[Darryl]

I finally got my car back on the road after a few months of tinkering and low and behold, an instant problem. On the way home from a local car show event, I slowly overheated...I was already in a mall parking lot, so I shut the engine down immediately since I did get a big red "!" and Engine Oil Light, coasted into a spot, and let it sit for a while with hood open. No steam, no signs of coolant leak, no oil leak, things exploding or milkshakes in my overflow tank, so I'm 99.9% certain everything is OK. Drove the car home with no problems and decided to continue diagnosing later that evening on a short trip to dinner and such very near my house, just in case.



Car seemed to be overheating due to the simple fact that the fan wasn't running. Once the fan would run, it was fine. I also didn't think the low speed was ever kicking on. Drove into work today with no issues, but temp would go up and down, but never into the red and at one point, I shut the car off and restasrted it at a light and I verified the fan was running.



Based on a couple of the other overheating/coolant threads, I've taken Pete's advice and purchased the lower temp (160 deg, NAPA #38) thermostat, but I wanted a recommendation on which fan switch to put in combination. There are three different fan switches:



75C

85/93C

92/102C



Which fan switch should I get? It's cheap enough to take a shotgun blast and replace the t-stat and all associated switches at the same time and I understand that there will be a slightly longer time to heat up in cold weather, but my convertible rarely goes out in the cold anyway, so I'm not concerned about lowering the operating temperature. Cooler is better, no? I've always thought these cars ran insanely hot anyway. Thanks!



- Darryl

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[flash]

cooler is better TO A POINT



this is really a two fold issue - nominal coolant temp, and variance under load



on the first, you want the coolant to stay as consistent as possible - on a gasoline engine, maximum power is generally provided at 190 - 195 degrees F of coolant temp



on the second however, you don't want to see more than a 20 degree variance in coolant temps regardless of load



in a perfect system, you would run 100 degrees over ambient temp or 195, whichever is lower, and 15 degrees above your thermostat point



cooling systems don't work like most people think they do - the coolant is supposed to be slowed down in the radiator long enough to cool down, and then gets let into the engine - the thermostat regulates when and how fast - it is supposed to open and close, varying with temp



too low of a thermostat, and you never get the coolant to slow down, and when you add a lot of load, you reach a saturation point - the same thing happens when you remove the thermostat



too high, and you have a limitation on how much cooler the "fresh" coolant is as it cycles through, therefore reducing the efficiency of the system, and causing you to run hotter under heavy loads and high ambient temps



too cold, and the mixture will run rich - at an extreme, this can fuel wash your cylinders - at a minimum, you will be wasting fuel, and burning dirty



much of this is affected by airflow over the radiator, and most importantly, negative pressure zones



pete and i differ somewhat on thermostat choices - i have not yet had the chance to play with the 160 to see what happens out here, but it goes against theory - there may be other factors at play though, so i'm withholding opinion until i get to see what the deal is for myself - there could be a fan speed/airflow issue that makes the 160 work - not sure yet



in my car, i am running the 180, but i have drilled a 1/8" hole in it, and am running the 85/93 - the result has been good, but still a touch more variance than i would like to see under extreme load, and it tends to run too cool most of the time (about 175 or so) - to bring it up to 190, i would need to INCREASE the thermostat point, but then i think i would run out of room in the efficiency of the system



if you are going to use the 160, i would try the 92/102 first - i think pete will say the same thing

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[Darryl]

Flash,



thanks for that long post...makes perfect sense. Since I probably don't push my car as hard as you do and when I do push it hard, it's not as often, so I'm looking to balance out the operating temperature as you described as best as possible.



Turns out, after posting the original thread starter, I went out and drove the car again and there are no issues whatsoever. Ran just above the bottom line, jumped up to just below the second line, fans came on and car ran fine. Fans continue to run after ignition off and all is well. I'm guessing my initial problem was simply a stuck thermostat from lack 'o driving...



I haven't installed the new 160C deg thermostat yet, so if there's a better combo of thermostat/fan switch for normal to moderate street driving, that's what I'd be most likely to choose. I'm sure I can swap the "NAPA #38" for a different one. The 92/102 fan switch is "OEM" - what is the "OEM" thermostat rating?



- Darryl

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[flash]

i believe the oem thermostat rating is 85C

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[Darryl]

A little more diagnostic info...



I decided to take the 968 out for a run on Sunday...about 160 miles round trip...and it ran fine. No temperature issues at all and fans seemed to be doing what they're supposed to and when. I do have one wierd thing going on now that my temp gauge has been repaired by the nice VDO people. It fluctuates quite a lot. I thought I read somewhere that a wildly fluctuating temp reading may indicate a blockage somewhere?



I decided to drive it home again tonight and sure enough, a few miles into the ride, the temp ran right up to the bottom of the red but not high enough for any warning lights. I pulled over, shut off the ignition and started the car back up immediately and all was well. I couldn't hear the fans running prior. So perhaps instead of a stuck thermostat, I've got a sticky fan switch. I'll order up the fan switch so I can replace the thermostat, switch, and new coolant simultaneously later this week.



Any previous experience and input always appreciated.



- Darryl

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[Winston968]

I've been meaning to ask the same question about "normal" temperature gauge behaviour.

I had my gauge fixed, and since then, my experience has been the following:

During a 10 kilometre (6 mile) commute to work, the gauge gets to just above the lower marker (about 8 o'clock).

On a longer drive, the gauge reads a little higher. After stopping at traffic lights, the gauge rises. Generally though, it doesn't reach the upper market before the lights turn green. While driving again, the gauge will drop back to about 9 o'clock.

There was an occasion where I was stopped for a long time, and the gauge read a little over the upper marker, but not into the red. I didn't hear the second fan start, but after a while, the gauge dropped down to around 9 o'clock.

I hope this helps.

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[xrad]

replace the dual temperature fan relay switch located under the driver side radiator hose (it is mounted to the radiator)

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[flash]

daryl - yours sounds like a fan switch



winston - yours sounds like a slow system - likely blockage from deposits - i would flush and bleed it - remember never to use tap water, and to only use distilled or de-ionized water, and only use phosphate free coolant - this should be done every 2 years anyway

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