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MG MGF Technical - Coolant Mystery
|I have an R reg VVC, 39,000 miles, and had the head gasket replaced 3 months ago. A month later the mystery started, after an hours motorway driving at 85mph I slowed down to a crawl and within 5 mins steam is pouring out of the back. First thought was HGF again, but the garage had a look at it, they blame the coolant cap and repressurise the coolant system.|
A few weeks later with no leakage, same again, but this time, I rev the engine to 3000 and the water temp guage drops within 20 secs to normal, I leave it to idle a while and up goes the water temp, I rev it up above 2500 and bingo, it goes straight down.
It only seems to happen if I have been driving at Motorway speed for at least 30 mins, i.e. everything is nice and hot, the oil temp is reading approx. 105 degrees.
The last time it happened there was water all over the top of the engine and to the side of the expansion tank, before I noticed and revved it up again.
|air lock in the rad?|
sticky thermostat? (my money here - but only becase I can still taste the coolant!)
|is the sensor-controlled radiator cooling fan working ?|
|not at motorway speed!|
Airflow is not an issue here, but water pressure or flow is.
Another thought - are your joggle valves clear? you could be collecting air in the top of the head, and at 3000RPM there isn't enough pump pressure to push it out. (really clutching at straws for an alternate explanation!)
|OK, course of action (all from cold):|
First open the boot and bonnet, undo cap and then go to the front and open bleed screw (plastic bolt in radiator (thru square hole in cowel) on RHS) stop when green blood appears. If air has escaped then check level of coolant, top up with water as required.
If it reoccurs:
With hands as nimble as a 5 year old, find the hose that leads to the inlet manifold (should be 1 cm in diameter and made of rubber, attached at the far right of the manifold). block the end to the cooling system and blow into the manifold (try not to get coolant on your lips - yuch). the air should hiss out, if you now run the engine a very tiny amount of water should dribble from the manifold. If so then all is well here.
If you still have the problem :-(
It's thermostat time! - if you want to do this job then buy yourself a ratchet 8mm spanner, they are expensive, but within 10 mins you'll be thanking this advice. Check back if you want to do this, it's not incredably difficult, but quite fiddly.
|>>>not at motorway speed!|
===surely he said this happens when in a crawl immediately *after* motorway driving. So hot engine, suddenly no airflow, fan sensor not working so temp goes through the roof. Rev engine, circulate water on pump, temp drops again. Or have I missed something?
|Do'h you're right, I was mislead by this line:|
>It only seems to happen if I have been driving at
>Motorway speed for at least 30 mins, ...
Right - Fan it is (probebly, but reving should not make much diffrence - ho hum)
Test: take engine bay cover off, start engine, disconnect brown sensor. Fan should start and engine speed incresse to 1200 RPM. If this happens you have a dodgy sensor or dirty contacts (spray with contact cleaner on plug and socket). If fan fails to work, check fuses, then look at fan (could be jammed).
NOTE be careful with the fan, it should be ECU controlled and may spinup with the engine off!
|>> NOTE be careful with the fan, it should be ECU controlled and may spinup with the engine off! <<|
I think that this will only happen on the Elise Will, as the rad fan power is controlled by the ignition switch...
Nigel, I agree with Will and Dave - this is very likely to be the non-operation of the rad fan causing these problems. The tests that Will mentions will determine whether it is a sensor/connection fault or a blown rad fan fuse.
More on these tests here: http://www.mgf.ultimatemg.com/hgf_pages/related_problems_overheating.htm
|>I think that this will only happen on the Elise Will, as |
>the rad fan power is controlled by the ignition switch...
I guess that that is because we have a seperate engine bay fan (thinks I must rewire the midget fan to use perm power).
TBQH You might find it quicker to look for the fuse first!
|>> thinks I must rewire the Midget fan to use perm power <<|
That's not a bad idea. I guess that the K-series mounted in the Midget will get more of a thermo-syphon circultion than an F - but you could consider using a low capacity water pump on the same circuit (thanks for that suggestion Bruce - still not had a chance to properly follow this up! Timely reminder methinks :o))
|Been thinking about thermosyphoning, and I not sure you get it on K's at all, on A's the thermostat is on the top of the engine, so it gets fed by the heat of the head (so will open, and stay open when stopped and hot), on the K it's on the inlet side, so the slowly moving water will cause it to clamp shut very quickly. This will stop thermosyphoning thru the rad, although you might get a very little thru the bypass tube - but then you'll get that on the F!|
|You're right Will- the thermosyphon circulation would be pretty much maintained in the block/ bypass circuit alone, and almost never through the rad :o( The VHPD derivatives of the Elise use a Bosch pump in the bypass circuit to keep things moving along...|
|>>Nigel, I agree with Will and Dave - this is very likely to be the non-operation of the rad fan causing these problems<<|
Could be - but then why does increasing revs bring the temp down ? I think Will was right - this may well be a coolant flow problem; either airlock, water pump or thermostat problem. Could even be due to gunge in water system after HGF.
Whatever it is, it needs fixing quick.
This is the most puzzleing thing mon-ami!
|Steve, you're right of course... I think I'd check out the operation of the fan first - because that is the simplest/cheapest thing to deal with. If the temperature remains persistantly high thereafter, then the cooling system as a whole is suspect :o(|
Water pumps often make nasty noises before they fail - but I wonder if this could be a suspect here, nes pas?
|water pumps _don't_ fail, water pump bearings fail (just you replace the whole unit in one go) There isn't anything alse to go wrong!|
|Which was what I was referring to: the pumps can leak coolant and seize in time - but I guess you're right: that in itself may not result in significant falls in flow prior to complete failure.|
|>may not result in significant falls in flow prior to complete failure.|
At which point, it jams, rips the teeth of the cambelt and locks the camshaft, the crank continues to drive the pistons punching the valves into the camshaft, denting the piston heads, destroying the valves, possibly damaging the camshaft, and generaly writing off the engine in the time it takes to progress about a meter.
In short it'll never cause an engine to overheat ;-)
|... unless an impeller blade or two falls off before hand! And no, I haven't heard of that happening either ROFL ;o)|
This thread was discussed between 21/04/2004 and 22/04/2004
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