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Discussion Starter #1
Is it possible to do a compression test with the turbo off? I bought a 2000 Jetta with 120K that had runaway hoping that the turbo may be the only issue. Previous and only owner was able to stop car within seconds with the key and then restarted and limped a mile home. Before I purchased a compression test by a local enthusiast gave 440, 390, 280 and 460 (1-4). The dealer replaced the turbo at 80K due to limp mode. After removing the turbo, the VNT failed the vacuum test described in forum FAQ. No apparent damage to turbo blades. Maybe a mm of play (across and along) turbo axis. Heavy build-up (10-15mm) found on intake and EGR. MAF sensor. A regional guru suggested that excess oil from the runaway could affect the compression test readings. I am trying to decide whether to replace the turbo if a chemical cleaning doesn't work or just ship to the local guru for an opinion. The other decision is whether to pull the head to check piston height to rule out bent rods while the engine is partially disassembled vs reassembly and then compression or leakdown test. Thanks
 

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turbo off test - yes.

If there's enough engine oil in the cylinders to combust, it will cause a pressure spike which will raise the compression gauge reading. I think the manual says to crank until compression stops rising which could be 10-15 cranks. This is different from a gasoline engine. Also don't add oil to see if it bumps the compression for the aforementioned reason, more info in the FAQ on compression testing.

#3 is low and needs more investigation. The runaway could have been minor enough to not cause damage. The question is where did the oil come from (or what caused the runaway) and why is #3 reading low.
 

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Discussion Starter #3
Runaway causes

Other than worn turbo seal, are there other causes of a runaway? To do a compression test with the turbo off is there a way to route the oil line for the compression test? Will cylinders hold excess oil after a runaway and is there a way to clear for a compression test?
 

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Older cars had a problem with piston blowby oil mist. I don't think it's a direct common cause of runaway on the TDI because this mist settles in the low spots like the intercooler. The problem is more indirect because the runaway starts when the pool of oil gets sucked up.

They sold an oil separator for the older diesels. Not sure how to route the oil line since it's under pressure. I believe any excess oil will burn out once it's running or it'll bleed off. Not sure.
 
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