Engine Puzzler

[cobber112879]

Here's the story. Had a car come in , a 2001 Kia Rio. SES light on and odd noise from engine. SES codes were LT and ST fuel trims out of range. LT fuel trim about 24%. Noise was a vacuum leak from around the dip stick.

There isn't supposed to be vacuum in the crankcase of this particular car.

So I check the vacuum routings, all O.K. I replace the PCV valve, no change. I go get the vacuum routing diagram to see if there is something I missed. Everything is still routed correctly. I put in another new PCV valve to be absolutly sure this isn't the problem. Still no change.

Why is there vacuum in the crankcase? Not too much mind you , but there should be a slight pressure, not vacuum. I did not hook up the vacuum gauge, but there was probably 5 to 10 inches of vacuum present.

Car has a slight loss of power as well so I did a compression test. No more than 15% variance. A friend at another shop suggested a leakdown test. Sent over for leakdown test.Cyl 1 74%, Cyl 2 10%, Cyl 3 27%, Cyl 4 24%. More than enough to indicate a problem. (This car is under warranty, customer is not paying for any of this.) Removed head, head O.K no leakage by valves. Pistons are scored a little, so I replaced pistons and rings as per procedure. Honed the cylinders and put it all back together.

Problem is now all gone. Here is what I don't understand. How did the rings possibly create a vacuum in the crankcase? I have come up with a couple ideas but I just can't see it actually happening.

Can anyone here offer some wisdom? Thanks in advance.

[Ducati]

Interesting dilema.
Without actually seeing the vacuum gauge readings and the conditions under which they were done (vacuum can change with temperature, I understand - at least with diesels it does) so my Guess is that there was enough blow by from the offending piston ring to create the vacuum and throw the flow of fuel off.

This is a wild stab at it. I dont know KIA's, and dont even know if they are really good little cars or peices of crap. I really have not heard great things about them. (Which is odd, being with all the kudos Hyundai is getting)

Electronic systems really baffle me. I would be inclined to do everything that the very worst Canadian Tire mechanic would do, and replace everything that I thought might be part of the problem. You did an exceptional job of diagnosis, in my opinion.

[cobber112879]

Thanks Ducati for the compliment.

Just an after thought and a side question:

1. Is it possible that the rings were sealing O.K going up but not going down, thus creating a vacuum in the crankcase? I don't see how this would happen though, just a thought.

2. You commented on vacuum reading with a deisel engine. I was not aware vacuum was even applicable with no throttle plate. If you wouldn't mind elobarating on that it would be great as I would like to learn a little from a deisel enthusiast. I know you have extensive knowledge on the subject.

[Ducati]

"Extensive knowledge" - I doubt it.
Made every mistake in the book - more like it.

The piston ring travels an infintessimal amount, that being the gap of the ring land. I cannot see that a piston ring would stick on the compression stroke and yet be free on the downstroke. My guess is that blowby of gasses created the vacuum reading irregularity and contributed to the upset that threw the fuel delivery off. Electronic sensors would have picked up on that small change in internal pressure.

This might be a question to ask one of the moderators, Alpine Autowerks, who actually does this for a living, and not some hobbyist like me who works on cars/bikes under a tree while slugging back multiple Pilsner Urquell's and smoking cheap Dominican cigars.

Diesels are basically giant air pumps (but, isn't any engine, really?) and vacuum figures big, affecting such things as the injection timing and, in Mercedes Benz' anyways, everything from my power windows to windshield wipers.
The biggest collection of bugs in my Mercedes diesel with close to half a million k on it is related to vacuum leaks, and I am too damn lazy to just replace every line in the car. So I have become accustom to listening to hissing, and spitting on stuff to look for bubbles.
Your boss would shit if he saw you horking on a customers vacuum lines.

[cobber112879]

Hey, no better way to learn than to make every mistake in the book.:thumbsup: