Oil cooling system questions 3.2L

[cbeers]

I have an '87 911, all stock, that I drive on a daily basis. Today coming home from work, about a 20 minute / 10 mile drive of mixed lights and expressway, I thought that the oil temp was a little high. It was rising over the first white thin line to about the 9:00 position. This seemed a little high for moderate traffic and outside temps.

When I got home I started feeling the oil lines to make sure that oil was flowing through correctly etc. What I found was that the oil lines leading to the aux. oil cooler in the front fender were not both hot. The sending line was hot to the aux cooler, but the return line was not. I thought the thermostat on the aux cooler was only for the fan and that the thermostat at the aux oil cooler relief valve (passenger rear fender well) controlled the flow of oil to the cooler.

So, my questions are how exactly does this system work? Does the aux. oil cooler pressure relief valve in the rear of the car block off the oil to the aux cooler until additional cooling is needed? Since one line to the aux. front cooler was warm with oil, what would cause the return line from the front aux cooler to be still cold? It would seem to me that if there was oil running to the cooler, it would circulate and return back to the engine.


Any insight or troubleshooting methods are very much encouraged



Thanks,


-Chris

[Chuck Moreland]

Likely, you haven't reached the opening temp of the external tstat. 9:00 isn't that hot. 20 minutes of running is often not enough to get up to temp - it wasn't even hot today in San Ho'.

The supply side of the tstat is not valved, it's just a through-pipe. Hence the supply side feels warm anytime the oil is warm. Only the return is valved off.

You haven't said anything that indicates a problem. But continue to monitor your temps especially as the summer wears on.

[Brother]

I'm looking into this as well. I have a similar commute and the trombone cooler is getting taxed in the already 100+ degree heat.

I was thinking mocal, but Chuck's site just about has me changing my mind to the widemouth coolers he sells. I tallied up the price with a fan and the aftermarket relay set and it was 911 dollars. That did it for me. Now I just have to save up a little more cash since it is about twice the cost of the mocal setup.

[vash]

$911 bucks? i can swing that . i need to go that route. chuck, look for my call.

chris, i dont think you have a problem. let's monitor it tomorrow at the AX. i will bring my infrared therm if you want. lemme know.

[Brother]

Vash,

Seemed way more reasonable than 1500. Pricey either way. If I can off load some old parts, maybe I'll be able to afford it before the summer is over.

[kqw]

Sent a PM

[kjb]

Hey Chris,

I would be surprised if the oil cooler return line was the same temperature as the supply line. The oil cooler is supposed to cool the oil, so oil flowing back will be colder than oil going into it.

/ Johan

[Bill Verburg]

There is a time differential between the supply and return line heating. just as there is a time differential between the internal and external loop heating.

Once the external t stat opens it is feeding hot oil to the cooler but it takes time for the return to heat up to its max temp. There will be a temp drop between the 2 lines but after the system reaches equilibrium both will be hot to the touch, the feed will just be hotter.

the switch at the cooler is just a fan switch.

[kqw]

Originally posted by Chuck Moreland
...The supply side of the tstat is not valved, it's just a through-pipe. Hence the supply side feels warm anytime the oil is warm. Only the return is valved off.

Chuck...how does this make sense? It is my understanding that the T'stat (supply) opens only when the oil reaches temp. threshhold allowing oil to flow to the aux. oil cooler. Once oil hits the cooler, it's cycled and loops back through the return tube.

[Chuck Moreland]

Here is how the tstat works:



Ignore the part about the purge valve.

The supply line to the cooler is always open. The valve only controls the return side. Because of this, the supply line gets warm well before the tsat opens. The return side doesn't get warm until it opens.

[Bill Verburg]

Because of this, the supply line gets warm well before the tsat opens. The return side doesn't get warm until it opens.

True, but...

the supply side ony heats by conduction untill the valve opens

then the supply heats, then the cooler heats then the return heats

not more than a couple of feet of the supply will heat by conduction, hot oil needs to be flowing for the whole supply line to heat up

[cbeers]

Thanks everyone, I ended up running it some more and monitoring it at the AX as well. It did rise again, but I guess it's just a matter of the warmer season coming on. It didn't seem too bad.

I do understand that the return line from the aux. cooler SHOULD be cooler, it was just that the supply line was very warm and the return was stone cold. No oil being returned as far as I could tell.

I think it was as Chuck and Bill said above: the supply line heats up to some degree from conduction. That was tricking me into thinking that the thermostat had opended and was sending oil to the aux cooler when in fact, it hadn't.

Thanks to everyone for educating me further about the system


-Chris

[masraum]

depending upon your temperatures and traffic the 9:00 area isn't "too hot". I drove my '88 daily, and except in the coldest weather my car always ran at the 9:00 area (about midway between the two thin white marks) except in extreme heat or stop and go traffic. Porsche designed your fan to come on when the temp hit the top most white line which is 245*.

[cbeers]

Good data point masraum, mine seems to run around the first thin white line in most conditions, so 9:00 seemed higher to me in moderate heat / traffic......

I won't worry any longer


Now, onto those spring plate bushings, THOSE worry me in the high speed sweepers!!!!


-Chris

[Chuck Moreland]

not to split hairs, conduction isn't the primary reason the supply line heats. In fact the return line is also connected to the thermostat is similarly being heated by conduction - yet it doesn't warm as fast.

The supply line heat is due to a mixing of the oil in the supply line with the heated oil from the engine. This is an open pipe and the molecules are free to migrate. The flow of oil past one side of the pipe only further encourages the mixing action. The oil is also full of air bubbles, making it compressible and further enhancing the mixing.

[cbeers]

Eeerrrr....what Chuck said


Sorry to mis-quote you Chuck


Either way, I worry too much!


Thanks again!


-Chris

[Bill Verburg]

not to split hairs, conduction isn't the primary reason the supply line heats. In fact the return line is also connected to the thermostat is similarly being heated by conduction - yet it doesn't warm as fast

This is going to be another one of those religion things.

My religion is that the supply line won't heat up for it's full length until the t-stst opens and fluid actually moves through the supple tube, yes this happens before the supply side, yes, quite a long tume before, because all that cool fluid first has to move through the whole length of the supply line, the cooler and the retun line, this does take time.

But you all may believe as you choose

[Bill Verburg]

One more thing, the situation is very similar to a domestic hot water setup where a separate hot water heater and tank are used. The main difference is that in domestic hot water the valve is at the end of a single run w/ no cooler inbetween.

observation 1
in a static condition there will be heat in the hot water supply line to the house up to a few feet from the tank
observation 2
the cold water supply will be cold

observation 3
it will take a period of time to get any hot water from the tap

[Chuck Moreland]

Indeed it won't get really hot for its "full length" until the thermostat opens. But it will be significantly hotter than the return side when measured at any point forward of the tstat, with the difference decreasing the further away you test.

In fact it is hot enough to fool one into thinking the tstat is open, as this thread demonstrates.

If there is any doubt on this point it's easy enough to observe. Go let your car sit and idle on a hot day, and keep feeling the lines until they both get hot. The supply will get progressively hotter, until the tstat begins to open - the return will then heat up very quickly.

I've done this test.

[Bill Verburg]

I've done this test.

Yup, everyone that owns these cars long enough will eventually do the touch test