Compression test and general questions

[mpahl]

So I'm starting an A4 overhaul. The answers to my questions might be in the Moyer book, but I don't have it yet, just ordered it last week, so bear with me...

Backstory on the engine: Engine is out of a 79 Oday 27. It has not been well kept, but has been upgraded some- it has an indigo electronic ignition and some kind of aftermarket alternator, so it isn't like it's been ignored, but it's caked with rust... I pulled the engine b/c the head has a leak, more on that in the questions. Since I'm gonna be pulling the head off, I figure I'll clean the whole thing up and do whatever else I ought to do...

So. I started by doing a compression test to find out whether I should be dealing with pistons and rings or whether the head is gonna be my only worry. I did the compression test cold (b/c I can't start it up right now) with all plugs removed and the throttle held open. Results seem fine for cylinders 2,3, and 4 - all about 100psi. The first time I tested cyl 1, I got 85 psi, but after testing the others, I thought maybe I didn't get the compression tester threaded all the way down to the block and I ought to give it another go, but this time I got 65psi.

Questions:
1) Why would my cyl 1 have such lousy compression compared to the others? Anything I can do with the engine cold to be sure it wasn't a test error? Why the change in compression over the course of 5 minutes?

2) When the engine was running, there was water leaking from the head where the red circle is in the picture. I figured it was corrosion and that if I took a wire wheel to it I'd see pinholes, but I found none. Any chance the problem is with the plug on the top of the head right there? Water was coming out in a slow dribble- more than a drip, but not a stream. Any solution to this problem besides a new head?

Pictures:

[lat 64]

I'll just address the variables in compression testing.

(1) Voltage drop from the battery can lower crannking rpm. Cranking rpm can affect readings: if the battery was drained near the end of the test, the readings would be lower.
If the jumper cables got loose or a bit dirty from arcing at the connection, you could see less juice to the starter too.

(2) Temperature can affect compression too.

(3) If there was any fluid(ie. water) in the cylinder at the first test it might read higher at first and then the water gets blown out and the effective volume is greater in the cylinder and so the compression is lower at the second test.

(4) Oil sitting in the cylinder would make a better seal for the rings for the first test and then again like the water, get cleared out and the rings might not seal as well on the second test.

(5) A valve may be starting to stick and give variable readings.

(6) If the throttle plate is not held open and is just flopping from open to closed, it can affect the compression enough to give different readings.

So you see there are lots of variables that affect compression test.I think it's good you got compression at all. Great to get a baseline, but I gather you are going to take this one apart anyway.

Happy wrenching,
Russ

[Dromo]

secound comp. test

Hi.

What I would do is take 2 sets of compression tests. Make a note of the first test reading .Then before you do the second test, I put a little oil into the cylinders via the spark plug hole and do another test. This should bring your numbers up by sealing any loss of compression from worn rings. Now you can compare the numbers to determine your next course of action.

Good luck Rick

[lat 64]

This was poorly worded:

(1) Voltage drop from the battery can lower crannking rpm. Cranking rpm can affect readings: if the battery was drained near the end of the test, the readings would be lower.
If the jumper cables got loose or a bit dirty from arcing at the connection, you could see less juice to the starter too.

I'll amend it.

(1) If the voltage from the battery is low, it can lower crannking rpm. Low cranking rpm can give low compression readings: if the battery was drained from cranking a lot, the readings would be lower near the end of the test.
If the jumper cables got loose or a bit dirty from arcing at the connection, you could see less juice to the starter and that would also slow down the cranking rpm.

Russ

[Dromo]

Just a thought I just noticed in your pic. you had removed the thremostat studs .this might affect the comprssion in #1 as the studs are used to torque that area of the head down.
Rick

[Laker]

Dromo , Good eye.

[Al Schober]

As to your leak, there is a plug in the top surface of the head in that area. Some folks call them 'freeze' plugs, as they tend to pop out when the coolant freezes. They're actually for getting the core sand out of the casting. Best bet is to remove the current plug and replace it. Hopefully, the water isn't coming through a porosity or crack in the block!

Al