RX7Club.com - Mazda RX7 Forum - Water temperature sensor: How to tap and install

Disclaimer: This will be overly detailed so as to try to give people more data with less questions as opposed to the opposite. The job is not hard, regardless of the detail.

Every aftermarket temperature gauge on the market requires a temperature sender to get it's readings. They work by using a variable resistor inside the sender that changes resistance based on water temperature. By sending voltage through the sender and comparing the returned voltage to the sent voltage can determine the actual water temp as the water temp varying the resistance results in a different output voltage. This is precisely how the gauge or control unit works to display the proper temperature (it has been calibrated to the response of the sensor already).

There are multiple places to mount the temperature sender such as the throttle body coolant lines, upper radiator hose, engine block, or thermostat housing. The goal is to obtain a location where one can have accurate readings but more importantly readings which are capable of a high rate of change. Ideally the best place would be in the water pump housing or in a block coolant passage right after the thermostat. Unfortunately due to physical design issues of the thermosensor and clearances, the water pump and/or block are a bad idea.

Almost all of the sensors out there are 1/8 NPT size threaded probes with the electrical harness on the external portion.

Pros/Cons of locations:

1. Throttle Body coolant lines.

Pros: Easy to do. You can use a freely available tee-line with a 1/8 NPT tap in the tee to install the sensor. Reasonably accurate block coolant temperatures regardless of the thermostat state.

Cons: Slow rate of change. Less room for installation when the TB coolant lines are commonly bypassed. No advantage over the other locations except for ease of installation. Due to the flow of coolant through the block, this is really only showing you temperatures indicative of coolant passing through about half the block, post-radiator cooled. Grounding issues.

2. Upper radiator hose.

Pros: High rate of change when the thermostat is open. Harder to find a tee-line adapter for the size of the radiator hose compared to the throttle body.

Cons: Somewhat useless when the thermostat is closed as the readings are more stagnant. This can be an issue when one has a stuck thermostat. In addition we are really interested in the sum of block AND post-radiator temperatures which this location isn't best for. Grounding issues.

3. Thermostat housing.

Pros: High rate of change when thermostat is open. Medium rate of change when thermostat is closed. Since the upper radiator hose is the one delivering coolant to the radiator, as regulated by the thermostat, changes in block coolant temps. are fairly immediate. No grounding issues.

Cons: Requires removal, drilling, and tapping. Stuck thermostat is still an issue, but less so due to the proximity of the sensor to the thermostat itself - basically you'll know when things are wrong before you warp the block.

Since the goal is to do things the right way and monitor coolant temps with the most accuracy and highest rate of change one should mount the thermo-sensor in the thermostat housing. It's a good position as it lets you gauge the rate of change in coolant temps _exiting_ the block, but pre-radiator. If you are seeing a fast rise from 185 to 220F, you should know fairly quickly that there is a problem and to reduce the load on the engine or go full heater/fans and shut it down.

---

Now I know the concept of "drilling and tapping" is scary to some, but it's really not that hard at all. When it comes down to it you're basically just drilling a hole and threading it so you can screw something into it; in our case, the thermo-sensor.

All of the tools required are available at any place selling halfway decent tools. I got mine at Sears as Home Depot and Lowe's have fairly bad selection when it comes to individual taps.

You will need:

1. 1/8-27 NPT pipe tap.
2. 11/32" drill bit.
3. Tap wrench.
4. Some kind of cutting lubricant (or kerosene).
5. Teflon tape.
6. Liquid gasket (I used some Hondabond I had sitting around).

All together it's about 15$.

http://www.anodized.com/~clayne/fd3s/tools_s.jpg

A good place I found to try a practice tap was in my stock Mazda scissor jack. Since it's a solid piece of aluminum you can tap the center of the bottom to do a decent test run. It won't harm the integrity of the jack.

Actual directions:

1. Drain a good amount of coolant from the radiator.

2. Remove thermostat housing by removing bolts. One bolt is by the airpump, so if you want to do this easily, loosen the alternator belt slack and use a long extension to get to the bolt. Otherwise you're stuck taking off the airpump which is an unneeded hassle. The gasket may make the housing stick to the block in which case some gentle prying or rocking back and forth will break the seal loose.

3. Remove thermostat and drill a hole using the 11/32" bit through the flattened out area (you can see it in the picture) right in front of the lower bolt location. Aluminum is soft and drilling through it is easy. Make sure to only drill through that one side and not all the way through both sides of the housing. We only want a tap, not a skewer mount for the t-stat housing. :)

4. Using the tap and the tap-wrench, being rotating the tap into the hole we just drilled, making sure to put cutting lubricant on it as you go. Initially the tapping will be easy, but as it gets harder back the tap out every hard turn about a turn or so to remove the metal shavings from the tip. Keep going until you have about 10% of the tap left showing. This is enough.

5. Throughly clean the housing in a sink and blow air through it to remove any metal shavings/fragments left.

6. Wrap one-half of the sensor or sensor mounting fixture (if DEFI) with teflon tape, leaving some uncovered metal for proper grounding. Screw it in to your new threaded hole and tighten until it's very snug but don't over do it as it is aluminum. If it's a DEFI sensor you should put teflon tape on one-half of the actual sensor itself as well, since this screws into an adapter which screws into the t-stat housing.

7. Apply liquid gasket to the mating surfaces of the housing and the mating surface around the rubber gasket of the thermostat. Secure the t-stat back into the housing and bolt the housing back up to the block. Wipe off liquid gasket bleed and let it dry for about 15 minutes.

8. Attach thermo-sensor to gauge or control-unit (if DEFI).

9. Replace lost coolant.

10. Check for leaks.

11. Start up car and blow out your o-rings. Just kidding. Do standard cooling system bleeding procedure.

http://www.anodized.com/~clayne/fd3s/sender_s.jpg

You now have a gauge which will show cooling system effeciency under average driving and track conditions. In addition, it can also save your engine's ass from overheating.

You should see around 180-185F (t-stat limited) while cruising and increasing temps under heavy load. The key here is to be proactive about rate of change in temperature. Don't do something when you see the temperature hit 250F. Do something when you see the temperatures rising quickly past 210F BEFORE they hit 250F.