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Unread 09-11-2013, 09:23 AM   #1
bluedaffy
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1989 YJ Wrangler 
 
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Location: Stevens Point, Wisconsin
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compression check

I want to do a compression check on my 4.2 YJ. I've read a few compression check write ups but I wasn't sure how they would apply to my vehicle in it's current state.

1. When doing the test should I have all spark plugs removed or just the one I'm putting the gauge into to be tested?

2. I have a mechanical fuel pump, does it need to be disconnected? I've read you should pull the fuse on the fuel pump but I don't think I have one.

3. Does it make a big difference if it's at operating temperature? I feel like that would get pretty hot doing the test and I currently have all the plugs out d/t some distributor stuff I was doing. If the test is pretty accurate even on a cold engine it would be much easier for me rather than putting all the plugs back in, connecting all the wires to let it run for a while just to take them all out again.

4. I currently have all the distributor wires removed as well as the coil plug that goes to the center of the distributor. I can perform the check in this state right? Or does the coil plug have to remain on the coil and be grounded or something?

Thanks, bluedaffy.

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Unread 09-11-2013, 09:39 AM   #2
Que89YJ
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I usually turn mine by hand so I pull all the plugs it makes it easier to spin . If you are using the starter then I dont see any reason to pull all the plugs just do it one at a time. The distributor and fuel pump are ok. You just dont want it starting, sparking, or dumping fuel. The engine temp is usually cold. Do the wet test when you do the compression test. It will tell you if you have ring or valve issues.
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Unread 09-11-2013, 10:50 AM   #3
kloppk
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From the shop manual...

CYLINDER COMPRESSION PRESSURE TEST
The results of a cylinder compression pressure test
can be utilized to diagnose several engine malfunctions.
Ensure the battery is completely charged and the
engine starter motor is in good operating condition.
Otherwise the indicated compression pressures may
not be valid for diagnosis purposes.
(1) Clean the spark plug recesses with compressed
air.
(2) Remove the spark plugs.
(3) Secure the throttle in the wide-open position.
(4) Disconnect the ignition coil.
(5) Insert a compression pressure gauge and rotate
the engine with the engine starter motor for three
revolutions.
(6) Record the compression pressure on the 3rd
revolution. Continue the test for the remaining cylinders.
Refer to Engine Specifications
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Unread 09-11-2013, 10:50 AM   #4
booboowv77
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I recommend doing it warm. (things expand and contract with heat and at temp is where you vehicle spends most of its running time)
Pull all plugs (if you don't you may not get the bleed through from another cylinder if it exists)
Disconnect power to the distributor (you don't want spark flying all over the place)
I just let my fuel system alone but have heard if you hold it wide open it will not let any fuel in.
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Unread 09-11-2013, 11:35 AM   #5
bluedaffy
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Thanks for the info! About getting rid of the spark, can I just take the wire that goes to the distributor cap from the coil completely off both the cap and coil or is it better to leave the coil side connected and ground it or something? Or take off the horseshoe connector on the coil? Also, I was wondering if (on a mechanically sound engine) the compression should shoot up to that cylinders compression on the first revolution? or if it is normal for it to build up to the peak compression after a few revolutions of the engine.
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Unread 09-11-2013, 02:36 PM   #6
jokerchief462
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3-5 revolutions is adequate to get the reading. If unsure go to YouTube and watch a video.

Sent from underneath my Jeep
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Unread 09-11-2013, 02:45 PM   #7
jaglusby
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just pull the wire from the center of the distributor and the coil and then just turn it over with the starter you should be fine and safe by just doing that
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Unread 09-11-2013, 03:18 PM   #8
Old4X
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When I do it, I pull the horseshoe connector off the coil and disconnect both harnesses at the ICM. Warm engine dry and wet.
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Unread 09-12-2013, 05:58 PM   #9
bluedaffy
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First of all, thanks for all the pointers. I'm always scared to do something I haven't done before with my Jeep. There was no diagnostic reason I wanted to test her compression, I just wanted it so I knew what it was and so I could write it down in the maintenance book that I keep.

Here are the results to follow up with my test:

Dry compression test: 150-156-155-150-150-155


I also did the first 2 cylinders with adding the oil (the wet test) and they were both in the 150-155 range. I plan to do a full wet test in the near future and compare it to the dry test I did today, but I started running low on time and I had to wrap it up to get to the University in time for chemistry (which I hate).

What do these results mean to me? What is a 'mechanically sound' engine supposed to have for compression?
Thanks again.
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Unread 09-12-2013, 06:25 PM   #10
Que89YJ
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It means your engine has very little wear and you have many more miles of fun in front of you!
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Unread 09-12-2013, 06:27 PM   #11
Old4X
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Looks good. No need for more wet test, that is only if one or more cylinders is way different from the rest. Tells if rings or valves
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Unread 09-12-2013, 07:25 PM   #12
bluedaffy
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Great! that's good to hear!

I guess I wont be in a rush for the full wet test, thanks for that suggestion. You saved me a half hour of my life!
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Unread 09-12-2013, 07:28 PM   #13
Que89YJ
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Now let's talk about the bleed down test....
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Unread 09-13-2013, 11:34 AM   #14
bluedaffy
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alright, shoot. What's the bleed down test?
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Unread 09-14-2013, 10:10 AM   #15
Rino1425
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Will someone explain the wet test? would this be like doing a leak down test?
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