The owner took a nice ride up to Erie Pa and did some
shopping at the wineries for the holidays. That gave him the trip that he
needed to run the fuel out of the truck, and have some real time for the
cleaner to do its job. Over the weekend he had a couple more shorter trips and
by Monday when he stopped by the shop the smile on his face was promising. He
reported that he could clearly feel a difference in the truck, he was certain that
its performance was notably improved.
The thing to do now was to collect some data with the very
same checks that were done initially. What was found was at idle the long term
trim was 24% and the short term was 19 for a total of 43%. That’s better but
only marginally. At 2500rpm no load it was 19% long term and 12% short term.
That was clearly better at 31% total trim but still not in control. At 1500rpm
in gear against the brakes it was also much improved and in the total range of
30%. Its important to note that the same two monitors were still not complete,
the evaporative and the catalyst. With our sudden cold snap, the evaporative
monitor not completing was no surprise since a number of the code enable
criteria have minimum ambient temperature requirements in the 40f range. The
catalyst monitor didn’t run because the fuel trims were still too lean to allow
that to take place at idle. While the
fact that the fresh fuel and the cleaner made a difference was promising, there
was still a pretty good chance that the truck might need new injectors. The
owner planned to return in two days so we could check one last time.
On Wednesday afternoon he returned only this time he
reported that the light was back on. However it was running even better than it
was on Monday. So what do you do with the light being back on? The right thing
to do is approach it exactly as any other repair visit, and that means forget
everything you already know while you are doing the diagnostics. With that plan
in mind here is what was found with the truck.
The code was a P0171 again, lean air/fuel ratio. The freeze
frame showed that it had set at idle. Starting the truck up the total fuel
trims were 40%, with the long term being 25% of that. Speeding the engine up to
2500rpm the total fuel trim dropped to 20%, with the long term at 19% and the
short term at 1%, that’s a big difference from the previous data! Loading it up
against the brakes at 1500 rpm the total trim was below 10% with the long term
at 7% and the short term varying around 2%. That’s good enough, not perfect,
but good enough. So why was it so lean at idle still, and what would you do at
this point?
False air, or a vacuum leak are quite probable now because
they will have a big impact with low engine speed and low cylinder charge
demand. Under faster engine speeds there isn’t as much time to impact the air
fuel ratio, under load against the brakes there is little to no manifold vacuum
so a vacuum leak can’t impact the air fuel ratio much at all. That means its
time to get the smoke machine out and check for a leak. With the mass airflow
sensor at the air cleaner, wrapping the air filter with plastic wrap will allow
all of the intake system to be checked. A few moments later smoke was seen
pouring out from under the resonance chamber that is in-between the MAF and the
throttle body. Pulling it off the breather hose wasn’t connected to the valve
cover, and the oil staining in the area suggested that it had been off of there
for some time.
Once it was secured back in place, the plastic wrap removed
from the air filter and re-installed,
the fuel trims were reset with the scan tool and it was time to
re-evaluate the fuel trims. By resetting the fuel trim, the short term trim
alone had to make all of the correction, at idle that was now 8%, 2500 rpm no
load 5% and against the brakes 5%. The truck was fixed but we weren’t quite
done yet, he still needed his emissions test.
In Pa he passes with one monitor incomplete, and in this case that was
going to be the evaporative monitor because of the ambient temperature as well
as needing to have a cold soak where the coolant and intake air temperatures
had to drop within the code enable criteria range.
The customer was provided specific instructions for how to
drive a cycle and about twenty minutes later he returned with enough monitors
completed to allow the emissions test to run and his sticker finally issued.
This truck and how it was diagnosed and repaired is based on
training case studies written into CTI classes by Jim Garrido. The best way to thank him for sharing this
kind of information is to attend some of his classes ASAP.
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