Low Compression - keep trying or rebuild?

  1. Charge the battery if the battery is not fully charged.
  2. Disable the ignition system.
  3. Disable the fuel injection system if applicable
  4. Remove all of the spark plugs.
  5. Block the throttle plate wide open.
  6. Start with the compression gauge at zero, and crank the engine through 6 compression strokes (6 puffs).
  7. Make the compression check for each cylinder. Record the reading.
  8. If a cylinder has low compression, inject approximately 15 ml (one tablespoon) of engine oil into the combustion chamber through the spark plug hole, recheck the compression and record the reading.
  9. The minimum in any one cylinder should not be less than 70% of the highest cylinder.
Interpreting the readings.
  • Normal: Compression builds up quickly and evenly to the specified compression for each cylinder.
  • Piston Rings Leaking: Compression is low on the first stroke. Compression builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.
  • Valves Leaking: Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you ad oil.
  • If two adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase the compression, the cause may be the head gasket leaking between the cylinders.
The above from > http://gregsengine.com/cylinder-compression-testing.html
 
petehoovie said:
  1. Charge the battery if the battery is not fully charged.
  2. Disable the ignition system.
  3. Disable the fuel injection system if applicable
  4. Remove all of the spark plugs.
  5. Block the throttle plate wide open.
  6. Start with the compression gauge at zero, and crank the engine through 6 compression strokes (6 puffs).
  7. Make the compression check for each cylinder. Record the reading.
  8. If a cylinder has low compression, inject approximately 15 ml (one tablespoon) of engine oil into the combustion chamber through the spark plug hole, recheck the compression and record the reading.
  9. The minimum in any one cylinder should not be less than 70% of the highest cylinder.
Interpreting the readings.
  • Normal: Compression builds up quickly and evenly to the specified compression for each cylinder.
  • Piston Rings Leaking: Compression is low on the first stroke. Compression builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.
  • Valves Leaking: Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you ad oil.
  • If two adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase the compression, the cause may be the head gasket leaking between the cylinders.
The above from > http://gregsengine.com/cylinder-compression-testing.html
Click to expand...
Thanks petehoovie. Had done the first 8 steps. The last 4 bullet points are really informative.

Regards, Jim
 
If it were me, I'd look at it like this:

I could pull the motor and rebuild it.

Or I could get it running and drive it.

Driving it will do one of two things.

It will improve with driving, any smoke or other bad symptoms will clear up and it will run as well as any other running flatty.

Or it will not run well, it smokes bad, runs rough, and smokes like a trooper.

If the latter, you pull the motor and rebuild it. You're no worse off than if you had done that in the first place.

If the former, you pat yourself on the back, compliment yourself on your judgement and revel in the fact that you saved yourself a load of expense and time.

It's a win / no loss situation, you may as well just fire it up and stop worrying.

Mart.
 
Mart said:
If it were me, I'd look at it like this:

I could pull the motor and rebuild it.

Or I could get it running and drive it.

Driving it will do one of two things.

It will improve with driving, any smoke or other bad symptoms will clear up and it will run as well as any other running flatty.

Or it will not run well, it smokes bad, runs rough, and smokes like a trooper.

If the latter, you pull the motor and rebuild it. You're no worse off than if you had done that in the first place.

If the former, you pat yourself on the back, compliment yourself on your judgement and revel in the fact that you saved yourself a load of expense and time.

It's a win / no loss situation, you may as well just fire it up and stop worrying.

Mart.
Click to expand...
Excellent advice!
 
pokey said:
Excellent advice!
Click to expand...

I have to agree. When I got my '40, 40 years ago. It seemed tired. I built a new motor, and before I made the change over, it got better and better. That motor sits on a stand still waiting for the other one to decline.
 
And when the test is done, don't forget to take the brick off the accelerator before pushing the starter button! Don't ask.....
 
Also, if it was mine, I'd pull the oil pan, clean all the crap and sludge out of it (there will surely be some) before you start it. In many cases the old oil will have coagulated and turned into a nice thick layer of goo (depending on the type of oil that was in it). Don't pump all that crap through the engine . . . or worse, ruin the bearings because they're not getting a good supply of oil on initial start up.

Water pumps: You'll need a good set anyway . . . so if it was mine, I'd have 'Skips' water pumps rebuild your originals -- he does an excellent job and you'll be very thankful that you used him. When I had the water pumps out of it, I'd try flushing hot water through the engine (from the upper radiator hose location on the heads). See how much rust and other crap comes out of it.

Ignition: - just take it off, clean the points with a point file, gap them and put a quality capacitor/condenser in it. Don't use an old vintage condenser - most of them have gone bad. Don't worry about losing your timing - the distributor used an offset "tang" so it will index when you put it back on. You may want to check the vacuum brake in it - taking it apart, making sure it is cleaned and operational, etc..

I would crank it over with 5 new quarts of oil in it, with the plugs out of it and do your compression tests. You can even crank it with 12 volts to really turn it over fast (I do this to get the oil pressure up). Before you start it - make sure you have oil pressure.

It may actually run quite well after it warms up a bit and things get a chance to "loosen up and seal". You will probably need a new fuel pump - I'd recommend having CharlieNY (on this site) rebuild your original --- so it has the proper springs and diaphragm materials in it. Most of the "new" pumps have the completely wrong pressure settings. Once you've done the compression tests, started it and ran it through a few heat cycles - then recheck the compression and see what you have.

You may be surprised on how this old girl might run . . . I know I was with the 59AB in my 32 Cab - it had sat in an unheated garage for 55 years - I cleaned the oil pan, put a new oil pump in it, pulled the heads and put new gaskets in it, new water pumps, new ignition and fired it up. It ran pretty well for the next two years that I drove it. (While I built a new engine for the car). One thing others have said - it is a LOT more fun having your truck running and driving, while you find another engine to build for it. Just having something to drive gives one a LOT of motivation to continue on the project! It ALWAYS helps me!
 
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