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Discussion Starter #1
Ok, so I read the how-to on compression testing, along with my tester manual. I tried both cold test and after engine warmed up. Readings were

(psi)
#1 - 130
#2 - 132
#3 - 118
#4 - 134

After the first attempt (I only tried cylinders 1 and 2, freaked when I saw the low readings, then read that the tester manual says to do it with a warm engine - so readings abover are from warm engine), I reconnected everything, turned the car on and let it idle for ~10 minutes. The CEL was on as soon as I started it and remained on after I completed the second (warm) test and put everything back together.

Question 1: Since all the readings are low, but fairly consistent with one another, how likely is it that I tested wrong or that the gauge is crap? I've heard of low readings on a particular cylinder or two, but don't believe I've seen across the board readings this bad before. At one point I posed a question about an engine noise when I turn it off - some suggested that this might be from the air filter/intake area. Any chance that a leak could affect the compression numbers? What about elevation? If less dense air is going in, would that lead to lower readings? I doubt to the degree I'm seeing (I'm at 1000' above MSL)...Someone, anyone, give me good news!

Question 2: I don't own an OBDII reader. Any likely causes for the CEL from running a compression test? Does this possibly indicate that I did the test incorrectly?

Maybe I'm grasping at straws here, but I need a scapegoat - otherwise my dreams of a turbo are dashed.

Background information: 2003, gutted precats (intact when removed), 74K. Gas mileage is normal (~25-30 mpg with top down, mixed driving). No other problems to report.
 

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Discussion Starter #4
go to auto zone and read the code you need it for help.

as for the numbers they do seem low but consistent are you burning oil?
Not burning oil that I can tell. Not enough to need to add any between oil changes or notice a low mark on the dipstick.

OBD readings were P0351, P0352, P0353, P0354 - ignition coil A,B,C,D.

I cleared them and my understanding is that it was reading a misfire from the spark plugs being removed.

Compression testers read gauge pressure, not absolute pressure, so altitude is not a factor.

Did you use a cheap gauge such as those available from Harbor Freight?

Does your gauge have a rubber tip that must be held against the head, or does it have a threaded end that screws into the sparkplug hole? If the latter, did you use the O-ring?

Did you allow the engine to turn four or more revolutions before reading the gauge?

Were all sparkplugs out during the test?
Sorry, I meant to state the model of the gauge - it is IEC #3614, purchased from O'Reilly Auto Parts. It has both types of adapters - rubber gasket and threaded. I used threaded and was sure to tighten it to the point where the o-ring sealed. All spark plugs were out. I erred on the side of accuracy and turned the engine ~6-8 times to be sure it had enough revolutions to get a good reading.

With respect to the "gauge pressure," yes, but if there is less dense air being pushed to the gauge, in theory wouldn't that lower the psi read by the gauge? I realize that even if this were the case, 1000' shouldn't cause such a drastic drop in reading, but I'm a sucker for wanting to know the physics behind it.
 

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A compression test done by the method performed will never yield accurate compression readings.
The best you can hope for is to spot gross variations and abnormalities to draw conclusions only.

When you do a compression test you must also depress the gas pedal to let the air in and I hope you did that.

Also if you find a cylinder with the reading of 118 psi what you should do is perform a wet compression test by adding a tea spoon of oil and check to see if the reading goes up by a significant margin.
What you really need to do is perform a leak down test and then the chambers can be accurately assessed.
 

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Discussion Starter #6
A compression test done by the method performed will never yield accurate compression readings.
Why not? What is an accurate method? The leak down test that you mention? I read up on it, and I'm guessing it's something that requires a shop or expensive equipment, since you need to force air into the cylinders and I don't have compressed air handy. Besides, I've read many reports of others doing the test I did and getting 180s and higher, which is why I wanted to get some opinions before going further.

When you do a compression test you must also depress the gas pedal to let the air in and I hope you did that.
Yes, I did. Suffice it to say, I did everything as stated in the how-to (http://spyderchat.com/forums/showthread.php?18141-How-to...-Compression-check&highlight=compression), with the one exception being idling the car first, based on the tester manual's recommendation.

Also if you find a cylinder with the reading of 118 psi what you should do is perform a wet compression test by adding a tea spoon of oil and check to see if the reading goes up by a significant margin.
This is next on the list, but wanted to get thoughts on the first readings before delving further.

Thanks for replying!
 

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A leak down tester maybe out of reach for the DIY crowed. What I would do is purchase a different brand compression tester and perform the same procedure.
The one that looks closer to what should be stock is probably more accurate. I would then return the one that seems to be off.
 

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Compression testers read gauge pressure, not absolute pressure, so altitude is not a factor.
This Statment is a Bad Statment..

Compression testers read Gauge Pressure.. yes that is Correct.. and if you are at a Low Starting pressure in the Cylinder, then your Ending Pressure will be Lower.. so altitude is a factor..


Leakdown test will be the Answer.. but as you pointed out.. the Numbers are Fairly Constant.. so this is strange..

For Grins.. Get a Different tester, and do it again.. I have Two Gauges.. ( Plus a Leakdown ).. and I use one of the Compression Testers to Check the Other.. for just such cases..

Im at 980 Feet.. and Got +200PSI on my Stock Internals 2001 1ZZ Engine.. so it sounds like yours has an Issue..

And your Cels were from having the Coils Disconected..

Cap
 

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The cheap compression testers use a mechanical bellows and spring set up to move the dial needle. If the thing got dropped it will go way out of cal. Borrow or buy another tester and do it again, autozone had them on their tool rental program years ago. If you get two or three testers telling you the same thing you have low compression.
 

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Discussion Starter #10
Thanks, all, for taking the time to impart your wisdom. I chose to forgo buying a second tester since my local shop would do compression and leakdown for $130 total.

Just got their numbers back - they got
Cylinder/PSI/Dry Leak %
1/205/18
2/205/19
3/205/17
4/204/18

Needless to say, I am ecstatic about the compression readings, although I am worried about the leakdown numbers. I believe I've read that anything above ~10% may be cause for concern. I plan to ask more questions when I go to pick up this afternoon to help root out the problem. I have read, for instance, that leaky valves can "dupe" the standard compression test but will not seat and close properly during leakdown testing. Any idea whether this is the most likely culprit? Anything else that would cause high leakage but not show up on regular compression test?
 

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You got New Numbers over 200 PSI and you are looking for Problems..

Stop Looking!!.. Your Fine.. Drive it..

My Last Compression Numbers were Over 200 PSI.. But that was 60 K Miles ago.. havent Checked Lately..

Cap
 

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Discussion Starter #12
Understood, Cap, and I'm fine driving it. My question is whether I should boost an engine with these numbers for leakdown. Anyone else have leakdown numbers for reference? I am fine keeping it NA if it's bad, or boosting if it's a solid motor. Just don't want to boost and have all hell break loose. I can tolerate the complications from turbos themselves, but not those that come from adding a turbo a bad engine.
 

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These Numbers are fine for Boost..

Doesn't mean you won't have engine issues ... Bad Rod Bearing #4 from running low on Oil, while doing a Long Right hand Sweeper.. Fatal

Cap
 

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Discussion Starter #14
That, I can accept. Would be kicking myself if it just crapped out because the engine internals were poor from the get go.
 

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Your motor is fine, take the real numbers and your oriley POS compression tester back to the store and get a refund. I guarantee the thing got dropped in shipment or at the store.
 

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Discussion Starter #16
Oh, trust me - I did. My favorite part was the first time I went in to check out the OBD codes and mentioned the compression results. Told the guy the readings and he said, "Oh yeah, that sounds about right." Mind you, I never told him what car I have...
 
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