I have a 94 Suburban with a 350 motor. Just in the past week, I've
noticed that after starting it (just when it's cold), the idle lopes a
bit. After it's warmed up, the problem goes away. What should I be
FWIIW, My 96 Sonoma's idle was irregular the time I left the
oil fill cap off (it even stalled a couple of times going to work
and back that once ~20mi). It seemed to stabilize after a while
running too (before I found the oil cap was off).
Thankfully the cap stayed near where I had set it down. I put
it back on and the idle was normal again. I figured it affected
the vacuum indirectly through the PCV, and that other vac
problems could easily have a similar effect.
It didn't set any codes either.
No, it didn't effect the vacuum indirectly...
Leaving the oil cap off allowed air to enter the engine that
wasn't measured by the Mass Airflow Sensor (MAF), thus, the fuel
mixture and timing were calculated wrong.
Because the PCM has no way of knowing whether the air is being
measured correctly other than the MAF sensor, and if you allow
air to bypass the MAF, it doesn't know it.
None of this is anything new.... Volkswagens did the same thing
back in the early 70s. (my first encounter)
Sounds like your description is more accurate to how the
computer sees it. I was thinking simplistic (ie: the PVC sucks
a vac onto the crankcase). It did seem odd that the PVC
could allow that much vac-leak - I guess not.
Neil Nelson wrote:
Air enters the breather tube which is plumbed after the MAF, it
is drawn thru the crankcase where it picks up the blow by and oil
vapors, then it goes thru the PCV valve and into the intake
IOWs, a standard PCV system except that the breather tube (crank
case inlet) is situated that air must go thru the MAF before it
reaches the breather.
If he vacuum leak is big enough, it can effect other vacuum
Got called over to another shop Wednesday.
92 Lumina with a 3.1 V-6. Runs like crap.
Low amps waveform of the ignition primary strongly suggests that
there is at least one shorted coil primary, shop owner replaces
coil pack, waveform now looks good but the engine still has a
miss-fire. (runs better but not right yet) We check the
injectors, good current flow there.
Connect scope to secondary, firing voltages sky high in the 40KV
range. Hmmm, has to be running lean and it's effecting all
cylinders, connect scanner to check the fuel trim numbers, yup,
it's off enough, MAP voltage is 3point something volts, I pull
the breather tube off of the intake snorkel to see if maybe I can
feel a stronger than normal vacuum that might suggest leaking
intake gaskets. The vacuum on the -breather- tube damn near
tears my thumb of it so strong.
Yesterday the shop owner tore the top of the engine apart....
Cylinder #3, a very large portion of the roof of the intake port
in the head along with the intake rocker and stud is just lying
there! That's probably the worst vacuum leak I've ever seen!
I've got a little more info on this problem. I've checked for vacuum
leaks and checked the PCV valve. Everything looks fine there.
The vehicle is hard to start when it's cold; I crank the engine to the
point where it feels like its ready to go, but then it just dies. After
it's been started it idles high and irregularly - the tach shows it
idling between 700 - 950 rpm. After its warmed up, just couple of
minutes, the idle evens out and falls down to under 500.
Could it be a problem with the injectors?
Steve Lewis wrote:
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