Kevin, I know you work on the turbos and your work is considered fine art, but what you've written here is a bit of a puzzle. The 15% ranging numbers tells me the test was not conducted properly. I guess blowbye is just a typo, but it suggests to me you were rushing your post, so maybe you can clarify what this has to do with turbo "leakage" (I presume you don't mean the turbos themselves, you mean the valves or rings under turbo compression levels?) And I don't understand why you suggest it's not possible to do leak-down hot? Just the physical problem of getting to the heads when everything is roasting? Once the engine has soaked up heat and normalized to operating temperature, it will have ample oil distribution, the rings will be doing the best they can and the valves will be seated as well as they're ever going to seal, so while I wouldn't don the fireproof gloves and jump into a "HOT" engine bay of a 993 Turbo, I think the objective is to have a consistent test protocol. What was the air line pressure at the regulator? I think a bungled cold test on a 15 year old turbo engine with 50K miles could well yield those "horrid" numbers (and yet the engine could well deliver 400hp for another 50K miles, no? : ) What's the history of the engine? Used to be a drag car running Nitrous? : )

Anyway, if nothing else, I'm not trying to lock horns with you on a minor issue, but I think Shrek993 should not worry too much about a 28K mile Turbo given the leak-down was so consistent -- and I'd be curious to compare the compression numbers. In any case, better to find a good shop that can perform the engine test thoroughly and properly -- on the presumption that there was some reason to check the health of the engine, so I imagine there's already some symptom or cause, but maybe it's just "tinkering."

 

I'm no expert but I think the turbo leakage refers to the crank case being pressurized by the blowbye (past the rings) causing increased back pressure for the turbo oil return, causing leakage of oil in the turbo past it's oil seal.

I've personally never done a leakdown but have changed my plugs, and seem to remember it being a pain in the backside, I think I had to remove the mufflers/wheels and some sheet metal ... would be a more of a pain to do a really hot engine.

 

^True definition of blowbye, by the time you do the test it's cooled down a lot anyway. I've never seen or done a leakdown on a hot engine, sounds painful.

 

Some feedback on the 993 TT in question. The original leakdown was done cold and produced 16-17% across the range. I took a chance and bgt the car as everything else was in order and had a hot/warm leakdown done back in Chicago by Midwest (some peeps who did "drivelikeyoustoleit" leakdown).
Previous posts mentioned you can expect to halve the results...this one did a little better than that with 3% as the result...needless to say at $65k i'm happy with the purchase altho as Kevin can confirm it was a late sleepless night beforehand!....thanks again Kevin for answering my questions at such a late hour.

 

Carrera GT>> First off, it is almost impossible to do a leakdown on these engines HOT. Warm is just a myth. Yes, you can put the car on the lift and start to take off the IC, and under belly pan and remove the passenger side tin work. The CATS and turbochargers are just to hot. Gloves on your hands>don't cut it.. It's not your hands it's your forearms reaching in and around the hot components.

All 993TT engines sold at wrecking yards and salvage yards provide COLD leakdown numbers..

I have engines that have sat for years on the shelf and have repeated 2 to 3% leakdown numbers.

To answer your "blowbye" statement.. I need to do a better job explaining the leakdown process.

When you do a leakdown test you ARE pumping in 100 PSI regulated with some very good precision gauges. I usually will drop my shop air with a very good "Sharpe" (used in body shops around the world) dryer regulator to 110 PSI My shop hose is 50'.. I have two gauges and a precision regulator that will lower the 110PSI to 100PSI.. Prior to doing a test, I have a quick disconnect that will allow me to open up the air line so that I can watch all the air dumped out of the tester. I will watch the regulated 100PSI.

When doing a leakdown, we want to roll the engine over to TDC (top dead center) on each cylinder that is tested. The intake and exhaust valves will be closed AND more importantly the 993TT piston will NOW be located in the combustion chamber. Again, please realize that the piston dome leaves the cylinder and head gasket and protrudes into the cylinder head 1/4"..

We want to diagnosis the intake valves, the exhaust valves, the cylinder head to cylinder gasket, and the piston rings.. Upon screwing in the "whip" into the spark plug hole (spark plug is removed) and connecting the quick disconnect to regulator we will HEAR air filling the combustion chamber, and a slight hissing. Typically we will see 1% to 5% loss on the gauge.

1) If you hear the noise out of the air filter you have a intake valve problem. Valve guide or valve to seat issue..

2) 90% of the time you will hear air coming out the tailpipes.. This will give you a indication of a worn valve guides. OR BENT VALVES.. When you miss a shift with these engines you will usually bend a EXHAUST valve FIRST.. It is very typically to have two bent valves vs ALL Six. A bent valve will usually give us a leakdown test at around 22 to 28% and greater.. So you can have a mild bent valve and a leakdown of 23%...

3) IF you hear noise grumbling in the basement or engine case, and BREATHER assy toward the rear intake, this will point to a failed ringset or broken ring. It can also be scored cylinder liners from something ingested into the engine>> ala a failed compressor wheel, or sand from poor air filter service.

To get to Carrera GT's assumptions> during the combustion cycle we have cylinder pressure at during the compression cycle when the engine is running, AND extreme combustion gas pressure at the point of ignition. These gases will PASS thru a failed ringset or scored cylinder liner. Where do these gases go? And what is the impact??

Since we have a scavenge system with a pump that delivers the turbocharger return oil into the engine crankcase. ANY positive pressure above normal operating conditions like windage and mild blowbye which is GASES escaping past the ringset. WILL directly impede the oil return from the turbochargers.. OIL will backup and begin to seep into the compressor housing and turbine housing. The reason is>> the oil level in the turbochargers is LOWER then the oil level of the crankcase.. Any restrictions due to backpressure, blowbye have a direct relationship to FAILED turbochargers..