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post #1 of (permalink) Old 04-10-2002, 07:38 PM Thread Starter
 
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Compression Testing: Correct Method?

What are the correct proceedures for compression testing my 258?

I have decided to add an FI kit so I can actually get this thing back on the trail. I don't want to spend $1000+ on this thing if it needs a rebuild.

I bought the cheap Harbor Freight tester. Now if someone can tell me how to use it!

I beleive I need to do a leakdown and a max pressure or something?
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post #2 of (permalink) Old 04-10-2002, 07:47 PM
 
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Re: Compression Testing: Correct Method?

disable ignition first!
mark the wires, remove all plugs.

get a notepad, screw guage into Hole#1 have someone crank engine over several times, record the highest number on guage ( guage should hold peak reading).

do all 6 or all 8 whatever your working with.

if any are low ( Lower than the rest by more than 10% or so) then ad oil to those cylinders, a squirt or a teaspoon full, reattatch compguage to plug hole and retest, if the low cylinders come up with teh addition of oil, then its a ring seal problem.

if the readings dont come up with oil, then its a valve problem, either burnt edge or, valve train not opening it ( worn cam lobe/broken rocker arm/bent pushrod)

you can find the weakest seal in your cylinder by bring whichever cylind you wish to test to TDC, put teh rig in NUETRAL, engage parking brakem, choke the wheels.

screw the compression guage hose into the plug hole, turn your air compressor to about 20PSI, attatch IT to teh hose instead of the guage and slowly increase pressure until you hear the hiss, you can easily hear exhaust, intake or crankcase.

poor mans leakdown test, not as accurate, but has told me alot of things in the past.


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post #3 of (permalink) Old 04-10-2002, 07:49 PM
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Re: Compression Testing: Correct Method?

The directions in my compression tester also state to open and hold the throttle of the carb wide open. It will give you two differnt readings with it open and with it closed.
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post #4 of (permalink) Old 04-10-2002, 07:53 PM
 
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Re: Compression Testing: Correct Method?

thats right hte instructions do say that!
BUT iver never noticed any difference?

for testing BLOCK THE THROTTLE WIDE OPEN
there must be a reason right?
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post #5 of (permalink) Old 04-10-2002, 07:59 PM
 
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Re: Compression Testing: Correct Method?

I just recently did a compression test on my 258 before tearing it all apart, and I got 2 drastically different reading with or with out the throttle wide open. With the throttle open it was giving me what I should have been at (about 140psi). With it closed I couldn't get over 115psi.

Shawn
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post #6 of (permalink) Old 04-10-2002, 08:56 PM
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Re: Compression Testing: Correct Method?

The throttle position should have NO affect on a compression test. One little item to remember when doing this test is you're cycling the engine without burning the fuel. If you crank the engine a lot during the testing the gas can wash down the cylinders. To minimize the problem I will:

a. ALWAYS pull ALL the plugs
b. Do a "Dry" test on each cylinder.
c. Only let the engine rotate about three revs to obtain the reading.
d. Do the same test "wet", or with a little oil sprayed into the cylinder to see how well the rings are sealing.

Compare the readings to spec. and see how low the compression is... ALL cylinders should be within 15% of each other's readings. No cylinder should be less than about 20% of the spec.

If you have a low reading and do a "wet" compression test and the reading does not come up then it's a pretty safe bet that there's a really big problem with the rings (unlikely) or the valve's are burned and air is passing by them and the head will need to be rebuilt.
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post #7 of (permalink) Old 04-11-2002, 09:26 AM
 
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Re: Compression Testing: Correct Method?

Got a dumb one for you folks, we did a compression test on a friend's Jeep and all the cylinders except #6 were fine, and came in about 150 psi. When we got to #6, it would bump up to about 100+PSI, but on the next turn of the engine it would drop to zero. It would repeat this behavior as long as we turned the engine.

The engine is high mileage (160K+) but for what it is, it runs/pulls fine, idles smoothly, and if we hadn't checked it, we would have never suspected any problems, so we were slightly puzzled.

Any thoughts on what might cause this?
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post #8 of (permalink) Old 04-11-2002, 02:29 PM
 
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Re: Compression Testing: Correct Method?

Something wrong with the compression testor's one way valve. A compression gauge has a one way valve in it, and should pump up after several revolutions to maximum compression generated. It's a one way valve, so it wont leak down to zero until you release the pressure. Somethign was wrong, not with the engine but with the test equipment I suspect.

I find they pump up faster if the throttle is open, because the manifold will not develope vacuum if hte throttle is open, and thus the cylinders fill more fully with air and the engine turns over easier as it is not fighting that resistance to airflow.
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post #9 of (permalink) Old 04-11-2002, 03:14 PM
 
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Re: Compression Testing: Correct Method?

While on the topic of compression tests.. I just finished testing a 86 Wagoneer w/ a 360.. Well I got 115 - 125 across the driverside, but the rear 3 on the passenger side was 140.. What do you guys think?? I was planning on swapping it into my CJ-7.. This is just barely within the 15% margin..
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post #10 of (permalink) Old 04-11-2002, 03:32 PM
 
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Re: Compression Testing: Correct Method?

proably carbon build up, or even if its wear and tear, an AMC V8 will start and run FINE with as littel as 40psi in all cylinders ( and even lower in a couple) it just wont make as much power.

Id fix what gaskets it needs, spec the oil pump and cavity, freshen the ignition and carb, and enjoy it for a long time.

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