2014 Mk7 GTI Coolant/Oil temperature issues

[je5343]

Backstory: In April my car suddenly had an EPC light and the coolant temperature quickly began to rise during a short test drive. It reached about 110°.

We replaced a couple of temperature sensors (one on the radiator outlet and the other on the right hand side of the block), that didn't make a difference and we eventually diagnosed it to a faulty thermostat and replaced the entire housing (brand new, OEM from ebay).

That fixed it! All was well, EPC light had gone, coolant never passed 90°.

Fast forward to last week, just set off from work, in stop-start traffic when I got a "Oil sensor: workshop!" error message on the dash. Odd. I rolled forward as the traffic moved and the message disappeared, came to a stop again and it came up reappeared (Stop start was enabled on the car when this occured). I switched the MFD to oil temperature and I joined the motorway (in eco mode and keeping the revs as low as possible), the oil temperature was slowing climbing and kinda balanced out at 111° but during the drive home, coolant sat at a constant 90°.

I have never seen the oil go above 105, even in my old 1.6 Mk5 golf so I was a little worried. I checked the oil level when I got home and it was within the marks on the dipstick, so it wasn't low.

Out of curiosity (maybe in hindsight, stupidity), I continued to run it. The oil usually sat around 111 on 20 minute drives and once maxed out around 114-115. The same "Oil sensor: Workshop" kept popping up the when stop-start engaged. Until one day, when it popped up as I was driving down a hill doing about 50mph and the temperature on the MFD was jumping from 83 to between 50 and "---" (it's hard to explain it's almost like the sensor was restarting and just pulling a value from nowhere, I have a video of similar behavior in the imgur link below).

Definitely looks like a sensor problem right? So I called my local VW dealer for a price on a new sensor, I though the oil temperature sensor was part of the level sensor (which is located in the sump), according to the sensor that's already in there (P/N 03F907660D), it's not.
Problem 1, I don't know where the oil temperature sensor is (is it the one that we replaced on the right side of the block?).

What's more interesting is, I recently ran it up to temperature, again oil climbed to around 110 but this time, the coolant didn't reach 90° on the gauge. It reached 70-80° and it started fluctuating, dropping about 5° before slowly climbing again. We left it idling and realized the coolant temperature was eventually dropping and reached about 62° before we shut it off. The coolant tank was hot enough to be well over what the gauge was telling me and had plenty of fluid it in it.

What would cause the coolant temperature to drop (according to the dashboard gauge) but the oil temperature to rise?

My only guess would be that it's the thermostat again but any suggestions or any steps to further diagnose this would be very much appreciated.


https://imgur.com/a/sNsB5aV

[Carbon VW]

Do you have access to a scan tool to scan for fault codes? That would be a good start. The sensor you replaced on the block is the engine coolant temp sensor, not an oil sensor. When you changed the thermostat housing, did you replace the water pump also, or just bolt the old one onto the new housing? It could be a number of things, but fault codes will help. Good luck.

[ajmoir36]

Could there be an airlock in the coolant too, which would cause oil temps to rise a little due to lack of cooling. Also might cause the temp to fluctuate.

[je5343]

OBD codes: Engine oil temperature sensor circuit P019600, range/performance (added an image to the imgur link).

We had a mechanic come round last night (it's booked in with them in a couple of week but he's local), we found that the radiator outlet temperature sensor was reading -48° so we disconnected the sensor and it still read as -48°, very weird. The water pipes leading into the top of the radiator were very hot after it ran up to temp but the pipes leading out were stone cold.

Today we pulled the radiator outlet temperature sensor out and tested it in hot water and cold water and the values did change so I think we can safely rule that out.
Next we checked the voltage on the cables connecting to the sensor and that all seemed good.

We've triple checked for airlocks (plus it's ran for a month and a half with no issues).

I'll check the wiring diagrams for anything else on the same circuit as the rad outlet sensor see if there's anything that could be interfering with that.

Aside from that, I guess we'll see what the garage says in two weeks.

I'll update this thread when it's resolved.

[je5343]

Turned out to be a faulty coolant temperature sensor (the one in the block), the garage replaced that and it seems to be running ok for now.
 

[Carbon VW]

Glad you got sorted. From your original post it seemed you had already changed out that coolant sensor on the block. Hope she runs well for another 10 years !