4AC AE92 Overheating, but head gasket is fine?

[stradlater]

I have a 4AC, AE92 and it's overheating, bubbling itself crazy, boiling water left right and center.

But I've done a compression test and the head gasket is fine, 150psi on each cylinder identical.

So what on earth could it be? It's not leaking any water, so I figured it's not the water pump and the thermostat moved easily when I pulled it out and moved it around.

Could the water pump be shagged but not leaking water through it's seal?

[wa5]

did you test the thermostat in boiling water to make sure it functions?

and yes, your water pump could be shagged, I think it was on this site that I saw a similar thing, no "blades" left on the impellor...

Radiator isnt blocked ?

[Hiro]

4AFC you mean?

[Casey-G]

Thermostat, water pump & radiator.

check them all.

[litldv8]

fan coming on?

[stradlater]

Thanks guys.

Yes - 4AFC

Thermostat - I haven't tried the thermostat in boiling water, I will try that - Thanks mate!

Radiator - The radiator LOOKS in reasonable condition on the outside, I was thinking of getting a radiator flush thing onto it though.

Water Pump - I haven no idea on it's state, other than to say it's not leaking water.

Fan - Yep, the fans are BRILLIANT and working a treat - they are what's stopping the thing from boiling it's nut off.

The thing that concerns me the most is - the temperature gauge shows the water as fine. This is the sensor that sits inside the thermostat housing, but on the HOT side of the thermostat, not the cold. Only when it gets really hot does this go up.

However - The water is bubbling it's nut off!!!

Plus - when I rev it, water GUSHES out like no tomorrow... Yet the head gasket appears to be ok? (150psi per cylinder).

WTF...?

[litldv8]

top hose hot and bottom cold or the other way around?

[beerhead]

Seems like you just worked out the problem, the motor is cooking, but the gauge isn't responding, and the gauge sender is on the output side of the external thermostat housing. Thermostat is shagged. Try taking out the thermostat and go for a drive. No wonder she's getting hot, your trying to cool your motor with a 18mm bypass tube.

[stradlater]

No no...

The temp sender is on the HOT side of the thermostat, not the cold side......

top hose gets hot (feed), bottom hose stays reasonably cool (return).

[Casey-G]

Still, take your thermostat out all together and go for a drive.. this will either confirm it or rule it out.

[jimmmayyy]

its very possible that it could be a cracked head if its overheating like that, of course after you have checked and ruled out thermostat etc. when its cold it won't leak but once it warms up and the head expands, water can seep through the expanded cracks into the cc. head gasket will be fine, and if you comp test it cold it will appear fine.

[stradlater]

I comp tested it hot though....

I comp tested it after cracking the shits with the water boiling itself.

[Casey-G]

Cracked head would show symptoms in the oil or coolant though..

[foamy]

Testing the thermostat in hot water will not always rule out the problem...you need to know how hot the water is...no good if it is 120 degrees hot and the thermostat opens
I hope the thermostat has failed, a blocked radiator is only the start of your problems...\


edit: is it just me or 150psi for a comp test seem a bit low? Did all of the cylinders get a similar result?

[jimmmayyy]

not necessarily, water can leak straight into the cc without it mixing with oil, just turns to steam, and out the exh.

could be somewhere in between that..once you've ruled out everything else.