Dead Cylinder Test

Date: Mon, 7 Oct 1996 08:25:41 +0000
From: "Mike Graham" 
Subject: Update on Sick & Ailing
To: TLCAL@tlca.org

> Note:  I wasn't able to properly tell if dead cylinders existed because of a
> ring problem or if I did indeed have valve problems.  

I think I suggested this, but maybe it was a reply to a different
message:  Do you have one of those $20 engine analyzer things with a
tach and an inductive pickup?  If you have one of those or a timing
light with inductive pickup (like, the kind you clamp to the spark
plug wire) then you can check for *ignition* dead cylinders.  Just
hook the timing light or tach up to each spark plug, one at a time,
and see if the light flashes or you get an RPM reading.  If the
light doesn't flash or whatever on #n cylinder, then that cylinder
isn't getting spark.  This is a very quick test, but it can speak
volumes.  You should be able to complete the test in about 10
minutes. If some of the cylinders aren't getting spark, then you have
an ignition problem, most likely your distributor cap, and my
personal guess would be a slightly out-of-axis distributor gear rod.
 This causes the rotor to chew into the posts deeper on one side,
and eventually the posts on the other side don't get hit.  This
usually happens with 4-bangers, but it could account for the
behavior that you are describing.  Just pop the distributor cap and
look inside.  If the posts on one side are chewed up deeper than the
other, then a new d-cap will get you back on the road for a few
months until it wears through again.  The fix is a new distributor.
If it's not the d-cap it could be the wires or plugs, but it's
unlikely that they'd all go at once.  Primary suspect is the d-cap.

If all of the cylinders are getting spark, then there is another
test you can do.  It's called a load-share test, or power balance
test, and you need that $20 engine analyzer again.  Hook up the
analyzer so you have an RPM feed.  Now disconnect the plug wires
|from the spark plugs one at a time, and see if the RPM drops (you
will have to switch the analyzer to another wire before you pull the
one that it's hooked up to).  Write down the RPM before and after
pulling each plug wire.  Only one wire is pulled at a time, then it
is reconnected before pulling the next.  Now what this test tells
you is this:  if you pull a wire, then that cylinder stops firing,
and it is doing no work.  If the RPMs don't drop when you do this,
then the cylinder must not have been doing any work *before* you
pulled the wire, and it must, therefore, have bad rings or stuck 
valves or something.  Do this test with a warm engine, and if you 
have a catalytic converter (don't know how old your truck is, though 
you probably mentioned it a few times. 8-) then don't pull the plugs 
for more than 15 seconds at a time, and wait 30 seconds before 
pulling the next one.  This lets the cat cool down again (because it 
will get uncombusted gas in it, which will burn and heat etc.)  This 
is highly unlikely, but just in case you get the idea of using this 
test on your new minivan or something, if you have electronic 
ignition, the plug wires should be immediately grounded to body after 
they're pulled (keeps the electronics happy).  I think that's all the 
caveats.  Oh yeah, wear gloves to pull them because there *are* 
20-30,000 volts in there, and it's a sharp shock (not dangerous, but 
unpleasant) and it might be worth pulling all the wires before you 
start the test to make sure they'll come off easily when you're doing 
the test.
  A good, healthy engine shouldn't have more than 50rpm difference 
between the highest and lowest cylinders.  More than that and you 
could have a problem.  If you've got cylinders that don't seem to be 
doing their share of the work, then do a wet and dry compression test 
to check for rings/valves.

//////////////////////////////////////////////////////////////////////////
//  Mike Graham 
//////////////////////////////////////////////////////////////////////////
//  1981 BJ42 24V Diesel LandCruiser, "New brakes, new problems."
//
//  "Smell?  What smell?"
//
//////////////////////////////////////////////////////////////////////////

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Date: Tue, 05 May 1998 18:11:09 -0600
From: Tony Bartlett 
Subject: Re: still burning oil

Rob, one thing you can do is (one at a time) put each cylinder at TDC on
the compression stroke, this will keep the valves closed.  Then hook up
compressed air through your compression tester adapter (if you have that
type).  Somewhere around 100PSI would work fine.  Then listen at each of
these points

intake hose (would indicate a leaking intake valve)
exhaust pipe (would indicate a leaking exhaust valve)
oil filler cap (would indicate leaking rings)

There is a tool for this called a cylinder leakdown tester.  It will let
you know how much air is leaking (a little is OK).  But you really do
not need the tool.

One thing.  when you put in the compressed air and are not directly on
TDC then it will turn the crankshaft.  If you need to, use a breaker bar
and have someone hold the crankshaft bolt, the closer to TDC the better
it will be.

Toyota Tony
http://www.digitalpla.net/~offroader


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