hey everyone
I am not necessarily sure I need to but - I was going to run a compression test to make sure the numbers are ok - I have no smoke but the plugs I took out looked a little rough- I had concerns of valve seals maybe not being their best - 69 k on a stock l98 - I had some power issues last year that new plugs cap and rotor seemed to fix. It was suggested here that a compression test would be a good next move to make sure before getting into anything.

I have never done a compression test on a vette - someone said I have to remove all of the plugs ? someone also mentioned something about a wet and a dry test? if anyone can give any direction words of wisdom for this I would greatly appreciate any help

thanks

Sean

 

Compression test won't directly indicate condition of valve seals. Rings and valve sealing mostly.
Remove all plugs lock throttle body wide open. Disconnect red wire from distributor cap and remove fuel pump fuse. You want fuel pump disabled along with no spark. Install gauge crank engine 5-6 revs. You need crank the same amount everytime. If you are watching the gauge should have 3 large jumps and then it will probably bump a little during last 2 or 3. Record the readings. Readings should all be within 10% and over 150 psi. A wet compression test a oz of oil is added to the cylinder(s) cranked a few revs then gauge installed. Reading compared to the earlier dry test. A large gain would indicate worn rings as cause for low compression. A leak down test is more accurate to determine cause of low compression and condition of rings and valve sealing.
YouTube has videos

 

thanks very much

 

Compression tests are Okay but if you are trying to find a problem I prefer to use the Leak Down Testing equipment. A leak down test will tell you far more than the Compression test will. I learned about leak testing the first time my Cessna was in for it's annual inspection. The info derived from this test will point you towards the culprit. If you hear air flowing out of the oil fill hole then your leak is around the pistons, if you hear air flowing out of the intake it means you have a bad intake valve. You get the idea as it tells you the overall health of the combustion chamber. They use this test on every piston type aircraft to verify the engine's condition. During a Leak down test to fill the cylinder with 100 psi and then you watch how much pressure is maintained in the cylinder, a Good aircraft engine should hold 75-85% inside the cylinder. The higher the holding pressure the better. You test each cylinder at Top dead Center for that particular cylinder, otherwise the valves are going to be an issue.

Kevova did a great job telling you how to perform the Compression test. The only thing I would add is to have a battery charger on the battery and be sure it is fully charged when you do the testing. This is not "Critical" but sure makes for the cranking speed to be consistent while doing the testing and that can have an impact on the final results. Without a battery charger the battery would be slowing down towards the final cylinder. The most important part of a standard Compression test is consistency in the testing. Whenever I do a compression test I do both Dry and Wet (oil) to be certain. Then I post the results in my Service manual for the particular engine. It is nice to see everything working properly and have records of the results.

I have done so many compression tests that I finally bought a digital unit which records the cylinder number and lets you play back the results. It is really handy as it came with all the necessary adapters to use it on virtually any gas engine. My Leak Down tester is the same One I used on my Cessna and it works great. They sell the leak down testers at Summit and Harbor Freight and are not very expensive, you will need a pretty good air compressor to supply the air for the test.

Performing a leak down test or a compression test is an great way to determine the condition of your engine. Accessing the Spark Plugs to do the test could be easier on a C4 if there was more room. Fortunately my "other" Corvette is a 1968 C3 with its 427 and it is great to work on as you can reach all eight spark plugs from the top of the engine.

I keep wondering how hard or expensive it will be to change the plugs on the new C8's. With a Mid Engine the mechanics time is going to add up just getting TO the spark plugs, maybe I will stick with cars I can work on myself. I guess I will just keep the C3 and enjoy the simplicity and not to forget that it too can go from 0-60 in under three seconds even with spinning the tires. They may handle a lot better but Acceleration is acceleration and that they don't have a big edge there.

Don't forget the importance of today in History! God Bless those Men who died in the Invasion! God Bless America! Now "God SAVE America"!

Best Regards,
Chris

 

thanks -

 

SEan, be careful you don’t chase a problem that doesn’t exist. If the car didn’t have a tune up for over 30k miles it would be normal for the plugs to look rough. The small block Chevy is a very durable motor and should be good for 200,000+ miles with regular maintenance. Instead of going through all the work of a compression test or leak down. Drive the car for 5,000 miles and check the plugs. You will probably find they look fine and you will have enjoyed the car for all those miles.

 

thanks jmeyer58 - I am trying to not overthink - and get into unnecessary things - I guess I just wanted to be sure - but, so far since the plugs have been changed it has felt fine - at the moment I think that is what I am leaning toward driving a little and see how it behaves -

if it seems down again and the plugs look bad - thanks to the forum - I now have a couple of very sound ways to approach the problem sensibly.

since I got the car it has received regular oil changes, air filter, plugs, distributor, rotor etc. trying to make sure that the basics are covered.

thanks again