Jørn

Hi

My 78 Euro 928 started running hot at idle this spring. By hot I mean it creept very fast up to second white line in slow trafic or at idle.

What I have changed thus far is, new waterpump and cam belt, new thermostat, new coolant resovar and cap, new viscous fan clutch. Nothing helped.

Since I can cool it down by reving it at 1500rpm at idle I was thinking of bad coolant flow therefore got the radiator checked. It had several channels clogged and I therfore changed it out with the new alu-rad from 928intl.

Status after this is that it now seems to run hot even at speeds creeping close to second white line unless I run it at high rpms.

I was beginning to worry about headgasket, but I had a shop measure for exhaust gases in coolant, and nothing showed up. Also my oil an coolant looks fine and engine runs good.

What am I left with now??? could it be air left from radiator change that is the problem?

Jetdriver69

Have you used an IR gun to measure the actual temperature at the water bridge and the inlet and exit of the radiator?

My temp gauge on a 82 5 speed was very inaccurate until I calibrated the gauge with a variable resistor and got rid of the engine driven fan.

I installed a very large single electric fan and deleted the viscous and aux AC fan, which helped a lot with the temps

Concerning the improper temperature gauge readings, my 82 with a new engine, new radiator, water pump and temp sensor showed above the second white line when temp were measured in the normal range (185F)

So I installed a 25 ohm potentiometer (Radio Shack or eBay and make sure it is a linear potentiometer) that is wired to one lead of the temp I sensor and then wired back to the sensor. Only use the center and EITHER of the other two leads on the POT or it won't work correctly.

When the engine is fully warmed up at 185F at the hose coming out of the water bridge and inlet to radiator, I simply turn the potentiometer until the coolant temp gauge reads in the middle of the two white lines. Mine took about 16 ohms of resistance to read correctly.

Now the temp gauge is calibrated to 185F in the middle of the two white lines.

This doesn't actually "fix" the gauge, it just now shows a "known" temp in the middle of the range and if the engine actually does overheat, the gauge will reflect that.

Ad0911

You can check the temperature on various locations of the radiator with an IR gun provide you aim at a painted area, not shiny metal area (the reflection will result in false readings).

Jørn

Hi

Yes I have an IR temp gun. The second white line is a bit above 194F. When it was climbing at idle I saw up to 212F at upper hose, at which point I did not want to see if it got hotter, and started cooling it down again. Might be that the aux. fan will keep it there as that kicks in around that area. I will do some more temp measurements during the weekend, with all the knowledge that is here. Problem before was I did not know what temps are normal.

Mrmerlin

I would suggest to install another thermostat, its possible its not opening enough or at all
and if your car has the rear seal in the water bridge replace it.
Though the early cars didnt have them.

What is the coolant mixture>? should be 50 / 50

I would replace the water temp sender connectors use heat shrink on the new ends.

To consider is that fact that the early cars seemed to suffer from degrading head gaskets.
if yours are original then replacement may cure the temp issues if swapping in a new T stat doesnt do the trick

You should be seeing about 185 to 195 at the top hose inlet then about 170 to 175 on the lower hose

mark kibort

the first thing you do is FEEL the radiator , top to bottom.....when this first starts to happen. see if the top of the radiator face is hot and the lower section is cool to the touch.
sometimes you get a sticking thermostat. or that seal in the thermostat housing is bad, this can effect flow and thermostat operation. (the T--stat doesn't seal against the engine).

feel the radiator and get back to us. this is the absolute first test. generally if the radiator is HOT from top to bottom, you don't overheat, unless the fans are not working. should be no issue at 40mph driving speed though, regardless of outside temp.

I run the race car and runs so cool on the track, even on a hot day, that the fans turn off and the temp is below the normal line

if you have a headgasket issue, then the overflow bottle should be over pressuring, and leaking water.


make sure system is bled.

Jørn

Hi

Wow, thanks so much for all the helpful inputs. I will do some measurements and get back to you. I do have one question, you talk about feeling the radiator from top to bottom. How should I go about that without chopping my hands off or do you mean to do it after I shut the car down? On the front side of the radiator I have the AC-condenser so no easy access from that side either.

I guess taking the thermostat out will mean draining the entire radiator again ?

Based on my symptoms I was really thinking about the rear seal Mrmerline mentioned, but from what I can see this was not introduced before model 1984? At least I can not find that seal in my PET for 78-82.

928nut

This brings back memories of my fight against overheating of my '79, several years back.

If it runs OK at speed/higher RPM the problem is insufficient airflow. In OSLO, you probably don't use A/C very often if at all and should your A/C be non-operational, pull the condenser out and watch the change.

I've replaced virtually everything in the system like you have and the problem persisted until I yanked the A/C condenser out. Not a hint of overheating ever since. I had to fabricate a "shroud" to extend the existing shroud of the electric fan, to bring it closer to the radiator, otherwise the airflow from that fan would get scattered all over the rad, without much air actually flowing through it. Try it, you and your rad may like it ;-)

erioshi

Have you tested the fan clutch to ensure that the fan is drawing air correctly?

Jørn

The fan clutch is brand new from Porsche, and gives the expected resistance against movement.


Yes, you are correct about me not using the AC much. However it still is fully functional and I like to keep things original. These cars was tested in desert areas when they were developed, so when it overheats in Norway something must be wrong! It should work even with the condenser in place.




So I did some driving and measurements these weekend. As long as it is moving the temp stays within normal range, dead solid at a fixed point slightly above middle of gauge at highway speeds. If I give it some stick, or climb steep hills the temp creeps a bit upwards but still not more than I guess 188F tops.

Now that it is bleed the systems acts as before meaning it only overheats at idle basically. After a long drive I stopped popped the hood and did some measurments with IR gun. From the readings the radiator seems to do its job.

So after popping the hood and getting out it started climbing fast,
Top hose 194, bottom hose 181
Top hose 201, bottom hose 185
Top hose 208, bottom hose 192
Top hose 213, bottom hose 203
When it reached 219 I reved the engine to cool it down again and stopped the test, as it did not seem to level out. The aux fan had not kicked in yet.

Seems that thermostat is opening at least, pehaps a leak down test is the next test to do?

Ad0911

Dumb question, but are you sure the thermostat switch is ok?

Jørn

If you mean the sensor for the aux. fan, the yes. I have seen it work in my previous rad but seems temps have to get real high for it to kick in. Anyway I thought that was sort of an emergency extra, that under normal operating conditions the temp at idle should not climb so high that the aux. fan needed to kick in.

Mrmerlin

at this point the the easiest thing to do is swap in another thermostat.
Also did you back flush rear to front the AC condenser maybe its filled with debris.
What is the make of the new water pump, Porsche or Laso is it new or rebuilt

928nut

I understand, if my A/C was functional, I wouldn't have pulled the condenser either, so you have to look for another option.

Look, the symptoms are telling you - your problem is "airflow" !!!

When you have airflow, driving at higher speeds or reving the engine at higher RPM, you don't have a problem. If you had a failure or malfunction of one of the components that regulate coolant temp. your problem would be even more evident at higher speeds.

With all due respect to Porsche design geniuses and their testing, the mechanical fan is next to useless at idle or slow, stop and go traffic, on a hot summer day. I am willing to bet, it draws virtually no air through the condenser/radiator sandwich under those conditions.

Your remaining option to provide adequate air flow through your radiator is electric "puller" fan(s), in place of the mechanical fan. I'm sure you're not crazy about this option, considering you've just purchased a new mechanical fan but you may have no choice.

Again, as others have mentioned, make sure the condenser and radiator fins are not clogged up with road junk - leafs, feathers, mud, etc.

Last point, I am not quite clear if your rad temp. sensor, (electric fan switch), is working properly or not. If you have serious overheating problem, your electric fan should be kicking in all the time, whether your A/C is on or not. If your fan isn't coming ON and you've verified both the fan and the switch are working, perhaps it is just your temp. gage that is that is off. However, looking at your temp. readings you've taken, the electric fan should be ON, at least for the last two readings. I believe the stock fan switch is 83 ºC = 181.4 F

Let me/us know, what your eventual fix will be, good luck.

mark kibort

is the aux fan coming on at mid point temp.... this is key and answers the radiator flow question. touching it down there will confirm there is heat in the lower part of the radiator. but the fan needs to turn on, If it isn't, then that could be the issue, or there is flow to get the hot water to the lower part of the radiator. sometimes a flakey waterpump with a loose shaft can do this.
I onece short circuited the radiator with clear hose to see if there was flow at idle. acutally, clear hose out the side of the car from top engine coolant fitting. and pouring water from the garden hose to keep up with the engine demand. its not much at idle, so an easy test. l also pegged open the t-stat to do this.
these cars should run way cool even in hot weather if all is working well. a single electric fan can allow me to run in traffic with no overheating. but,if it stops. the car will overheat.