Leakdown test?

dtw · 09-24-2012, 08:21 AM

Hey guys,
Looking for any tips on how to do a leakdown test on the 928. I can find TDC ignition for cylinder 1 by setting to Z1 on the harmonic dampener, no problem. However, are there are other marks/notches on the dampener or the crank pulley to help locate the TDC positions of the other cylinders? I tried finding TDC on the other cylinders by checking the height of the piston as I rotated the crank - but that's a little too imprecise. Thoughts?

Sorry if this is in the WSM - I scanned through it and didn't see anything.

dtw · 09-30-2012, 12:38 PM

OK, so no takers on this, so I just dove in again this weekend.

After careful examination of the harmonic balancer as well as the crank pulley, there are no other markings for TDC on other cylinders. If I had been thinking of it, I would have marked the balancer while it was off for the timing belt job.

After getting the engine good and hot, I found TDC ignition on cylinder 1 and started working my way through the firing order. I used a long 3/8" extension to monitor each piston as it approached TDC. When I hit TDC, I put the transmission in gear and tested the cylinder while counter-holding the crank pulley so the 100 psi from the tester wouldn't rotate the crank (yes, overkill).

When I did the test last week on the cold engine, I got values from 5-15% leakdown. When I did it again on the hot engine, I got 0-4% leakdown. I also did compression and got 145-165 pounds. 145 was the last cylinder - the engine was cooling off and the battery was getting a little weak. The rest of the cylinders were 155-165.

If you are looking for good testers, I am really happy with the OTC test sets I picked up. They are on Amazon. Apparently they are identical to the Snap-On testers at a fraction of the price. The compression tester even came with spare plungers and o-rings.

MarkRobinson · 10-01-2012, 07:35 AM

You don't need TDC for leakdown.

Run engine until hot.

Remove all spark plug boots from the plugs.

Take a larger vacuum line, blow out all sand from spark plug sockets prior to removing plugs.

Remove all plugs: engine will spin easier so battery will last longer, & faster for a more true compression result.

145-165 are normal numbers for the pre-85 cars. A little higher for '85+ cars. The one low-ball # could be a small chunk of carbon that hasn't left the valve seat yet: wouldn't sweat that one.

Mark

dtw · 10-01-2012, 07:58 AM

Mark - you're thinking compression test. TDC is definitely needed for leakdown testing.

For the compression test, I would agree with everything you said. It is also recommended to completely open the throttle while performing compression check. As I had already disconnected the throttle console and my pedal was useless, I used a vicegrip to chock open the throttle at the WOT position.

MarkRobinson · 10-01-2012, 08:00 AM

Ok, the gauges I used show both: compression + you can test for leakdown. Difference in gauges it appears.

forgot that: WOT.

Yes, WOT is good.

Pete R · Pete R's Garage

TDC only works for 1 piston on a leak down. I would just use a light or long stick to touch the top of the piston (per desired cylinder) and see when it stops coming up. If it leaks down at 100% it's on its exhaust stroke and just go I more revolution

dtw · 10-01-2012, 08:33 AM

Quote:

Originally Posted by Pete R

TDC only works for 1 piston on a leak down. I would just use a light or long stick to touch the top of the piston (per desired cylinder) and see when it stops coming up. If it leaks down at 100% it's on its exhaust stroke and just go I more revolution

Yep, that's exactly what I did using a long 3/8" extension (see 2nd post). I started on #1 TDC ignition and just followed the firing order after that.