Leak down test.. What results? - Page 1 - 911/Carrera GT - PistonHeads UK

Hi, I hope to be looking at a near 85,000 miles 996.2 GT3 in a day or two. I will be having a PPI and a leak down test, but what do I need to be seeing the results at to see if the engine is ok? Thanks,

I think a good engine gives single figure readings. Below 5% is v good.

Variation between cylinders is all you can look for. You want them all to be much the same. Leak downs are a nice way to watch for developing issues but to do so you need to do them regularly and establish a trail of them so you can watch for a change that may give you a warning of impending problems. In the absence of that you need to just look for consistency across the readings. Ive seen brand new Cup engines give double figure leak down results and fresh engines making good power give readings that would suggest they should be toast. Edited by fioran0 on Tuesday 20th May 20:56

Is consistency more relevant to compression testing than leak down?

Thanks. So would you recommend me having one?

Dan911 said: Thanks. So would you recommend me having one? You should not rely upon either or both tests. Neither test will tell you anything really about how the engine runs, or if it even runs at all. Of course, whether you want to have one or both done is up to you. In the past a compression test was preferred over a leak down test. The leak down test was generally used to help ID why an engine failed or delivered subpar results in a compression test. With these engines and their controllers the engine's health is monitored all the time. The DME constantly measures the acceleration imparted to the flywheel by each of the cylinders during a power stroke. If the acceleration is not within spec -- too low (or too high) -- the DME will flag this as a misfire, turn on the CEL and log one or more error codes. Given the above, then, the best you can do as part of your checking out the car is to run the engine, drive the car, experience the engine and the car in its natural state and confirm there are no obvious issues. Briefly, visit the car cold. Turn off the A/C. Turn on the key and verify all warning lights come on, especially the CEL. Then crank and start the engine and let it idle. Be sure all warning lights go off. Listen to the engine idle while cold and while it warms up. Get out of the car. You should have the engine compartment open if you can. (With a Boxster/Cayman this take some work but you want to view the engine.) Pay attention to the engine as you walk around the car and look at body panel finish, fit, gaps, etc. The engine should sound healthy and its idle smooth out and slow down as the engine warms up. Continue to let it idle as you continue to inspect the car. After the engine has idled say 10 minutes, have the seller take you on a test ride. The route should cover 15 miles or so and be varied so the driver can demo the car in a variety of situations. You want to experience the car as you will use it. In stop and go traffic, slow speed cruising in town, higher speed cruising on the highway. Try to get the driver to subject the car to a hard acceleration from near standstill and the engine at near idle to near redline. It is better if the acceleration can continue through at least into another gear so you can experience the engine's behavior under hard acceleration when the throttle is let up and the transmission upshifted and the throttle shoved to the carpet again. The engine should pull strong and smooth and afterwards should return to normal idle with no drama. After the test ride back at the starting point take the car out as a driver. Follow the same route. Drive the car the same way. Have to add you are checking out a very expensive car. Focus. Leave the cell phone off and keep talk to a minimum. You want to focus on the car. After your test drive if you still like the car give it a thorough used car check out. Check everything. After all of this if you still like the car, believe you can buy it for a sum that is agreeable to you and the seller, arrange for a PPI. I have to tell you that were I the seller I'd be very reluctant to have the engine subjected to a compression or leak down test. I do not like to reuse the old plugs so new plugs would have to be used. And I wouldn't let just any old auto shop do the job. I'd want the shop -- the dealer's service department -- where I take my car for its servicing to do the job. However, the techs there would probably try to discourage the shopper from either of these "tests" as they simply do not deliver high confidence results. Anyhow, the PPI looks the car over and with the benefit of the car having been driven around 30 miles and the engine having been run maybe an hour. If there are any leaks they are more likely to be visible after the test ride/drive than before. Then DME overrev counters are read long with the engine run times and total engine run time so you can confirm the engine has not been subjected to any scary number or levels of over revs. The engine run time can be used to sanity check the odometer. Other checks that are appropriate can be carried out. For instance the oil filter housing can be removed and the oil and the filter element checked for any scary metal or other debris. A Porsche diagnostics computer can be used to check the cam timing readings are in spec and not bouncing around. A good PPI'er can fill you in on these and more.

Why do Porsche insist on a leak down test if over revs in certain ranges have been recorded before warranting a car?

Had mine done at its service a couple of weeks ago. MK1 GT3. Curiosity was getting the better of me as the car has now covered 62k miles. My leakdown results were 3-4% across the board. I was very surprised. I expected 10% on the bright side, maybe more. I think of them like this- Upto 5% is excellent and pretty much as new. 10% is perfectly fine. 15% wear you'd expect in a well used engine but should be ok for a while yet if no bad symptoms. 20% and above, you'll need to spend some money in the not too distant future. When I was looking for a 964 c2 to go with my 964T a couple of years ago, one car I had inspected had 28% leakage! I rejected it obviously, then a guy I know from up my way off 911uk bought it. I couldn't believe it. I told him about the motor but he's still driving it around perfectly happy so I guess it depends on how fussy you are about everything being tiptop. One more thing, the leakage in itself isn't the problem if it's across the board. It's if one bank or one cylinder is way above the other/others, then something is probably amiss. ETA - I answered before reading the whole thread and see some of the above stuff has been mentioned.

Alpinestars said: Why do Porsche insist on a leak down test if over revs in certain ranges have been recorded before warranting a car? It's because an over-rev can cause internal damage depending on the scale of it. If you clip a valve etc. I still don't understand it either really though. As in most cases you'd have other symptoms like noise, smoke, rough running. Nice little cash cow though and they can demand what they want come warranty renewal. Glad I've never had OPC warranty really.

marky911 said: It's because an over-rev can cause internal damage depending on the scale of it. If you clip a valve etc. I still don't understand it either really though. As in most cases you'd have other symptoms like noise, smoke, rough running. Nice little cash cow though and they can demand what they want come warranty renewal. Glad I've never had OPC warranty really. Yes I would have thought it fairly obvious if the car's been run sufficiently long after any over revs. Nice little earner for Porsche I guess.