What's up with my compression!?!?!?!?!!

95Vert

New Member

Aug 19, 2004

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Apr 27, 2006

#1

• Apr 27, 2006

• #1

Ok, so I can hear my rockers. I freak out for a minute and then calm down. I take it to a local shop and the guy tells me that it's my oil pressure. Gives me a 20min speech about how the cam bearings and rings all crap must be worn out and I NEED to rebuild my engine. I freaked out at first and went home. I called up a couple of other friends and one told me to remove the VC and start the car to see if oil was traveling up to the rockers. I only did it on the passenger side, and it looked fine. So today, I take it to another shop for a compression test and I get this C1=75, C4=85, C5=75, and C6=75 I wasn't sure what it was supposed to be at but I knew those numbers were bad. The guy also checked my oil pressure and I was at 50lbs at idle. WTF could cause these compression numbers. My plugs were free of oil, my car isn't smoking, I have no power loss, and I'm clueless how my compression could be this low. Any advice?

 

94-302-vert

Active Member

Aug 16, 2004

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NE CT

Apr 28, 2006

#2

• Apr 28, 2006

• #2

OK, here are my thoughts:

I am not sure how much an ecam affects the compression, but I know cams typically can give lower values.

What did the other 4 cylinders test at?

Is this the stock bottom end? How many miles?

 

Swarzkopf

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Sep 23, 2004

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Apr 28, 2006

#3

• Apr 28, 2006

• #3

I am not sure how much an ecam affects the compression, but I know cams typically can give lower values.

Click to expand...

They can? How? The valve still closes, compression shouldn't change...right?


As to the original post...

You need to check your compression yourself. **** what the shop says. Buy a haynes manual for $15, buy a compression gauge for $30, and do the test.

If those numbers are accurate, your motor IS toast.

Test all the cylinders. The lowest test cylinder should be 75% of the best cylinder.

Generally, I'm under the impression that anything over 120 is "okay". Stock 5.0s tend to test between 145-155 from what I recall..I believe that my own car's lowest cylinder was 135 when I last tested it a few months back.

Good luck.

 

bill302

Active Member

Nov 2, 2005

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alabama,gadsden

Apr 28, 2006

#4

• Apr 28, 2006

• #4

click on link info. on compression testing and do your own test as Swarzkopf stated above. wal-mart sells the compression test guage and hose for about $20...

http://www.pumaracing.co.uk/comp.htm

 

94-302-vert

Active Member

Aug 16, 2004

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Apr 28, 2006

#5

• Apr 28, 2006

• #5

Actual compression only starts once the valves are both closed. If the intake valve stays open too long to better fill the cylinder then you have less gass to compress and therefore lower numbers.

Now it is also possible that the gagge itself was horrible and hence the need to know what the other 4 cylinders were. How was this tested? Was the engine turned by hand (single compression stroke) or allowed to spin on the starter until the pressure maxed on the gage (multiple strokes)? Remember the gauge adds to the overall volume of the cylinder by a very high %.

 

Swarzkopf

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Sep 23, 2004

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Apr 28, 2006

#6

• Apr 28, 2006

• #6

Actual compression only starts once the valves are both closed. If the intake valve stays open too long to better fill the cylinder then you have less gass to compress and therefore lower numbers.

Click to expand...

I've never heard this before...for all I know you could very well be right, but this just doesn't make sense to me.

Unless one of the valves is open when the piston begins in compression stroke, the compression reading should not change whatsoever. Is it common for an aftermarket cam profile to keep the intake valve open as the piston begins the compression stroke?

Otherwise, wouldn't the valve staying open HELP in compression as the intake stroke draws more air into the cylinder?

 

bill302

Active Member

Nov 2, 2005

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alabama,gadsden

Apr 28, 2006

#7

• Apr 28, 2006

• #7

could your cam timing be off a tooth or more....

 

final5-0

Mustang Master

Apr 6, 2003

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DFW Texas

Apr 28, 2006

#8

• Apr 28, 2006

• #8

94-302-vert said:

OK, here are my thoughts:

I am not sure how much an ecam affects the compression, but I know cams typically can give lower values.

What did the other 4 cylinders test at?

Is this the stock bottom end? How many miles?

Click to expand...

Very good point

Duration can bleed off some squeeze

Like you ... don't know if that is an issue with the e cam

Those values do look low.

Maybe the valves are too tight

Shims/different length pr's ... gotta love the peds

Grady

 

95Vert

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Aug 19, 2004

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Apr 28, 2006

#9

• Apr 28, 2006

• #9

It was a multiple stroke test. He only did 2 on each side. The pressure would jump from 30 to 75 and then stay there.

Need more info on what you're talking about Grady, please.

 

95Vert

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Aug 19, 2004

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Apr 28, 2006

#10

• Apr 28, 2006

• #10

Oh, and my lower end if off an 98 Mountaineer(Explorer) and it was 67k miles when I got it. Don't know if that matters. Our compression ration should be at 8.5:1 right?

 

94-302-vert

Active Member

Aug 16, 2004

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Apr 28, 2006

#11

• Apr 28, 2006

• #11

I think you are fine.

I would get a compression tester and test all 8 cylinders yourself.

Are the rockers adjusted properly?

 

95Vert

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Aug 19, 2004

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Apr 30, 2006

#12

• Apr 30, 2006

• #12

94-302-vert said:

I think you are fine.

I would get a compression tester and test all 8 cylinders yourself.

Are the rockers adjusted properly?

Click to expand...

Not sure. I just tightened them til they were snug. I'm going to install new lifters with a friend, and he's going to check all of that. And what makes you think my car is fine?

 

95snoozer

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Dec 14, 2002

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RCR

Apr 30, 2006

#13

• Apr 30, 2006

• #13

if your rockers are a lil too tight, they could be opening the valves enough to bleed compression off

 

95Vert

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Apr 30, 2006

#14

• Apr 30, 2006

• #14

95snoozer said:

if your rockers are a lil too tight, they could be opening the valves enough to bleed compression off

Click to expand...

That could be the reason why this is happening. I mean, it's weird that my compression is so low when my car feels strong and is not smoking or burning or leaking oil. I may be leaving power on the table, but it still doesn't feel "slow".