Compression Testing for Dummies

[OvaBoost]

I just picked up a high mileage 87 GN on Friday. I want to do a compression test, but I have never done it before. Just picked up the gauge and wanted to know if someone can walk me through the best and simplest way to do it.

A Compression Testing for Dummies would be nice!

Thanks,
Bill

 

[Dabigguy]

Engine Compression Testing

 

[Nick Micale]

Good idea both for you to learn more about your car and a teaching experience for you!

First disconnect the computer power orange wire at the battery.

Remove all the spark plugs.

Turn the engine over about 4 times for each cylinder and note [write down] the first reading on the gauge and also the final reading.

The first reading should be about 90 psi, final about 150 psi, and all cylinders should be within 10% of each other.

 

[OvaBoost]

Just to clarify...

Ok, so disconnecting the ORANGE wire at the battery will disable BOTH the spark and fuel.

Is that correct??

I'll be doing by myself, so should I be pressing Accelerator completely down to the floor (WOT) to open the Throttle Body while cranking??


Thanks for the all the help!!!

 

[S10xGN]

Ok, so disconnecting the ORANGE wire at the battery will disable BOTH the spark and fuel.

Is that correct??

I'll be doing by myself, so should I be pressing Accelerator completely down to the floor (WOT) to open the Throttle Body while cranking??


Thanks for the all the help!!!

No on the computer (orange) wire, it will not disable either of those. I'd pull the FI fuse and the spark plugs, as Nick says, then block open the throttle body with a shim (or anything else handy) and proceed as directed.

 

[Nick Micale]

No on the computer (orange) wire, it will not disable either of those. I'd pull the FI fuse and the spark plugs, as Nick says, then block open the throttle body with a shim (or anything else handy) and proceed as directed.

OK, maybe the wire is not orange, maybe red, but it WILL remove power from the ECM which disables both spark and fuel as I just verified this with a compression test on a T-Type an hour ago.

Pulling the ECM fuse will also power down the computer.

Also, I find almost no difference in readings with the throttle open or closed.

 

[OvaBoost]

Thanks!

So I did the compression test tonight. All cylinders measured ~150.

Disconnecting the orange wire at the battery disabled the ECM. That was all that was needed to kill the fuel and spark.. Making the test simple enough for a dummy like me..


Thanks again for all the help!!!

 

[RIFF RAFF]

This is for duh me's

Take a turkey baster & remove the vacume/plunger ball. Remove spark plug and insert baster tube. Place one small party balloon over the end of baster tube & crank engine for three seconds. Remove balloon and tie end off. Repeat all steps till all balloons are filled . If you have some smaller balloons thats ok . But ya never know . Could be bad . Might need to seek some profesional help..

 

[ChristianKustom]

Call me old fashioned but I prefer to just disconnect the connector that plugs into the coil pack module using a 1/4" socket size. This will take any chance of any accidental spark. Plus it completely disables the electrical system from getting charged. It only takes a few more seconds to do but I prefer to do it this way.

Also make sure you are using a quality gauge. I have found through the years cheaper gauges will read inaccurately. Find a good name brand one like SnapOn or Cornwell. It's worth it to get an accurate reading.

I start by removing one plug at a time. I insert the gauge end in and screw it down tought. I crank until the needle on the gauge stops moving. I do this cause many time it will take more cranks to reach maximum PSI on the cylinders. Example: Cyl. #1 could take 4 cranks to reach max psi while cyl. #5 could take 6 or 7.

As a general rule of thumb anything lower then 150 (such as 130, 135 psi etc) psi you should consider replacing your valve springs and valve seals. I did this with mine and compression went from 128 psi to 170. All my cylinders are above 170psi. Works pretty much all the time unless you have something major going on in the engine.

Lastly make sure everything is within 10% of each other. While you are at it since you already had to remove your plugs...replace them. I would recommend the Autolite NON-platinum 23's. They seem to work best. Learned that trick from Bob and JD. I gap mine at .025. You could go to .030 or even .035. I run mine at .025 to prevent blow out. I am running at 26psi for now and as soon as my tuning is a bit better I am going up to 30psi. I hope this helps. Good luck to you.