Leak down results after rebuild
 

Hi all,

today I performed a leak down test for the first time and it was on my rebuilt 2.4S engine. New cylinders, pistons, rings, valves etc etc

All are between 31% and 42% leakage. Leakage comes through the crankcase

Now, either I'm doing something wrong when performing the test, or the figures are what they are and either they mean bad news or more break-in is needed (ring seating).

The engine has approx 60 miles on it.

:confused:

Would like to hear your advice!

Does not sound good. Did you measure your cylinders for roundness? Is your ring gap in spec? Another possibility would be piston rings installed upside down.

I did none of those. I cleaned and installed the cylinder/piston set straight after receiving them from Mahle.

Maybe I'm not using the tester correctly? I need to be sure about that first.

I think I need to run a compression test as well, to check if the leakage tester is faulty? Would leakage figures like this show a high compression, I guess not

Maybe I'm not correctly measuring at TDC, but then I would expect to hear air coming from the inlet/exhaust valves.

Hmmmm

The proper way to leak test is with the piston at TDC on the compression stroke (valves closed). You will probably need to stop the engine from turning over as the compressed air will try to push the piston down if it is not perfectly on TDC.

Yep, that's the way I did it

You need to get the rings seated,....:)

There are specific procedures for doing this and after that, re-do the leakdown test to confirm they have seated. If you did everything properly, you should see 2-4% leakdowns.

Allow at least 500 miles before retesting.

OK Steve, so you are saying that these values are not abnormal given engine has only 60 miles on it?

I will go and search for the ring seating procedures for sure!

Maye a bit of context: I'm having the dreaded high-idle issue, so I'm going through CMA. One of the first steps is de leak-down/compression. What would be the right course of action. Continue CMA or perform 'ring seating procedure' first. Can the ring seating procedure be performed if the engine has not gone through CMA?

OMG, yesssssss,... :)

You MUST follow a precise, strict regimen to get the rings seated properly with the engine in the car.

Do that before CMA!!

Are the break-in procedure the ones that are in Waynes 101 Projects book? I'm also finding some info on the Web that promotes hard driving in the first miles....

What I'm not getting is this: how can I push the engine for break-in if it doesn't run 100% smoothly. I'm getting hestitation here and there and some pops/bangs. Driveable, but no star performer yet.

I've not read Wayne's book but I think you can find this info pretty easily.

The engine will not run right until it has proper compression and cylinder pressure so this is the first priority. After the leakdowns are normal again, you can tune the engine for smoothness.

OK, will focus on getting the break-in first

Thanks so far!

Leak down numbers seem pretty high, even at this point. What are the compression numbers?

Sounds like you installed a new mahle p+c set and you said you cleaned them before installing.

Maybe this is a dumb question but did you remove the pistons from the cylinders and reposition the ring gaps staggered correctly?

I ask because I have a new sealed box mahle 3.3 liter p+c set and when I opened it and removed a piston from one of the cylinders the ring gaps were all lined up in a straight line - from mahle.
If they were installed like that there would be plenty of leakdown past the rings.

Also what kind of leakdown tester did you use?

Thanks so much for your help guys. I planned 2012 to be the break-in and CMA year, so I'm in full need of it!

I haven't been able to check the compression yet, as I first need to fabricate some tool to get the compression test plug in. Will do this next weekend

Yes, I've carefully positioned the ring gaps according to Wayne's instructions (book). I also hear people say that the rings travel, but I nevertheless positioned them as needed.

I'm using a Gunson leakdown tester. It gives me 'made in China' feelings:rolleyes: . Maybe I need to upgrade the tool just to be sure.

Anyway, I've been fiddling with the car all day and this is were I stand now:

- per Steve's instructions I'm focusing on the engine break-in instead of trying to achieve perfect CMA results
- nevertheless, my concern is this: I assume one needs some minimum engine performance in order to not damage things during the break-in. I mean for example: there needs to be -some- correlation, right? To help me deal with this 'contradiction' I installed an AFR gauge (bung in both SSI's) to at least monitor the AFR so I can see whether things are somewhat OK
- furthermore, I set the dwell (35 degrees after much fiddling, just in spec) and set the timing again. I have 5 degrees before (vacuum hose attached) and after (vacuum hose disconnedted) TDC at approx 850 rpm.
- Been driving around to get some miles on the car. I've taken note of the AFR and this is the current status:
- I've been doing quite a lot of WOT between 3000 rpm and 4500 rpm, as well as decalleration coming from 5000rpm back to 3000 rpm. The AFR between 3000rpm to 4500rpm at WOT is between 14 and 15 (increasing) and there are no pops or bangs/hestitation
- after 4500 on WOT the AFR goes up quickly, ranging from 16 - 18, which made me stop using WOT and not exceed 5000rpm. The engine obviously doesn't like this and hestitates quite a lot
- below 3000 rpm I need carefully feed the 'power' until I'm past 3000rpm as there is quite to hestitation as well. The AFR is in range 12 - 13
- at idle (warm) (rpm around 1300-1600), the AFR is pretty low, 11-ish, sometimes even 10+. (edit) I get quite some bangs in the exhaust at warm idle (I guess due to the rich mixture)
- when coasting at 3000rpm, the AFR is 12-13
- I've briefly taken off all MFI linkage during warm idle and then the idle drops some 300rpm, so definately something to gain here. I can actually -see- the right bank rise slightly from the stops when connecting the linkage (mind you, the linkage was set up by a pro shop...).
(btw: on my previous drive this season I forgot to install the thermostat heating pipe and the engine performed a lot better, I guess because the mixture was richer over the entire range)
- (edit) I noticed that in corners, the car seems to hestitate more than during a straight line.

So, this engine needs both miles and setting up. My take is that while I continue driving as above, I can perform some individual (sequential) steps from the CMA to get closer to what it needs to be.

Here are some details for those who are interested and to enable the hunt for faults:
- new p&C set Mahle, standard 2.4S
- heads done, new valves, some new valve seats
- throttle bodies cleaned, butterfly axles bushes replaced for new ones
- I think I recall seeing some light around the butterflies, but not sure how serious this was (or if it needs to be interpreted as such). Will need to recheck ones I get to that CMA step
- I took the distributor apart and found that the thin 'skid plate' that sits under the weights was broken into pieces. I replaced this by temp-resistant teflon material (read this somewhere). I'm not sure about the distributor performance, I would not be surprised if some issues are caused by it. I guess this will be covered by some CMA step
- MFI pump resealed with new seal kit, oil feed disconnected and running on own oil (using breather). Timed correctly, but will for sure check this again
- leads, spark plug are new as well as cap and rotor. Spark plugs obviously show signs of used because of the break-in
- I know not all intakes drew an equal amount of air at idle, cause I checked using the synchro-tool. I need to reset this at the correct CMA step
- injectors are old, I have had the spray pattern checked 2 yrs ago (200 miles ago or something, before the engine rebuild). Not perfect but acceptable according to the shop that performed the test
- (edit) I know I once couldn't resist turning a setting on the mfi pump when it was on my bench. Just to feel the clicks. Not sure if I ever put it back in the 'correct' position, but I for sure would like to check the pump settings once I get to that CMA step (yes I know, don't touch it now)
- (edit) the mfi pump rack moves freely. Although it doesn't go back with a 'snap' (not sure if it needs to do that), is returns immediately
- (edit) I have a dual pipe exhaust (Dansk dual outlet, one on both ends)
- (edit) I moved the petrol pump to the front of the car, volume and pressure were OK last year
- (edit) the cut off solenoid on the mfi pump has been removed per advice of tuning shop. I does work however (i tested it)


Any thoughts on how to proceed are highly appreciated!

Did you remove the rings for cleaning, and, if so, are you certain you put them back on facing the right way?

No, I didn't as I didn't feel comfortable (I tried one ring and was concerned about either breaking it or scratching the piston).

I did clean them with break cleaner though (sprayed a lot behind the rings)

Any thoughts on the immediate next steps? Compression test is my guess

Hi all,

I checked the linkage setup today and noticed the following:
- pump rod 1.3mm too short
- no slack on rod coming from gaspedal, so has a slight pull on the central shaft, which pushes forward the other rods coming from central shaft
- left bank rod coming from central shaft pushes butterflies slightly off the stops
- right bank rod the same but some 2 mm off the stops
- left bank rod and right bank rod length difference 3.8mm

So I -think- that the mixture will be impacted by this, I guess it will cause a lean mixture as the pump is 'under'correlated to the throttle bodies. And the right bank will run leaner than left (shouldn't this be especially noticeable during operations of only slight opening of the butterflies, this would explain the bad transition during 1000rpm-3000rpm)?

At least I now have something to focus the improvements on

Hi all, As I haven't recieved much further replies on my post & because I've implicitly changed the subject to MFI setup, would it be better to start a new 'help elflamo with MFI' topic in the General Technical Forum?

Check that your head bolts are still tight. Not uncommon for them to come lose causing air to leak from the head gasket. Discovered mine were all lose after 1,000 miles.

Chris
73 911 E