How to Prove an Engine Misfire
Posted to Technical Discussion Forum on 12/24/2009
197 Replies
I am putting up my information here for all to see. It took
a while to put all of this together. I hope I didn't make
too many mistakes.
I am admitting that I could not prove what was causing a
dead miss on cylinder #2 on a 95 Chevy S-10 Blazer with a
4.3 liter V-6. The vehicle has 153,000 miles and is a first
time customer. What I am trying to accomplish with the help
of everyone here is to develop a test that can be performed
to isolate this type of failure without undue or unnecessary
testing. This is the 4th vehicle in about 2 years that we
have seen this happen on. We have a mechanical problem that
I am unable to prove with any specific tests in my (limited)
arsenal.
You can refer to my original post located
http://members.iatn.net/forums/read/msg.aspx?f=forum2&m=236670&fv=4&ar=2143
to see what many others thought about this particular
situation.
This engine uses a single fuel injector that feeds 6
separate poppet nozzles which snap into the lower intake
manifold, aimed at the back of the intake valve for each
cylinder. The manifold is designed as such that the
possibility of reversion feeding fuel from one cylinder to
another seems impossible to me.
It has a single coil with a high voltage switch
(distributor) and the spark is triggered from the crankshaft
position sensor. I believe it was GM's first use of the low
profile distributor cap that has become known to have a high
incidence of crossfire internally.
The customer's comment was a rough idle and low power and a
CHECK ENGINE light that was on. The vehicle exhibited a
typical "dead miss" on the very short test drive into the
shop. She stated it happened all at once. The misfire was
evident at all engine speed and engine loads. The trouble
code that was stored was a P0302 (misfire detected cylinder
#2). Fuel trims were reportedly at +20% at idle after
resetting and running for a while. No problem right. Easy
diagnosis right?
Just a little background for clarifications sake. I am the
shop owner, and I let my techs do all the work, unless it
becomes a problem child, then I put them on something
productive while I do the testing and show them what I
found. This lends some reason to some of the redundancy of
the tests that I am going to present.
This started out as a simple misfire. My tech swapped the
plug and wires to see if the misfire moved to a different
cylinder after checking for good spark at the plug wire with
and ST125 spark tester. Nothing changed. He then did a
compression test on that particular cylinder only and
nothing was out of the ordinary. We then used some
carburetor cleaner into the throttle body to see if the
misfire would go away. One time we thought it did, the next
time we thought it didn't. The test was inconclusive to us,
so we called the customer and sold upper intake removal to
inspect the fuel injector poppet nozzle. My techs performed
the poppet nozzle test while I was away and said everything
looked the same when comparing #2 cylinder with #6 cylinder.
I then instructed them over the phone to swap the nozzles to
a different cylinder to see if the misfire changed, as
seeing a 10% difference would be impossible. After swapping
the nozzles, the misfire remained on cylinder #2.
I talked to my techs about how they had performed the poppet
comparison and found out that they were not allowing the
fuel pump to run during the test and that they were doing
only a visual test. During this time, the first tech became
sick and since the shop was slow, I instructed my second
tech to swap the injectors a second time, just in case
something got mixed up the first time with the testing,
multiple techs, etc. After the second time of swapping the
poppet nozzles to different cylinders, the misfire remained
on #2 cylinder. This rules out fuel delivery to me 100%.
Here is a picture of the intake manifold with the top
off
I then proceeded to have tech #2 do a running compression
test on the car, as the first tech was out sick. He checked
the cranking and running compression on 3 different
cylinders. #2 (the problem cylinder), #4 the neighboring
cylinder, and #1 on the opposite bank. The cranking
compression, in numeric order was:
#1 = 150
#2 = 155
#4 = 165
The running compression was:
#1 = 70
#2 = 60
#4 = 70
Using a manual gauge with the Schrader valve installed and
taking the reading at the same rpm after the needle
stabilizes, then bleeding the gauge and repeating the same
pressure every time at least 5 times in a row with no change
in the pressure. My thoughts at this time, was that we had a
valve sealing problem. I was elated due to the fact that
this is another one of those that were rare, the customer
was in no hurry as the wife was out of town, and I wanted to
try out my vacuum and pressure transducers to be able to
narrow down what was causing the problem. My only concern
was that I was going out of town.
Initially, performing secondary ignition testing with my
scope, I could not isolate which cylinder was causing a
problem. The secondary pattern was such that, not being one
who uses secondary analysis a lot, I could not even isolate
which cylinder was actually misfiring. I discussed
procedures with my techs and they both assured me that
cylinder identification was done manually by disabling spark
to each individual cylinders and that the computer had
identified the correct cylinder. The next day (Thursday) I
was scheduled to leave town for a long weekend vacation, but
I wanted to get some pressure waves with high resolution so
I phoned a technician that owns a Pico scope and has and FLS
sensor and explained my predicament. Due to some
communication issues and my inability to be here while he
was present at the shop, we didn't get any FLS waves of this
engine. He did mention the fact that fuel trims can tend to
cover up lean issues, so I cleared the codes the next
morning and scoped this thing. The secondary confirmed the
cylinder and showed what was happening in the cylinder.
Since I have had problems with my scope in the past, I
assumed a scope issue, because even after running a while I
could still see the problem very readily.
Secondary parade pattern
Single cylinder view
Another Secondary parade pattern
Jason Parkin has seen and diagnosed this type of secondary
pattern before here:
http://members.iatn.net/forums/read/msg.aspx?f=forum2&m=236837&fv=4&ar=2144
And confirmed an exhaust valve that was too tight.
I performed my own running compression tests and came up
with:
#1 = 75 @ 680 rpm, and snap to 150
#2 = 70 @ 680 rpm, and snap to 150
Dang. Only 5 psi difference in idle pressures with a dead
misfire. But my techs results showed 10 psi. I had recently
talked to an automotive instructor about the problem I had
on the previous three engines proving a valve sealing
problem and he assured me that he has tested thousands, and
his spec was anything over 5 psi difference in running
compression indicated a valve sealing problem as long as
engine rpm was within 50 rpm when compared. According to my
scope, it was less than 10 rpm difference. Still shows a
valve sealing problem, but I would never have thought that
was enough to cause a dead misfire. Low power contribution
maybe, but a dead miss?
I instructed my service writer to sell teardown and
inspection of the cam and lifters with the potential of
performing a valve job on this car. They were instructed to
remove the RH valve cover and back off the rockers slowly
first to see if they could make the misfire stop. But since
my first tech was out sick the rest of the week, no repairs
were performed before I came back. This was great for me as
I wanted to get some waves to prove what was happening to
this vehicle.
Tired of reading yet? You wanted the info right?
Anyway, Monday morning I'm back at this thing cold, after
setting for 4 days. This thing has been at my shop
approximately a week due to scheduling, sick tech, etc. and
has missed ever single solitary revolution since it has been
here. So I didn't even start the car after setting. I wanted
to see if there was any difference between a hot and cold
engine.
My thought process is this. Perform cranking compression and
leak down tests cold, then proceed to check them hot. I
performed the compression tests and leak down tests, then
proceed to the running compression test on a cold engine.
The leak down test showed each cylinder holding
#1 = 80%
#2 = 90%
#4 = 85%
(This part was from memory as I didn't write it down). On a
dead cold engine. What I am sure of is the 90% on cylinder
#2 was the highest of all the cylinders tested and it was
5-10% higher than the others.
I start the thing up and it is running perfectly. Rev the
engine, torque it, etc, and it runs great.
DAM. DAM. DAM. Just in case you are wondering, that is not a
misspelled curse word, that DAM is there to help me hold
back my anger VBG :^)
So now I have this thing in here with all this time spent. I
know it has a mechanical problem, but I don't have enough
data yet to prove to myself what it was except for the
running compression tests, pressure waves, and scope
patterns. I am not convinced the running compression tests
are that conclusive, but it is all I have to go on. That and
past experience. My tech's running compression test shows a
10 psi difference. My running compression test shows a 5 psi
difference. We do have some running pressure waves that
compares good and bad cylinders and most of them hover
around the 5 psi difference between a good cylinder and a
bad cylinder.
Missing Cylinder #2 before
Cylinder #2 After Valve Job and lifters
Good Cylinder Before
Good Cylinder After
Vacuum Wave Before
Vacuum Wave After
Another Fixed Vacuum Wave
Good Vacuum Wave After
So this begs the question. Do you sell a 4 figure repair
based on 5 psi of running compression? Or was it 10 psi?
At this time, I removed the valve springs from both of the
#2 valves to test for any sticking, binding or lack of
sealing. Applying 10 psi of air and up to 120 psi of air. I
can feel nothing wrong with either valve, they spin free and
seal immediately regardless of the air pressure applied. Oh
well, I know the problem was there so we sell a valve job
and replace both lifters, just in case. The car runs great,
but then again, it was running good just before the
teardown, even though it had been running bad for a week
prior.
It is my firm belief that we had one of two things
happening. Either we had a lifter that was not collapsing
properly or we had a high pressure valve sealing problem. If
the valve could hold cranking and running compression but
still had a dead miss, my thoughts are that it was not
closing completely or that it could not hold the pressures
of combustion and would leak under high pressure. For this
we rely on prior experience outlined below.
We were working on a 2004 Chevy Impala with a 3.4 liter V-6.
It had similar problems except that it would only miss at a
slow idle. It would not miss if the engine was revved up
even slightly above warm idle. That one, we could find
nothing wrong with the original leak down and cranking
tests. The only thing we could find was the running
compression was 10 psi lower that two other good cylinders.
We performed a valve job only to have the exact same
conditions. We found on that one, that the intake lifter was
stuck and not collapsing. Loosening the intake rocker arm
1/4 turn fixed the misfire on that vehicle, so we replaced
all of the lifters and fixed the problem. We were able to
confirm that the lifter would not collapse in a vise. After
disassembly, it looked perfect. After reassembly and pumping
it up manually, it would retract normally. We found the
camshaft lobe was not perfect, but the customer declined
repairs.
Here are some waves from that vehicle.
Missing #2 cylinder pressure
Missing #2 cylinder with BDC mark
Good cylinder pressure
Good cylinder with BDC mark
Vacuum Wave 1300 rpm #2 leaking intake valve at idle
Vacuum Wave 1481 rpm #2 leaking intake valve at idle
Vacuum Wave 1500 rpm #2 leaking intake valve at idle
Vacuum Wave 780 rpm #2 leaking intake valve at idle
So, what I am looking for from the greatest minds here is
some type of test that can be performed that will prove to
me what is wrong when I have a mechanical problem. Some say
5 psi difference running (at idle) compression is enough. I
would like to believe that, but I haven't checked enough
engines to know. Until about 2 years ago, I have never seen
a mechanical problem that I could not prove. I am relatively
new to the running compression test thing and now I have
these high tech procedures at my disposal to analyze them,
but I still can't seem to prove it. Before you answer,
consider the fact that when this vehicle was fixed a repeat
of the running compression test revealed that #1 cylinder
which had 75 psi before the repairs, now has 70 psi running
compression. The exact same pressure that was measured on
cylinder #2 when it was missing prior to repairs. But #1 is
running perfectly and not missing.
Is there a minimum pressure that is required when running? I
am thinking this test as well is inconclusive. Anybody out
there been doing a bunch of them? What have you come up
with? I have loads of captures on this vehicle. Before and
after. I know we had a mechanical problem. I don't know what
it was.
I have some pressure waves on the 2004 Malibu, and some
vacuum waves. We know for a fact that it passed all cranking
compression tests and passed a leak down test. The only
difference we could find was 10 psi running compression,
measured at idle with a manual gauge screwed into the
cylinder with the Schrader valve installed. There is a
discrepancy between the manual gauge and the pressure
transducers, not the least of which is caused by using
different hoses and the schrader valve.
Scott from Missouri