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compression check, what all can it tell you? by georgie
Started on: 01-27-2014 07:10 PM
Replies: 11 (186 views)
Last post by: Neils88 on 01-28-2014 06:45 PM
georgie
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Report this Post01-27-2014 07:10 PM Click Here to See the Profile for georgieClick Here to Email georgieSend a Private Message to georgieEdit/Delete MessageReply w/QuoteDirect Link to This Post
I have my 85gt 2.8 it run's strong an I'm not having any prob's at all, but I am going back home to key west, fl. From south california in a few so I'm trying to cover all my base's if you know what I mean. I know that if a out board motor has poor compression then it's just a matter of time till it's dead because low compess mean's bad bearings not just bad ring's. So this is my deal, what is the norm compress for the 2.8 an how much can you learn form checking it?
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rogergarrison
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Report this Post01-27-2014 07:22 PM Click Here to See the Profile for rogergarrisonClick Here to Email rogergarrisonSend a Private Message to rogergarrisonEdit/Delete MessageReply w/QuoteDirect Link to This Post
Condition of rings and pistons, cylinder walls, valves. I dont see how it can tell you anything about bearings.
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fieroguru
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Report this Post01-27-2014 07:34 PM Click Here to See the Profile for fieroguruSend a Private Message to fieroguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
A compression check is more for comparing cylinder to cylinder to see if there are any low ones. The low cylinders (20% down) indicate a wear issue somewhere in that cylinder (scored bore, worn rings, leaking valve seats, bad head gasket - normally 2 low cylinders side by side). A leak down check can help confirm if the compression "leak" is in the head or in the cylinder.

In an automotive engine, compression has nearly zero to do with bearings... unless the piston is down about 1/8" in the bore because the bearing is completely gone in the rod - but there would be a very audible knock, which will be more of an issue than concerns about compression.

As long as the car idles smoothly and doesn't pulsate or feel like there is a slight miss, I wouldn't bother with doing a compression check before a long trip.

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[This message has been edited by fieroguru (edited 01-27-2014).]

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Gall757
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Report this Post01-27-2014 07:38 PM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
Uniform compression readings in all cylinders tell you about general wear. They start out life as high as 150 and can go down to 70 with the engine still working fine.... maybe not as peppy.

different readings in the cylinders tell you something else....bad valve in a single cylinder or bad gasket...etc.
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georgie
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Report this Post01-27-2014 07:46 PM Click Here to See the Profile for georgieClick Here to Email georgieSend a Private Message to georgieEdit/Delete MessageReply w/QuoteDirect Link to This Post
If the bearing's are going then the rod is lose so it can't compress fully so the compress is low, at less that's the theory. It sounds a bit off but I do know when I check out board's for compress and is bad an thay still run it it allway's blow's the motor. Iv had only one motor that had no compress it was a ford bronco 289 an it just stoped running . That's why I'm asking. I have no reson to think my motor is bad but it does have some miles on it an key west is 3000 mile's of driving.
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georgie
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Report this Post01-27-2014 07:51 PM Click Here to See the Profile for georgieClick Here to Email georgieSend a Private Message to georgieEdit/Delete MessageReply w/QuoteDirect Link to This Post

georgie

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Oh, also what is the norm compression for the 2.8?
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georgie
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Report this Post01-27-2014 08:06 PM Click Here to See the Profile for georgieClick Here to Email georgieSend a Private Message to georgieEdit/Delete MessageReply w/QuoteDirect Link to This Post
Thank's, you all have cleared up what I was thinking. I have no leak's with the head's an it run's smooth at 850/900 idel an if I step on it it jump's an has no stammer to the motor at any rpm. I most likly just a bit worred of the long dive!!
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georgie
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Report this Post01-27-2014 08:51 PM Click Here to See the Profile for georgieClick Here to Email georgieSend a Private Message to georgieEdit/Delete MessageReply w/QuoteDirect Link to This Post
Once again, thank's to all for the info an hopefully she will run fine all the way an more!
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BrittB
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Report this Post01-27-2014 09:08 PM Click Here to See the Profile for BrittBSend a Private Message to BrittBEdit/Delete MessageReply w/QuoteDirect Link to This Post
One thing to remember when doing a compression check on the 2.8, disconnect the relay for the fuel pump. Hard to get good numbers if you wash away the oil in the cylinders!

[This message has been edited by BrittB (edited 01-28-2014).]

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Lou6t4gto
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Report this Post01-28-2014 12:56 PM Click Here to See the Profile for Lou6t4gtoClick Here to Email Lou6t4gtoSend a Private Message to Lou6t4gtoEdit/Delete MessageReply w/QuoteDirect Link to This Post
Change your OIL and filter before you leave. do you have Towing on your policy ? Long Drive. If you're going soon, you sure picked some Weather for a trip ! weather won't be ok 'till you hit Fla !
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2.5
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Report this Post01-28-2014 02:13 PM Click Here to See the Profile for 2.5Send a Private Message to 2.5Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by rogergarrison:

Condition of rings and pistons, cylinder walls, valves. I dont see how it can tell you anything about bearings.


Because he said outboard he may be referring to 2 cycles.

[This message has been edited by 2.5 (edited 01-28-2014).]

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Neils88
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Report this Post01-28-2014 06:45 PM Click Here to See the Profile for Neils88Click Here to Email Neils88Send a Private Message to Neils88Edit/Delete MessageReply w/QuoteDirect Link to This Post
If you do a compression test (called a "dry" compression test) and find one or more cylinders have a variation in the readings, then you can move on to a "wet" compression test. Squirt about a tablespoon of lubricating oil (normal engine oil) into each cylinder and then repeat the compression test. If the pressures increases appreciably, then there is likely piston ring and/or cylinder and piston wear. If after adding the oil, the pressure does not go up on the cylinder(s) that have reading variations, your valves may not be seating properly (worn/damaged seat or valve).

[This message has been edited by Neils88 (edited 01-28-2014).]

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