Compression test during valve adjustment

MrScott · 09-23-2009, 06:05 AM

Being that I'm in the middle of a valve adjustment the oil is drained. Doesn't seem safe to turn the engine over so many times with only residual oil -- it's been sitting with the cam covers off for about a week.

Is there disagreement on this (safe vs unsafe) or am I being overly cautious?
Maybe I reassemble and swallow that pill if needs to come apart again...

docrodg · 09-23-2009, 06:24 AM

NEVER turn over an engine without oil in it.

BK911 · BK911's Garage

Do a leakdown. Turn the engine by hand.

MrScott · 02:17 PM

Just to be clear, the engine isn't totally dry, it was running before I drained the oil and took off the valve covers. So I've already turned it over a bunch of times adjusting (and re-adjusting) the valves...

A leakdown is beyond my means -- no compressor, no gauges, etc. The motivation for the compression test is I noticed one valve was extremely loose during adjustment. Unfortunately I hadn't read the tech article cautioning against spark plug removal... I'm worried a piece of carbon might be responsible for that particularly loose valve making my adjustment off.

Hendog · Hendog's Garage

Quote:

Originally Posted by BK911

Do a leakdown. Turn the engine by hand.

The engine should be warm when doing a leakdown.

As for the OP; turning the engine by hand should be OK without oil for your compression tests.

MrScott · 09-23-2009, 07:54 AM

Alright thanks. I was going off the the tech article for compression testing using the starter.
Also read the rennlist post in this thread suggesting using the starter (or a short-skirted assistant

) to turn the engine over during a valve adjustment.

Hendog · Hendog's Garage

Actually, you may have to turn the engine with the starter to do a propper compression test; in which case I would WANT oil in there.

I've not done a compression test, so my thought on turning by hand is likely not the right way to get a compression reading, it just won't hurt the engine to turn by hand.

BK911 · BK911's Garage

Quote:

Originally Posted by Hendog

The engine should be warm when doing a leakdown.

I've done dozens of leakdown tests on warm and cold engines. The difference wasn't that much. Plus you're only looking for a disparity across the cylinders so as long as all are tested in the same conditions you'll be fine. Imho of course.

Hendog · Hendog's Garage

Quote:

Originally Posted by BK911

I've done dozens of leakdown tests on warm and cold engines. The difference wasn't that much. Plus you're only looking for a disparity across the cylinders so as long as all are tested in the same conditions you'll be fine. Imho of course.

Your experience supercedes my "knowledge-by-reading-only"

. I see your point on the disparity.

MrScott · 09-23-2009, 10:49 AM

Alright, so the consensus is button it up, compression test w/starter and spring for the extra oil if I have to drain the sump again. Thanks guys, I'd be lost without this place.

911pcars · 09-23-2009, 11:03 AM

Cranking the engine should be fine even with oil drained. However, the engine should be at normal operating temp. before performing a compression test (same with a leakdown test).

Finish adjusting the valves, run the engine, then perform the compression test.

Sherwood

MrScott · 09-23-2009, 01:31 PM

So say if an exhaust valve weren't fully seating, bringing the engine up to operating temperature and then running the compression test wouldn't hurt it? Sorry if I'm being dense, but my lack of experience combined with the cost/downtime of even a top end rebuild has me nervous...

I'm not looking to evaluate the overall health of the engine -- did that a couple of months ago with the PPI (which showed max 6% leakdown cold) -- just trying to verify I've adjusted the valves properly despite removing the spark plugs beforehand and possibly dislodging bits of carbon.

911pcars · 09-23-2009, 01:35 PM

Quote:

Originally Posted by MrScott

So say if an exhaust valve weren't fully seating, bringing the engine up to operating temperature and then running the compression test wouldn't hurt it? Sorry if I'm being dense, but my lack of experience combined with the cost/downtime of even a top end rebuild has me nervous...

I'm not looking to evaluate the overall health of the engine -- did that a couple of months ago with the PPI (which showed max 6% leakdown cold) -- just trying to verify I've adjusted the valves properly despite removing the spark plugs beforehand and possibly dislodging bits of carbon.

Even if the valve weren't fully seated, the engine would have to run for awhile before injuring the valve.

However, if you didn't adjust the valves properly, e.g. too tight, a leakdown test might be a more accurate test to reveal the source of a leak.

Sherwood

MrScott · 09-23-2009, 02:13 PM

Quote:

Originally Posted by 911pcars

Even if the valve weren't fully seated, the engine would have to run for awhile before injuring the valve.

However, if you didn't adjust the valves properly, e.g. too tight, a leakdown test might be a more accurate test to reveal the source of a leak.

Sherwood

Excellent, just what I needed to know. Ideally I'd do a leakdown test but I've got no compressor and no gauges. It was really only one cylinder I was concerned was misadjusted/maladjusted? since it was so out of spec. Leakdown was consistent beforehand, which I'm assuming means compression was consistent so I'm looking for an outlier here. If none, I'm solid, if one (or more!) drive/tow to someone who knows what they're doing.