Thermostat replacement 110TDi

Sounds like your coolant temperature sensor (CTS) needs replacing. If you have VCDS this can be confirmed by checking the coolant temperature reading.

Had it replaced on my TDI 110 a few years ago, the gauge started giving strange reading then stopped work completely. Also had the thermostat replaced just for good measure since the car was 10 years old at that point.

Didn’t cost much at a back street garage, it’s a fairly easy job if you’re going the DIY route.

Sadly a standard OBD scanner won’t show any details on a failed CTS.

The 110 TDI has the AHF engine code and was used by VW, Audi, Seat & Skoda so you’ll find guides on line and YouTube.

Mine was dropping into limp mode above 3k revs in 3rd and 4th gears. The ECU limits the turbo boost in 1st and 2nd gears which is why it isn’t over boosting. But I wouldn’t recommend taking the engine to red line in 2nd gear often and certainly not in 1st gear. There’s lots of information online about limp mode on TDIs but a basic outline:

The turbo uses control vanes to adjust the air flow so it responds well at low and high engine speeds. If the vanes get stuck then the ECU will trigger the limp mode to stop the turbo over speeding. Check the VNT control arm on the turbo, if it’s sticky clean the turbo. If it moves freely check the vacuum line and control valve.

My limp mode problem went away when I started doing a 70 mile daily commute, the turbo was getting hot enough to burn off the exhaust soot/carbon.

If you’re losing boost check the air tubes and connectors on the turbo and intercooler to see if any are loose or split.

If you don’t know the history of the car then it’s worth either cleaning or removing the EGR. I’ve had the electric wires unplugged from the EGR vacuum control valve for 12 years. I’d recommend cleaning the EGR then disconnecting the wires from the valve – make sure you do the right valve otherwise you’ll have no turbo response

You could flush the coolant system and replace with new coolant.

But my coolant has been brown for a few years and goes a bit frothy (like coffee) when the engine is running. Scared me at first and had a garage check for a blown head gasket, they said it was fine so it’s still in the car.

Happened sometime after the CTS and thermostat were changed which maybe related or just that the car is 15 years old.

chriswales, the engine in a TDI 110 Mk.1 Leon is more likely to be ASV rather than AHF. The only differences I know of are that the ASV has oil cooled pistons, different injectors and different software in the ECU, the latter two to meet newer EU emissions standard (D3) compared to the AHF.

Limp mode is likely to be stuck turbo vanes, a reasonably common problem on engines that are used mostly for short-distance communting - diesels in general are not best suited for such work as they never get warm enough to work properly. They are *much* better for longer-haul work, with a decent amount of running at "cruise", 50-70 mph.

Yes Muttley the MK1 110 is normally a ASV, but it’s often better to search for the old AHF code when looking for guides and videos online.

I got limp mode when my commute was 10 miles each way. After a forced move of work locations, I now spend an hour on the motorway. No more limp mode and the car is running better. So yes diesels much prefer to be used a lot, even more so if you have a newer one with a DPF.

The turbo on the 110 seems to respond well with the oven cleaner fix. Although if the car is mainly used on short trips (less than 30 minutes) the problem is likely to return.

Or an ALH as in my 110.

If it's never been cleaned then the EGR would be my first check. Easy to undo the connecting hose and look for loads of lovely black carbon build-up.

If it's there then odds are the turbo will have some and the inlet manifold as well. Luckily it's a fairly easy job to dismantle all if you want a thorough job done and use of a couple of cans of poundland oven cleaner. Just get new gaskets in for reassembly.

Doesn't look that bad to be honest. Often on the newer cars big lumps of carbon and oil are blocking it restricting the air flow.

It'll always have oil in there because the PCV is connected air intake.