Irregular idle

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 looking at to diagnose the problem?

Thanks

Reply to
Steve Lewis
Loading thread data ...

Have you scanned the computer for any error codes?

formatting link
Doc

Reply to
"Doc"

Reply to
Elliott

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)

Reply to
Neil Nelson

Reply to
Elliott

Reply to
Elliott

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 manifold. 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.

Reply to
Neil Nelson

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:

Reply to
Steve Lewis

Reply to
Elliott

If he vacuum leak is big enough, it can effect other vacuum circuits.

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!

Reply to
Neil Nelson

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.