Leakdown test

philfran · 05-05-2003, 04:17 PM

I was at the Porsche Visit 3.0 at the Corvette Museum in Bowling Green this last weekend and had a chance to see Bruce A do a visual PPI on a 80's car.

At the end of the session he said that it was waste to do a leakdown test. A compression test was the only thing needed.

He said that he had seen many a car with poor leakdown that ran strong for an extended time. And he had seen cars that had great leakdowns but when the motor was torn down he found a broken ring in each cylinder.

He talked about leakdowns taking you down a wrong track.

Thoughts?????

BTW the trip was great. Only about 80 cars due to weather, it was cool Saturday but at least it was dry, but that was fine cause it gave me time to spend an hour or so with Bruce.

Another item of note was that out of the 80 or so cars there were four from Martin, Tn (my home). Not a bad turnout from a city of 7,000!

ChrisBennet · ChrisBennet's Garage

That is what I've heard. It's useful for determining where something is leaking, just not for determining the actual health of a motor. A leak down is just done at the top of the pistons travel at pressures far below those of a running motor.
-Chris

RoninLB · 05-05-2003, 04:57 PM

leakdown should be contributing info.. not the only info, IMHO

snowman · 05-05-2003, 05:24 PM

I have done leakdown tests on many engines. I have found that a leakdown test is far more informative than any simple compression test. For example, broken rings. The compression test was normal but leakdown way way bad, ie 75%leakage or more.

Leakdown can pinpoint bad intake or exhaust valves, ie tell you wihch one(s) are bad. It can spot broken rings very very well. It can detect head gasket leaks and deterimine that it IS the head gasket leaking. I kind of think the guy had it backwards. A race car will start showing poor lap times about the same time leak down exceeds about 15%, compression test will be unchanged.

Anyway thats my experience.

philfran · 05-05-2003, 05:24 PM

Thinking back on his statements I come up with the following.

For a PPI do a compression test. If it fails pass on the car. If it passes then there is no need for the leak down.

I guess if you were troubleshooting a poor compression test on your own car you would want a leak down so you would have an idea if a valve or ring was leaking.

How does that sound?

But it does bother me that he said a leakdown was a waste in troublshooting since it could lead you down the wrong path.

I have used them in the past and will probably continue to do so in the future.

HarryD · 05-05-2003, 05:28 PM

I think each test has it's place. The compression test gives a good view of gernal health of an engine. The leakdown helps to isolate specific problems before you teardown the engine.

In my case, the PPI showed lower compression in two cylinders. I asked for a leak down and found the problem to be vlave not ring related. For me, at the price I was paying, this was acceptable as it meant I would have a top end in my future.

YMMV

304065 · 05-05-2003, 05:30 PM

I don't get it: what information would a compression test reveal that a leakdown would not? Or expressed another way, a compression test will tell you if you have bent valves (causing a leak) or broken rings (causing blowby) because the compression will not be as high. A leakdown test will tell you the same information, and as a bonus, will also tell you WHERE the leak is coming from (by listening to airbox, exhaust pipe, or crankcase breather).

How can a compression test be better than a leakdown? If the rings are broken, which would show in a compression test, why wouldn't the leakdown test show the same thing?

A compression test is certainly easier than a leakdown and doesn't require an external air source, which is maybe better for doing the initial look, before you take the car in for a full-blown PPI.

philfran · 05:46 PM

Remember that this is the guru of Porsche that we are all disagreeing with!!!!

I look forward to seeing what JWW and Wayne have to say.

ChrisBennet · ChrisBennet's Garage

Quote:

Originally posted by john_cramer
I don't get it: what information would a compression test reveal that a leakdown would not?

If the leak only occured part way down the cylinder wall, a leakdown wouldn't show it. A leak down only tests the piston in one position for better of worse.
-Chris

Wayne 962 · Wayne 962's Garage

As I said in the new book, no test should be done in absence of other data. The leakdown test is one test - it does not always indicate good or bad engine condition. It should be used in conjunction with a compression test, emissions test, and a good test drive.

-Wayne

snowman · 05-05-2003, 05:56 PM

[QUOTE]Originally posted by philfran
[B]Remember that this is the guru of Porsche that we are all disagreeing with!!!!

Thats true, but did he really hear the same question that the person reporting it heard? He may had but also misinterpted what was asked, he may had to much to drink the night before, whatever. ITs usually just mis communication and if a request for clarification is made there will likely be a reasonable reason and the net response will change. In other words do not get hung up because he is an expert. IF I were there I would have jumped on this and asked a dozen hard quesitons to clarify his position if it were true.

philfran · 06:06 PM

There was a group of us around and I didn't want to act the prick so I didn't get too carried away with questions. In fact there was not even a question asked of him. He brought it up on his own. Keep in mind we are talking about a PPI. I did not question him on a difference between a PPI and a test on a car of your own.

I have always thought that a leakdown will compliment a compression. As close as I remember he was talking about PPI on an engine and he stated that a leakdown was a waste of time. It will take you down a wrong road, at which point he pointed his arms left and right.

He made the statement about a leakdown not picking up on a engine with a broken ring in each hole. He also talked about cars with good compression but poor leakdown that ran for extended periods of time without a problem.

I still like a leakdown since it can show me more specific items than a compression check I will continue to do them both but what he said just got my interest up.

Zeke · 05-05-2003, 06:09 PM

Leak downs are useful on engines that are out of the car and can't be spun by the starter, only turned over by hand. It's not the best circumstances for any test, but it's better than nothing. A total engine leak down is valuable as well, not just a cylinder by cylinder test. This will find any induction leaks. I have never performed this on a 911 engine, so if it won't work here, excuse the thought.

philfran · 05-05-2003, 06:18 PM

I sent Bruce and email, it will be interesting to see his response.

Bruce,

Just a note expressing my thanks for the time you took with us in Bowling Green. It was a very enjoyable time.

I do have one question about your comments on leakdowns.

Do you mean that you have no confidence in them at any time. Or, do you feel that they are a waste of time when doing a PPI. Could you please clarify.

Thanks

Phil Francoeur

snowman · 05-05-2003, 09:52 PM

IN response to Chris,

All the action happens at the top of the compression cycle. all the wear, all the damage. Why would you check anywhere else? And even if you did, the problem becomes insignificant almost instantly as you get away from the top of the cylinder.

rcwaldo · 05-05-2003, 10:03 PM

I have attended a few seminars hosted by Bruce Anderson. When he mentions leakdown testing and how they are (sometimes) misleading, he explains because of the flat arrangement of the engine, the valves are more horizontal than vertical (I dont know the exact angles). The exhaust valves have had tendencies to trap bits of carbon between the valve and the valve seat. This would temporarily disrupt the combustion seal and give erroneous results.

Bruce mentioned seeing many engines give poor leakdown results only to have them turn to great results after Techron treatments and some spirited driving.

I believe thats why he feels this way.

Chris

philfran · 05-06-2003, 01:45 AM

Here is Bruces reply:

Hi Phil,

I have seen leak down tests do more harm than good so all I use them for is
once we know we have a problem is to pressurize the weak cylinders and listen
to where they are leaking... oil tank, by the rings... out the tail pipe, by
the exhaust valve... and in the intake, by the intake valve. And that is all
that I would use them for.

Bruce

RoninLB · 05-06-2003, 10:28 AM

911 leakdown is a little different than a solid block of pistons and cylinders.. not exactly a dangling participle, but close enough. I read hard info about it a while ago, but can't find it now. it is a good topic....Ron

snowman · 05-06-2003, 09:41 PM

Sort of sounds like an interpertation problem, ie the tech is not trained well enough to properly interperet the results, leading to bad diagnosis, leading to un necessary work, leading to excess cost to the factory trying to support the car. The basic compression test is black and white, companies like this. This is in fact quite plausable and a genuine reason for the FACTORY to avoid the leak down test, but a highly qualified independant, and well trained tech should be another story.