(Home) Air in cooling system

I have a 92 3L engined hiace supercustom.
Have air in the top hose which I remove each morning
For example - Last night took it for a 25 mile blast and this morning removed an amount of air equivelant to the amount of milk in a couple of cups of tea from top hose. On this engine the top hose is (I think) the highest point (possibly the front heater matrix being a little higher)
Historically I have tried different thermostats and 5 days back flushed the coolant through and renewed.
If I remove the pressure cap there are no bubbles when engine running cold - increase revs a little and a small fountain of (air free)coolant will spurt up - increase some more and the level sucks down with no/little turbulance.
The toyota pressure cap (few months old) is begining to get a black stain on underneath and the thermo housing is black inside - don't know if this is soot or muck but when I purchased I thought it looked as though the head gasket may have been replaced before but no way of knowing for sure.
Does anyone have experience of air locks, getting rid of them, how long the air can stay in the system etc.
The vehicle uses no coolant to speak of and does not shove it out the overflow on the expansion tank - but the top hose does appear very pressurised when hot (alot more than our other vehicle)
I dont understand why this 'pressure' is'nt relieved more by the pressure cap/exp tank
I did renew the coolant very slowly and the day after its 1st run it did lose/use most of the expansion tank contents - but wonder if this was sucked into the system on cooling rather than blown out.
So I am hoping that its a case of trapped air finding its way out (how long might this take?) and dreading the thought of head gasket probs (or worse).
Its hooked to a multimeter with temp probe and this is either making me paranoid or giving me good info - the viscous fan has been inconsistant temp wise leading me to suspect impaired flow through the rad (been on the motorway with engine cover up and viscous performs ok and operation improved a little after renewing coolant) -
Could the air in top hose interfere with coolant flow through the rad?
Getting fed up and wondering whether to 'call it a day' with the vehicle
What temp might an iron head and block safely function at? Errors accounted for, mine will vary between about 92-100 (sometimes 100+) at the block
Guess the acid test would be a run of 100+ miles.
Is it possible that I am experiencing the early stages of head gasket failure? and could gas from a head gasket reach the coolant only when engine hot?
Should I have the pressure cap off when hot to look for air or may this allow more to be drawn in?
Seem to be driving more miles trying to sort it as I am for pleasure currently.
If anyone could offer me advice, or recount solving air acumulation in cooling system problems it would be a big help?
Chris
- (#10058) chris turner, 27 Jul 03 8:36

Chris. With a pressurised system, I should have thought that the operating temperature, as measured by your probe, will be well within the "safe" parameters. You appear to have done everything that is required but, have you got a seperate rear heater matrix on the Hiace? With my Masterace, I found that the rear matrix was a bit of a debris and air trap and, for a while, ran the vehicle with this matrix "out of circuit", ie, the inlet and outlet hose connected together with a piece of 15mm copper tube. This did seem to make things a bit better so I took a lot of trouble to flush this matrix and bleed the air at this part of the system. If your coolant system is as complex as the Masterace, ie, with oil cooler, turbo cooler, rear, low mounted heater matrix, it does need a VERY slow fill and much bleeding at various points. If you have done all this, perhaps the next thing is to get the coolant system pressure tested. Have you ever taken the rad out and really flushed it, ie, a backflush with a hosepipe? This might reveal where any suspected partial blockage might be.
Also, a test of the possible "crossover" between cylinder pressure and coolant space is to check the turbulence, at the filler neck, when running the engine from cold and lifting the throttle slightly. This is best done just on start up from cold but it can be done with a warm/hot coolant but, you have to be very careful and, with the cap off, the coolant system will not be pressurised and the results might be a bit misleading.

- (#10059) John Davis, 27 Jul 03 12:22

Thanks John - thats helpful - had not considered the rear matrix as a possibility but 'assumed' that any trapped air would find its way to the highest point.
Removing the radiator? would like to do this but having stared at it from numerous angles on many occasions I have'nt figured out how yet.
Does seem a little better since coolant flush and quite happy with temp today (except the 104 deg on a long steep hill).
The old coolant had only a few flakes in so it does suggest a proprietry coolant flush may help at some stage.
I have reached the opinion that variable driving conditions (stop, start, uphill/downhill + differing speeds) are better at shifting the air around the system than steady cruising on motorways(at least thats what the sound of the viscous suggests).
I'll check the rear matrix and keep bleeding daily.
Am I right in assuming that if I squeeze a hose and can hear a slosh this = air? ie
coolant can't 'slosh' if no air present?
Thanks again John - its reassurng to know that its still a 'possibility' that I won't be digging too deep cost wise

- (#10060) chris turner, 27 Jul 03 16:31