Coolant sensor fluctuations

Noticed over the weekend, my water temps drop 3-5'c when I lift of the throttle.

is this normal? im worried I have a weird voltage issue.

Is it repeatable? Where is the temp sensor?

it seemed pretty random, sensor is in the coolant neck, so about 5cm from the head.

What temperatures is it running when it happens?

105'c and the drops to 102-98'c

That's running hot! it could be when you lift off your rad is getting enough time to cool the coolant properly. Personally I'd be looking into why it's running so hot

Chris250 is right, that's a pretty hot ECT to be running consistently. I like to see ECT sitting in the 85-95 deg C vicinity. That aside it's unusual in my experience to see the ECT change instantly like you've described. Understandably it takes some finite amount of time for the water temp to change. That being said I have seen ECT do some pretty strange things in situations where you've got the head lifting under boost or something similar.

Given your description I would be looking at your wiring and in particular making sure the zero volt side of the ECT sensor is connected cleanly to a sensor 0V and the power grounds for the ECU are sound. This also requires proper grounding between the engine and chassis.

engines out the car at the moment (cracked no2 ring land we suspect).

but was looking at old data and yeah It consistently drops 6'c in around a second....

Its using a Toyota loom so they actually group all earths on the back of the manifold. (both sensor and other earths) I had a 2 gauge from the battery straight to the transmission then had a 4 gauge earth from the trans to the body. (body is also earthed in the boot).

thought that would have been enough but I will investigate.

datalogue for those who are interested.

Most fluid temp sensors have a thermal mass that will not allow for the sensed temperature to change that rapidly, to get that sort of change in the coolant temp, you would have to pull over 200kW of energy out of the coolant in that timeframe. It may be that when the throttle closes, you have an change in either the supply voltage or your 0v that moves the read value from the sensor, thus giving the change.

ok so how would I begin diagnosing to rule out either a supply issue (alternator?) or a 0v issue?

no body can give me ideas where to start?

What ECU/Logging do you have?

its a haltech PS1000 using HT-010300 Coolant Temp Sensor - Small Thread.

using a haltech short loom spliced to the original Toyota harness.

Does the Haltech allow you to monitor and log the 0V and sensor voltage values at a high rate?

can log battery voltage, sensor voltage.

but cannot see an option to measure 0V.

issue is car has been stripped for rebuild so makes it hard to do running testing..

edit also it is a 5ms sample rate (20hz)

so had a look at the wiring diagram for the engine loom and all the earths are spliced from the same earth (back of the manifold).

the fact both sensor earth and also general earths all meet in the same point cause a sensor fluctuations.

interestingly, the sensor would read hotter if the voltage went up. this seems inverse to what I was experiencing.