Hi, my car was recently diagnosed as having 30% compression in one of the cylinders. I don't have to get smog certificate until next May. Power seems fine, it just idles funny. No smoke, nothing. What might I expect as I drive it? Will smoke start to appear? Can it suddenly go kerplunk? Or will I start to see signs it's about to go kerplunk?

Just wondering if anyone has prior experience/knowledge about such a situation.

Thanks, John.

 

A leak down test gives you a percentage figure 30% leakage is way above normal - but what are the actual compression readings in PSI? And was it determined where the 30% loss was going? Pistons or valves?

 

compression can only go out three ways. Rings, Valves or head gasket. If you have an NA engine, good compression is 180 PSI,for a turbo its about 150. If you do your own compression test, start with a warm engine, then pull all the plugs. Crank for 5-10 turns of the engine, which should be good enough to get a stable gauge reading. If you find one that is low, you can drop about a tablespoon of oil into the spark plug hole, refix the gauge and try again. If it bumps to normal, that points to the rings. If its still low then you can do a leakdown test or get an inspection scope to look for a burnt valve. If you suspect a head gasket issue, there's other tell tales - loss of coolant, mix of oil/coolant, or in this case, high coolant system pressure or exhaust hydrocarbons in the coolant. You can buy an HC coolant test kit at Autozone etc. for about $40.

 

Thanks for replies.

Mechanic did not say, and doubt he went into it enough to determine if rings/valves/gasket.

Of the three scenarios, which is worst on the bank account?

Is it possible the car will drive until May and the failed smog test and state buy back?

 

OK - why did he do a leak down test? Does the car run bad - do you have a complaint? Did he just get a new cool tool and want to try it out on someone? (They are fun to play with)

A leak down test takes more time than a compression test - and you need to make SURE the pistons are in the correct position to have the valves closed. That not as easy as it sounds - especially on a 5 cylinder engine.

So - the whole point of doing a leak down is to find an answer to a complaint - and in your case there is an unusual outcome (your 30% leakage) - but there's no point in spending the time doing a leak down test if there's no diagnosis afterward.

Oh Dude - you got 30% on one cylinder - OK wtf does that mean? Did someone do this for grins or did you pay for this 30%?

 

The complaint I took it in with was a weird idle that had the engine bouncing around slightly. Thought it was going to be a coil and plug replacement. He called two/three hours later and said one piston had 30% compression. Told me a used engine or rebuilt would cost $2000 either way.

 

A rough idle could certainly indicate a miss firing cylinder due to compression ignition or fuel problems - unfortunately compression is the most expensive to fix. If you have a valve failure it may get worse quickly and cause a constant miss-fire.

 

For a non-turbo:
30% compression (180psi X .3) = 54 psi. That isn't enough for the fuel to ignite. You'll be driving around to next May sending unburnt fuel to the cat.
That'll likely burn it up.

mt6127's post lists things you should do..

$2,000 for a junkyard engine or rebuild installed(?) sounds like a great deal.