V70 2.5 tdi slow to hot start . Coolant temp sensor?

[Neilmathews]

Looking for a simple explanation! My age defying pride and joy, 230k, has become a sod to start when hot. Even had to resort to a sniff of easy start in the air intake once or twice during the hot weather. Cold...starts first time even if it's been standing for weeks. Having read loads of threads n theories....water coolant temp sensor seems to be a possible culprit (one of many)...so I have ordered one..cheap at £15 and easy access....but out of curiosity I unplugged the temp sensor, and started the engine. It hardly even turned before firing up, and the cooling fan came on. Stayed on when I stopped the engine and did so until I plugged the coolant sensor plug back in AND ignition back on. So my question, if unplugged, this simulates a very hot engine given the cooling fan came on (?). It started sooo quickly, hardly even turned over, what happens to the starting system when the sensor is unplugged? Does it bring in glow plugs (I assume not if th fan comes on, indicating it's hot?) ...can a wise person explain the impact on starting caused by unplugging the temp sensor?

[C70 98]

I had this on a T5, replaced the inlet pressure sensor (MAP) and it starts as should. Don't know if that could be it on a TDi.

Same T5 had rough running when cold and revs all over the place at idle when warm, cured by replacing the inlet temp sensor.

[Martin Cox]

Presumably the dynamic pump timing is correct. This can have a significant effect on starting if it's out.

Martin

1998 V70 Tdi auto

[Neilmathews]

Presumably yes ☺....but process of elimination also. I now believe that if the sensor is unplugged, the cooling fan goes into default mode...no signal so assumes worst case scenario 'hot' so the fan comes on. However, the opposite with the diesel system...when water temp sensor is unplugged...The ecu or pump adopts 'must be cold' default,and the cold start cycle kicks in including raised tick over. Plug the sensor back in tick over drops to usual idle. So...The coolant temperature sensor has a big impact on starting. I fitted a new sensor today...top front of the block...use a deep socket or 19mm open or ring spanner...Bit fiddly...does not need to be particularly tight. ...engine started fine. However too soon to assume it's fixed...need a few fast short runs then off/ restart when hot...see if it behaves. Given that when the temp sensor was unplugged it fired up immediately when hot....and that I'm on the ferry to france sunday..........I'm taking some connectors, wire and switch, so if it misbehaves, I will interrupt the live to the coolant sensor, run it back to the dash somewhere,so I can switch off the sensor just to hot start (without getting out of the car/opening the bonnet etc)...and flick it straight back on soon as it fires up. Heath Robinson maybe but buggered if I'm gonna have to pop the bonnet half way up a ferry ramp if something happens!!!

[rogerb]

I have this found on a few of these , usually with a bit more miles, that they can be hard to start hot.
I found that manualy operating the glow plugs for 5 seconds would make them start easy every time.
Disconnecting the coolant temp sensor may have the same affect. As the glow plugs are ecu controled. As the engine would think it is cold and so switch them on.
The fan running is a safety feature, and can stay on for up to 10 minutes or so, as the ecu doesn’t know what temperature the engine is.
Fuel wise it assumes a cold start and then adjust it self over a set time. But just in case it runs the fan.
The long turn cure was a set of replacement piston rings, or short engine as the compression was down a bit.

Hopefully this is not your problem.