M20B20 Overheating problems

[jkarran]

It's a standard 1990 M20B20 abused for navigational rallying. Basically it wont regulate its temperature, on long downhill runs it gets cold (just above the blue) but under load and even at idle it shoots up into the red.

First instinct was broken fan or mud on the rad (it's used off road) but they look fine, the rad looks nearly new.

The coolant looks like coffee but the oil's clean (well, it's filthy but it hasn't got water in it) so I'm assuming it's blown a head gasket.

Are there any known M20B20 problems to look at (thermostat, waterpump, airlocks etc) before I pull the head off it? Only other thing I can think of is the sumpguard, it curls up the front to keep deep water out of the engine bay. I've recently had to pull it up against the valance obstructing a couple of the small air inlets but surely they're not that sensitive to airflow, the kidney grilles are huge

<edit> guard is visible in avatar for an idea of which holes it blocks up. I made it that big after a smaller one scooped 10s of kilos of cow poo into my belts and dizzy

Cheers,
jk

[daimlerman]

While you have the head off have it crack tested,they go at the back of the cam valley above 5 and 6.Other than that,all you say would suggest a radiator partly blocked,or a fan coupling not working.Are you winning any prizes?

[jkarran]

While you have the head off have it crack tested,they go at the back of the cam valley above 5 and 6.Other than that,all you say would suggest a radiator partly blocked,or a fan coupling not working.Are you winning any prizes?

I think the rad's new just before I bought the car but I'll flush it through with a hose, it could well have some crud in it.

The fan is spinning though how much air it shifts at high rpm I don't know. It certainly moves air at idle but it'll overheat if left ticking over so I guess that's not the issue.

Odd thing is it runs very cold under light cruise/no throttle conditions which it never used to, it used to be rock solid, mid-dial no matter what

I'll get the head skimmed and checked if It has to come off.

Prizes? No, just having a hoot going sideways in the trees
jk

[325icab]

check the return pipe in the header tank is not blocked, that will cause it to overheat, basically pull the thin pipe off the rad and blow into it, if its blocked u will know