DIY: Temp gauge for front oil circuit

Old 05-07-2005, 11:38 PM
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Join Date: May 2001
Location: Raleigh NC
Posts: 264
Default DIY: Temp gauge for front oil circuit

Hello, everyone.

Last fall, on a cool weekend at Grattan Raceway, my oil line separated from the thermostat housing and dumped oil all over the track. I posted a note about it (read about it here),, and there were many theories about why this happened.

After some sleuthing, the likely explanation is that I got strongly into the throttle before the front oil cooler circuit was open. I have an auxiliary oil cooler in the nose of my '95 993 (the large Cargraphics cooler that fits behind the front bumper), giving me a large reservoir of cold oil (more viscous on a cold morning). Even though the oil temperature in the engine was adequately warm, I was beginning to push hard before the thermostat opened to the front oil cooler. When it did open, I was probably near the redline, and the hot oil (under higher pressure at high revs) encountered the large volume of cold, sluggish oil. So the oil line pushed its way out of the thermostat block. It is only held in by a small bracket.

Here's what I've done. First, the oil line was remounted with a beefed up bracket to hold it in place. Next, I'm careful to make sure that the front oil circuit is open before getting aggressive with the throttle. As many of you know, you can tell when this happens: First, the oil temperature rises, then falls as the front oil circuit opens, then rises again to the final operating temperature. BUT. It isn't always convenient to sit and watch the temp gauge. It can take quite a while to warm up fully, and it's difficult to watch the temperature gauge on the track. So I was thinking that what I need is a way to measure the temperature in the front oil circuit, independent of the rear oil circuit.


I've fashioned an easy-to-install temperature sensing system that tells me what's going on in the front oil coolers. A quick glance at the gauge tells me whether the thermostat has opened or not. It's great!

You can buy a thermocouple that terminates in a hose clamp; perfect for wrapping around an oil line fitting. I found one that comes with 10' of braided steel cable, perfect for running it to the cockpit. I also found a 9V battery-operated temperature unit. Here's how to do it. See the photos at the end of this post.

1. Buy the following from McMaster-Carr (

(a) #38855K82, Magnetic-back thermocouple thermometer type J ($86.95) (this is the ░F version; they also make a ░C version)
(b) #37045K621, Mini-plug thermocouple with surface probe type J, flat-pin connector, 1/2" to 1-1/2" hose clamp ($26.90)
(c) #3869K33, Thermocouple and RTD connector male plug, type J ($2.89)

2. Remove your passenger-side headlight. Easy - just open the trunk, rotate the lever near the side of the headlight, and pop the whole assembly out.

3. Attach the thermocouple to the inlet to the stock fender-mounted oil cooler using the hose clamp. Remove the mini-plug from the other end to make the cable easier to route the cable into the cockpit. I could remove mine without damaging it too badly, but it worked out better to put a new mini-plug on later (see item c above).

4. Pass the cable through the rubber grommet that the headlight cables travel through, into the trunk. Route and zip-tie the cable through to the cockpit using whatever access point you can find. I passed mine through a hole in the firewall that had been used in a stereo upgrade.

5. Re-install the mini-plug (or install a new one; see item c above).

6. Mount the temperature head unit where you like. I used a piece of metal to fashion a bracket that allowed me to mount it above the floor console. (See photos) I used Radio Shack super velcro to mount it - that stuff is strong. I removed the magnetic backing plate on the head unit, since it is not glued on well, nor does the magnet provide a secure enough mounting force.

Done! When driving, you'll see the new oil gauge register ambient temperature for quite a while, then suddenly begin to rise when the thermostat opens. It'll settle in to 190 ░F or so thereafter. You'll always know when your system is ready for full throttle application.

For track folks: Let your car idle in the paddock for a while before going out on the track. If the front oil temperature still reads ambient when you go out, take it easy (<4000 rpm) until the gauge shows some action. Then have fun!


View through headlight hole, showing the thermocouple/hose clamp attached to the inlet to the stock auxiliary oil cooler:

Closeup of same:

Head unit:

Head unit showing mounting bracket. The on/off switch is just above the mini-plug:

Let me know if you have any questions.
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