Rosesbabe

Recently got stuck in traffic and crawled for close to an hour – engine temp slowly but progressively rose to the red line along with my nerves - played with fan on and off and then A/C to see if I could effect temp - rose to a point when the warning light came on. Weather was sunny and close to 80 degF. Quickly dove to the side and shut her down for 20 mins to let her (and my nerves) cool. No blow-out from the reservoir, but definately HOT. After-market elect fan ran the whole time. Start her up OK when engine cooled, but entering heavy traffic again, same thing happened. Instead of pulling over this time, traffic miraculously cleared within a minute of the red light triggering and I was back down to running at a normal temp. when I got going and reached Hwy speed. Now terrified of traffic. Coolant is -40 antifeeze and reservoir full. Normally runs at lower white line on guage when at speed, goes to mid-point in "regular" traffic.

Any suggestions as to where I should start my investigations? Coolant or airflow?

I read a few overheating threads, but none seemed to show these symptoms. Had overheating problems before, but that was due to my electric fan not coming on at all (fuse kept blowing when it engaged – upped the wire thicknesses and fuse amperage to cure). Not sure if this is normal or maybe symptomatic, but my fan always runs for a considerable length of time after the engine is shut down (2-5 mins up to 10 mins depending on outside temp or spirited driving), and the air being pushed is huge (crazy hot air being pushed out when I open my door to get out).

Thanks in advance.

Mrmerlin

is your fan blowing the correct direction ( air moves in the from the front and comes out the back) of the radiator??

dprantl

If the car is at normal temps when driving above 30mph and only overheats in stop-go traffic, it is almost always an airflow problem. Your viscous fan clutch may be giving up.

Dan
'91 928GT S/C 475hp/460lb.ft

Mongo

Did the '85 have the automatic cooling flaps that close? Those things caused my S4 to almost overheat in traffic one day. I pulled the fuse on them when the car was off, keeping them open to airflow. Never had an overheat issue in stop-and-go after that. I am also using a lower temperature thermostat to keep the temperatures down to the 1/4 mark at speeds over 55mph.

dprantl

No, an '85 has no cooling flaps. It has an electric fan in front of the radiator and the belt driven fan with viscous fan clutch behind the radiator.

Dan
'91 928GT S/C 475hp/460lb.ft

Landseer

Seal behind stat can do this if decomposing.

Rosesbabe

No slippage as no mechanical fan - PO replaced with new electric from 928motorsports - I replaced this original fan with a new one from 928Motorsports as I thought the bearing may have gone. Definately pulls front to back as this is what I first suspected was the culprit (high torque demand needed if fan blowing wrong way) when the original 10A fuse always tripped on fan start-up, before I replaced with larger fuse (30A). Fan seems to come in at right temp also. I have a hwy commute which is 15 mins @ 70mph and every day when I stop, fan is running. (not linked to ignition) Seems like lots of air is moving. Any more air and I would think I wouldn't be able to open my door!

dprantl

Ok, well that changes things. First thing I would do is change the thermostat, both seals and radiator cap. Also, what ratio coolant:water are you running?

Dan
'91 928GT S/C 475hp/460lb.ft

Rosesbabe

Thanks Dan. This car's history is all in Alberta, Canada, where winter temps go down to -40 deg C so I doubt if there is ANY water mixed in there for fear of freezing - when I had a blow-out from the previous fuse problem and the fan quit, I topped up with neat antifreeze (suitable for all engine types) . Are you thinking the thermostat may be stuck closed? What is the reasoning behind replacing the radiator cap?

JHowell37

Keep us posted on your progress. I've got an '85 that does the same thing.

dprantl

The reason I ask about ratio of antifreeze is that straight water cools much better than straight antifreeze. Even at -40 deg C I would expect there to be some water mixed in there... there is a chart out there somewhere that shows mixing ratio with lowest temp freezing protection.

Thermostats do not last forever, and yours may very well be on its way out. But the real reason to replace it is that it's cheap and when you are replacing the thermostat seals, may as well do it as well.

Radiator cap can fail and not hold pressure properly. A cooling system that doesn't hold the correct pressure will not cool as efficiently as it is supposed to. Again, it's easy and cheap to replace, so why not.

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

make sure the radiator is REAL hot to the touch, top to bottom, when it starts to climb to that level. seal behind the radiator could be the culprit as was said, or the T-stat (or clogging of the radiator).
fans working??? they need to be in traffic.

WICruiser

Sounds like your radiator may not be up to capacity - lot of air flow but not enough cooling.

As indicated by Mark, check the radiator surface temperature to see if there are cool spots or just pull it and have it cleaned - both inside and out. There can be a lot of debris build up between the fins restricting air flow externally and gunk build up internally preventing either flow or heat transfer.

Landseer

So far, I've not tangled with a blockage in the radiator. Others have, and it might be helpful.

Low coolant level has caused high temp for me. So has degraded inner seal (on 3 different cars).
If you are not familiar with this seal, its function might surprise you, so check out Dwayne's post below and maybe it will help.


https://rennlist.com/forums/928-foru...-trip-out.html

tveltman

I don't know about aftermarket fans, but I know that on my car ('88 S4), the fan control unit sometimes goes a little haywire when the battery has been disconnected for unrelated service. The fix I discovered is to pry off the fan control module caps while leaving the plugs still connected and jumpering pin 7 on plug 1 to pin 1 on plug 2 (i think that's right, you may want to check the pinout to confirm that, or I can go check my reference sheet in my glovebox after work). What this does is makes the fan control module think that the engine temperature is infinite (because there is no resistance between the two points, which come from the temperature sender in the radiator, and because of the design of the sender, lower resistance means higher temperature), and the fans kick on to cool the engine down. However, once the fans start up and the jumper is removed, they turn off, however the control module then appears to function exactly as it should. I'm not sure if there is some logic circuit that gets hung up inside and just needs a "reboot" or why this works, but I know for a fact that it does, and I've done it several times and I have no adverse affects to report as a result of doing so. If someone with more electrical know-how wants to chime in and tell me I am dangerously foolish, I'll accept that criticism, as I don't precisely know what I'm doing. If your fan is running, then maybe this won't help you, and since I don't know about the control functions of those fans, I don't even know if the advice is applicable, but its my $0.02, and if it helps someone, that's all that matters. Good luck!