My 2002 wrx has a history of misfire codes (e.g. P0302) every 30,000 miles
or so and the solution has always been to replace spark plug.
Right now, it has P0302 code when the car is driven for half an hour or
more. If I turn off the car for a while, it runs fine again.
Also, the idle speed does not increase when A/C is turned on. I wonder why.
What is the mechanism that increases the idle speed when A/C is turned on?
Does the A/C button sends a signal to the ECU to increase idle speed, or
does the ECU senses the extra load on the engine and increases idle speed to
Yeah, I noticed that neither my current 2008 Tribeca, nor my old 2000
OBW had their idle speeds increase when the AC was on. I think the AC is
probably running in a very low power mode during idle. Or even perhaps
disengaged during that time?
Well, with mine I can faintly hear the clutch engaging and the air coming
out of the vents gets cold so it is certainly doing something. With the
clutch engaged there really is no 'low power' mode - it is either pumping
Freon or it isn't. It seems likely that the engine computer is simply very
good at keeping the idle speed where it wants it to be. This wasn't so in
the old carbureted days when there was an idle speed solenoid to keep the
engine from stalling.
Correct - the engine speed does not NEED to increase with the AC on.
The throttle needs to open, or in the case of a GDI engine, more fuel
needs to be injected to increase the power output enough to MAINTAIN
engine speed. The ECU on today's cars is extremely powerful and fast.
On carbureted engines, generally there was only on or off for the
Yeah, it's more than likely that the idle speed is already optimized for
Interestingly, I found out some time ago, that the AC needs to operate
in the winter to defrost the windshield, as it is used to dehumidify the
natural moist air coming through the air vents.
Subaru messed up big time on this one. My Subaru would have a noticeable
drop in power whenever I had the misfortune to be compelled to turn on
the AC. As they say, they ain't building them like they used to. Subaru
could easily program their control system to replicate their cars of the
80s but it's this lack of attention to details that will be the downfall
I used to notice a drop in power on my old 2000 OBW too, but that only
started happening after it had gotten very old (on its 11th or 12th
year). I think it had more to do with natural engine compression loss
reducing the power overall. I used to actually turn off the AC on
certain uphill climbs around the end.
I had bought an ac cutoff back in the 80s but never got it installed on my
datsun. That automatically turned off the ac when you wanted maximum
acceleration. That was vacuum operated as well as the ac idle increase. I
don't notice much change in a slow steady acceleration in my outback, but
it was more pronounced in my cavalier. Yes, I do the test in every car I
drive. It's easier to check up hill.
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