[Lear35A]

For awhile my water temperature gauge has been running low, typically halfway between the thick mark at the bottom of the scale and the first hash mark. This morning I got out the Clark's garage troubleshooting guide for the fans and the 968 wiring diagram (Sheet 3) for this. I suspected the coolant temperature switch, but did the diagnosis step by step as the troubleshooting guide suggested. On step 7b I confirmed a bad switch. What actually happens is that the high temp point (which should come in second and is supposed to be 102dC) switches on at a very low temp (I'm estimating about 70dC with an infrared gun). What should be the first switch to close (92dC) comes on second (probably about where it is supposed to). Nevertheless the net result is that the fans are on full blast about all the time, and I run too cool.



So, how big a problem is this? Changing the sensor is a pain because it's hard to reach, and I'll have to drain some coolant to do it. If I can get to it from the bottom, it will be a lot easier because from the top a lot of supercharger stuff is going to have to be taken off.



I will order a new one Monday and eventually I'll have time to change it. But is it a big deal to run at lower water temps? Any long term consequences possible?



The good thing is that I confirmed all the other stuff in the fan circuit works as it's supposed to. At ~$30 this is probably the cheapest thing that can fail.

Partial Post: Please Login or Register to read the full post.

[flash]

if you order a new one, order the 85/93 and not the 92/102. i recommended this at the onset of the supercharger project.



if it is indeed running that cold, then you are not fully burning the fuel. gasoline wants to be warmer than that to burn fully. the potential harm is to the catalytic converter, as well as a reduction in fuel economy.

Partial Post: Please Login or Register to read the full post.

[Lear35A]

thanks. do you have a p/n for the 85/93? don't see it in the PET.

also isn't it a little contradictory to recommend a lower temp range switch in one line and then list the problems with running cold in the next <img src="/forum/images/smilies/968/huh.png" class="smilie" alt="" /> ?

Partial Post: Please Login or Register to read the full post.

[flash]

lol - the lower temp switch doesn't make it run too cold. it won't run where it is running now. the lower temp switch merely makes the temp more stable, and essentially parks it just above the 8 o'clock line



i don't have the part number handy, but it's here in a few places, as i know i've posted it before.

Partial Post: Please Login or Register to read the full post.

[Lear35A]

This weekend I replaced the bad fan switch. Accessed from the bottom and only had to remove the plastic underpan; the splitter and metal rear plan stayed. I spilled a minimum amount of coolant, but Porsche's design with the plug directly above the tray holding the radiator could be more friendly. After removing the fan assembly it was pretty easy to unscrew the switch with a cut off 1 ⅛" open end wrench I happened to have.



Now the fans don't run all the time, but the water temp while driving still never gets above the point shown in the picture in post 1. So actually the fans don't come on at all. I only know the switch does its thing because I left it at idle after everything was buttoned up, and eventually the temp gauge got to mid scale (9 o'clock position) and the fans came on at low speed. I'm not going to change anything else because there really isn't anything else to change, but I am curious.





So why so cool? I put in a new 71dC thermostat when I first started chasing this low temp, and there was no change. I am using 100% Evans NPG+ coolant (no water) with a lower pressure cap. Could that stuff be so good the engine won't get up to design temp?

Partial Post: Please Login or Register to read the full post.

[flash]

according to cooling system theory, 71 is too low. that may well be the problem. the system would only run at about 170-175 degrees F (10-15 degrees F above the thermostat), which is just below the low temp fan switch point, and well below the stock fan switch.



the thermostat point should be about 82. basically you want to achieve about 190-195 during operation. a bit cooler won't necessarily hurt anything, but it doesn't allow full atomization of the fuel, which means that your mixture is probably off and you could be down a bit on power.



what will happen here is during load and arduous use, the temps will swing a lot. that can make for head gasket failure, as the materials are changing temp too quickly and unevenly.



i went through this with the mgb. it took me a while to learn about what makes things work in a cooling system, and why low is not always a good thing.

Partial Post: Please Login or Register to read the full post.