P38A overheating

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rover75lover

rover75lover

Active Member
Posts
210
Location
newport.wales.uk
P 38 1996 auto,diesel 2.5 165,000 miles
Recently replaced my head with a tested head. New BMW gasket and new torque bolts.
after 300 miles shes now started overheating ,so ive changed for brand new, the radiator,viscous fan,water pump for a metal impellered type , thermostat.Flushed the head out .
Problem im getting is that she sits lovely above cold but below the first white segment. Now after about 40 miles the needle gets to half way holds for about 10 miles then suddenly needle swings over red light on . If i lift the bonnet theres water all around the expansion cap and if i try to release it loads of pressure.and i have to add about a litre of water and she will climb back down.
HOWEVER if i travel with the heater on hi but not have the fan blowing she seems to run ok and doesnt get over the middle indication. When I stop let her cool ive lost no water.
Is the heater always online so to speak and the blend motor just directs the air with a flap or is the heater controlled by a water valve which switches the water flow off and on? It seems that if i leave the heater on im getting sufficient water flow from and away from the head so to speak
Alternatively
Is this head cracked cracked and im running on borderline
or is my block cracked?
Many thanks
 
That sounds like an airlock to me. Ram air effect with heater on high just enough to keep it safe but when heater is set to cold, it's got no cooling.
Have you checked if the rad is hot all over? Top and bottom hose both hot?
Not an expert on the diesel, just experience with other cars.
Could it be your rad cap is blowing off at too low a pressure and the extra cooling of the heater keeps it just below where it releases?
Just thinking out loud, hope it helps.
@wammers is the expert on these. I'm sure if they do airlock, he'll know about it and point you in the right direction.
 
Think the water flows through the heater regardless and temperature is controlled by blend motors.

Thinking airlock, dicky thermostat or possibly no baffle in the radiator. Was it a Nissens rad?
 
you say you have changed the radiator.
Are you sure you have the correct one? It should have baffle in the header tank...
??
 
Did the thermostat have a small drilled hole in the outer rim, and is the small pipe from the rad to expansion free from blockage
 
I'd try a new expansion tank cap. Might just be releasing at too low pressure dumping a litre of coolant then overheating.
 
Static test rig check needed here. Check cap does not vent until 15 PSI. Pressure system cold and check for leaks. Fit unit unpressured cold and run engine, check for undue pressure build as it warms. If it takes 40 miles for the needle to reach mid point i would think there is no stat fitted. Under normal circumstances mid point (normal temp) should be attained after less than five minutes driving.
 
i fitted a new stat when i fitted the new rad,viscous etc ,it had a hole with a ball bearing fitted which i imagine is some sort of one way /air bleed valve.
She used to sit at about 11 oclock and you can see the water flow when the stat comes in as the needle rises then falls. she usually gets to normal after about 5-8 minutes on the road . after about 40 miles is when the needle rises above the midpoint then keeps going into the red.
if the heater is permanently in circuit so to speak ,am i effectively using the heater matrix as a heat dump?
im thinking that ive either got a blockage somewhere in the head ,or possibly my new stat is duff and not allowing enough water flow over or through the head.
i did run her stationary and when warm the one hose read 82 degrees and the other 59 degrees so assume the rad/fan are doing their job.
mind you thinking on it,many many years ago i had a diesel tractor that was fine around the yard but as soon as i went to work her i.e mowing,chain-harrowing she would lose water and boil . That turned out to be a hairline crack through the block and head (x rayed ) by our local college .
there are similarities as the rangey seems to misbehave when i put her into work i.e after about 40-50 miles.
does anyone know how the double thermostat works ,i know the stat opens up to allow water to flow but does this affect the lower part of it in respect to water flow around the head?
wammers ill try the pressure tests you suggest tomorrow (hopefully) as a tad busy with my regular day job.lol.

BIG THANK YOU TO everyone replying
 
rover75lover said:
i fitted a new stat when i fitted the new rad,viscous etc ,it had a hole with a ball bearing fitted which i imagine is some sort of one way /air bleed valve.
She used to sit at about 11 oclock and you can see the water flow when the stat comes in as the needle rises then falls. she usually gets to normal after about 5-8 minutes on the road . after about 40 miles is when the needle rises above the midpoint then keeps going into the red.
if the heater is permanently in circuit so to speak ,am i effectively using the heater matrix as a heat dump?
im thinking that ive either got a blockage somewhere in the head ,or possibly my new stat is duff and not allowing enough water flow over or through the head.
i did run her stationary and when warm the one hose read 82 degrees and the other 59 degrees so assume the rad/fan are doing their job.
mind you thinking on it,many many years ago i had a diesel tractor that was fine around the yard but as soon as i went to work her i.e mowing,chain-harrowing she would lose water and boil . That turned out to be a hairline crack through the block and head (x rayed ) by our local college .
there are similarities as the rangey seems to misbehave when i put her into work i.e after about 40-50 miles.
does anyone know how the double thermostat works ,i know the stat opens up to allow water to flow but does this affect the lower part of it in respect to water flow around the head?
wammers ill try the pressure tests you suggest tomorrow (hopefully) as a tad busy with my regular day job.lol.

BIG THANK YOU TO everyone replying
Click to expand...

Did one last year for nephew exactly same symptoms. Small crack in head between number two inlet and exhaust valves. Would run for over an hour at idle with cap off. Lots of small bubbles in header tank. With cap on twenty minutes then up to red and an almost explosive ejection of coolant. With tester on showed no sign of leak down cold. But steady rise as engine heated. With tester on, with needle at mid point pressure should be around 5 to 7 PSI and go no higher at idle.
 
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