Compression Test
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Joined: Sep 2006
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Compression Test
I plan on doin a compression test tomorrow on my truck to find out what excately is wrong with her. Everywhere I read says that I should get the truck warmed up before I do it. Unfortunately the truck is pretty disassembled right now so that is out of the question. Can someone give me a step by step way to do this so that I dont screw anything up in regards to the results that I will be getting? I realize this is a pretty simple thing to do but I really want to make sure I am getting accurate results. Also what should the compression be? Thanks guys I really appreciate it.
Elder User
Joined: Jan 2009
Posts: 712
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From: Blue Eye, mo. hwy 86 & JJ
while we're on the subject, how do you do a compression test exactly? i understand you are testing to see that all cylinders are compressing the gas/air mixture properly but how exactly do you do that? please dont flame me
Junior User
Joined: Oct 2006
Posts: 69
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From: Madison,WI
You have to use a special tool, you remove a spark plug and screw it the fitting into the spark plug hole and crank your vehicle over 4 times. Repeat this on all of the cylinders.
Do you have to un hook a fuel line at all for doing the test? I am used to FI where you just have to pull the fuel pump fuse.
Do you have to un hook a fuel line at all for doing the test? I am used to FI where you just have to pull the fuel pump fuse.
Junior User
Joined: Oct 2006
Posts: 69
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From: Madison,WI
I forgot you can also you a press in style, but you would almost need a second person to do it, here is what they look like:
Screw in style:
Press in:
Screw in style:
Press in:
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Moderator
Joined: Nov 2007
Posts: 22,415
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From: Denver Metro Area, CO
No.
I first remove all spark plugs, then do a dry test followed by a wet test. I've never first
warmed up the engine, exhaust manifolds get hot.
Both styles of gauges are cool for different reasons; the hose is flexible and gives you a
bit of freedom to move around. The press-in one is fast to use.
Get or make yourself a remote starter activation switch with a 6- or 8-foot cable, no need
for a second person. 150 PSI isn't really a whole helluva lot, you won't need to use ALL
your muscle for this.
-ct
I first remove all spark plugs, then do a dry test followed by a wet test. I've never first
warmed up the engine, exhaust manifolds get hot.
Both styles of gauges are cool for different reasons; the hose is flexible and gives you a
bit of freedom to move around. The press-in one is fast to use.
Get or make yourself a remote starter activation switch with a 6- or 8-foot cable, no need
for a second person. 150 PSI isn't really a whole helluva lot, you won't need to use ALL
your muscle for this.
-ct
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Methanoholic
Joined: Mar 2009
Posts: 6,442
Likes: 8
From: Maine (NorCal Native)
Dry then Wet Compression test Procedure.
If you don't have a Remote Starter, Get a friend (or the Good Woman) to assist.
Preferably Remote Starter for better control! They're cheap!
Make sure you have good full batteries, low batteries can produce funky results! (Tapering Results)
Personally, I keep a charger hooked up!
Set Parking Brake.
Put Transmission in Park or Neutral.
Gasser.
Compare Dry Numbers, Should be no more than a 10% difference.
Compare Wet Numbers to Dry Number, Significant rise in pressure indicates bad rings.
Personally, I think the only Compression Gauge to use is a screw in type!
I have an Oil Squirt can that I have modified with 1/8" copper line with the tip plugged and 3 holes around the end of the copper tubbing. This squirts oil out to the Cylinder walls versus on top of the piston.
-Enjoy
fh : )_~
If you don't have a Remote Starter, Get a friend (or the Good Woman) to assist.
Preferably Remote Starter for better control! They're cheap!
Make sure you have good full batteries, low batteries can produce funky results! (Tapering Results)
Personally, I keep a charger hooked up!
Set Parking Brake.
Put Transmission in Park or Neutral.
Gasser.
- Pull All Plugs.
- Unplug IP Pump.
- Pull all Glow Plugs.
- Install Compression Gauge.
- Turn Motor over 5 compression strokes.
- Read Gauge and Record.
- Release Pressure in Gauge.
- Remove Gauge.
- Add a little new motor oil to cylinder. (a table spoon or 2)
- Install Compression Gauge.
- Turn Motor over 5 compression strokes.
- Read Gauge and Record.
- Release Pressure in Gauge.
- Remove Gauge.
Compare Dry Numbers, Should be no more than a 10% difference.
Compare Wet Numbers to Dry Number, Significant rise in pressure indicates bad rings.
Personally, I think the only Compression Gauge to use is a screw in type!
I have an Oil Squirt can that I have modified with 1/8" copper line with the tip plugged and 3 holes around the end of the copper tubbing. This squirts oil out to the Cylinder walls versus on top of the piston.
-Enjoy
fh : )_~
