Compression difference in a 345?

STEP 13: Make sure the test results are no more than 10% apart. The difference between the highest and lowest cylinder rating should be no more than 15 to 20 PSI. Big pressure differences are a sure sign of engine problems. Note which cylinder has the low reading to uncover the source of the problem. You may also see multiple cylinders with low readings, which could alert you to more serious issues.[12]
For example, a series of low readings could indicate that the valves between cylinders have worn out. It could also be a sign of overall engine failure.
 
STEP 14: Retest cylinders below 100 PSI after adding engine oil to them. Pour about 1 teaspoon (4.9 mL) of fresh engine oil directly into the open cylinder. Then, hook the pressure gauge and hose adapter to it again. Repeat the test by turning over the ignition a few times. When you’re done, check the readout again to see how it has changed.[13]
The PSI typically rises when you do a wet test instead of a dry one. The change can help you determine what is wrong with the cylinder. If the test worked, a large change could mean the piston rings have worn out.
 
STEP 15: Fix the engine if it doesn’t appear to be functioning correctly. The engine is vital to your vehicle, so don’t ignore poor readouts from the compression test. Identifying and fixing engine problems can be very tough to do on your own. If you need help, get the car to a qualified mechanic as soon as possible.[14]
If the low reading is from a single cylinder, check it for worn-out piston rings. If the test results were both the same, the cylinder could have a bad valve.
If you notice 2 nearby pistons with low PSI readings, you most likely have a blown head gasket to replace. The gasket is between the 2 cylinders.
Low compression in all cylinders could mean your engine needs a new timing belt. If that doesn’t work, the engine may need a tune up.
If the engine no longer runs very well, you may be better off replacing it with a new one. Fixing an engine can be expensive, so sometimes getting a new engine from a scrapyard is more cost-effective.
 
TIPS:
Higher PSI readings from the test usually indicate better engine performance, although the ideal reading varies between vehicles. In general, the PSI will be somewhere between 100 and 150 in a well-functioning engine.
Keep parts labeled as you remove them so you know where they belong when you’re ready to put them back. Consider taking pictures with your phone before disconnecting parts.
Removing all of the spark plugs will not harm your vehicle or affect the test. It does give you a chance to check the parts for damage and then test all the engine cylinders.
 
WARNINGS:
Working on a running engine increases the risk of electrical shock. Make sure you detach the ignition coil or its fuse to prevent the spark plugs from carrying a charge.
Hot engines cause burns, so always wear protective gear like heat-resistant gloves and safety glasses.
 
TOOLS YOU WILL NEED:
Compression gauge
Compression gauge adapter
Socket wrench
Extension handle
Spark plug socket
Heat-resistant gloves
Safety glasses
 
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