KS03 CHRA or Headgasket?

Hi Guys,

AUQ 130k miles

Driving, coolant light came on, headed home as close (no overheating), checked next day lots of coolant but brown in colour (rather than pink)

Took to garage, Block test passed and compression test passed.

Garage now thinks headgasket but it hasnt overheated at any point. (I've seen block test can be passed at low rpms but when at 4000rpm exhaust gases were detected on another car)

Oil cooler replaced and two 20 minute test drives all good, idled for pre mot for longer than 20 minutes and oil started showing in coolant again.

I have been doing some research,apparently the coolant channels in the turbo internals (CHRA) can be breached by the higher pressure oil leading to oil in coolant but no standard head gasket mayo under oil cap and dipstick which are my symptoms.

Has anyone any experience of this and has anyone fitted a new CHRA to a turbo?

AI thinks the ks03 has coolant channels, others who have removed them said not.....

Any ideas as want to rule out if possible before either fix or scrap

AI: "

Yes — confirmed, it's water-cooled. I checked the part-specific data rather than relying on memory.

The exact turbo for your engine — K03s part number 5303 988 0052, OEM 06A145713F, the 180HP AUQ/ARZ unit — is explicitly listed with a water cooled bearing housing. That's the K03s fitted across the Mk1 Audi TT, Seat Leon Cupra, A3 8L, and Mk4 Golf/Octavia with the AUQ engine.

TurboTurbos

Backing it up: on the stock K03s, the line on top of the turbo running around the timing-belt side is the coolant return, and the coolant supply comes off the back of the block. The AN-line kits sold for this engine include both a turbo water feed line and a turbo water return line alongside the oil feed — two separate coolant lines to the CHRA.

VW Vortex

MAMBA TURBO

And the tell-tale that people deliberately remove it: you can run the K03s oil-only, but you have to plug the old water lines to do it. If it weren't water-cooled from the factory, there'd be nothing to plug.

VW Vortex

So the diagnostic logic stands: your AUQ's K03s has oil and coolant galleries sharing the same centre housing, which is exactly what makes an internal breach able to push oil into the coolant — and what makes the coolant bypass test valid. If yours has already been swapped for an oil-only turbo at some point (worth a quick look — are there two small coolant lines actually running to it?), then the turbo drops off the suspect list entirely and you're back to head gasket / cracked head. So before the bypass test, just eyeball it: two small-bore water lines to the CHRA = stock water-cooled unit, test is valid."

Thanks,