PPI advice: myths about leakdown/compression test - needed or not?
 

Hi all,

I have received conflicting advice regarding the need for leakdown and compression tests. My understanding from those on this forum, and in a speaking with a couple of shops, is that the leakdown/compression tests are a must.

I was recently advised that if a car is a lower mileage example (<40K miles), in good shape, it's sufficient to gauge the engine's health through the smog results. In other words, the smog reading won't lie, and will tell you if the engine/valves/etc. are working properly.

I was also advised by one shop that if they do a leakdown and compression test, they would need to change the sparkplugs. I'd never heard that before.

Finally, can doing the leakdown/compression test damage -- in any way -- to an otherwise sound engine?

This leaves me a bit uneasy. I don't want to forego a test that people recommend, but if the shop tells me I don't need it (and will charge me $500-600 to do it as described above) - I'm not sure what I should do. I like the fact that the numbers give me a basis of measuring whether the engine is performing at par, and whether the cylinders (?) are all working similarly (i.e. one isn't measuring unusually lower than the others). I don't see how the smog test would tell me this.

Obviously, I want to know what I'm in for.

Advice?

-bg

Not meaning to be facetious, but maybe you should find another shop.

Not true. A smog test wont show a spun bearing, A smog test wont show a broken head stud... There are mechanical items that if are bad wont show up on a smog test.

You have to pull at least one set of plugs, but unless they need to be replace there isn't any reason to change them.

Not really, I guess some could put a long reach end in a short reach hole and have a piston collision, but IIRC they are long reach anyway. In any even, no, not unless you are a complete moron could you damage the motor doing a compression test.

Lastly, you only need the leakdown if the compression test fails. The compression test will tell you if it leaks or not, the leakdown will tell you where it's leaking and give you an idea how bad it is.

First, you should do a search if you haven't. There is a ton of stuff on the board. I decided to do both because the marginal cost was small on a big expenditure. That said, $500+ seems high for these tests. I'd recommend searching or asking for a shop that other R-Listers have used in your area. I've found that experience is worth a lot of $$$.

I disagree with just about everything you've said.

For one, a compression and leak down test won't be telling you about a spun bearing either...but thats easy enough to tell without any tests.

You can damage the motor during compression tests. When the tech puts the plugs back into the head, he strips the threads. You could certainly timesert it, but thats no fun either.

A good compression test doesn't necessarily mean there is no cylinder leakage because they are different tests. One is static and the other is dynamic for one. Hell compression tests can be swayed by a weak battery as well.

If you've got the plugs out, slap the cylinders at TDC and do the leakdown, its only prudent.

I had both done during my PPI last month and it was worth it only because it gave me the piece of mind that I was getting a good, tight motor. Like quadcammer already said, they are different tests, so get them both. However, 600 bucks is ridiculous. All in I paid 385 (that is PPI and the compression/leak down on top of it).

I personally wouldn't bother with a compression/leakdown on a low mileage motor that was running perfectly, had service records, etc. 993 motors are pretty damned bulletproof except for valve guide wear, and you can only get a read on that from looking at oil consumption.

A car that has higher mileage, been tracked, modified, chipped, etc, is another story of course. Then it pays to verify.

Not "only." Oil consumption is your best measure of this, but you would have to rely on the seller for this information if it was available. A good PPI guy can also tell from a manual/visual inspection of the valves (which a good PPI guy would do during a compression/leakdown) whether they are loose and there may be a problem.

OP,
Search this forum....this has been covered a 100 times. You can find a bunch of opinions on this particular topic in a few minutes, rather than waiting for responses to this thread. A quick search for:
ppi leakdown compression
returns 6 pages of results. There's your evening's reading...and then some! :)

Well...to measure valve guide wear, you have to remove the valve covers and disassemble the valve retainers, springs, etc...has nothing to do with a compression/leakdown test. But you're right, it is the best (and only) way to measure guide wear if you don't have any data on oil consumption.

IMHO, on a 993, many of which develop worn guides at very low mileage, PPI money is better spent on measuring valve stem play than doing compression/leakdown.

Let me disagree. A valve with worn guides is A) going to beat up the seats and B) may not sit well in the seat. While nothing is foolproof, a car with leakdown numbers under 5% would generally indicate that the cylinders are sealed up well.

Yes, disassembling the valvetrain and doing the "wiggle test" on the valve stem is a great option if you have the time, money, and a understanding seller. But leakdown and compression give a good understanding without all the cost and invasiveness.

Now then, oil usage is not the only sign of bad valve guides. A car could use little oil and still have bad guides.

Agree with you for the most part -- except that a 911 motor will burn oil due to worn guides long before it gets to the point where the valves beat up the seats. Or put another way, the guides have to be really shot to cause seat damage, so shot that your motor will be drinking oil at an alarming, unmistakable pace.

Happened to me years ago on a 2.7 motor, come to think of it...one of the guides in particular wore so badly that the motor was making a thunking noise at idle. When I tore it down I found that all the guides were worn to hell, but only that one in particular was worn so badly that the valve head was beating up the seat (and making the thunking noise).

I chose to forego the leakdown/compression test on my car because the PO had documentation of the tests within 3 years and the PPI mechanic said it would be a waste of time because everything else was good including the OBD2 readings which would have shown something if there was a problem. Had I not had the previous documentation, I would have gone through with it as part of the PPI.

And I agree that a compression test is not necessary if you have a leakdown

clearly you have no idea what you are talking about static and dynamic are different mathmatical ways of calculating compression ratios, which in no way should be mistaken for a cylinder pressure test. That is what a compression test really is. It wont tell you squat about CR all it tells you is pressure.

hysterical.

however, given that "static" and "dynamic" are words in the english language, they have meanings aside from compression ratios.

In a leakdown test, the piston is static (i.e. not moving) at TDC while the test is conducted.

In a compression test, by cranking the starter, the piston is moving, the valves are opening/closing, etc.

The tests do tend to give corresponding readings, but it makes no sense to do one without the other.