I recently bought a Manual 97 Mazda Lantis 323 1.8L DOHC 16 Valve BP.

I found the following wire unplugged on the drivers side of the engine, next to the water intake from the radiator:

enter image description here

It plugs into the grey connector with the gold base. I plugged it in and the car wouldn't start. So I unplugged it and was able to get it to start after giving it some gas, but now the idle is erratic, falling almost down to around 200 and then jumping back up to 1000 ( previously it was fast idling just below 2k) . The engine also has a clear shake as if the timing on the pistons has gotten screwed up. I've also got a red light with an explanation point in it on the dash to the left of the speedometer.

A little history: the car had a head and ring job a few years ago, and the throttle body was replaced six months ago with a TB from an automatic due to idle and stalling problems. Not sure if the original TPS was kept or not. Previous owner claims he drove every day with no problems since then, but the first day I had it I noticed it idling around 2k after the engine warmed up and revving caused black smoke out the tail pipe. Getting about 10 mpg.


After finding a repair manual online I think that that thing in the picture is the Water Thermosensor. I'm wondering if this is the source of all my problems? Maybe the guy who did the work for the previous owner changed the throttle, saw it didn't solve the problem and then just disconnected the water thermosensor to get it working?

2 Answers 2


This absolutely could be the issue. On fuel injected vehicles, the coolant temperature sensor is one of the key sensors used (O2, MAP, & MAF are others) in calibrating the fuel map stored in the computer. Without the computer knowing the temperature of the engine, it has to run in a "limp-home" or "open loop" mode (depending on how the vehicle manufacturer does things). This mode is usually very rich. It does this so that the engine will not encounter a lean state which can cause damage much faster than a rich state.

I would suggest that yours is shorted internally, thus the reason why the car is not starting when plugged in. And like I said, it goes into a limp home state when unplugged. You can definitely try testing it, but I think you are on the right track. If it isn't the thing, it is definitely a thing which is wrong and needs to be addressed.

Here is the actual test you want to use.

Here is what the part looks like when pulled out:

Image Pulled from autozone.com

  • What I don't get is why before it was idling fast and running rich, and now after plugging / unplugging this sensor it's bouncing between nearly stalling and about 1k rpm. Commented Oct 17, 2014 at 14:48
  • I would bet if you disconnected the battery right now (before a sensor replacement) and let the computer reset, it would go back to the way it was before. Commented Oct 17, 2014 at 17:03
  • Looks like I'm going to go out tomorrow and buy and O2 sensor and get some MAF cleaner, then if that doesn't work I'll replace the coolant temp sensor too... Commented Oct 18, 2014 at 18:59
  • @RobertS.Barnes ... That isn't a bad idea, but wouldn't it make more sense to replace something first which you know is probably bad? Or is it just hard finding this part where you are at? Commented Oct 18, 2014 at 19:28
  • I've heard that cleaning the MAF has helped other people with weird idle problems and the temp sensor looks very clean like it's been replaced in the past few years already. If the other two don't work then I'll replace the temp sensor. Commented Oct 19, 2014 at 3:26

you're going to need to adjust the idle. look inside your repair manual for the idle speed adjustment section. you should be able to use a paper clip or jumper wire for the dlc part for getting it in diagnostics mode.

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