Hi All,

I suspect I might have broken another valve spring on cylinder number 2. The engine suddenly runs badly again and clearly lopes while idling wanting to die and the automotive stethiscope sounds different above that cylinder.

This same scenario happened to cylinder number 5 about six months ago and I developed exactly the same symptoms. It actually took a while of daily running the engine trying to diagnose everything from a vacuum leak to bad timing to bad injectors or wiring before the engine sound became clearly bad and I was able to distinctly "hear" the difference when listening to all the cylinders with the stethiscope.

I went back to the FSM and realized that I had neglected to perform a cylinder compression check because I didn't really know how. I actually have a compression tester but don't know EXACTLY how to check. I would have found the problem a lot sooner if I had done that.

I searched the forum but most posts that mention compression checks assume that we would know how to do it.

Can someone chart out exactly how to do it? I mean what to unplug electrically etc. taking ther plug out and plugging in the tester is easy but I'm not sure about the rest.

The engine is a 383 L-98 in a 1986 vette.
Thanks in advance!
Lee

 

There are two ways.

Method one:

Remove all sparkplugs. Depressurize the fuel system. Pull the fuse for the fuel pump. Pull the coil wire off the distributor cap. Install the compression tester into the spark plug hole for the first cylinder to be tested. Turn the key to run but not start and verify that the fuel pump is not running. Watch the gauge as your friend/wife/trained cat hits the starter. (repeat for each cylinder) If you find a cylinder lower than the others pour a couple of table spoons of engine oil into the plug hole and retest it. If the compression goes up it's rings. If the compression stays the same it's in the head.

Method two:

Find a shop with an electronic compression tester. They put a few sensors on the ignition wires and battery and watch the car crank. The compression is calculated based upon changes to the current the starter draws as each cylinder comes to TDC - compression stroke. And it is pretty accurate too.

 

Last time I measured squeeze was on a carb'ed motor.

Pull the spark plug, screw in the guage, and start her up.

Fuel injected motor? Do you have to unplug the injector too?

I doubt it.

ed.:
Holy cow '94doifn78skld!!! That looks like neurosurgery!

 

For method 1, be sure to crank the engine over the same number of times for each cylinder. I spin the engine over 3 times for each cylinder. I also pull the electrical connector off the injectors. Even with the fuel pump not running, the injectors will open/close during cranking and any residual fuel in them will drip into the cylinders. Probably not a big deal but easy to avoid.

 

Thanks for the procedurw guys

 

I know I'm jumping into an old thread here, but I had a few follow-up questions. I'm about to do a compression test on a 1990 L98 I recently purchased to assess the overall health of the engine. Here's what I was wondering:

• If instead of using the ignition switch, I connect a remote starter switch to the starter solenoid so I can crank the engine over from under the hood, do I still need to remove the distributor wire and/or run down the fuel pressure, etc. That is, won't the key being in the "off" position disable all those things, or do I still risk some damage and/or fuel leakage into the combustion chamber?
• I think an additional step to get accurate results is to lock the throttle body open, correct? Is there a preferred way of doing this?

Thanks in advance for any insight on these additional details.

 

You can pull the FP fuse to keep fuel out of the equation and the two injector fuses to disable them too.

I used the ign. to crank the engine but can't remember if throttle was full open or not...
Try it both ways to check for difference.

 

Youll want to disable the distributor, pulling the "IGN" wire is one way

OP have you pulled a valve cover to inspect the springs? Most times you can see the break visually or see if the valves are going all the way up or not. Not a bad time to inspect the pushrods to see if ones bent or not that can give you a crappy idle.

 

I was taught to perform a Compression test this way. You need everything to be "consistent" to get accurate readings and this is why I use a battery charger on the battery while performing the test as well as securing the throttle wide open.

Remove the fuel pump fuse to disable the fuel system on Fuel Injected Engines.
Remove all spark plugs and relocate the spark plug wires so they are out of the way. (So they won't hit you with a really nasty spark)
Secure the throttle "wide-open" with a tie wrap or whatever.
Attach a battery charger to the battery and charge the battery for a few hours before doing the compression test. It is important to have a "fully charged" battery during the whole test. If the battery voltage keeps going down so will your compression numbers.
Make a little chart and then crank the engine for 10 seconds on each cylinder with the compression tester screwed in snugly. Record numbers as you go, right away so they are not forgotten.

After taking the dry test of all eight cylinders then decide whether you need to try it with oil. Generally I do this when there are big differences between cylinders and want to know more. If needed you simply do the test over again after squirting maybe an ounce of oil per cylinder. Just squirt the oil in the particular cylinder you are testing, one at a time. I used too much oil one time and my spark plugs got fouled out when I started the engine so be careful.

The difference between the highest and lowest numbers should be within 10% of each other. If you find a larger margin than the 10% then you might want to consider a Leak Down test. The leak down test is where you put say 100 psi into the cylinder (@ TDC) and it is able to hold 85 psi suggesting a 15% leakage. The Leak Down test can identify bad rings, valve guides, valve seals, head gaskets very easily. If you hear air coming out of the intake then you have a leaky intake valve guide or seal. Air out of the exhaust suggests a faulty exhaust valve, valve guide or valve seals. If the air is coming out of the oil fill hole then you have leakage past the piston rings. This test is very useful if done by somebody who does them regularly and knows how to interpret the findings. I was taught by my A&P (Aircraft and Power-plant, Not the grocery stores up north) Mechanic as she did one every year on my old Cessna 172 during the "annual" maintenance.
I have seen others do the leak down test after loosening up the rocker arms so the valves are as tight as they can get. It is another step and not so necessary unless you have a problem with a particular cylinder.

The compression tester I have is a digital one that has a memory so I tell it this is #1 and so on. It catches the highest PSI the cylinder is able to achieve and makes the test a one person affair. Another "gadget" albeit a useful one!

I hope this helps a bit, if not there are probably several you Tube videos explaining how to do the compression test. Good Luck!

 

Start the engine after removing fuel pump fuse to reduce pressure in fuel system until engine stops or drain gas out of shrader valve on fuel rail.

Do this prior to performing other steps.

Also hold throttle blade wide open when cranking engine.