Henry LaViers wrote:
> { snip of Mike D's very interesting re-post here on DiRT of Shane Moseley's
> posting from the Dakota Mailing List} <snip>
> After I read Shane's post, I was reminded of something that I learned from
> the ScanTool diagnostic scanner readout.
>
> When you first turn the key of your Ram to start the engine - in the brief
> moment before the engine is turned over - the PCM reads the MAP sensor very
> quickly and stores the result.
>
> This 'pre-start' MAP reading is used to calibrate the computer's
> programming. It is a 'barometric' reading in inches of mercury similar to
> that given by the weatherman on the television news. <snip>
This is true. Think of it as a way to get a baseline for all the subsequent
calculations to be based on. It is sometimes referred to as the current or
"ambient" pressure also. This is why if you start the vehicle at a low altitude
and climb to a much higher hill/mountain the vehicle will feel sluggish up
there. If you pull over and turn the engine (and keyswitch) off and back on
(resetting this calibration) you will regain the lost torque. Too bad the
factory was cheap and didn't provide a second MAP used esclusively for
periodically updating this calibration. On the other hand - I'm sure its not
much of an issue for those who live/travel in the flatlands but we mountaineers
have a problem. Could this be part of the "high-altitude" package???
> If my Ram's MAP reads too low at both places, then my computer is getting
> bad information. The MAP is telling the PCM computer that the air is less
> dense than it really is. The PCM then makes the mistake of injecting too
> little fuel. This makes the 'real world' air/fuel mixture too lean, and
> lean mixtures are more prone to ping at wide open throttle.
I don't think the problem is with fuel delivery. It should be correct as it is
constantly updating the pulsewidth based on feedback from the O2 sensor (except
during cold-start and WOT). This behavior can be verified using an Air/Fuel
meter. During extreme pinging the A/F meter shows correct fueling by bouncing
(rich,lean,rich,lean,etc) around stoich (14.7:1) due to being in closed-loop
mode.
The problem is more (if not all) related to the timing advance. Once the MAP
shows that a large load is being placed on the motor - the computer should be
retarding the timing a good bit according to factory testing/calibrating on a
dyno. With no load the timing can be as high as 36 degrees advanced and when a
high load is noticed on the MAP - it should be retarded to around 12-15 degrees
advanced depending on current rpm and map values. When the MAP is not giving
accurate reading - this much need retarding doesn't happen and the vehicles ping
like crazy. No matter how good the gas is - it will still ping. Kinda makes
you respect the older mechanical advance systems - they didn't have this
problem.
Anyway, I hope this helps. Oh yeah - borrow someones scan tool if you want to
watch this behavior in realtime while pulling a large hill. Its a good way to
confirm this as well.
Latr,
Shane
-- '96 IndyRam-HisIndy-MPI/TB/Pulleys/Coil/Wires/CompTAs '96 IndyRam-HerIndy-numbered(#142)"Track Truck" '74 Triple-Black Dodge Challenger Rallye 360 home-brew EFI R&D vehicle '68 Black Corvette Convertible 427 (For Sale)
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