does a compression test have to be done if a leak down test has already been preformed?? Do i really need to do both?

 

I would think that if all you readings are OK in all cylinders you wouldn't have to bother. If one is below normal then you need to do a leak down test to see where you are losing compression. Valves or rings are leaking and the test will tell you which area is the culprit. My .02

My bad I re-read your post!! If the leak down test is OK then I wouldn't bother with the compression test other than just for your information and piece of mind.

 

I did a cranking compression test then a leakdown......The leakdown showed far worse than the compression so I would just stick with the leakdown

 

thanks guys.. I'm having a local speed shop that specialize in ls1/ls6 cars, check a z06 out.. I'm looking at buying a 2002 zo6.. they will check it out with a fine tooth comb, the owner said he would do a leak down test.. so i wonder if he's going to do a compression test or not.. or what is standard procedure.. I'll be sure to clarify everything with them tommarrow.. thanks for the input

 

As an aircraft mechanic and aircraft engine overhauler with over 30 years in the business, I've done countless leak down (differential compression) tests. We only on occasion do a cranking compression test due to safety concerns. Nothing like a 100 lb. 74 in. prop spinning a few inches from your head to scare you. Care must be taken when analyzing results from a leak down test. Temperature of the engine is very critical during a leak down test. A few degrees difference can make a big difference in the test results. Quite often, we will reinstall the spark plugs and leads, run the engine again to regain some heat and retest. An engine with a cylinder head temp. of 375 F will usually give much higher results than it would at 275 F, even though both temps are in the normal range. By the time you get to the last cylinder on an engine, it is usually considerably cooler than the first cylinder you tested.. Also remember, rings are designed to seal to cylinder walls when in motion, not sitting still, and are designed to seal at normal combustion pressures, which are much higher than then the pressure of a leak down test. A leak down test is a very good way of finding where compression is lost if the results of a cranking compression test are unsatisfactory. On several occasions,(comparing 2 engines) we've seen the engine with the lower leak down results have a much higher cranking compression than the engine with better leak down results.

In summary......... good leak down test results are great and the engine should have good cranking compression test results then. Poor (lower) leak down test results should be investigated further, but may only be indicating normal wear, especially if the cranking results are good/normal.

Now that I have probably really confused the issue, I hope it helps a little.

 

Good info.

 

 

I prefer to start with a cylinder balance test....This test is done when the engine is at idle..(about 800 rpm)...rather than the cranking RPM that a compression test is done at.....Kill one cylinder at a time (either with a cylinder balance tester or by pulling one plug wire at a time..(hopefully you wont get a JOLT!!)....see how much the rpm drops.....All cylinders should drop the same amount of rpm....If you have a cylinder that doenst drop RPM then you have a "dead" cylinder....either due to a bad valve, piston ring(s), of faulty fuel injector (if multi-port port fuel injected such as the C5)...Once you locate the "dead" or weak cylinder then you can jump to the compression test or leakdown test to determine if it is a mechanical problem (valve or ring) or an injector problem....Also, dont forget if you have a dead cylinder to make sure you are getting spark to that particular cylinder....The C5 uses a coil for each cylinder...You can have a bad coil and it will result in a dead cylinder, as will a bad plug wire or spark plug...
Dan Samento
Canon City, Co
www.dansamento.com

 

An engine with a bent rod, bent pushrod, broken rocker, or damaged cam lope will leak down perfectly. Only a compression test will detect a problem. Best to have both done.

 

Thanks silver bullit,nnial8tr and ptindall.. Good stuff