Intermittent Coil Warning Light on Mk5 GT TDI

[ozrcboy]

Fault came up on the wife's Mk5 GT TDI, approx 120K on the clock.

Coil warning light flashing saying with MPD saying "Engine Fault - Workshop".

Drove into town to fetch her, when I got there I saw the car exhibit the same behaviour.

Checked service history - possibly the car is overdue for a fuel filter but that's about it. Popped the bonnet and lifted the engine cover - not sure what I was expecting to see. Nothing untoward. Reinstalled engine cover, closed bonnet. Went back and turn on ignition. No flashing coil warning - no "Engine Fault - Workshop".

Wife drove car home - seemingly no problems.

This isn't the first time we have had the coil warning light behave like this. Whenever we have taken it to VW in the past we have been told there are no stored fault messages.

Car seems to run fine - maybe a fraction rough if anything (hard to tell on the 125TDI MkV engine - it sounds like a truck).

My two thoughts are dodgy sensor, or perhaps I need to decarb the engine intake.

Can anyone offer any thoughts? As ever thanks in advance.

[Umai Naa!!]

Get it scanned. It's the only way to know why the light is coming on.

The are nearly 3,000 different fault codes that could trigger the light.

[Greg Roles]

...but most likely to be a failing EGR flap / valve, so an intake decarb is a great idea, and certainly can't hurt. Second best guess is a failing battery, and on a TDI even though a battery may seem fine, a older one often causes tons of weird problems. Does it start fine?

[ozrcboy]

Interesting thought on battery thanks Greg - is starting to get a little harder to start and is still still on the original battery (so just turned 8 years old)

[Greg Roles]

Borrow a battery, SO many times TDI problems have been an otherwise "fine" battery, my belief is the heat from all the regens kills them pretty quick, yet they still "seem" fine.

[benough]

8 years on a battery!

My GT TDI is 8 years old and is on it's 4th battery.

I get the "Engine Fault - Workshop" light all the time. Mainly due to my intake providing too much air or something else. I just turn it off and go again. These cars are robust (mine has 220,000km) but they sure do moan a lot!

[ozrcboy]

Did the intake manifold clean tonight using the liqumoly. Might be confirmation bias but idle seems to be smoother and throttle response a little improved. Pretty easy process. Yes benough - I figure that battery company has long since gone out of business. Only thing that is done that might help the battery is the car gets driven most days (but not every day). Tried to look for a replacement battery at costco today - apparently they only have batteries to fit up to Mk4. Don't tell me these have some kind of special complex battery.

[benough]

Nah, just get a Varta Black and make sure it has the right amount of CCA.

[Greg Roles]

It's all about having the poles the right way around, and the correct CCA rating. My last one was a Century about $175 from Supercheap.

[ozrcboy]

So a little more on the diagnosis side.
The coil light is now going off, but is then coming on when the engine is cranking. That's obviously a high demand time for the battery - if it is the manifestation of a low voltage condition of some sort that would make sense.
Also found out something interesting with the helpful guys at SCA - if you don't have VCDS you can go to an SCA store and they will hook your car up to their bosch diagnostics kit and do a read for $14.
Priced a replacement century at SCA - $275 - ouch. Any other thoughts - don't want crap by any means but that seems a lot (I know - the guy who has gotten 8 years from a battery probably shouldn't complain...)