Engine Error Code 9: Water Thermistor

Need some help to trouble shoot error code #9 for the Water Thermistor
I've been out of town and letting my daughter drive my Miata (91 BRG) to
school (all the guys think its cool that she can drive a manual). Anyway, I get home and find out that the check engine light has been coming on. It comes on when the engine is first stated and then goes off after driving for some time. I had thought it may due to fouling somewhere since my daughter's drive to school is very short. However, I checked the diagnostic code and it shows #9 for the Water Thermostat.
Miata.net does show where the thermistor is located (behind the engine where you can't see it), but doesn't talk about how to check or replace the thermistor. Would appreciate some advice on how to check and replace the thermistor.
Thanks,
Gus (91 BRG)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Testing and installation of the water thermosensor starts on page 5:40 of Rod's Miata Enthusiast's Shop Manual (Amazon.com product link shortened) If you're going to be doing this kind of work on your Miata, do yourself a favor and pick up a copy.
The ESM says you need to detach the coil assembly to get to the sensor. Disconnect the sensor wiring and unscrew the sensor, *retrieving the sealing washer*. Pretty simple, just real tight. Rod recommends ignoring the low temperature check at -4 degrees F ("Let's face it, if you are driving around with your cooling system at -20C, you've got bigger problems to deal with than incorrect thermosensor resistances." - this witty aside alone is worth the price of the manual) and go for the easier to obtain 176 & 68 degrees F checks. At 176F resistance should be 0.29-0.35 k-ohms; at 68 it should be 2.2-2.7. Oh yeah.....remove the radiator filler cap to make sure the system is depressurized before removing the thermosensor.
Could it be just a loose wiring connection? -- Ken Lyons '97 Brilliant Black/'90 Classic Red Inside the Beltway [Remove the first two digits to reply]
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Ken,
Thanks for the reply. Sorry I didn't get back to you sooner, I was out of country for a couple of weeks. The check engine light was on again when I got back after 3 weeks. It cleared as soon as I got the engine up to temperature. I then did a couple of red line runs to clear out the engine. All back to normal.
I'm going to try asking my daughter to let the car idle once she gets to school until the temp gauge comes up near normal range.
I still think the problem is that she is driving such a short distance at low loads that the engine never really warms up. With out the thermostat opening, the block gradually drying out (e.g., water out but no water back in).
Gus 91 BRG
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.