Rough Idle on Every Start Up! Help!

KevoBox2010

Original poster
Member
Dec 3, 2018
1
Mississippi
2003 GMC z71. 5.3 Vortec. 239k. Ok, so for over a year now I've had running issues. All kinds. Started backfiring thru the intake. After a new fuel pump, filter, 2 bad coils, new plugs and wires ended up being stopped up injectors. So hope that helps if anybody is going thru that.
After the injectors the truck ran great for a while. Then one morning it started rough idling on start up. But it rough idles every start up,whether cold or running temp. It lasts between 30 seconds and a minute. Then smooths to about 800-1000 rpm. It will not take any gas without spitting and sputtering. Forget taking off. You can cut off and start right back up and it does it again. Every single start up!! Then back in the summer when it's hot outside if it sits for a while idling the temp hand will warm to probably 230° (210° normal) but wont yo higher. When you drive away it falls on its face and misses while back firing until it cools down some. Then it smooths back out. Even replaced the coolant temperature sensor. Then tonight I was driving and "Low Engine Power" displayed. Pulled over had an extremely rough idle. Replaced the Throttle Position Sensor and seems like it solved that issue. But still has the rough initial idle...any ideas on what to try next? I've checked grounds and replaced all vacuum hoses. 3 bad oxygen sensors. The mass air flow sensor has been cleaned. I've sprayed the entire engine down with carb cleaner trying to find a vacuum leaks...pcv valve replaced. New air filter. literally everything...any guesses?
 

mrrsm

Lifetime VIP Donor
Supporting Donor
Member
Oct 22, 2015
7,639
Tampa Bay Area
To Quote @djthumper in Paraphrase…

Codes… Codes… We NEED The Codes...”

In the Absence of Codes... You might Start with looking at THIS....

TS (Thermostat)--- This seemingly obvious mechanism Brass or Copper Poppet vs. Spring Pressure Mechanism for REACTING to changes in Temperatures is NOT one that should ever be ignored when High Temps are getting Sampled by the Coolant Temperature Sensor(s) and pegging the Temp Gauge on the Dash Panel.

If Combustion Chamber Temps exceed the limits set for this motor... They can get so HOT ...that the Engine will begin to experience Very Destructive “Pre-Ignition” Events. These are uncontrollable Explosions/Backfires that can occur instead of having the F/A Mixture burning smoothly and at the proper Compression Level to provide an even “Power Push” on all of the Piston Heads.

Excessively HOT Combustion Chambers caused by Lean Conditions occur when either the Fuel being added is TOO Little versus the amount of AIR being allowed into the Combustion Chambers... or... when an added amount of excess, Un-Metered Air is entering the IM Air Stream and Leaning Out the Mixture. The presence of higher Engine RPM at Idle can be a resulting Clue. Replacing the Old Coolant only with the proper 50/50 Mix with Distilled Water and DexCool should always accompany any repair (if it proves necessary) and ONLY using an ACDelco Thermostat during such an R&R ...is Paramount.

... and to continue on with a complete description of all the players concerning what the "Low Engine Power" Event Triggers might be::

MAF (Mass Air Flow Sensor)—- This Sensor actually WEIGHS the incoming air and in concert with the IAT Sensor... alerts the PCM as to the MASS of the air coming in in minute; weighed out in tiny Gram ‘chunks’ to compare against the Fuel/Air Tables in ROM and calculate how much Fuel to add via the EFIs at any given moment. In the Early Model Tbs & Envoys… the MAP (Maximum Air Pressure) Sensor performs this task. Cleaning the MAF Does NOT necessarily make up for one that is simply NOT working. Broken, Stretched or Damaged Wiring in the Harness or Bad or Loose Connectors will directly affect these matters.

IAT (Inlet Air Temperature)---- This Sensor measure the Temperature of the Ambient Air passing through it because the density of Air changes as function of Temperature and Altitude… and rather than rely upon the infinite variability measurement of the outside Air Pressure using a Barometer… the IAT works in concert with the MAF to do a pretty good job of telling the PCM what it needs to know about adding in the right amounts of fuel...whether you are on Pikes Peak… or down in the low hot desert of Death Valley. The MAF and the IAT are Side By Side ...and a Poor Connection at the MAF Harness or having any Damaged or Pulled Wires in the Harness will make these Sensors fail. Start by Pull the Connector(s) off of the MAF and check ALL of the Wires.

ACC (Accelerator Pedal)---- Unlike the ‘Old Bicycle Brake Cable’ style of controlling the position of the Throttle Body via an analog braided wire… Modern Vehicles use a Position Sensor that transmits Class 2 data using One High and One Low Voltage signal...at the same time ...to the PCM and essentially uses a Table to guess what the intent of the Driver when pressing on the “Gas” Pedal. The Throttle Body on Modern GM Engines plays the next part in communicating the actual Butterfly Blade position and remains responsive via a built in Electric Motor to re-position the TB opening in concert with the input from the ACC Sensor. So if THIS Sensor is bad and you “Step on The Gas”… and get a poor response... You will need a sophisticated enough Code Reader to look at the High and Low Voltage Variances to determine if THIS ACC Sensor has failed.

O2S (Oxygen Sensor(s)--- This Sensor is designed to one thing: Make sure that the LEAST amount of O2 is present in the exhaust stream at any given moment in Both Banks If the Exhaust Stream is Rich… the Long Term and Short Term Fuel/Air Balance will throw an O2 Sensor Code. If the Exhaust Stream is Lean… The O2 Sensor will alert the PCM to “Add More Fuel”...a condition that can become obvious when the engine RPM rises inexplicably...when you are not stepping on the ACC Pedal as the first clue. Usually there are TWO Sensors… The first being (Upstream) ...right off of the Exhaust Manifold and the second being (Downstream) aft of the Catalytic Converter to measure the CAT’s ability to efficiently break down the Exhaust Stream into non-noxious Carbon Dioxide, Water, Carbon...and the LEAST amount of SMOG components possible. Use the STFT and LTFT to figure out if any Un-Metered Air is getting past the TB Control to confuse and confound these sensors caused by a Vacuum Leak around the Intake Manifold. Use a Smoke Machine to locate IM Leaks or… Very carefully ... spray small amounts of Starter Fluid Vapor around the IM and listen for the Engine to automatically raise the RPM in response. Do NOT overdo this...and NOT in the Garage!

TB (Throttle Body)--- Instead of having the familiar Dual Cable Analog linkage between the Driver’s Gas Pedal and the Throttle Body Intake Butterfly (Baffle) to control the amount of Air coming into the Engine… The Later Model Throttle Body works in conjunction with the “Fly By Wire” ACC Pedal Sensor to more or less… make an educated guess as to what the Angle of the Butterfly Plate is in at any given moment to factor that restrictive port from as low as Idle all the up to WOT. If the built-in Control Motor is damaged or the Motor was abused by being “pushed in and out with finger pressure” during clumsy efforts to “Clean the Throttle Body”… the covered, in-dwelling Plastic-Nylon Big and Little Gears can either crack or lose teeth and create very sketchy operation.

Unlike the Carburetors of Old… all that the Throttle Body does on Modern Engines… is either be permissive or restrictive with Air Flow. The MAF, IAT, ACC, O2s and the PCM all communicate the right measurements to figure out the OTHER part of this equation of “HOW MUCH FUEL IS NEEDED?”...and whether or not to Advance Timing up and down the scale to handle varied RPM and Torque requirements. If ANY of these Air vs. Fuel Sensors fail to “hold hands” during the engine running…. It will set a “Low Engine Power” Code… and seeking other Codes that might help narrow the cause down from list is the next place to look.
 
Last edited:
  • Like
Reactions: Redbeard

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Before pulling out the howitzer of shotguns, lets wait for codes.
 
  • Like
Reactions: Redbeard

Forum Statistics

Threads
23,271
Posts
637,476
Members
18,472
Latest member
MissCrutcher

Members Online