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

[Aaron Kulkis]

Robert Meier wrote:
> 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.
>

Perhaps your Mass Air Flow sensor is reading high (engine
computer thinks more air is coming in than actually is,
and thus directs for more fuel than is needed).

By different means, a dirty air filter can also cause
an overly rich mixture.



75,000 miles is getting up there.
When was the last time the following services were performed:

Fuel Filter Replacement
Spark Plugs Clean and Re-gap OR Replace and re-gap
Spark Plug Wires Replace
PCV valve Clean or Replace
Throttle Body Clean
Air Filter Replace

Spark plugs are cheap ($2 each), so I replace those ever
12-24 months. Aging spark plug wires can result in weak
ignition, even if you don't have any misfiring.

A gummed up PCV valve allows the inside of the engine block
to hold high pressure gases (and also lots of nasty stuff
that you don't want in your oil).

Fuel filter... it's amazing how a used up fuel filter can
rob you of both mileage and power. I'm not sure if it could
cause overly rich readings, however.



> 		    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,
>