Airlock or what?

stoutgoose · Post by **stoutgoose** » Sun Feb 22, 2009 7:40 pm

Rover V8; was 4.0 litre, now 5.0 litre. I suffered what I thought was a cracked liner last year and have replaced the block. I had my heads pressure tested before refitting.

The cooling system utilises a large radiator which is slightly lower than the top hose. The top hose has a filler cap at its highest point with a blank cap fitted. From this is a small bore hose expansion take-off to the expansion tank.

This hose enters the bottom of the tank and thus has a head of water. There is a 15 psi pressure cap built into the top hose, beyond the expansion take-off.

My symptoms last year were the same as I am experiencing now. I have fully bled the system. I have attached a pressure gauge to the top of the expansion tank. On starting from cold, pressure is seen to slowly build, reaching around 14 psi, which is around the time the fan cuts in at 88 degrees. As soon as this happens, pressure is seen to drop significantly. When the fan shuts off at 77 degrees, pressure rises again, and then drops when the fan kicks in, and so on.

If I release the pressure cap, and the refit whilst hot, and keep the engine running, pressure reaches around 5 psi before the fan kicks in and reduces it to around 1 psi.

I have sniff tested the system for HC’s which proved negative. The fact that exactly the same symptoms are occurring as on the old block suggests to me that this is not a cracked liner issue, however what is it?

I’m thinking that this is the mother of all air-locks, but where? I have fully purged the system.

Would fitting a bleed take-off to the header tank from the bottom hose, with the top air-bleed running into the top of the tank help in this situation and is it worth trying?

The system is very new, so no blockage issues and the radiator cools excellently, both with the fan and when running on the road.

Any ideas before I completely pull my hair out?!

Post by **Rossco** » Sun Feb 22, 2009 9:18 pm

What vehicle is this in?

RoverP6B · Post by **RoverP6B** » Sun Feb 22, 2009 10:53 pm

Does your thermostat have a jiggle pin?

If not, has a hole been drilled into it at the 12 Noon position?

A lot of thermostats don't have the pin, nor come with a hole to prevent such an occurance.

Thermostats without such can be very problematic when used in a Rover V8.

Ron.

Post by **katanaman** » Sun Feb 22, 2009 11:34 pm

Just thinking out loud here really but what your seeing I would say is perfectly normal barring its reaching too high a pressure.

Pressure is caused by the expansion of the coolant so when its expanded and you release the pressure it will stay low because its already expanded. When it cools you will get a vacuum, suck in air and the cycle will start again.

If you had an airlock I would think the pressure build would be less because air is easy to compress therefore pressure wouldn't build so high.

Because of this fact I am thinking you might not have enough of an air gap in the top of the header to act as a spring/damper.

I don't know what tank your using but it could be a miss match with the large radiator (more coolant, more expansion) leaving you with not enough space. You probably wont notice such a large pressure rise moving because the rad will remain cooler so that lot of coolant hasn't expanded so much.

Solution might be simply filling the tank less or changing it for a larger one allowing you to have the same amount of extra coolant but have a larger air gap. I would fill the existing one with less coolant short term just to see if it helps but keep an eye on the coolant levels.

Its not like your having cooling problems as your not overheating. in fact it looks like your temps are pretty stable, you just have excess pressure which when released stays low so I think leak by is ruled out as well as stat problems.

Does that lot make sense?

stoutgoose · Post by **stoutgoose** » Mon Feb 23, 2009 8:11 am

Yes, it does. My only thought is that water expands by around 4% between cold and say 90 degrees; with my 12 litre capacity thats around 0.5 litre.

I just can't see how the expansion of 0.5 litre will cause such a large compression?

With regard to the stat, I have drilled 3 holes around it to allow for air movement.

Another thought occurred to me. The radiator is a serpentine type; i.e. it has baffle plates in both of the side tanks (1 on the left about one third of the way down and one on the right around two thirds of the way down) so that coolant is forced to travel through the radiator in a snake like fashion from top to bottom in order to exit the rad.

I'm wondering if I have air trapped in the middle section, so I will try jacking the car up on either side to try and shift it.

ChrisJC · Post by **ChrisJC** » Mon Feb 23, 2009 8:41 am

As Marki says, I'm not sure it's a problem.

If it's not overheating, and not using water, it's probably OK.

Chris.

stoutgoose · Post by **stoutgoose** » Tue Feb 24, 2009 2:44 pm

I left the engine to cool and then tipped out the remainder of the coolant in the expansion tank (about a cupful).

The pressure has dropped by around 5 PSI; I suprised at how much it has come down by!

Thanks for the advice; time for a larger header tank!

Post by **katanaman** » Tue Feb 24, 2009 6:16 pm

glad we could help