About to give up on my MGF - I think I have tried everything.

So - my MGF overheats.
If you let it idle - it climbs continually until it over heats. If you rev it for a small amount it rapidly returns to temp.
When I drive it sits happy at 85-88C (got a OBD2 gauge that I constantly monitor) if you stop for traffic a blast on the accelerator can keep it in check.
Until.... sometimes it just shoots up rapidly - for no apparent reason at all - total thermal runaway while cruising within 10-15 seconds it goes from 88 - 120C. Then I pull over - turn off wait for a few mins - start it again and temp comes down and it is fine again.
I have been battling this for a while - and I have changed a lot of things - each time I think I have got it.
One thing to note is this symptom only started appearing after I noticed a lot of blowby pushing past the dipstick.

Let's go with the obvious ones
- New Rad cap
- New thermostat
- New water pump
- flush and clean of cooling system - multiple times - used dishwasher tabs
- bled and bled till she wont bleed no more
- no oil in water
- no water in oil
- cleaned egr pipes
- added o-ring to dip stick (reduced a lot of the gas blowing past)

Things I am now pondering
- is it the start of a HGF (had it done using multi gasket a few years ago)
- is it liners
- is the blow by part of the above.

I have probably spend as much as I want to to resolve this - I just cant think of anything else to try with out stripping the whole motor down.
The rest of the car is in mint condition so it will be a shame to give up. I am in Australia so things like an easy motor swap just can't be done for the right $

Thoughts?

Try putting a pressure test on the coolant system first to see if there are any obvious leaks.
If it holds pressure then you could be looking at a blockage. Is there any return flow back to the header tank, try running the car with the cap off when cold to see if the coolant is returning ok to the tank.

Pressure test kits are quite cheap on ebay.

Sundance

The only time you would normally get rapid temp change Indicated is when you have gases in the coolant. When the temp sensor is sensing not coolant but a bubble of gas.

Positive pressure in the crankcase also suggests gases are escaping the combustion chamber and entering oilways somehow. This could be head gasket, worn rings, or cracked liner.

SundanceUK wrote: Try putting a pressure test on the coolant system first to see if there are any obvious leaks.
If it holds pressure then you could be looking at a blockage. Is there any return flow back to the header tank, try running the car with the cap off when cold to see if the coolant is returning ok to the tank.

Pressure test kits are quite cheap on ebay.

Sundance

System holds pressure - no leaks. System flow appears to be fine although interesting it seems to take a long time to circulate through radiator on idle. It can be overtemp and rad pipes are cold till reved a little. Heater is warm

David Aiketgate wrote: The only time you would normally get rapid temp change Indicated is when you have gases in the coolant. When the temp sensor is sensing not coolant but a bubble of gas.

Positive pressure in the crankcase also suggests gases are escaping the combustion chamber and entering oilways somehow. This could be head gasket, worn rings, or cracked liner.

Im starting to think that it is one of these options - maybe opening up suddenly at time of the quick rise.

Where in Oz are you? If you're in Melbourne you can bring it around to my place in Burwood, and I'll systematically check over it for you.
It will in all probability be something simple. Even if it is drastic you'll know for sure, before you give up on the car.

Cobber wrote: Where in Oz are you? If you're in Melbourne you can bring it around to my place in Burwood, and I'll systematically check over it for you.

Really appreciate the offer but I'm up at the Sunshine Coast

What sort of things would you look at - only thing I really havent done is a compression and gas test - was concentrating of the symptoms too much I think.

I had exactly the same I ran round the houses trying everything to no avail, eventually I had one of our mobile techs Jon from Rough luck to look at it, he inserted a camera into the bores via the plug holes and number 1 cylinder was wet, so over heat was due to gasses although a sniff test said otherwise, I also had a multi layer head gasket that han done less than 4000 miles liners were ok, just the gasket had let go, Jon changed the gasket for an N series gasket and all well 4 years hence.


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Things to check:
Are ALL the fans kicking in? don't forget the engine bay fan.
Have you back flushed the radiator?
A cracked liner should show up in a compression leak-down test, a sniffer test should also show this.
A compression test should show a failed head gasket.
Just because parts are new, doesn't mean they're not faulty, check over all the new parts, test that thermostat and check that it is the correct one.
Pull the spark plugs, is one super clean in comparison to the others? Coolant getting into the combustion chamber will often "steam clean" the plug. An overly oily plug will indicate buggered rings, a gasket failure to an oil gallery or a liner to crank case seal.
Sometimes just filling the cylinder with compressed air at TDC will indicate a problem, is the air leaking trough to the crankcase? or bubbling the coolant?
Are al the coolant hoses going where they should? Sometimes the minor hoses can be connected to the wrong nipples, this can cause interruption to the proper flow.
Lean fuel/air mixture can cause hot running, check for vacuum leaks, check sensors particularly temp sensors are correct, the wrong plug can be on the wrong sensor, so check all the plugs are on the right sensors.
Don't assume that anything is correct until its been properly checked.

Cobber wrote: Things to check:
Are ALL the fans kicking in? don't forget the engine bay fan. - both fans running fine - even programmed ECU to get fans to come on a bit lower temps to try and help
Have you back flushed the radiator? back flushed all cooling system - removed radiator and checked - its in very good condition.
A cracked liner should show up in a compression leak-down test, a sniffer test should also show this.
A compression test should show a failed head gasket.
Just because parts are new, doesn't mean they're not faulty, check over all the new parts, test that thermostat and check that it is the correct one.
Pull the spark plugs, is one super clean in comparison to the others? Coolant getting into the combustion chamber will often "steam clean" the plug - all plugs look good with light grey. I did notice when cyclinder 4 spark lead was removed less blowby was evident.

Responses above.

Steve wrote: I had exactly the same I ran round the houses trying everything to no avail, eventually I had one of our mobile techs Jon from Rough luck to look at it, he inserted a camera into the bores via the plug holes and number 1 cylinder was wet, so over heat was due to gasses although a sniff test said otherwise, I also had a multi layer head gasket that han done less than 4000 miles liners were ok, just the gasket had let go, Jon changed the gasket for an N series gasket and all well 4 years hence.

Would a HGF give the sudden increase of temp then resolve or just continue to be a heating problem?

I had the same issue years ago - for me it was a clogged toggle valve in the inlet manifold. Try to clean it.