shaaark89

kevin wrote
"Could air in the system cause such an extreme condition?"

i've had this happen on more than one occasion even though i was certain i had burped the system well. i've found that air can collect at the top of the water bridge where the temp sensor is and may give a false reading on the gauge that is high. loosening the temp sensor allows the air to escape and solved the issue.
still would check the t-stat. if the outflow from the rad is 165 and the tstat is working, the gauge shouldn't be reading 210.

Bill Ball

I appreciate the posts about the t-stat and burping. The 928 cooling system is supposed to be self-bleeding, and I have always maintained that it is. I just squeeze the hoses a bunch and run with the heater on after changing coolant, waiting until the return hose fills from radiator flow. Then just top off the reservoir as needed. Never had a problem with that. But I'm having a problem here, so I'll try Tom's sensor removal tip.

Is the sensor reading the return flow? If so, this is a false alarm.

However, the T-stat is controlling inflow to the radiator, so I was expecting 170F (75C T-stat = 167F) inflow or something close to it. Granted, I've seen a lot of 928s with 75C T- stats and they run hotter than that on warm days underload (uphill grade), but should run about 170F on a cool day.

shaaark89

bill,
i might be totally wrong on this, but i thought the direction of flow was from the top hose through the radiator and back to the motor via the bottom hose. in that case the t-stat would allow flow back from the radiator when open.

if thats not correct then obviously what i said about the temps is incorrect as well.

Bill Ball

You're sure right about direction of flow and where the T-stat is located. I need more coffee.

OK, so it seems I've never thought this all the way through. So, the T-stat controls where the return flow goes. Jeezuz, I can't make sense out of that flow pattern this morning. Coolant comes back...hits the T-stat. If cold the T-stat is open and it goes...into the block? It's called a bypass. If it's hot, the T-stat is closes off the bypass. It can't just go back to the radiator as that would create a closed loop with no flow to the block. What controls flow into the radiator? Eh, I need to find the cooling flow diagrams.

Bill Ball

OK, here's the flow diagram from the 78MY book. Have fun - it's 928 complicated. It does indicate there is a bleeder. It says the T-stat closes the return from the radiator when cold. Huh? I don't think that means what it says. When I squeeze the return (lower) radiator hose on a cold motor there appears to be flow into the bridge, but part 19 looks schematically like the return radiator hose.

shaaark89

haha!
i just went and pulled the tstat housing off a spare motor to figure it out.
anyway, based on how it works, the temp sensor doesn't see the return flow from the radiator, but rather the mixture of coolant from the engine and the radiator after it passes through the waterpump. still would be interesting to see if there is air in there.

Bill Ball

I agree about the sensor seeing a mixture. Anyway, I need to get out to the garage and get my hands dirty. Getting the T-stat hosuing off with the blower in the way is going to take some strange tools.

BTW, do you know which fan lead is primary? I want to wire the 4 muffin fans to primary and the pusher to secondary and I'm not sure which fan lead is which. Oh, wait, I can figure that out by pulling fuses - they have separate ones.

Vilhuer

Bill, thermostat does both. When its closed it blocks lower radiator hose and also opens bridge hole. When thermostat is open it opens lower radiator hose and also closes bridge hole. Its important that special round seal behind thermostat inside bridge is there and in ok condition. Also its important thermostat has bridge hole closing plate which is large enough to actually close the hole and also that it extends far enough to reach the seal.

Bill Ball

Update:

Car is running fine. One stupid thing on my part seems to have been most of the overheating. No surging now either using the coned boot. Not sure why, but I will try to explain later. Fans don't seem to like being ganged on one circuit. Otherwise it's about ready to go.

Off to install George's SC.

BC

You are a great guy Bill, and have helped many beyond the call. Unrelated to you, some other issues on this thread get a big

Mike B

Great job Bill...I knew you would figure it out!

shaaark89

t-stat o-ring?

aggravation

It looks like he's saying the fans were the problem due to being wired together.

Cliff Ruckstuhl

Bill,

In your findings did you find the car stumbled/surged when one of the screens was removed? I am missing a screen and have ordered a new one and I have a very slight stumble. It is very slight and I am wondering if this wiull cure it?

87 $4 Murf #44

Cliff

Tony

If its possible it almost seems like this boot needs some sort of guide vain inside to help air make the turn a bit more smoothly?