[mdlug-discuss] [TECH SUPPORT REQ] OBD-II typical readings

[Robert Meier]

Aaron,

> Bigger question... what exactly is the car doing that is
> driving (heh!) you to monitor its behavior so obsessively?

The car is a Saturn SL2 2001 with 75Kmi.

In late July, I had new tires put on the car.
The prior tires were Firestones with 75Kmi on them.
The new tires are same radius Cooper tires.

As always, I kept track of gas mileage,
and expected to lose a percent or two simply
due to widing of tire with new tread.

The actual gas mileage dropped from about 30mpg to 25mpg (~20%).
The car also seemed to be struggling up some hills I remember
having no trouble climbing and otherwise appearing slightly "sluggish".
After a second tank of gas confirmed the mileage drop,

I tried pressurizing the tires to 4psi over the door plate figure,
as the old tires had been.  There was no noticeable change.

I was too busy trying to satisfy a long-hours job, and so couldn't
readily take the car to the mechanic until about 1month had passed.
I confirmed the presence of adequage oil, transmission fluid, coolant,
and my inexpert eye could not find any disconnected/broken hoses/clamps/...

Taking a coworker to lunch on M-59, the speed topped out at 45mph
and the tach was ~4000rpm.  I scheduled to take it to the mechanic
the next day.  On the return trip, the speed topped out at 15mph,
with everything I could think of to reduce engine load.
A few days before, on the weekend, I'd used it with only slight
sluggishness compared to pre-July performance at Detroit highway
speeds.

I dropped my coworker off and took it directly to the mechanic.
Enroute to the mechanic, I tried pushing the accelerator to the
floor (when traffic permitted) to engage the "flood mode" switch
that leaned the fuel injection (to permit starting when flooded).
After three such operations, the cars top speed climbed to 45mph
at which I made it to the mechanic.

The mechanic originally said the computer reported multiple
trouble codes (the MEL was NOT lit), and that the transaxle output
speed sensor was not reporting.  The mechanic assumed that
most drivers would not have noticed that the speedometer was
reporting 0mph.  I would have noticed a speedometer reporting 0
and did not at anytime see such.

The next day, the mechanic said that the speed sensor was working
and the misdiagnosis was due to misuse of the monitoring equipment.
They did change the fuel filter (which was already reported to
need replacement (50% blocked) when next convenient).  The
mechanic reported that the car behaved fine in multiple test drives
and they could find nothing wrong.  The mechanic recommended that
I merely watch the car until I could learn something deterministic.

The mechanic did suggest watching the transmission change points
and said the torque converter should be bypassed at 47mph.

Since then, despite the time limits of the above-mentioned job,
I stepped up my long-planned efforts to install a modem (already
assembled) in the car and begin monitoring (planned for baseline
measures, now unfortunately for diagnostics).

I have found that its misbehavior is intermittent,
usually but not always when the outside air temperature (not the engine)
is hot.  (Driving the car in the cool morning, cool evening, on
a cloudy day, or after sitting in the shade, it seldom misbehaves.
After sitting on a hot parking lot on a sunny day, it frequently
but not always misbehaves.)

When it misbehaves, the tach rises above 3000rpm without downshifting.
When it is not misbehaving, it seems less peppy than I remember,
and it takes several times as long to accelerate to I-75 speed,
as I remember when I was commuting daily to/from downtown.

At no time have I seen the MEL lit, nor have I found any trouble
codes in the queue.  I've been unable to identify conditions
under which it reliably misbehaves.

I've verified accurate dashboard speedometer reading with a
gps receiver on several occasions.  On two occasions, when
I had neither palm nor gps, I observed my dashboard speedometer
reporting less than posted speed while keeping up with traffic
(once on M-59 and once on I-75).

The monitoring tool is OBD-Gauge running on a salvage (for 5$,
suffers total amnesia every few days) Palm.  It reports and graphs
(not uploadable) throttle position, tachometer, vehicle speed,
calculated load, ignition advance, coolant temperature,
manifold air pressure, short term fuel trim, long term fuel trim,
manifold air temperature, oxygen sensor (pre-catalytic),
oxygen sensor (post catalytic).  I understand 0 fuel trim
indicates stoich burning; negative readings like I see,
indicate rich mixture.

		    normal observation		misbehaving
tachometer:	    2000-3000			2500-3500
vehicle speed:	    (matches speedometer)	(matches speedometer)
calculated load:    ~33 in drive with brake	
		    ~25 in park
coolant temp:	    ambient->95<->105oC		same
manifold aid press: ~100KPa engine off
		    ~40KPa idling
		    ~20KPa under load
oxygen sensors:	    250-750mV (almost white noise) same
short fuel trim:    ~0 to -40/99 averages -13	same
long fuel trim:	    ~0 to -23/99 averages -9    same

On one occasion, I observed an OBD vehicle speed of 0, while the
car was in motion, and the dashboard speedometer was apparently
reading correctly.  (This may have been loss of data cable connection.)

Reporting,
-- 
Robert Meier

  "Always go to other people's funerals; otherwise, they won't go to yours."
     -- Yogi Berra