I was called to a shop with a complaint of a hard start & hesitation on acceleration on a 97'Toyota Camry
with a 7A-FE engine (Figure 1)
.The
engine would start up and run slightly rough and as you would leaninto the throttle to rev the engine it would cut out. The shop had already installed a TPS, MAF sensor,
checked for an exhaust restriction and checked the timing belt. It was becoming more of a “feels like”
diagnostic run for the shop and we have all been in this situation before. Failure pattern diagnostics
can sometimes pan out but there are situations like this one that can eat up some valuable time and not
to mention cutting into your parts profit if you can’t sell everything you already have into the job.
The shop did not know where to turn next so I was called in to give them a second opinion.
When I scanned the system I discovered that there were codes P0120 and P1300 stored in memory (Figure 2)
but these codes were generated by the shop during their attempt to fix the vehicle. It not uncommon to
find codes generated by a prior repair shop or even the car owner himself. These types of stored codes
can throw you for a loop leading you in the wrong direction. I find it best to just write down the codes
stored along with the freeze frame information and clear them to see which ones come back. Then you have
to start the interrogation process to find out how these codes were generated and see if any of the codes
now stored held value to lead you in some type of direction.
The engine was experiencing a low engine vacuum condition at idle (Figure 3)
and ran poorly like it was out of time. In my experience over the years I can tell you that this was always contributed to late
ignition/valve timing or something as simple as a stuck open EGR valve. This was not an EGR problem because
this engine would not respond well when the engine was throttled up. A stuck open EGR would usually be more
related to an idle only problem. The quickest way for me to validate a valve timing or ignition timing
problem at the same time would be with the use of a pressure transducer in the cylinder chamber coupled
with a synch probe. This would save me valuable time in disassembling components that may not be the source
of the problem.
I removed a spark plug from cylinder #1 and placed my pressure transducer in he cylinder chamber using
the proper adapters. I next used a spark tester on the plug wire for the #1 cylinder and placed a synch
probe around the plug wire. I started the engine and captured a waveform. You can see by the waveform
(Figure #4)
that the valve timing was okay. The exhaust valve opening was about 30 degrees
before BDC of the power stroke.This you can see after the peak rise in the waveform where the pattern begins to fall and
rise again at the next large purple cursor representing 180 BDC.Each smaller purple cursor represents 30
degrees of crankshaft rotation.I usually find exhaust valves to open about 30-45 degrees before BDC of the
power stroke so I was satisfied knowing that the valve timing was correct for this engine.
The ignition timing however was occurring about 10 degrees after TDC of the compression stroke.This you can
see by the first rise of the synch pattern after the peak rise of the compression waveform. This vehicle
had an ignition retard problem which most likely was caused by a crank cam correlation problem. The crank
sensor read off the trigger wheel on the end of the crankshaft and the cam sensor was located within the
distributor housing. The crank trigger wheel was secured to the crankshaft journal by a key and the distributor
was non adjustable. It was more common for a trigger wheel to move due to a damaged keyway and cause a cam
crank correlation error that could result in late or early computed ignition timing. A corrupted or wrong
signal signature could also play a role in this problem as well. I needed to get a closer look at these
signals by using a scope.
I tagged the cam and crank signals along with the ignitor trigger signal. You could see from the waveforms
(Figure #5)
that the ignitor signals were not equidistant across the screen. The ECM was
having a problem controlling the ignitor. The cam and crank sensors seemed to be creating good patterns but I could not be
sure of their proper correlation because I did not have a good known cam crank correlation pattern for this
vehicle. Having a good known pattern makes life a lot easier and it is always a good idea to save as many
as you can into a database for later retrieval. Sometimes with a little soul searching you can actually
find one on the internet through databases created by certain organizations such as iATN that share
information gathered by other technicians around the globe.
I knew from the compression waveform that the timing gears were properly indexed so I had to concentrate
on the validation of the cam and crank signal signatures. As I took a closer look at the crank sensor
waveform it did not seem correct. It seemed to have 2 very distinct synch patterns when most crank sensors
in the industry that I have seen usually provided only 1. These synch patterns did not seem to be placed
in an equidistant pattern within the trigger wheel and they did not follow the same signature. One was an
imposter telling lies while the other was the real deal.
I instructed the shop to pull the timing cover so I could inspect the crank trigger wheel. As the engine
was slowly turned by hand I discovered a broken tooth on the trigger wheel (Figure #6)
.
This missing tooth was causing all the problems with the incorrect ignition timing. The ECM was confused into thinking there
were 2 synch signals and this caused the ECM to restart the ignition timing sequence each time it saw the
synch signal come around. The odd thing was that the ECM never set a code for the crank sensor. The ECM was
only doing what it was being told by the sensor. The fix here was to replace the trigger wheel with a new
one.
What a turn of events in this diagnostic arena to find a chipped tooth on a crank trigger wheel. I would
love to know how that even happened in the first place when this trigger wheel was securely hidden behind
a plastic timing cover. That is, unless someone damaged it during as prior labor operation and did not tell
anyone about it. I am sure plenty of us have done this before! The good news is that the owner of this
vehicle will be one happy camper to finally get his car back and the shop can now move on to more work to
make up for this nightmare.