Thursday, September 15, 2011

When engines run too fast, or too lean . . . valves break

Last week, we received a 1990s Range Rover Classic that ran rough and made some noise.  A local garage had changed the plugs and wires, to no avail.  They did a compression test, and found one dead cylinder, with 25 PSI compression.  A normal reading on a truck like this is 175.  That's when the owner decided to ship the rig to us.

We did a leak down test, where we put air into the cylinder through the spark plug hole, to see where it comes out.  Air came out the exhaust as fast as we put it in.  There was only one thing to do - we pulled the head.















We found just what I expected - a burnt exhaust valve.  You can see the two valves in the cylinder head photo above.  The intake is on the left; exhaust on the right.  See the ragged edge on the right side of the exhaust valve?  That's the failure.  Here's the valve, removed from the head:















In this photo the failure is unmistakable.  That was where the story got complicated.  The owner said, "The engine only started skipping after I got the truck back from having a new transmission installed."  I asked why he had the transmission changed.  "It stopped shifting gears," was his reply.  "It stuck in low."

I decided to look at little closer.  Look real close at this photo of the piston.  Note the little line just below center right:




















It looks to me like the valve touched the top of the piston, ever so slightly.  BTW, the liquid in the cylinder is residue from disassembly.  It has nothing to do with the repair.

I thought about what the owner had said, and I concluded one of two things must have happened.
A - He ran the engine too fast because the transmission was stuck. As a consequence, the valve "floated" and hit the piston, causing it to fail
B - The engine was racing way too fast with a light load because the transmission was stuck.  As a result, it ran too lean, and the lean running caused that cylinder to overheat because its injector was a little marginal.

I don't know which explanation is correct, and of course one of you readers may suggest something totally different.  For now, I am going to use my best judgement and recommend we change that injector, and fix the valve.  Changing the injector covers both the bases for us.  The trans repair took care of the over-revving, and the injector will take care of possible leanness in that cylinder.

As you can see, there is no sign of damage on the cleaned up piston:



















The valve looks just as fried as ever.  We'll fit a new one, and new springs just to be safe.  We will also touch up the valve job on all the other cylinders and clean up the seats.




















If you go back to the top photo, you'll see there was black carbon all over the valve.  What does that tell us?  It says someone tried to drive this truck a ways with the failed cylinder.  That suggests it may have failed earlier than the owner thought.  However, it does not change the diagnosis.  

Is there a moral to this story?  I think so. When your car starts to fail, or act at all unusual, park it.  Don't drive it home and then say, "it sounds funny."  By the time you get there, it may be too late.  This fellow started with a transmission repair, but when he kept driving, it became an engine repair too.

In his defense, I must say that we see this all the time. Modern cars have loud stereos and lots of sound deadening around the motor.  Most of the lights and gauges we had 20 years ago are gone.  It's easy to drive today's vehicles until they go up in smoke, and then ask ourselves, "How did that happen?"

As owners (I am an owner too, not just a repair guy) we have to be extra observant because the sounds and smells that used to warn of danger are no longer there for us to see, unless we look real close.  Check your fluids every now and then, and if you ever feel something may be amiss, slow down, open a window, and listen and smell.  It it ticking?  Does it smell hot?  Is that gauge pinned in the red?

In the end, if our cars fail, we pay the price.  So it pays to be vigilant.

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