What does a compression test tell you?

[Spencer]

I’m looking at used e30’s and I know that when people get used cars inspected they often get a compression test. What does bad compression mean? Major work?

TIA

Spencer

[UNHCLL]

bad compression means you have leakage past what should be sealed parts of the engine.

IE: leaking past rings... requires rebuilding bottom end
leaking past valves ... rebuild the head

[Spencer]

I think I remeber hearing about M20's and cracked heads. Is this something that can easily be tested for?

-Spencer

[Ryan Stewart]

Its not really where the compression will tell. Usually its valve seals or rings like he said.

The place where the weakness is on teh back where the camshaft seal is. Its hard to see too, its the worst place for a problem like that.

[Spencer]

What needs to be rebuilt if the head is cracked? $$$

-Spencer

[Ryan Stewart]

Ohhh, dunno about rebuilding it at that point. It would probably be easier to just get another.

I think Metric Mechanic welds on a thick aluminum plate on the back there to reinforce and support the part below the camshaft seal to prevent the crack. I guess you could do that to cover the crack but I wouldnt want a crack there at all....

Why, is your engine having problems?

[Spencer]

I sold my eta this past week and i'm looking at an 88iS.

It's going to be inspected I just want to know more about the possible problems and what is involved in fixing them.

-Spencer

[UNHCLL]

What needs to be rebuilt if the head is cracked? $$$

-Spencer

throw it away if its cracked.
its a POS boat anchor.

remedy for cracked head = buy new head.

compression, leakdown are the 2 main things to do when inspecting a car for purchase.

[Spencer]

What's leakdown???

TIA

Spencer

[e30Matt]

A measure of the inability of a cylinder to hold compression. Instead of measuring pressure, like a compression test, a leakdown test measures pressure loss.

[e30evolution]

and you can tell where it is coming from too......

you need compressed air for this test, and the equpmet for the test is much more expensive than a compression tester

[Rob]

this will be in the FAQ section soon

[Spencer]

Is a leakdown test something that any specialized German car shop will be able to perform?
If the car I’m looking has shows bad leak down should I walk away?

Thanks for all the help guys...I don't want to buy a lemon :?

-Spencer

[UNHCLL]

Is a leakdown test something that any specialized German car shop will be able to perform?
If the car I’m looking has shows bad leak down should I walk away?

Thanks for all the help guys...I don't want to buy a lemon :?

-Spencer

Yes on the shop question.
Not necessarily on walking away.
Look for less than 25% leakage. However, *most* e30's have ~ 25-30% on at least one cylinder.
EDIT: be suspect if they do the leak down and its all good, except for 1 cylinder. IE: 12%/12%/15%/35%/18%/19% that'll cause problems later on. Its better to have uniform leakage/bad compression across the board, then to have 1 cyl that is very bad.

They will be able to tell you if its past the rings, or through the head.
One way to tell for yourself: blue smoke on COLD start-up = leaky valve guides.