Cell Mate From Hell
I was called to a shop with a complaint of an engine bucking problem on a 1994 Volvo 940 with a 2.3L
engine (Figure 1)
.
The car would start up and run well but when the engine was accelerated it would
start to buck and misfire. The garage performed a basic tune-up on the car by replacing the cap, rotor,
wires and spark plugs hoping to correct the problem but the problem still prevailed. The shop also replaced
the ignition module but this did not help either. The crankshaft sensor was already replaced by the
same shop a few months ago for a no start no spark condition so they decided to try another sensor
just in case the prior one was defective. The shop was well equipped with a Snap-On Modis and an Alldata
information system but car was an OBD I compliant vehicle and they did not have access to a Volvo factory
scan tool to put on this car. They believed that the vehicle might have an electronic control module
problem but at the price of replacing a computer they decided at this point to call me in for technical
assistance.
When I arrived there I started up the vehicle and it ran fine with no problems. As I snapped the throttle
it would hesitate and buck like a bad TPS or MAF sensor. I also heard a weird tinny noise coming from
the motor at times. I asked the shop about the engine noise and they told me that it was probably a loose
plate near the transmission bell housing. At this point I decided to hook up my Volvo scan tool to see
if there were any codes stored in memory and to make sure my readings were within spec. I did not see
anything that was jumping out at me to grab my attention but it just seemed from the way the engine was
running and misfiring that it was more of an ignition problem then a fuel problem.
I placed my scope on the crank sensor, coil primary, igniter trigger line and the injector signal. These
injectors were all banked together so all I needed was one scope lead to check the operation of the
injector driver circuit. As I started the engine the patterns seemed to be okay (figure2)
.
The injectors were pulsed for every 2 coil firings as seen on the scope. As soon as I raced the engine you could see
that one coil firing would drop out (Figure 3)
.
At the same time the coil primary dropped out I was loosing the igniter trigger signal from the ECM. This was not the igniter causing the problem but rather the ECM.
Why was the igniter trigger being lost to the igniter? Was this the fault of a bad ECM, bad igniter
trigger line or was the ECM getting bad information from the crank sensor at higher engine operation.
The resolution on my scope was not set to where I wanted to get a better view of the crank signal so
I set the time base lower to hone into the crank signal pattern (figure 4)
.
This pattern did not seem right to me by having two synch signals within one revolution of the crank. They were not even equidistant
from one another. To get an even better view I imported the crank signal pattern into the measurement
portion of the scope and zoomed in closer (figure5)
.
This flywheel had to have a bad tooth that was broken off the flywheel creating the pattern I was seeing. At this point I instructed the shop to pull
the transmission so that we could inspect the flywheel for damage and I would return to perform a further
inspection.
I left the shop and within 15 minutes the shop called me back to tell me that they fixed the Volvo and
it never ran better. I was in disbelief because there is no way they could have replaced a flywheel that
quick. The head tech then told me that this Volvo had a small inspection plate for the flywheel where they
were able to see the teeth of the flywheel and so they turned the flywheel by had to inspect every tooth.
Then they were shocked by what they had found with the flywheel. There was a bolt stuck in between two
teeth of the flywheel that was jammed in place. The bolt was the cause of the tinny noise when it came
in contact with the inspection cover under certain revolutions of the crankshaft. They pulled the bolt
out and the engine ran fine. They saved the bolt for me as a memory of what can go wrong in this business
(figure6)
.Okay, I was sold on the bolt but how did this bolt ever get into the bell housing in the first place. The
garage then told me that this car was towed in a few months ago with a no start no spark condition and his
mechanic replaced the crank sensor. During the process of replacing the sensor his mechanic dropped one of
the crank sensor bolts in the bell housing and decided to let it be where it came to rest and found another
bolt to take its place. After a few months of being kicked around in the bell housing the bolt finally
wedged itself into the flywheel only to create havoc with this car. This poor bolt only wanted out of its
jail cell of hell away from the flywheel cell mate who was unrelenting to constant abuse. I am sure this
bolt made its presence known by making a lot of noise banging around in the bell housing but this poor little
soul still did not receive the attention it needed. Its only escape was to obscure the integrity of the crank
signal to a point that would force the driver of the vehicle to visit a shop thus regaining its path to the
outside world.
I just can’t believe how a mechanic could drop something in an engine and just let it be. I could remember
the old days when guys dropped jets down a carburetor while trying to partially rebuild a carburetor on an
engine without taking the time to pull the carburetor off the engine and doing it on the bench. They would, during
those days, pull the carb and take time out to retrieve the jet. I guess in this fast pace world we need to be
more careful during a labor operation and if something is dropped in the wrong place it may just come back to
haunt us. I was just a little disappointed in myself that I disregarded that tinny noise as a potential problem.
I was unaware that this particular vehicle used an inspection plate to provide a view of the flywheel teeth.
I could have had a great picture of the bolt lodged in the teeth of the flywheel.