Healeyguy

Yes, I know that this topic has been hashed before. But, here's the thing.

A PPI gets done on a low mileage 993tt. Compression test of 145 accross all cylinders. Leakdown of 9-10% accross all cylinders. There's a huge variety of opinions on the leakdown. Most stating that t 10%, while not terrible, warrants extra investigation and that a 5% or less being good and >15% being bad.

what would you do after receiving these numbers?

Paul

Quadcammer

Paul,
first question is where was the leakage? exhaust, crank case or intake (intake is rare).

Most likely you have some exhaust valve guide wear.

If the car runs well and oil consumption is low, I'd try to negotiate a lower price.

10% leakdown is at the stage where you could likely drive it for another 20k miles without a problem, but its still a sign that there definitely is some wear.

Felix

At roughly 60K the leakdown results on my engine were 14%, 10%, 26%, 8%, 12% and 12%. A teardown revealed worn exhaust valve guides with everything else within spec. The 26% figure came from a piece of swarf caught between the head and barrel, presumably factory installed.

So, to answer your question, IMHO budget for a rebuild in the next 10-20K miles.

LexVan

Assuiming the PPI is healthy, otherwise, and the leakdown was done properly. Negotiate hard, buy the car for a competitve price. Have a liquid slush fund of about $12K for a future top end. Enjoy the ride. Monitor and log your oil comsumption. When you need to do the work, the money is already set aside. No worries.

We'll all be doing top ends some day. It's just a matter of when.

Kevin

Be careful. At 12% your turbochargers will start to show oil migration from the higher crankcase base pressure.

Valve guide wear is the likely culprit. Please note that incomplete combustion increase the heat at the exhaust port and exhaust valve. You risk cracking the ceramic liners in your exhaust ports. The ceramic sheds in small pieces and makes contact with the exhaust valve and exhaust seat. These small ceramic pieces also impact the turbocharger turbine wheels (hot side) and damage them. I have seen three engines in 2012 that have needed new cylinder heads because of this. You can't repair the head, you can only buy a new one at $2300.00

With that said, the 10% leakdown could cost you $20K plus in a few more miles..

Please note, that if I was looking to purchase "any" 993TT, I would remove both heat exchangers to inspect the exhaust port ceramic and valve guides. One could take pictures of the ports and keep them as a sales record.

Mike J

What have people seen as variation on technique and instruments? When I do a leakdown, I do it a few different ways with the device, I calibrate it to "zero", ie. holding pressure not hooked up at a few different places in the gauge and then test, usually three times to make sure I am not reading weird. In just about all cases, I get good low readings that are barely within the accuracy of the gauge, but that means the leakdown is very low. i also cycle the engine over a few times and retest at the next tdc.

I am working with Paul on this, the car is very low miles (<14,000), and the only thing in the PPI that is weird is the leakdown. The interesting part is that the leakdown is very consistent, so I thought it was either the same bad technique/bad calibrated instrument on each piston, or there is a problem. No indication from the mechanic on where any leaking sounds are coming from.

There are no records of oil consumption to go by, as you can tell the car was barely driven.

I like the idea of dropping the heat exchanges to check the ports, but I suspect that most people will not allow that in a PPI, given the risk of breaking a stud. If there is lots of crud/black stuff at the base, then yeah we have valve wear.

Kevin, you have scary comments!

Cheers,

Mike

Kevin

The leakdown gauge instrument thing is cr@p. Folks in denial search for any reading material to disprove this method of testing. Carbon in the valve seat, engines that aren't hot, engine runs fine, the sellers tech did a leakdown and it was OKAY (but I never seen the numbers).. We do NOT care about a HOT engine to test the rings. The rings are usually OKAY. The wear is at the valve guide/valve stem OR head to cylinder. Look at Felix 26% number. You either have calibrated gauges or you don't. Everything is based off 100PSI. The first gauge is set and regulated at 100PSI. The engine is rolled over on TDC with the cylinder ready to fire. The nice thing about the 993TT is the piston is above the head to cylinder sealing ring. The combustion chamber is in the head. So the valve wear at leakdown REALLY points to the exhaust valve 99% of the time. As for the carbon in valve to valve seat messing up the test>> roll the engine over with 100 PSI connected.. The exhaust valve will open quickly and carbon WILL be displaced. You can even convince yourself to improve the numbers by spraying in WD40 into the spark plug hole. The numbers will usually improve by 2%. When you roll the engine over TDC the liquid will clean the possible carbon that is in the valve seat/margin.

If the leakdown numbers are consistant (10 PLUS) then he has consistant VALVE GUIDE wear.. At 6% leak down you have alot of air coming out of the turbocharger. At 10% leakdown you can see the turbine wheels turn! At 12% you will have turbocharger passing oil, passing oil, and smoking at startup. This is years of statistics. Customers, shop owners, Porsche dealers.

Disclaimer, if you have only ONE hole at 12% or 14% leakdown, and the OTHER 5 holes at 4 to 7 % RUN the ENGINE. You have thousands of miles of life. Every three years or when you change your plugs, check that bad hole. If it moves towards 20% you might want to think about doing at least one bank "overhaul"

Pull a heat exchanger on ONE bank, spray a little dish soap on the exhaust guide/valve seat. It will foam up instantly. It will look like a foam generator.

Mike J

Thanks Kevin! Pretty funny in that I know that the tech in this case says is the values are fine and consistent with their dealership - holy crap!

I did a PPI on a Turbo a few months ago, all cylinders were 2% except for one - at 25%! You could hear the howl through the exhaust. I was doing this for a friend, so he phoned the owner and gave him the bad news. Well, in this case, the car had 18,000 miles, had a problem with a rod bearing so was totally rebuilt and I was testing the car 50 miles after the rebuild. Never mind that the tech did not check the valves or replace the guides while the engine was totally apart, he actually told the owner 25% was in acceptable range and tried to use the "carbon on the seat" arguement!!

Ok, sounds like consensus (and I concur) is to either walk, or negotiate a price appropriate for likely mechanical work.

Thanks Kevin, interesting test with the spray solution to look for leaks at the bottom of the valve seat - on a brand new head, I guess there should be no bubbles at all then?

Cheers,

Mike

Healeyguy

I should have stated eariler that the car in question is very low mileage with only 14k. So, I'm a bit stumped as to how the leakdowon could be 10%

Healeyguy

One thing I'm struggling with is why the leakdown would be 10% across all cylinders? Hard to imagine all guides failing on a low mileage car

Mike J

I thought we were talking about the car with less than 14,000 miles and a leak down of 10% across all pistons?

Healeyguy

We are...my mistake on the mileage...It should have read 14k

Kevin

Healey you have a hole at 19% leakdown and 5 cylinders at 10%?

People who have had 0W and 5W oil fills will get you the accelerated guide wear on a aircooled turbo engine. Do some research about the soft valve guides that Porsche has used in the past.

LexVan

And why you recommend Mobil V-Twin 20W50, and smart Turbo owners follow.

Healeyguy

Hi Kevin, Not sure I understand your question or comment. The car has 145lbs compression across all cylinders and 9 or 10% leak down. The car has 14k miles. what do you mean OW and 5W oil fills? Thanks