My coolant sensor is EVIL... please help me kill it!
 

I'm running a Hydra and FMII on a 99 1.8L. It has the single coolant sensor for the ecu and gauge with the triangle connector at the back of the motor.

Issue: Coolant temp shows accurate in the Hydra when the key is in the on position, and for 1-2 minutes after I start the car. Then it starts getting erratic with some crazy temp swings, then it goes to a whopping 3.2 degrees F so the car runs like crap. Gahh!!! I have a brand new temp sensor so I put that in. Nope, does the same exact thing. The factory temp gauge appears to be working fine which makes it even weirder.

The only thing that has changed on the car since this issue started is me fucking with the grounds on the front of the motor. I didn't move any of them, but I did chop the factory brass wire crimp there and put the four wires on a ring thinking it would clean it up a little.

Could that really be what caused this? I didn't think it was possible to create a ground loop issue by taking wires using the same ground and simply putting them together. I attached a pic so you guys can see what I'm talking about.

Does anyone have a pic of the grounds at the front of a 99 1.8L? I can't find a pic showing the ground wires so I can put it back to stock.

https://cimg9.ibsrv.net/gimg/www.mia...f6e0fc66a4.jpg

Sounds like you might need to burp the coolant.

Wouldn't it read a somewhat normal temp though (ambient?) even if it was in an air pocket and not 3 degrees F?

Hmmm good point. I am not an NB guy, that seems like quite a few grounds to be connected to the IM. Are you using a phenolic manifold gasket?

Yeah, I was thinking the same thing. They were there when I bought it. I need to find someone with an NB to inspect their front grounds.

Just looked at some pics, there are a lot of grounds on the front of the IM. Hydra may default to 3F if the connection is broken or the sensor quits working. It could be a fail safe.

What does hydra read when you disconnect the sensor?

1.4 F, and it immediately changes when I disconnect it.

There are three all black ground wires that should be on the front of the motor from the wiring diagram I am looking at. Any idea what the two extra black/yellow wires are or where they should be grounded?

The coolant sensor gets voltage on one wire and outputs a different voltage depending on the temperature it senses. It simply changes resistance which changes the voltage.

Since you said the factory gauge appears to be working fine the issue could be with the ECU. To test this, measure voltage on the coolant temperature input at the ECU.

On the NA, the gauge and ECU use different sensors.

was the car running and the coolant reading wonky on the Hydra when you confirmed this?

ninja

Correct, car was running. Did it a few times just to confirm with both of my coolant sensors, one being brand new.

Found an old thread on m.net where the guy had the same exact issue and replaced his fuel injector harness to fix it. I don't think the grounds are the issue since the car was running fine with them essentially the same way they are now. It makes sense if it is the injector harness, it carries all three of the coolant sensor wires to the connectors on both sides. I found one used for about $40. I'll give it a shot and report back.

Thanks. That's good to know. However, the test remains valid for confirming the ecu is getting appropriate info from the sensor.

Or you could use a $10 DMM and troubleshoot the issue before throwing parts at it.

Honestly, I kind of enjoy throwing parts at the car. I've replaced damn near every component and sensor on the car and it has been fun. It's not my daily and it's too hot to drive with no ac right now so I enjoy having stuff to replace and fix on it. I also just pulled the injector harness and opened it to find cobwebs and spider eggs that look about a decade old.

Just saw your advice on checking it. Good stuff and I need to pull a Hydra wiring diagram to figure what wire to measure voltage at.