V8 compression test queries

[markuskj]

Hello guys,

I own a 96 V8 and just got the results of a compression test back but am not sure how to interpret them. Any help would be most aprpeciated:

"125 115 90 90
140 115 120 130 psi"

What is the expected result for a healthy engine?

The mechanic mentioned that the lower readings were irregular and may be due to "piston crown errosion, common lotus engines".

Any thoughts or advice greatly appreciated!

Mark

[cnh1990]

Thats quite a spread.

Are the low cyl reading the ones with the wet plugs?

>> Edited by cnh1990 on Saturday 28th May 14:37

[zhastaph]

I think the manual specifes 150 psi +- 15 psi as being a healthy engine.

Though a more true comparison is look at the spread of compressions and if any look inordinately low compared to the average then start to worry a little. This would generally imply that the 2 90s don't look all that healthy


Edited to add;

Can I ask, are you actually having a problem that you are trying to diagnose with a compression test? Or are you just compression testing for the sake of it?

>> Edited by zhastaph on Saturday 28th May 15:13

[markuskj]

cnh1990 said:

Thats quite a spread. Are the low cyl reading the ones with the wet plugs? >> Edited by cnh1990 on Saturday 28th May 14:37

Good question mate, I will confirm that.

Does anyone have any thoughts as to the casue of the low compression, or the treatment I need to seek?

[markuskj]

zhastaph said:

I think the manual specifes 150 psi +- 15 psi as being a healthy engine. Though a more true comparison is look at the spread of compressions and if any look inordinately low compared to the average then start to worry a little. This would generally imply that the 2 90s don't look all that healthy Edited to add; Can I ask, are you actually having a problem that you are trying to diagnose with a compression test? Or are you just compression testing for the sake of it? >> Edited by zhastaph on Saturday 28th May 15:13

Thanks for the info on the ideal psi, should all readings be roughtly the same?

In answer to your second point, yes - the probelm was raised here: www.pistonheads.com/gassing/topic.asp?f=17&h=&t=181257

I am not sure if the two are connected but even after the sparks were dried and waterproofed, the engine still goes through bouts of great driving to lumpy in the same day.

Thanks again for all help so far!

Mark

[zhastaph]

I looked through your history to see if you had a prob and didn't notice that

I recently compression tested mine {I was having similar probs}, I think that there was probably the best part of about a 30psi sweep between the best and worst readings {appox 120 to 150}. Sort of similar to what you have excluding the 2 90's - which I have t say do look grim

Is she smoking at all? Losing water? Overheating?

It's also worth repeating the compression test on the 2 suspicous cylinders after the car is nice and warm and been run a bit in case they are erroneous.

Possible causes are piston rings, head gasket, cracked head, dodgy valves/seats. Head gasket is prolly the favourite particularly with 2 adjacent cylinders.

[markuskj]

zhastaph said:

I looked through your history to see if you had a prob and didn't notice that

No worries

zhastaph said:

I recently compression tested mine {I was having similar probs}, I think that there was probably the best part of about a 30psi sweep between the best and worst readings {appox 120 to 150}. Sort of similar to what you have excluding the 2 90's - which I have t say do look grim Is she smoking at all? Losing water? Overheating?

None of the above mate, this is the strange thing. No other signs we can work out, and since I got her back she feels pretty good to drive, thought still lumpy - just less pronounced - could this be caused by poor compression on 2 cylinders?

zhastaph said:

It's also worth repeating the compression test on the 2 suspicous cylinders after the car is nice and warm and been run a bit in case they are erroneous.

Good suggestion, I will do that. I am contemplating purchasing one of the pc diagnostic tools for the engine - would this help diagnose this sort of issue.

zhastaph said:

Possible causes are piston rings, head gasket, cracked head, dodgy valves/seats. Head gasket is prolly the favourite particularly with 2 adjacent cylinders.

This list contains some of the more costly repairs I know but at least it gives me a reason to open her up and confirm if there are any deeper issues - any suggestiosn on costs for this sort of work? I am ame knowledgable of baisc engine mechanics ut but not yet up to delving.

Thanks so much for your advice so far!

Mark