirk or racing then you shouldn't need an oil cooler.
In fact it will make the oil too cool! :-)
Paul Nothard

Mobil 1 will not sve you if your head gasket goes and you do not stop immediately ( assuming you look at the gauges all the time and not the road) the first HGF was recognised because the heater stopped working the second time too late engine oil boiled and B&G said the engine was ruined despite Mobil 1 !! After new engine and a very poor chap i am obsessive about the gauges, what is needed is an audible device to warn of over heating.

The data for the correction of some wrong indicating oil temp. gauges at MK2=MY2000 models of the first manufactured batch (1999) have IMO not been reported until now.

Who cares, I've got it :)
First instruction is to let this works do under warranty at the dealer (registered MG workshop)

The works are to install a 15 Ohm 1/2W resistor between the oil temp. sensor and (IMO) the ECU.
The location for it is a plug behind the ECU fixture bracket.

Haven't got the wire colour to hands but can state it later in next week.

PS. have problems in identifying the connection to the new ECU at MY2000. Different wiring schedules.
Dieter Koennecke


Very surprised that your engine oil boiled. I would you'd get serious pre-ignition first, or at least melt all the plastic parts on the engine.

Where did it boil?

N837 OGF

Sorry Hugh
I cannot substantiate the statement that my oil boiled
It did seep through the gaskets and the engine was extremely hot, but did not seize. It all happened on the M11 probably doing 70 mph I hoped that the Mobil 1 might have stopped irreversible damage but B&G said it was ruined.

My oil hit 150 today. A number of times. Ooops...

Track day at Donnigton. <grin>

Oil cooler next methinks...

Paul Nothard

125-130 deg is about normal running temperature (according to the oil temperature dial) at legal speeds (except in Germany). If pushing a little harder it can easily creep up to 150. Personally I will ease off if it gets to 150, but this should not be a problem and should be well within the safe temperature range.

The only problem I have with the oil temp sender unit is it's unprotected location, I had one broken by a stone, causing a small oil leak. Anyone have any ideas regarding some kind of guard for these unprotected parts.
Tony Smith

150 on a track day is still probably a bit high IMO. On track days i have not been anywhere near that level, 120 is the max i think (if 120 is the next mark on from vertical) and at Donnington on the 15th i think Rob Bells oil temp was even lower than mine.

I think i'd a stopped if my temp got that high (or did you).

but apart from that, great circuit init

Quite a few times on thread about oil temp i have noticed though that there seems to be a wide range o ftemps that peoepl are seeing. I haven't got a clue how well set up/calibrated the guage is, but could it be that they are infact very poor and should be used on a, is it higher than normal basis, rather than using the figure it reads.


Matt, 150 is the mark past vertical so it sounds normal. On my dial it shows marks for 50 first, the next line is unmarked (I think), then 120 (just before vertical), then 150 (past vertical) then a small gap to where the red line starts and runs up to 170.

From cold by just driving to the Motorway and getting up to speed I reach this temperature and it is pretty constant running around 120-130 kph (70-80 MPH Approx).

According to the manual the sender was changed at Vin 001018 and just appears to use larger resistors but this change was not very far into production as you can see by the vin number. This should not be a problem for most of us.
Tony Smith

Mobile 1 is a good oil, and being synthetic it performs differently than mineral oils.

The main difference is that when cold it is vey thin, but unlike mineral oils it will not get substaintially thinner with temperature.

I had a problem on a car, not an MG, a few years ago when using Mobil 1 with several minor oil leaks which could not be cured. When I changed oil to Castrol GTX magnatec, also a synthetic oil but thicker grade, the oil leaks disapeared.

I personnally would always go for synthetic oil and also use a flushing oil at each change to remove all sludge etc.

I used to only change oil in several of my previous cars at 12,000 miles using this approach not the recommended 6,000 with no ill effects, but i am not sure I would want to change the oil in an f at 24,000.

This is vet interesting.

I had my 36,000 service and the used Shell Helix, a synthetic oil, the next day i drove back to the dealer as the oil temp gauge was higher than it had ever been. It was past 120 and heading to 150. Usualy 120 was the limit regardless of driving conditions.

The dealer said this was within normal operating conditions however I put it down to a change in oil. The car is new to me so i dont know what was in there previously. Any opinions ?


I don't know at what temperature oil boils, but the maximum operating temp for a good mineral oil would be about 300 deg F, and for a PolyAlphaOlefin (PAO) synthetic such as Mobil 1 around 400 deg F plus. Even with a relatively low sump temperature, piston ring and cylinder wall temperatures can exceed 500 deg F.

Synthetic oils are perceived to be 'thinner' than mineral oils because their cold (W) rating viscosity can be lower, Mobil 1 being 0W (in the UK) for instance. Synthetic oils can be engineered down to this lower viscosity because they are not so volatile: a 0W rated mineral oil would quickly flash off the lighter elements to produce an unbalanced thicker oil. The great advantage with a low viscosity oil is that it circulates far easier, and thus faster, when the engine is started from cold.

The high temperature SAE ratings are just that, and apply to both synthetic and mineral oils. An SAE 40 rated synthetic will be just as thin, or thick, as a 40 rated mineral oil at 100 deg C. What should be added to this is 'When the oil is new,' as the mineral oil package degrades more quickly than the synthetic.

Personally I wouldn't use a flushing oil. I don't think that a modern engine run on semi or full synthetics need it. GTX Magnatec is a semi-synthetic, by the way.

Shell Helix Ultra is a full synthetic, 5W-40 I believe. I have run my car on both Magnatec and Mobil 1 and I think (but I may be kidding myself) that the oil temp gauge registers just a little lower with the Mobil 1, at 90 instead of just over 90. It has only ever touched 100 deg once, I must have been having a wild moment. Synthetics should run a little cooler than minerals (just), and fresh oil should also be at its peak, so I don't know why you're getting a 150 reading Chris. No oil change should produce a difference of 30 deg!

Of all the advantages of synthetics, some
applying to extreme conditions I would never wish to experience, the two which sell them to me are fast cold start circulation and the ability to perform without significant degredation for the lifetime of the oil.

Regards, Kes.

Kes, is oil your specialist field?

Very useful, thanks,

I will have to check my magnatec, I brought half a dozen cases when it was launched as it was on special and still have a case left, I thought it was synthectic, but could be wrong.

Flushing oil is out of favour these days, but I like to think it helps, I supose the best option is to flush the engine with new synthetic oil, but I will need to get better bulk buy deals before I can afford that.

Incidently while on about oil I used to buy Genuine Ford Motorcraft oil for about 5 a case of 4x5l, I used it on any non performance cars. I then made the mistake once of taking a car for a service to a Ford dealer (I was too busy to do it for a while and service was well overdue) and was amazed to see them charging me about 5 per litre for the oil. They tried to con me that their parts department charged that per litre, which was correct for buying a single litre, but they had a 205litre drum they used for the servicing,

Whats the most a Rover Dealer has charged for oil in an f?

BTW. do not think you can get round dealers by giving them your own oil for the service, a friend of mine took his pride and joy to a local independant with 4l of that Castrol RS stuff, they did the oil change, did not quibble about the arrangement, but put cheap oil in instead, you can only guess where the RS stuff went.


I too have seen the oil leak issue appear with one oil and vanish with another of the same viscosity range, also not on an MG engine.

One of the main reasons that flushing agents are out of favour is that many current engines run hydraulic tappets, and especially a higher mile engine, there is a real risk of the flush dislodging crud that then finds it's way to blocking tappet oil galeries.

Roger Parker

I understand what your saying but would the crud not find its way into the galleries anyway in time. Is it not better to flush at every oil change to prevent build up in the first place.

Graham Robson

This thread was discussed between 27/04/2001 and 11/05/2001

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