Low Compression across the board - Manifold Vacuum Test?

Silverback2w2 · Post by **Silverback2w2** » Mon Apr 07, 2014 11:44 am

Quick question before I go buy leakdown tester.

How accurate is the Manifold Vacuum Test on Pg 103 of 9-8014?

My compression stinks. 1 = 80, 2 = 90, 3 = 70, 4 = 75.

THe book says to re-torque the head bolts and try again, which I'll do then re-test.

Hopefully the compression comes up.

All valves have been properly adjusted.

Here's whats going on. Engine sounds like it's missing and is putting out real rich smelling exhaust with a little white smoke. Timing light says it's right on. Engine fires right up but runs rough. Fuel pressure is good. All vacuum lines are present.

1955 M38A1 - Clock says 58K miles, but who knows what it really is.

BullRun · Post by **BullRun** » Mon Apr 07, 2014 12:12 pm

From my own experience and not saying it applies here... these engines will continue to run even though they are technically worn out.

The upside to this is if you decide to rebuild this engine there are fewer questions to ask about its condition and they are fairly simple and cheap to rebuild.

The white smoke sounds like some kind of water leak... Is the engine oil cloudy? That would indicate water getting into the oil. And conversely is there oil in the water?

Silverback2w2 · Post by **Silverback2w2** » Mon Apr 07, 2014 7:26 pm

Did the oil in the spark plug hole trick.

Got 1=95, 2=110, 3=80, 4=100

artificer · Post by **artificer** » Mon Apr 07, 2014 10:38 pm

Has your engine been dusted [run with no air cleaner]? This is indicated by the compression wet test....provided the initial test was done correctly.
Vacuum gauge readings are a very good indicator of engine condition.
See here what rings going out is indicated by http://automotivecars-info.blogspot.com ... -test.html

BullRun · Post by **BullRun** » Mon Apr 07, 2014 10:55 pm

The M38A1 that I have that is in similar sounding condition also blows oil. If you put a piece of white paper near the exhaust pipe droplets of very fine black oil will splatter over it. My jeep engine is probably clogged with carbon build up too. Just something to check for as it is hard to tell. Not saying your engine is worn out. This is just my experience when there is low compression on one of these engines.

On my jeep it will run but has no power anymore for hills and runs erratically so I ended up parking it to not damage the engine.

On my engine the valves definitely need redone and probably the rings. The crank seems to be OK. However I hope to do a full rebuild eventually. A local machine shop will do the machining for about $650 to $850 and with all the other parts it totals about $1750 is the quote I got. All I have to do is assemble it. Turn around time is about 4 to 6 weeks.

As Wes has pointed out if the engine is painted red or blue it has been rebuilt. There might be a rebuild tag somewhere on the block that will list the current piston sizes and or how much the crank has been ground down.

A tip my machinist gave me is that the more thermal cycling that has occurred the smoother the engine will run when rebuilt as the metal is more stabile. So old engines do have a few advantages.

Xamon · Post by **Xamon** » Tue Apr 08, 2014 6:53 am

You did set it to 10BTC and not the 5 of the manual, Wes has said many times if I am remembering right that it runs better on modern gas.