When you get a PPI done...
 

Hi all,
Just curious when you get a PPI done do you always get a leak down test done as well? I can understand with a P car it is more critical as a replacement engine is lots of $$$ but would you do the same for a Miata? Also is a leak down test the same as a compression test? I am guessing for the extra few $$ it is worth it?

Cheers, Guy

Not the same. Depends mileage and condition. For a Miata, if it has a history of potential neglect. For one that I would want to hold onto for awhile I would do a compression, and if anything came back abnormal. I would do a leak down.

Compression test is just as it sounds. Hook up a pressure gauge to the spark plug hole, and crank it over. Highest reading is the compression for said cylinder. All cylinders within 10% is normal. basically looking for all cylinders to be in the same range with no outlier. Basically checks things like Head gasket, burnt valves, condition of the rings.

Leak down is a little different. You add compressed air into the cylinder and measure how much leaks out. You can also get an idea of what's wrong with an engine through the leak down. Ie. bubbles in the coolant, or hissing coming from intake/exhaust.

Great response thanks.:)

Be sure that you expect an actual document / list where items are checked by the technician to include a print out of any fault codes. Recently, when having a PPI done at a BMW dealer out of state on a 528XI that was represented by a small independent used dealer - They would not send me an actual printed document!

The service writer claimed since I did not own the car, they could not give out any negative information. I replied that I am purchasing the vehicle predicated on the PPI's outcome. After some back and forth............I said I own the CC that is paying for this. If I do not receive what I have asked for, I'll contact my CC company for a charge back..............


Ultimately the issue was resolved, but be aware PPI's differ from each shop.......

You know what would be great to have? A PPI check list with all crucial items that should be checked as a matter of course in inspecting a car . We should all pool our experiences together and make a comprehensive PPI checklist . For Porsche and non Porsche cars .
I look over stuff all the time for potential buyers, and what I do varies from car to car. I would not even bother to check compression or leakdown on a chevrolet pick up, or a toyota camry that I was looking over for someone, I would be looking at all the important stuff that I commonly see fail, and potential upcoming need items.

In every shop that I have worked in, they had an existing document (their version) already at hand..................such as "Our 150 point inspection = $120.00" or something like that. I'm sure that there are shops that may not have anything in place - I would avoid those....

There's a pdf floating around somewhere on the rennslist for a 928 PPI. Very detailed, with plenty of space to write comments.

You should.... Case in point. I'm selling my 68 912. The car runs and looks flawless. PPi said as much, compression perfect, but the buyer insisted on a leakdown. We found great numbers (2%!) except...one cylinder, #4, was terrible.

Turns out the valve was not seating properly (installation error on a rebuild) and it had 80% leakdown on the exhaust valve. I promise you that you could not tell by driving the car, or oil consumption, or anything... It ran superbly and way stronger than my other 912! Maybe it sealed better warm, no idea... Even my mechanic tried the test a few times because he could not believe it based on how it ran and the other #s... We've repaired it (not bad, one valve) and it's all good. Still from now on I might do a leakdown on a PPI, even if something the results are inconclusive..

What? Passed compression and failed leakdown miserably?

Do you have any pictures if that exhaust valve? That's crazy

On a pcar or something special I always get a leak down. On utility cars, compression is fine if the engine isn't too $$$$

I do but they're not very crisp and revealing (just a quick pic I took in passing for the buyer), in person you can tell the valve was not seating totally concentrically and there's a little swarf on the outside of the head from the valve rubbing... caught it in time according to the mechanic, no damage. I'll ask for details later of how that's technically possible - I'm not an engine builder, I draw the line at adjusting valves - at the moment I don't really care (other than to recommend a leakdown to the OP) because this is an odd deal :

The guy wants my pretty 912 bad and wants to drive it home down the coast so it needed to have this issue addressed first, and also is financing a small portion so he had to have it professionally inspected in running condition to get his loan $, so he pushed for an immediate repair. So much so that he is paying for the repair out of his own pocket BEFORE he's even officially bought my car. We just got a tentative deal in writing (!!) That's a new one to me! Phoned his CC to my mechanic and said to go ahead and fix it (I made sure of that)! It came out cheaper than expected but that's ballsy, that or my character inspires tremendous trust ;-)

If the deal doesn't go through for any reason, he fixed my car for me ! Of course I would pay him back (minus a small portion for forcing my hand into getting it fixed now, I was not ready to proceed) but he's put himself out there...

But yes, the car had great compression on all 4, 2% leakdown on all 3,and 80% leakdown on #4. And I promise you under you enjoy listening to hissing sounds with a stethoscope, you'd never have guessed by driving the thing. I'll getcha a better answer when I talk to my mechanic again !

I don't understand how you could have such conflicting information from the same engine.

If you have 80% leakage, how can you have high compression?

Either they made an error, or there was deposit on the valve was temporary?

Operating cylinder pressure is about 10 times what you get from shop air- helps seat that valve

That helps seat valves when you are driving, that might explain how the drives well despite a poor leakdown result.

But a compression test is only 150 psi, and a leak down should be about 100 psi, so the disparity in the results is unlikely. You are being told the cylinder seals well enough to create 150 psi, yet the leakage is huge.

The only explanation I can see is that his air supply was limited on the leakdown.