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Want to perform a compression check. One question

R

RadioShackSpecial

NAXJA Forum User
Location
Michigan
Hello.

Specs: '96 w/ 4.0l, aw4.

I would like to perform a compression check on all the cylinders. So my question is, what is the best way to disable the ignition? Should I just remove the wire between the Coil and the Distributor and then perform the check as normal?

Simple question, just dont want to mess up anything. :thumbup:
 
The coil wire will work, but normally you pull all the plugs at one time to allow the engine to turn over easier.
 
If you crank the engine without the ignition, you can end up wet fouling the plugs. It also cranks a lot slower. It is best to pull all the plugs.
 
Disable ignition: pull the ASD relay or unplug the CPS/CKP crank sensor.

Pull all the plugs. Run each cylinder first dry and then wet. Make sure you get three compression strokes on each.
 
Pull all plugs.

Disconnect the INPUT wires to the ignition coil. Cranking with no load on the coil can cause to fail (or so I've read, it's never broken when I've done it so YMMV).

Technically you're supposed to also disable your fuel pump and unplug the injectors. I've never done that. But again YMMV. If you catch on fire, don't say I didn't warn you.
 
here is my suggestion. 1) Remove all plugs. THis allows the starter to crank easier, and work faster, and drain the battery much less. 2) I would disconnect the power wire to the coil. 3) i would pull the fuel pump fuse OR remove each one of the fuel injector connectors. 4) enjoy doing your compression test. Take note most often its said that a hot/warm engine causes slightly better readings....i haven't found this myself. Also you should hold the throttle at WOT when cranking... i haven't noticed a difference doing that either. ANother thing to note is that its more important how fast the gauge jumps up on the first crank, rather than the total pressure.

For example if one crank jumps to 130psi and then the next 3 cranks goes to 150 psi. that would be much better than a cylinder that takes 7 cranks, but hits 160psi.

Finally i've found that you should compare the numbers in respect to each other. Rather than your # vs what the book says (to a point) Since many of these gauges aren't super accurate.
 
Thanks for all the help and info. Just finished with the test.

I pulled the coil wire and fuel pump power. Removed all the spark plugs before starting at the first cylinder. Cranked the engine over three times per cylinder and took note of how fast the gauge jumped and what reading it gave. Here is a break down of my results.

Cylinder 1; On the first crank the gauge jumped to 130psi, for the second it hit 150 and third a little under 160.

Cylinder 2; On the first crank it jumped to 110psi and for the second it went to 120 and third hit 135.

Cylinder 3; Was much like the first. Hit 130, then 145 and then 155.

Cylinder 4; Again like one and three.

Cylinder 5; First crank hit 125, second hit 140 and third hit 155.

Cylinder 6; First crank hit 130, second hit 145 and third hit 160.

So if I am not mistaken everything is within spec. Thanks again.
 
RadioShackSpecial said:
Thanks for all the help and info. Just finished with the test.
Cylinder 1; 160.
Cylinder 2; 135.
Cylinder 3; 155.
Cylinder 4; 155.
Cylinder 5; 155.
Cylinder 6; 160.

So if I am not mistaken everything is within spec. Thanks again.
Click to expand...

Cylinder two is low. Spec is 90% of the high cylinder which would be 144 at a minimum.
 
90% (10% variance).

Do a wet test on the low cylinder and see if it comes up. If it doesn't, run some MCCC or Seafoam through the intake and retest dry.

I wouldn't panic over it, it is well above 110 psi so it will keep firing. A little carbon buildup on a valve, the compression rings are more aligned on that cylinder.
 
FWIW I've found a wet test of dropping like a teaspoon of oil in the cylinder ALWAYS sky rockets the readings. on a good cylinder or otherwise. YMMV but thats what i've always seen...
 
Banditt007 said:
FWIW I've found a wet test of dropping like a teaspoon of oil in the cylinder ALWAYS sky rockets the readings. on a good cylinder or otherwise. YMMV but thats what i've always seen...
Click to expand...

Not when there is a burnt/bent valve. The wet test is to help determine if the problem is with the valves/head or the rings. FWIW.
 
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