<VV> Banging Noise from Engine

[Jim]

Several members of my local CORSA chapter [ACORN] and I have been trying to
track down the source of the noise on a new member's car, a '65 Monza,
110-PG ['64 engine].  Here's what's happened:

 

BACKGROUND

Late last fall, our new member bought the Monza from out of town.  He was
going to take it out for a drive when he found that the brakes didn't work
properly while going out of his driveway [he lives in the country and has a
long driveway].   So he drove it back to his garage and put it away for the
winter.

 

This spring, one of our members visited and helped him sort out and replace
the brake parts that were causing the problem.  They were getting ready to
take the car out for a test drive.  When they started the engine, it began
making loud, irregular banging at idle:   BANG! [irregular pause] BANG!
BANG!  [irregular pause]  BANG!  [irregular pause]  and so on.  These were
LOUD metallic bangs that occurred on an irregular frequency and an
irregular number at a time and were all the same; no screeching, grinding,
squealing, or any other sounds.  Sometimes it would idle for a minute or two
with no banging.  In between the bangs the engine seemed to run okay.  No
stalling, no decrease or increase in the engine idle speed.

 

The sound was not that of a dropped seat [I've had that happen to one of my
Corvairs before], and none of us had heard a sound like this before.  It was
scary, and we didn't want to rev the engine, no knowing what the source was.

 

INITIAL DIAGNOSIS

The sound was coming from the driver's side of the engine.  To check the
cylinder operation, we began pulling each plug wire off at the distributor,
one at a time as the engine was idling.  Pulling the wires off of cylinders
2 and 6 caused the engine to slow, as you'd expect.  Pulling #4 did nothing,
so something was wrong there.  Also, when the engine was idling with all the
plug wires in place, there was a "chuffing" sound at the tailpipe which
indicated that a cylinder wasn't doing its share.

 

After shutting off the engine and letting it cool, we pulled the plugs from
#4 and #6 and took a look into the combustion chambers while turning the
engine over by hand.  #6 looked normal:  We could see the top of the piston
come up to the top of the cylinder and then go back down.  #4 came up and
appeared to ENTER the combustion chamber before going back down.  Broken
piston?

  

DISASSEMBLY FINDINGS

The parts necessary to remove the head came apart easily, and the heads
apparently had been rebuilt or cleaned at one time.  The previous owner had
used Viton seals, etc., and it looked like the work was well done.

 

After removing the top cover, we inspected the crankcase and could not find
anything amiss there.   Looking at the top of the piston in the cylinder and
turning the engine over by hand while keeping the cylinders in place didn't
show that anything was amiss there either.  

 

We removed the piston and cylinder as a unit and inspected the bore of the
cylinder while moving the piston back and forth.  The cylinder wall looked
pristine; the piston moved without difficulty.  HUH?

 

NEXT STEPS  

At the time of the disassembly, we didn't have a ridge reamer or ring
compressor to remove the piston from the cylinder, so that will be next as
well as removing the two valves from the head to confirm that the seats are
okay [Just to be sure!].

 

QUESTION

If we don't find a broken piston, what could cause the banging?  [Can a
broken piston even cause such a noise?]  PG torque converter?  

 

Your thoughts and suggestions would be appreciated.


Jim Bartasevich