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Weird compression test numbers


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23 replies to this topic

#1 X35 Performance

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Posted 15 September 2007 - 09:18 PM

Car: 2000 RS 2.5 Coupe
2.5 engine

I just ran a check of compression and here's what I get:
1 - 140-150 psi
3 - 170-180
2 - 220-230
4 - 220-230

Everywhere I've looked so far says the normal range is about 180ish, so how is the 2/4 side reading so high? I redid the test several times because I didn't belive it. All plugs out, Throttle full open, 5-6 cranks per test.

The 145 worries me as well. The engine has 123k. I bought it at about 103k a year ago. It runs fairly well, but shakes a little at idle.

Any interpretations of these numbers?

#2 nipper

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Posted 15 September 2007 - 10:49 PM

Timing belt off a tooth. The 2-4 is on the same side of the engine.


When was the last time you did a timing belt change.


nipper

#3 X35 Performance

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Posted 15 September 2007 - 11:37 PM

A little follow up -

I did a leakdown test with a crude tester. #1 and #3 leak into the crankcase, 2&4 is much less noticeable.

Using the crude gauge I had..:

1 - about 5-7%
3 - about 3-5%
2 - could not tell movement on the gauge
4 - could not tell movement on the gauge

Timing belts are lined up correctly

#4 nipper

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Posted 15 September 2007 - 11:40 PM

A little follow up -

I did a leakdown test with a crude tester. #1 and #3 leak into the crankcase, 2&4 is much less noticeable.

Using the crude gauge I had..:

1 - about 5-7%
3 - about 3-5%
2 - could not tell movement on the gauge
4 - could not tell movement on the gauge

Timing belts are lined up correctly


No they arent, check again. The only way you get over 180psi on this engine is if a cam is out of time by a tooth. And since both high numbers are on the same bank, thats where the issue is.

Forget leakdown. What are the numbers wet vs dry.

Also you didnt say, how old are the timing belts.

nipper

#5 grossgary

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Posted 15 September 2007 - 11:58 PM

timing marks are dead on? how about the timing tensioner? it may seem fine at rest but have too much give under load. i've seen this happen on an XT6.

on older soobs i've seen numbers read high due to headgaskets leaking, but you should notice coolant loss if that were the case. i suppose liquid in the combustion chamber may cause a high reading in some cases, just a guess.

#6 nipper

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Posted 16 September 2007 - 12:00 AM

timing marks are dead on? how about the timing tensioner? it may seem fine at rest but have too much give under load. i've seen this happen on an XT6.

on older soobs i've seen numbers read high due to headgaskets leaking, but you should notice coolant loss if that were the case. i suppose liquid in the combustion chamber may cause a high reading in some cases, just a guess.


Which leads to the question ...

Did you have all four spark plugs out when you did the compression test.

nipper

#7 X35 Performance

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Posted 16 September 2007 - 12:36 AM

Ding ding - Prize goes to Nipper. :banana:

Upon closer inspection, the DS cam is about 1.5 -2 teeth ahead of where it should be.

I guess I have a Sunday project now...better check the rollers/tensioner and realign the belt and then retest the compression and see what we get.

Oh to the other questions -
1. Yes all plugs were out, and throttle fully open,
2. The belt is about 2 years old with less than 30k on it.

Thanks a bunch for the help!
I'll follow up as soon as I finish putting it back together.



No they arent, check again. The only way you get over 180psi on this engine is if a cam is out of time by a tooth. And since both high numbers are on the same bank, thats where the issue is.

Forget leakdown. What are the numbers wet vs dry.

Also you didnt say, how old are the timing belts.

nipper



#8 bulwnkl

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Posted 16 September 2007 - 09:15 AM

Another victory for the USMB members! Glad you found it X35.

#9 stevetone

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Posted 16 September 2007 - 10:01 AM

...Upon closer inspection, the DS cam is about 1.5 -2 teeth ahead of where it should be...


So the obvious question is: How did that happen? Was it always off, or did something happen to get it to skip?

Check the tensioner, the belt itself, etc. before putting it all back together.

Did you change the tensioner with the last belt change?

Steve

#10 OB99W

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Posted 16 September 2007 - 10:27 AM

[...]The only way you get over 180psi on this engine is if a cam is out of time by a tooth. And since both high numbers are on the same bank, thats where the issue is.[...]

In case anyone is interested in how valve timing being off can increase the measured compression, here's the explanation: A piston reaches bottom dead center (BDC) of the intake stroke and reverses direction, beginning the compression stroke. Under normal circumstances, the intake valve closes a short time after BDC is achieved, which causes some loss of compression back to the intake. If the valve timing is off in the "right" direction, the intake valve closes a little sooner, at or closer to BDC. When that happens, more cylinder content is compressed (the loss to the intake is lessened), resulting in higher pressure.

#11 X35 Performance

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Posted 16 September 2007 - 02:28 PM

Ok got it back together and reran the tests. Now we have (cold engine):

1 - 170-180 :headbang:
3 - 170-180 :headbang:
2 - 150-160 :mad:
4 - 200-210 :confused:

Checked the belt alignment several times upon reinstallation. The tensioner and rollers are all fine, so why so high in #4 I'm not sure..sticking valve maybe?

I'm going to give it the seafoam treatment a couple times this week and re-test it next weekend with the engine warm to see what I get in comparison.





Ding ding - Prize goes to Nipper. :banana:

Upon closer inspection, the DS cam is about 1.5 -2 teeth ahead of where it should be.

I guess I have a Sunday project now...better check the rollers/tensioner and realign the belt and then retest the compression and see what we get.

Oh to the other questions -
1. Yes all plugs were out, and throttle fully open,
2. The belt is about 2 years old with less than 30k on it.

Thanks a bunch for the help!
I'll follow up as soon as I finish putting it back together.



#12 nipper

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Posted 16 September 2007 - 04:48 PM

Get a vacume gauge and tell us what that reads and what the neelde does. I still thjink that you need to replace the idlers and tensioner. You cants always tell that they are good without a load on them

Also let us know if the valve timing jumped again. It is possible that the belt is damaged and its jumping.

nipper

#13 bulwnkl

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Posted 16 September 2007 - 05:43 PM

I know some folks have had results with Seafoam. I've not tried that particular thing. One thing I have used and fixed compression troubles with on my XT-6 is Auto-RX. If the troubles have anything to do with deposits, carbon, or 'dirt,' it will clean it SAFELY and thoroughly.

I'm with nipper on checking idlers, etc. Still, I suppose it could be carbon deposit build-up in the cc? If you could determine that, it'd be something I'd throw Fuel Power or Renewable Lube's injector cleaner at for an extended period, or possibly Redline's SI-1.

#14 nipper

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Posted 16 September 2007 - 05:50 PM

If this was my car, i would replace the belt, the idler and tensioner. Even if the belt is not that old, the belt can be damaged by jumping. There is no reason to risk a 5000.00 engine on a 60.00 belt.

nipper

#15 grossgary

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Posted 16 September 2007 - 08:06 PM

Did you have all four spark plugs out when you did the compression test.
nipper

yep, always do. plugs out, throttle wide open....

#16 nipper

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Posted 16 September 2007 - 08:11 PM

yep, always do. plugs out, throttle wide open....


Just checking.

nipper

#17 sea#3

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Posted 16 September 2007 - 11:13 PM

Attached is the compression test procedure out of the manual, It says to do the compression test with engine warmed up and injectors disconnected.

Hope this helps

SEA#3

Attached Files



#18 nipper

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Posted 17 September 2007 - 09:18 AM

Attached is the compression test procedure out of the manual, It says to do the compression test with engine warmed up and injectors disconnected.

Hope this helps

SEA#3


heheh ideally it should be a warm engine. Most the time a compression test is being done because the engine wont start. A warm engine is important to compare the results to the factory specs. Fort diagnostics (as it should be) its not necassary.
I foten thought, gee the guy has a no start condition, what does he do, take the engine out and put it in his oven for an hour at 200 degrees?

hehehe

nipper

#19 sea#3

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Posted 17 September 2007 - 11:12 PM

Yes I agree warm is best but not always a option , but more important is either you crank the engine over with the injectors disconnected or you use a "bump" starter switch off the starter so that you don't pump fuel into the cylinder , which will give you faults readings

SEA#3

#20 WAWalker

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Posted 19 September 2007 - 09:21 AM

Were the valve cover gaskets leaking oil into the spark plug tubes? If so when you pull the spark plugs that oil is running into the cylinder.

#21 nipper

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Posted 19 September 2007 - 09:38 AM

Were the valve cover gaskets leaking oil into the spark plug tubes? If so when you pull the spark plugs that oil is running into the cylinder.


That might raise the numbers, might not (if the rings are good), and wouldnt bring it over 200psi.

nipper

#22 hankosolder2

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Posted 19 September 2007 - 08:47 PM

That might raise the numbers, might not (if the rings are good), and wouldnt bring it over 200psi.

nipper



Nipper,
I think the point here is that if you add a large enough volume of oil to a cylinder, it effectively reduces the volume of the cylinder heads (by taking up air space with a non-compressable liquid) and can raise the compression ratio tremendously. This is why sucking water into your engine can hydrolock and destroy it! As to whether the quantity of oil stored in the spark plug recesses would be enough to cause this- probably not, but I don't know.

Nathan

#23 nipper

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Posted 19 September 2007 - 10:09 PM

Nipper,
I think the point here is that if you add a large enough volume of oil to a cylinder, it effectively reduces the volume of the cylinder heads (by taking up air space with a non-compressable liquid) and can raise the compression ratio tremendously. This is why sucking water into your engine can hydrolock and destroy it! As to whether the quantity of oil stored in the spark plug recesses would be enough to cause this- probably not, but I don't know.

Nathan


Its possible but doubtful, as it would be running down the side of the valve cover. It has to get over the lip of the spark plug seat.

Also your overlooking ont thing, that extra oil would go up in the compression gauge. It would be extreemly noticble in the hose, and when you release the pressure. Also that would be a few ounces of oil, so its highly improbable.

nice try though :)

nipper

#24 WAWalker

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Posted 20 September 2007 - 12:59 PM

Ok,

2000 Outback 2.5L SOHC. (same engine as in question) Very neglected car w/ 183310 miles on original engine.

Came in with dead miss on cyl#4.

Performed 105k service (Timing belt & vlave adjustment, which had never been done). All exhasut vlaves were tight, some with no clearance. Set valve lash, performed compression test.....................valve cover gaskets leaking oil into spark plug tubes.

Compression readings- 1/195 2/190 3/195 4/200

Not what I would have guessed to see from a dry compression test on a 183310 mile engine that has been poorly maintained all its life. But it wasn't a dry test it was a wet test.
A little more oil and you can bet the readings on at leat one cylinder would have been over 200.

When performing a wet test less than .5oz. of oil is added to the cylinder. A teaspoon or two at the most. This is a small amount of oil and will not end up in the compression gage, but will raise the compression #'s noticably in an engine with good vavles and rings.

If I wanted to spend more time looking I could find more documentaion of these engines that I have preformed tune-ups on that had oil in the spark plug tubes when I pulled the plugs, and had 200+ compression readings.

P.S. #4 dead misfire ended up being a bad fuel injector. They were lucky it wasn't a burnt valve.




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