Compression Problems

[david_taylor_waypoint]

My A 4 has lost a lot of power, the prop is clean, carb has been rebuilt, filter, fuel lines, etc. so I decided to check the compression. 48 hours prier I put 1 oz. of mystery oil in each cylinder. Upon removing each spark plug I noticed #1 &#2 had dark black soot on the plugs, and the exhaust has shone signs of burning oil. 2 days later I installed the plugs and started the engine. She ran, idled fine, but no power under load. After warming the engine I followed the directions on the compression test kit, loosened the plugs one turn, installed the kit on cylinder one which read 70 psi number two 60 psi number 3 105 psi number 4 105 psi. Next I put 1 oz. of oil in each cylinder and retested each cylinder which all showed an increase of 30 psi. My conclusion with the oil burning and low compression in adjacent cylinders would be a head gasket. what do you think?

[Don Moyer]

David,

I wouldn't be too quick to suspect your head gasket. While 60 and 70 psi is
somewhat below the 80 psi we consider minimum normal compression, with the
other two cylinders being good, I might expect you to report a bit of
roughness in powering under load, but not a general power loss. The fact
that the compression came up 30 psi after adding oil (I assume to 90 and 100
psi) is also an indication that the head gasket must be tight.

I don't know what the condition of your ignition system is, but your power
loss could easily be explained by a breakdown in the secondary system
(problems in the primary system would affect all of your cylinders). Poor
combustion resulting from weak spark will quite quickly deposit carbon on
rings and valves (just as on your plugs) causing poor compression as a
result and not a cause of your loss of power. Unless your secondary
ignition components are all relatively new, before replacing the head gasket
I recommend replacing your secondary components starting with the cap and
plugs, and then the rotor and plug wires.

Regards,

Don

[T27_478]

Any resolution to this thread? I'm having a similar problem and am now suspecting that running on 2 cylinders for so long has gummed up the two stuck valves?

[Carl-T705]

From your test, my conclusion would be weak or stuck piston rings. I would ad some SEA FOAM available at auto parts stores to the oil and gas and run this engine with fresh spark plugs in the weak cylinders and then after a couple of hours running retest. You could also pour straight SEA FOAM into cylinders and let it soak overnight before running. With bad valves, the compression rarely increases by adding oil into the bores. Adding oil generally helps to seal rings, increasing compression numbers.
I also have an A4 with two weak cylinders side by side, like 20 and 40 numbers, adding oil did nothing to improve compression. This told me stuck valves or head gasket. I'm going to pull the side cover and look at the valve stems for stuck valves or broken springs. I'll also pull the head to view the valve seats and the bores, but I enjoy tinkering with these little jewels and will most likely lap all the valves. The other 2 cylinders are over 110 on the numbers.

[roadnsky]

Originally Posted by T27_478

Any resolution to this thread? I'm having a similar problem and am now suspecting that running on 2 cylinders for so long has gummed up the two stuck valves?

Note that the original post and Don's response are in 2008.
Also the OP only made ONE post, so he's very unlikely to return...