How to know when to do a compression test / leak down test? Please Help.

[Nick.M18]

Hi everyone, I'm hopefully going to be new to the family as I am looking at purchasing an 03 cobra with 74k miles. I went to see it this past weekend and it looked and ran great. I immediately fell in love. Quick background, I'm a huge car fan, but I don't know much about the internals of a car. I have been trying to do all the research I could to make sure the car was in good shape and to make sure I'm getting a good deal. I keep reading on these forums though about compression and leak down tests. With 74k miles, should I be concerned about doing such a test? I called a local mechanic, who has done really good work for me in the past, and he said that he didn't recommend doing it. He said with that mileage he is almost certain the car is fine. He asked me if the car was a 4.6 V8, in which I said yes. Being a 4.6, he seemed concerned about damaging something when removing the spark plugs to do the test. Wish I could be more specific than "something", but he lost me in the mechanic lingo. He said he'd give the car a look over so I may just have him do that.

Should I get a compression and/or leak down test?

Are there any signs that one would be needed prior to just doing it?

If test results came back with a leak, how "critical" is it? Should I stop driving the car altogether until fixed or is it okay to cruise around still until I get the money to do so?

What repairs need to be made with a leak?

Am I being over paranoid???

Any help would be appreciated.

 

[cj428mach]

If you already bought the car its pointless to do the test now. Might as well drive it.

 

[Nick.M18]

Haven't bought the car yet. Just agreed on a price, which is why I'm looking for information.

 

[cj428mach]

A leak down/compression test may not be necessary on your standard 4.6 but its not a bad idea on a supercharged 4.6.

 

[DSG2003Mach1]

sounds like hes confusing these with the 2 piece spark plugs in some of the 3 valve engines (i thought that was just 5.4 truck motors though) that tend to break in half and need a special tool to remove them.

mileage doesnt have a lot to do with it, a bad tune or mechanical issue that went unresolved can do a lot of damage in single WOT pull.

if the compression or leak down tests show a problem odds are its going to be a big repair bill to pull the motor and dig into it.

info on whats supposedly been done to the car would probably help with feedback.

 

[jrgoffin]

You'll want to get educated on the heads as well. Your mechanic may be using the word "damage" to go along whatever knowledge he may have of the problems that can arise when the heads with only four threads per plug get beat up.

This will help:

http://www.terminator-cobra.com/castings.htm

 

[Juiced46]

@ 74k miles ANYTHING is possible. A leakdown and compression test will tell alot about the engines health regardless if its running fine. Even if its running fine, there may be issues that will not show its face right away.

The spark plug thing could be a bunch of things. Some people blow the plugs out of the heads because they didnt check them or tighten them properly. Some times they get fused to the threads in the head and pull the threads out. I would for sure want to know if I can pull the plugs out of my newly purchased car since you will eventually have to replace them.

Next, is the car modded? Even if not 74k miles on a supercharged car anything can go wrong. I have seen near stock cars with leakdown problems @ 17k miles on 03-04 Cobras. Like I said, anything is possibly. If its your standard bolt on, pulley, tune car, I would want to check it even more. Even if the dyno sheet looks good, that does not mean the tune is spot on. You can tell alot about the engines health by reading the plugs as well.

First tell tale sign of a possible issue a small puff of smoke on a cold start. This sometimes can be masked by catalytic converters depending how severe or not the situation is.

Doing the leakdown and compression test is a very good idea for your investment. These cars can have blowby issues and the car will run "fine" but eventually can self destruct if you do not address it. Oil in the combustion chamber causes detonation, detonation causes the slow or fast death to your engine depending on certain circumstances.

 

[hotcobra03]

History helps..yet @74k I wouldnt expect much on repairs..more on modded than un modded to sell..


I will be the 1st to say this car can run almost flawless on 7 cyls.

ours has been a daily driver since new..

have had most basic Issues giving use..

however the head issues is another. .

I keep up on oil changes and keep engine clean..only fix what needs to be, but stay on top of the small things..

Ive only done compression test on mine..seems like when car doesnt run right and its not a plug or fuel filter issue , I do compression test..

3 times ive had issues on drivers side...valve seat drop and cracks.

both issues showed on test as very low or no compression..

180psi to 195 is what my last test was @225k on engine ..

I did all the work as a 1st timer with aid of this site ..

I havent had any plug issues..but did have issue with starters going bad during compression test

 

[Nick.M18]

Thanks everyone. I'm a lot more knowledgeable with all of your help. I'll definitely do the compression check before purchase.