VAG 1.8T engine problem.

( |-| |2 ][ $ · 3 Apr 2014 at 17:56

Can anyone help me attempt to diagnose a problem with my Leon 1.8T 2004

I'm currently raging as I took a day off work to give it too the local stealers to fix, only for me to hear at 5pm that their "diagnostics machine is broken and can they have it over night".

Anyway the symptoms:

- On cold start the secondary pump doesn't come on, otherwise it behaves fine.
- After that is runs perfectly for my journey.
- After stopping (and turning off the engine) it takes a few extra turn-overs to start, then runs with a high idle as if it's running rich for 2mins then it's all good again.
- No engine lights

I think the coolant temp sensor might have failed. (am a right in saying the one for the dash gauge is separate (because that works fine)

Your thoughts?

miniyazz · 3 Apr 2014 at 18:08

Coolant temp sensor shouldn't give those symptoms and AFAIK there is not a separate one for the dash gauge.

Pabl0 · 4 Apr 2014 at 07:23

Okay,

Firstly why did you take a 04 Leon to the stealers, get it in your local indie.

You need to get it scanned with vagcom or get yourself a copy of vcds lite and a cheap eBay cable.

I'd say MAF but really need to scan it or you will be throwing money at the car.

benneh · 4 Apr 2014 at 11:54

I had an issue with turnover/rough idle.
Replaced the coolant temp sensor, removed my MAF and TB, cleaned the MAF carefully with switch cleaner, and mauled the TB with carb cleaner and a toothbrush. Put everything back together; which took an hour or so overall, ran vag com to clear any fault codes, and within an afternoon it was running perfectly. Worth a shot.

And the dash takes info from the coolant temp sensor (on my golf anyway, which afaik is the same sensor/setup).

Firestar_3x · 4 Apr 2014 at 11:59

A broken CTS can make the 1.8T hard to start, the CTS is four pin irrc, just because the dash gauge is working doesn't mean the sensor is feeding the ECU the correct value.

For £6 it would be my first port of call.

Matt82 · 5 Apr 2014 at 15:09

Doesnt sound that unusual, lots of cars runs a warm up cycle high idle when started. I wouldnt expect it on a baking hot engine but its not that out of the ordinary

MissChief · 5 Apr 2014 at 15:17

The dash gauge runs off the thermostat on the front of the engine, not the CTS. Mine is currently shot but this WILL throw a code error and the check engine light. It also makes the engine run cool on long runs.

Have the coil packs been changed as there was a recall on most of them. Call your local dealer again and get them to check as it's a free swap.

( |-| |2 ][ $ · 5 Apr 2014 at 15:52

Just changed the CTS, I tried a start after a run and it started much better but still idles a little high (for a short period) and stutters when blipping the throttle. (I think running rich). I've had the car 8 years, it never used to be like this.

So the CTS has seen an improvement but it's clearly not the problem. (still no secondary air pump from cold also).

Thanks to others for your replies, in response, yes coil packs changed 2 years ago via recall.

Will book it in an indy for vag com check.

miniyazz · 6 Apr 2014 at 02:26

How does the dash temperature run off the thermostat? The thermostat doesn't, AFAIK, have any sort of electrical supply but opens based on temperature, i.e. mechanical.
In any case if the thermostat were shot the engine would run cool as you say but based on my own experience with a mkIV golf it wouldn't throw an error code - and the engine temperature will appear cold on the dash readout - surely if the thermostat were electrical and therefore not detecting a drop in temperature so not closing, and the thermostat controlled the dash readout, this would not show a low temperature?

Dogbreath · 6 Apr 2014 at 09:33

MissChief said:
The dash gauge runs off the thermostat on the front of the engine, not the CTS. Mine is currently shot but this WILL throw a code error and the check engine light. It also makes the engine run cool on long runs.

Two separate things. The thermostat regulates the engine coolant temperature, but it is (on most cars) a purely mechanical device and is not connected to the temperature gauge.

Most VAG engines of this era use a 4 pin coolant temperature sensor, which is effectively two separate temperature sensors in one device; one for the ECU and one for the temperature gauge. Using a code reader with live data capability (e.g. VAG COM) will show what the ECU believes the coolant temperature is.

I had a similar fault occur on my wifes Octavia, it has different engine (2.0L 8v) but the secondary air injection stopped working and idle control was ropey and it was caused by the coolant temp sensor. However, this did throw a fault code IIRC.

Matt82 · 6 Apr 2014 at 10:38

Dogbreath said:
Most VAG engines of this era use a 4 pin coolant temperature sensor, which is effectively two separate temperature sensors in one device; one for the ECU and one for the temperature gauge. Using a code reader with live data capability (e.g. VAG COM) will show what the ECU believes the coolant temperature is.

Thats the truth

With regard to the stutter, id get the impression its overfuelling. Could be lambda, could be fuel pressure reg (ie manifold vacuum) or a post maf air leak (most likely pcv system)