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This is a question for those more experienced with leakdown and compression tests. (Hard to find those that are less experienced than me...)
Is it possible that if cylinder #6 misfires, the unburned fuel washes the oil off the bore walls and rings? If this is possible, is it also possible that the leakdown test reads higher for the #6 because there's no oil on the bore walls and in the ring gap? That is, even if the #6 bore and rings have the same amount of wear as other cylinders?
Thinking along those lines, is it possible that if cylinder #6 persistently misfires while the car is being run a lot of miles, the fuel washes oil off the bore walls and this causes the rings and bore of #6 to wear prematurely? Then the larger (worn) bore will increase the ring gap and increase the leakdown reading?
Those are all good questions, I don't have the answers.
Originally Posted by ptuomov
Is there a way to figure out whether the leakage goes thru the valves (because of a damaged valve/seat) or thru the ring gap? How do you diagnose a leaking valve? Does this show up obviously in the compression test? If so, would the correct conclusion here be that it's not a leaking intake or exhaust valve in this case?
Yes, you can hear the leakage either through the crankcase (in this case, see posts #22 & 23 above) or from the intake or exhaust in the case of leaking valves.
Swap the injector locations and see if the problem follows the injector. As said above, swap the distributor and rotor too with the other side. Swap whatever parts you can to see if the problem follows it. Then if still #6 is the issue, the problem is mechanical.
Based on the leakdown I would jump the fuel pump relay and look down #6 to see if the injector is leaking. If it's not leaking look for oil and scoring on the cylinder walls.
I do not issue flow charts with my injectors - never been asked for them.
Quick injector test would be swap it and see if the problem goes with it.
A simple node test will tell if the injector is firing - or use a mechanics stethoscope/screwdriver.
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Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission? George Layton March 2014
928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
Got the car back together and Jason picked it up last night.
1) Checked the #6 fuel injector wiring with the noid light and it was fine
2) Swapped the #6 injector with another.
3) Swapped distr caps and rotors from drivers to pass side
4) Installed new o2 sensor
5) Installed wideband A/F ratio monitor
6) Installed stock (hotter) spark plugs.
He is going to drive it around and see if anything changes. He is also going to log some A/F ratios and post them here to get opinions.
You might consider pulling the LH out and sending it Rich at EFI and ask him to bench test it.
He is the best in the biz at this. I think the test is $300. At the end of the day you can eliminate this off of your list
As soon as he gets a baseline on what the A/F is showing, I plan to swap the LH from my S4 over to his to see if there are differences. I also want to do it with a good MAF.
Yes, washed-down and dry walls will cause more leakage. You can squirt a little oil into the low cylinder, put the plug in loosely, spin the engine for a few seconds, and retest. Improvement shows dry or damaged cylinder walls.
so after switching the injector from no.6 and flipping the rotors/caps, no.6 plug still looks like there is an issue. These plugs are bout a week old and are the OEM temp plugs. As you can see no.6 looks black, and the other looks clean & bright.
the car is batch fired, so look at the injector... swap that out or do the compression test to see if you have an issue vs the cylnder with the clean plug
Mark, I already swapped the injector back on post 37 and a few other things. Post 22 had the compression numbers and leak down. Compression did not show anything, but the leakdown on that cylinder was 13% versus 3-4% on the others.
07-05-2016, 11:18 PM
George Layton March 2014
