Primarily the pinging and knocking that are somewhat common on the 93 GTS's. I am going to install Greg Brown's crankcase vent kit and wanted to get a baseline, too.

 

I'm trying to think of an additional way a compression test could be done wrong and nothing is coming to mind.

Full charge on the battery with a good charger hooked up.
Engine warm.
All plugs out.
Throttle wide open.
Same number of chugs for each cylinder (5 to 7 typically).

 

My compression result:

cylinder 1 : 190 195 195
cylinder 2 : 190 190 190
cylinder 3 : 185 185 185
cylinder 4 : 190 190 195
cylinder 5 : 205 195 195
cylinder 6 : 205 200 200
cylinder 7 : 190 190 185
cylinder 8 : 185 185 185

 

Awhile back I tested my 86.5. I couldn’t believe how consistent they were. I was skeptical so I checked them all again the next day and got the same results; here they are:

 

It is normal for there to be some deposits in the combustion chamber, and an engine with high mileage, or bad rings/cylinder walls, or poor fuel/air mixture control may have more deposits.

When you remove a spark plug, you often break loose a few flakes of those deposits. When you spin the engine for the compression test, it is normal for those flakes to blow out the exhaust - but sometimes, a flake may get trapped between the valve and its seat. This will cause a false low compression reading for that cylinder.

The best practice for a compression or leakdown test is to loosen each plug a few turns, then reinstall it. After all plugs have been loosened and retightened, crank the engine and rev it a few times. This not only blows any loose deposits out, and removes any that might have gotten caught in the valves, but it also ensures that the cylinder walls and rings have normal lubrication.

Then remove all plugs and check compression. Remember that the 928 ignition system has extremely high voltage output - be certain to pull a primary (small wire) connector from both ignition coils, or disconnect the ignition amp on the front panel, or do whatever is required to kill the ignition on your model.

Harbor Freight has their leakdown gauge kit on sale for $39.99 during March.

 

Couldn't find it. How do they have it listed / what do they call it?

 

Don't use Seafoam, order Kreen from Kano Labs

 

Kano Labs.

Are you saying Seafoam is bad, or Kreen is better?

 

Don't waste any money on either, bottle of water hooked up to a small vacuum line is all one needs.

 

Sorry, Kano Labs Kreen is really good stuff and no BS it works

 

Something like this?

 

Not sure I will do that ...

 

My GTS compression numbers ranged from 190 - 210.
Engine rebuilt 4 years ago, and then she was run on some super thin oil until I got her. So wear could be that of a 70K motor.

 

Performed compression test again Sunday. Removed all spark plugs, engine temp around 120 degrees. Throttle wide open. Used Mighty vac digital compression tester:

#1 205
#2 211
#3 211
#4 212
#5 189
#6 145
#7 211
#8 211

So, #6 is 30% below the highest value. Stuck valve or worse?

 

A leak down will tell you what's going on.

A simple test to figure out where the compression is going is to pressurise that cylinder at TDC to about 100psi, with throttle held wide open, air filter out, oil fill cap off, and coolant tank cap off, then listen to where the air is going. Intake valve will allow major leak out intake, exhaust valve major leak out exhaust, rings will give huge blowby out the oil filler, head gasket will pressurise cooling system or adjacent cylinder. Test a known good cylinder if in doubt about what you see and hear. The difference in blowby between good rings and one broken or not sealing is massive, and can't be mistaken.