HELP! The Cert Porche Tech said...

[Nine9six]

Guys please help...
The certified Porsche tech that is performing my PPI claims that a leakdown and compression checks are just a waste of money. He states that if there is leakage past the valves it will be obvious in HC and CO reading from the tailpipe.
Yet some people swear by leakdown #'s, and say to walk if beyond certain percentages.

plymouthcolt
Senior Member

Registered: Jul 2004
Location: Gaithersburg, Maryland
Posts: 108
Fom 1 to 8 or 9 is ok. Stay away from anything 10+.

Basically the lower the better. You also want to have no more than 10% variation between the numbers, if you do, again walk away.

Signed,
More confused than ever
Please HELP!

[ubiquity0]

Find someone who knows what they're doing. From what he's saying you might as well just take the car to the local Shell station smog check for a 'PPI'.

[john walker's workshop]

it's true that the HC would be higher if there was a significant enough problem to leave some fuel unburned. unburned fuel (HC), is caused by many things though. what you are trying to detect with a leakdown is often not a large enough leak to cause high HC, but worn guides could have caused the valve to beat the hell out of the valve seat, resulting in a high static leakdown of 30% or more. without a leakdown you would never know.

[82SC]

hmmm...I'd trust the likes of John and TRE's Dave when it comes to these matters.

I am sure this guys knows lots about cars...but the certified porsche tech title would only make me trust this guy over the guys mentioned above is if I was having problems with my brand new 997 GT3...John and Dave's experience with older cars and the condition of older cars makes their opinion stronger in this matter (in my humble opinion)

[Sniady]

Why would he say that?

either he hates doing leakdowns, but why when he can make some coin. or he does'nt know how to do one.

[obrut]

leakdown also allows you to listen for the leaks cylinder by cylinder - ie. hissing out the muffler = bad exh valve and hissing into the intake manifold = bad inlet valve

[Por_sha911]

This "Certified Tech" is fine if you want a PPI on a 996. He may not have known how to spell Porsche when your car was built. For air cooled, find someone who has been around a while and know about more than just plugging in a computer tester and reading the code.

[sprbxr]

He is also working under the flat rate system. He will probably get paid the same amount of money no matter how long it takes to perform the PPI.

[speeder]

Originally posted by sprbxr
He is also working under the flat rate system. He will probably get paid the same amount of money no matter how long it takes to perform the PPI.

Um, no I don't think so. There is no way that a dealer would have only one price for a PPI no matter what it included. Some might charge for an hour of shop time, (or a 1/2 hour), but when I bought my car the dealer had a set price plus extra for compression/leakdown.

I do agree that a complete leakdown test on an otherwise healthy motor w/ good compression is a big waste of time and $$, (and I've heard shop owners say as much), also the mechanic is right about the HCs being high if you had a bad (leaky) cylinder. He might in fact be an excellent mechanic who actually knows how to read a diagnostic on a car, and knows that a leakdown test would be a waste on the car that you are looking at. Or not. (Keeping an open mind here).

Remember, there is no hiding a weak/bad cylinder, and there is no way possible for a motor to have bad "leakdown" and good compression. You can have a broken stud, (make that 2), w/ good compression, though. Ask me how I know.

The important thing is to have someone good look at the car, people around here, (and elsewhere), sometimes just parrot the same recycled advice and info w/o really knowing what the hey it all means, requiring a complete leakdown test of any prospective used car would be an example of this.

[john walker's workshop]

i would have to disagree on that. i've seem plenty of cars that had decent compression and had bad leakdown on one or more cylinders. it takes very little leakage between a valve and seat for instance, to get a leakdown reading of 30%. that slight leakage would not amount to much during a compression test where the piston is quickly and repetedly pumping air into the gauge. too quickly for much air to escape thru the slight leakage between valve and seat. you might get 10lb less compression in a 30% cylinder, or more or less, but a leakdown test measures the % of air that leaks past whatever will let it pass by, with the piston at the top of the compression stroke, valves closed, nothing moving, so a small leak, which is what most of them are, are much more noticable. it would take a pretty well burned valve, like with a pie notch burned into it, to actually be felt as a miss, or show up dramatically low on a compression test. so what i'm saying is just because an engine seems to run well and sounds good, doesn't necessarily mean that it will pass these two tests.

[Nine9six]

DAMN! Thank you Mr. John Walker!!! Thank you all on Pelican BBS who bring about this type of conversation and fact!!! In essence, a big fat THANK YOU to all who contributed, and stimulated this topic.
This is precisely the type of sound reasoning I was looking for.
With my best regards,
Paul

[speeder]

I will definitely defer to John's opinion since he has a ton more experience w/ these tests, but could a car w/ 30% leakdown still run smoothly and evenly, even at idle? TIA.

[tshih]

Bruce Anderson has stated that due to the flat orientation of the cylinders in a 911 or similar flat config. engines doing a leakdown test with a spoonful of oil inserted into each cylinder may be a bit tricky and the results taken with consideration of possible poor technique rather than poor engine condition. It's just more data to weigh about the inner conditions of the engine without actual tear down. I would trust the PPI if it includes a good leakdown result and get a second opinion if the leakdown is "bad".

[LongDuckDong]

I just got a PPI done a few weeks ago and the mechanic told me he same thing about the smog check. I know this sounds stupid, but I had passed on an older 911 a week earlier because it failed a rather extensive PPI. I REALLY wanted this one to pass, so I didn't have him do the leakdown check and trusted him. They did note a couple of oil leaks, which I was told is pretty common. In hindsight I wish I had the leakdown done, but I am hoping all goes well!

[Wayne 962]

Hogwash. That's the silliest thing I've heard all day. A poorly tuned fuel injection system can cause HC to go off the chart - there's no way to know if the motor itself is good or bad.

Get the real facts, read this first:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-1.htm

For what it's worth, I'm usually happy if the compression check comes back fine.

-Wayne

[imcarthur]

Numbers:

Car #1 Compression & Leakdown results:
cyl 1 140 Leakdown: 0%
cyl 2 140 Leakdown: 0%
cyl 3 110 Leakdown: 55%
cyl 4 120 Leakdown: 50%
cyl 5 125 Leakdown: 10%
cyl 6 145 Leakdown: 0%

Car #2 Compression & Leakdown results:
cyl 1 165 Leakdown: 2%
cyl 2 160 Leakdown: 4%
cyl 3 165 Leakdown: 24%
cyl 4 160 Leakdown: 6%
cyl 5 165 Leakdown: 3%
cyl 6 170 Leakdown: 0%

Car #3 Compression & Leakdown results:
cyl 1 182 Leakdown: 5%
cyl 2 179 Leakdown: 3%
cyl 3 179 Leakdown: 2%
cyl 4 180 Leakdown: 2%
cyl 5 180 Leakdown: 5%
cyl 6 180 Leakdown: 5%

I walked from Car #1 & Car #2. Both wrenches performed the test twice with similar results. Was it just carbon? Could they have been improved with a good high-speed drive? With a long distance PPI, it was not worth the risk.

I bought Car #3.

Ian

[imcarthur]

Oh, all cars above ran great. The wrench for #1 - a respected Houston independent - was very surprised. He said it sounded & drove great. Ditto #2, the wrench - not quite as respected in Pensacola - said the same. I did have one wrench in Long Island who didn't want to do a leakdown/compression. He claimed he could 'tell'. The car had other issues anyway . . .

Ian

[Andras Nagy]

Here's another "real world" experience, for what it's worth:

Prior to rebuilding the engine (lots of oikl consumption, some of oil through the tailpipe, etc), we decided to check the engine on a chassis dyno. Results were 156 HP at the wheels, which translates to about 184 at the flywheel. Stock configuration of my car (per the Owner's Handbook) is 180 HP. So power was not a problem, but the oil consumption and oil burning were.

Upon teardown we discovered: six broken piston rings, three broken piston lands, two leaking cylinder-to-head interfaces, medium "coking" on the top of pistons, worn valve guides, and some other minor stuff. Engine was running "fine" prior to the teardown.

What does this tell us? That you never know what's with the engine until you inspect the internals. Test are "indications", not "proofs" of certain symptoms. Just like the surgeon has to "unbutton" the patient to "see" the cancer, so you have to "unbutton" the engine to see problems.

[CarreraS2]

Originally posted by speeder
I will definitely defer to John's opinion since he has a ton more experience w/ these tests, but could a car w/ 30% leakdown still run smoothly and evenly, even at idle? TIA.

There's a guy trying to sell a Carrera cab that was tested at 70% leakdown on one cylinder, but he says the car runs perfectly. So, maybe?

[speeder]

In Ian's examples, a simple compression test would have eliminated car #1, and wouldn't *zero % leakdown* indicate that the operator didn't know how to perform the test(?)

I just cringe at the thought of people spending their $$ on a long-distance LD test on a car w/ a dead cylinder. (Or two). Any drunken monkey should be able to tell when a motor has a dead hole, he should have told you for free. Or just hooked it up to any ancient exhaust analyser machine and said, "SEE?"

And no, a motor w/ 70% leakdown would not "run fine", unless you consider a 911 running on 5 squirrels to be "fine". With 1 dead squirrel flopping around in the treadmill. It's pretty simple.

And Wayne, we all understand that other causes can give a bad HC reading, the point is the converse: a motor w/ bad leakdown could not give good HC #s. No matter how many cool collars are installed. An expensive LD test could be eliminated on most of these turds by a simple sniffer. And I agree that a compression test is sufficient. I wouldn't even bother w/ that unless it passed the sniffer. (With mixture adjust/etc..).