NEED HELP Idle speeds up when trans downshifting to a stop

jhenry999

Original poster
Member
Jun 28, 2015
27
Okay, here's the facts: bought this VERY NEGLECTED 05 Envoy 4.2., 149K miles.
Running on 4 out of six, replaced 2 bad coils, Replaced unbelievably dirty air filter, sprayed MAF sensor really well, removed throttle body and cleaned years of hardened carbon off. Runs pretty smooth now...slight vibration which I think is motor mounts. But.... IDLES faster than it should.

My other two 4.2s (Trailblazer and Rainier) Start from COLD around 1200rpms and very quickly calm down to 650.
Not this Envoy. Starts up to 2000rpm and slowly gets down to 800.or 900. Lowest idle I've seen is 800.

When warmed up and stopping for a red light the rpms SPEED UP as the transmission downshifts.

It's now idling around 1000 while I'm stopped at the light and holding it back with the brake.

In other words;; IT BEHAVES LIKE A CARBURATOR ENGINE WITH THE CHOKE STUCK ON.

It's tempting to try to Blip the gas pedal to make the choke come off and the speed come down, but, of course, there is no choke.

Transmission fluid and filter change tomorrow.

ANY IDEAS?

Thanks!
 

mrrsm

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Supporting Donor
Member
Oct 22, 2015
7,639
Tampa Bay Area
In a PCM Computer Controlled environment... it all comes down to Four Players...Not necessarily responding in this order:

(1) When the Oxygen Sensors (O2) detect ANY Extra Oxygen in the Exhaust Stream... If there is any present... The O2 Sensors interpret the Extra O2 as "Needing Fuel" and respond by advising the PCM to provide it via Fuel Pump pressurizing the EFI set up with commands using Pulse Width Modulation (PWM).

(2) A Dirty or DAMAGED Throttle Body (TB) mechanism with a Bad Motor driving the Butterfly Baffle and/or stuck open will allow that engine to GULP air and if stuck open at WOT... the PCM responds as listed in (1).

(3) The Accelerator (ACC) Pedal Sensor responds via TWO signals; One High and One Low to signal the Throttle Body AND PCM as to How Much or How Little Air to let in... A Bad ACC Sensor = A Confused Throttle Body.

(4) Your vehicle is equipped with a Mass Air Flow Sensor (MAF) located at the "Source of Nile" for Air Intake. It literally weighs how much air there is that passes through it and tell the PCM this information constantly. If the MAF is as FILTHY as was the Air Filter you R&Rd ...or the Connector Plug is FUBARed or Shorted Out... Then PCM has NO idea how to read its Fuel to Air Ratio Tables and will find it impossible to find anything close to "Stoichiometric Perfection" of 14.7 Parts of AIr to 1 Part Fuel. (Start by pulling the MAF and Cleaning it ONLY with CRC MAF Spray Cleaner).

So between what the (MAF) reports to the PCM on what the amount of (MASS) of the available Oxygen is + What the ACC Pedal Input is + What the actual Throttle Body Position is + What the O2 Sensor(s) are reporting determines in the PCM's BRAIN "How Much Fuel Should I Allow into the Engine?" moment by moment.
 
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Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Is the CEL on with stored codes?
 
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