MrLexse

Well, I'm finally able to resume work after being displaced from my garage many, many months ago. My mother in law moved in and claimed she needed a bigger bedroom. The only way to expand was into my one car GARAGE. I was left with the GAR and she took the AGE. I was also left with boxes of parts and three orphans. I've been juggling them around for many months while trying to set up a new place to work. It's not finished yet, but I can't let the cars go any longer, so it's back to work.
I've got MY 87 S4 auto (which hasn't been run in 18 months) down to the short block as of this weekend. All the cylinders I pressure tested would not hold pressure. This seemed strange to me because the car was running before I disassembled it. Anyway, I pulled both heads (just in case). After pulling the heads, and not seeing anything obviously wrong, I read the instruction sheet which came with the pressure tester. It said to warm the car up to operating temperature before pressure testing (see avatar). Could testing the engine at room temperature have caused such rapid pressure losses?. I was watching the cams and there was no observable valve lift in the cylinders being tested. Any advice would be appreciated.

SharkSkin

Did you have each cylinder at TDC, firing stroke when you did this? You have to move a V8 engine's crank to 8 different positions to perform a proper leak-down check.

MrLexse

I had the covers off and was watching the lobe position at each tested cylinder. Lobes were pointing away from the lifters , so I'm pretty sure that wasn't the cause. Thanks

heinrich

DAMN Lex I had thought you were now a Cadilac driver as I hadn't seen hyde nor hair from you in what -- 2 years?? Welcome back Dude

WICruiser

Not sure what the "rapid" pressure loss was but clyinders do not hold static pressure due to leakage past the piston rings. If you used a leak down tester that provided a leakage rate or a percent leakage those numbers would be helpful.

MrLexse

Thanks Heinrich. Feels like I've just awoken from a long sleep.

I did use a leak down tester but the pressure loss was so rapid, it seemed instantaneous. I tried a squirt or two of oil in one of the cylinders, and it slowed the rate of leakage considerably, but it still wasn't holding at all. I have a set of newly rebuilt heads which I can bolt on and then re-test to determine how rings are holding. I was curious if anyone knew what effect the cold test would have on leakage rate.

heinrich

Lex ... I'm sorry you missed SITM. Let's see about Sharktoberfest? Or zip out here and let's drive Godzilla together

WICruiser

Cold vs. hot should not make a hugh difference. The oil addition is a strong indication that the leakage you were seeing was past the rings. If the cylinder bores look OK I would recommend bolting on the heads and recheck. Make sure you have enough compressor for the leakdown tester to work properly. Basically you are not expecting the cylinder to hold pressure, you are trying to measure the leakage rate and without enough air supply the air will leak out faster than you are putting it in and the pressure will drop as you are reporting.

MrLexse

Compressor in my shop is a 25 hp twin screw rotary, which can pump the equivilent of a 40 hp reciprocal. It's the most overdone thing in the shop, so it can handle any leakage rate. If I bolt on a new set of heads with new head gaskets, can the head gaskets be re-used? I'm trying to get the heads and rings tested and repaired/replaced in preperation for a twin screw install at 928 Specialists.

WICruiser

You should not reuse the head gaskets once installed but for a test you could use the old ones if you still have them, you are not sealing combustion pressures and yo do not have the heat, etc. associated with running engine.

I gather the cylinder bores look OK, can you check them for out of round and size?

Leak down testing is not a great way to check piston ring sealing because the ring dynamics are so different on a stationary leak down test compared to a running engine.

When the engine was running previously was there any oil control issue or lack of power that would lead you to believe the rings might be bad or are you just trying to make sure the engine is sound prior to the twin screw install?

MrLexse

Before disassembly, the car ran okay. The kick down wasn't working, so all driving was from second gear. If I remember correctly, old leak down test showed six cylinders within range, one mildly above range and one cylinder greatly above range. I don't remember what the rate of leakage was. At this point, I'm more concerned about building a solid foundation for the supercharger installation than I am worried about any existing problem. I will bolt down the new heads using the old gaskets and re-test. I would rather go through whatever I have to now, than have to do it after something doesn't hold under boost.

MrLexse

Heinrich,
Thanks again for the invite. I'm working on zipping to the curb right now, but with a little help from my friends, who knows.

WallyP

One possible cause for your problem is oil drainage off the rings. Depending upon the oil that you are using, and how long it has been since the engine was run, the rings may be almost totally dry. This will cause serious leakage, as the oil helps a great deal in sealing the rings to the ring gaps.

This can give a very misleading impression of wear and condition.

I had a friend whose old Ford V-8 would run fine if his wife drove it at least once a week. If she let it sit for three or four weeks, the compression would be so low tha the engine wouldn't start. A couple of squirts of oil into each plug hole would increase the compression enough that the car was usable again.

MrLexse

Thanks Wally. I appreciate your input.