[rhudeboye]

Got my plugs and wires yesterday. Looking to install Friday or Sat. While I'm removing the plugs I figured I would check the compression. I never did it before but the concept is simple enough. Would this tool be fine for the job? it's pretty cheap as far as these units go.
http://www.sears.com/shc/s/p_10153_12605_0...20070921x00003a


Also. What is the optimal compression PSI

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[flash]

should work fine

200 plus at 8 cranks is new - anything over 185 is "ok" - anything below 180 is really ready for some work - even though it will probably run and seem "fine", it will be missing enough compression to seriously affect the power output

do the test with all plugs out - crank the engine and watch the gauge - crank it until the needle stops climbing - 8 rotations should do it, and frankly should not take that many

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[rhudeboye]

Thanks Flash.

I have also heard of doing the test one plug out at a time. Would this not give an accurate reading?

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[flash]

not if you had a leak between cylinders

it's also much harder to turn the engine with the plugs in - very free-spinning with them out - much easier on the starter and the battery

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[Cloud9...68]

Also, and this may just be a case of "...because that's the way I've always done it...", I've always done my compression tests with the engine fully warmed up. For some reason, my readings were crazy - all over the map, from a low of 140, to a high of 200 - when I did the compression test on my 968 cold. Warmed up, the cylinders were all within a few psi of each other, though on the low side (174 - 178 psi).

I can't think of a fundamental reason why doing the test warmed up is inherently "better" than doing it cold, but something about having the oil at operating temperature, and thus providing the level of sealing between the rings and cylinder walls that they would in actual operation, that feels right. I'm sure I read in a manual somewhere 30 years ago that the compression test should be done warm, so I've stuck with that methodology. Anyway, I would be interested if anybody else had the same experience I did with my cold vs. warm readings, and whether they might mean anything.

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[flash]

ring expansion

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[rhudeboye]

These are the steps I'll be following. note step 1.

1 Make sure the engine has been warmed up before beginning the test, to ensure that the oil has been warmed up. A cold engine will not test correctly.
2 Disable the ignition module or coil.
3 Insert the compression tester into one cylinder spark plug hole at a time.
4 Hold the throttle to full open position to ensure the engine gets adequate air intake.
5 Crank the engine continually for at least five to 10 full revolutions to obtain an accurate reading on the compression tester.
6 Record the reading for each cylinder. If any of them vary 10 percent or more from each other a problem may exist in one or more cylinders. If the variance is greater than 10 percent, specialized testing equipment may be required to fully diagnose the problem.
7 If all cylinder readings are within 10 percent of each other, no further testing is required and compression is considered optimal.

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[durtkillon]

Here are some diagnostic techniques. This is straight out of my compression tester instruction manual, which I don't think I've ever read until now. [img]style_emoticons/<#EMO_DIR#>/rolleyes.gif[/img]

DIAGNOSIS
1. On a normal cylinder, compression should increase
quickly and uniformly during each compression stroke
of the engine until a top (or peak) reading is reached.
2. If compression is low on the first stroke, builds up on
following strokes, but does not reach normal value, the
piston rings may be worn or leaking.
3. If compression is low on the first stroke and does not
increase on following strokes, leaking valves may be at
fault.
4. If the compression value is higher than the
manufacturer's specifications, carbon build-up may be
present in the cylinder or on the piston.
5. If compression readings on two adjacent cylinders are
20 pounds (or more) lower than the other cylinders, the
cylinder head gasket may be defective.
6. Recheck cylinders with low compression readings by
injecting approximately one tablespoon of mediumgrade
oil into each cylinder (through the spark plug
hole) and retesting. If compression readings increase,
worn, broken, or poorly seated piston rings are
indicated. If compression readings remain the same,
leaking or damaged valves may be at fault.

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[rhudeboye]

Somebody, more skilled then I, should do a DIY on this.

To a novice like me, this is good stuff. So simple, yet important

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[flash]

not bad - the one thing missing is to remove all 4 plugs before doing the test

as i said, this will eliminate the possibility of air moving between 2 cylinders due to a leak between them which would thereby contaminate the results

it will also take the load off the starter and battery

also, i generally do not have a problem with a cold test, though it is better to do it warmed up

there is also another trick to determining ring issues, but you'll smoke like a freight train afterward for a while - you put about 2cc of oil down the spark plug hole - this creates a temporary "seal" around the rings, and if the numbers go up, you found the problem

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[rhudeboye]

yup, Step 6 in Durts post

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[flash]

lol - i didn't read that far - ADD i guess - good catch

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[rhudeboye]

[img]style_emoticons/<#EMO_DIR#>/laugh.gif[/img]

Well, I guess it shows you know what your talkin about.

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[Cloud9...68]

I just did a compression test on my engine again (after replacing the o-ring on my compression tester), and got readings in the 170s. But after reading through this post, I realized I did one thing incorrectly - I forgot to hold down the throttle while doing the test. Does anybody have any experience, or at least an educated opinion, as to how much, if any, difference in compression pressure might result from the fact that I wasn't letting the maximum amount of air in the cylinders? I'd prefer not to redo the test, as I hate to take the plugs in and out of an aluminum head any more often than absolutely necessary, but if nobody has a definitive answer to the question, I may go ahead and redo the test with the throttle floored, and report the results. Considering my car doesn't consume any oil, or smoke, ever, I was hoping to see somewhat better numbers. But of course, I could have a leaky head gasket... Thanks.

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[flash]

if the throttle plate was set correctly, not much of a difference - if the car was idling properly, then the plate was probably set correctly (a small gap - not totally closed)

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[MLB]

I'm reviving an old thread here getting ready to do a compression test tonight.

After reading on the forum and elsewhere it looks like I want to disable the DME relay, G5, and probably also the fuel pump fuse number 34, what about the control unit DME, fuse #7? Maybe fuse number seven is only for automatic transmissions?i've got a six speed manual so I guess I can ignore that one

The engine is warmed up,and all four spark plugs out...

Survey says:

Cylinder one 192

Cylinder two 190

Cylinder three 187

Cylinder four 197

I guess those numbers are fairly decent but a little bit of a wide margin between cylinders three and four

This is reassuring as I prepare for a cross-country trip in a couple weeks

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[flash]

lol - make sure it's out of gear and brake on

 

seriously though, spin the engine until the needle stops climbing.  it should be about 6 to 8 rotations.

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[MLB]

Thanks flash

WOT and

Yep,reached a stable reading after six cranks stopped at eight cranks

Odometer shows 91,000 but the PO indicated the odometer was broken for a while "whatever that means. But he said he got the odometer gears fixed within a year or about 5000 miles on the clock.Head gasket is original

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