First Time Poster, Long Time Lurker just saying "Howdy!"

[RD Joe]

Hey all, long time lurker, first time poster. This is my '03 Envoy SLT 4.2 w/3.73 rear end that I got last year from my uncle's estate when he passed away. It had 114,000 miles when I got it, now with 121,000. She's all stock except for removal of that hideous "baby coffin" resonator (had a muffler shop just weld on a piece of pipe with some subtle bends to keep it tucked-up about even with the lower edge contour of the bumper). It's mostly a weekend grocery getter and frequent 200-mile freeway round trips to see family which is where this vehicle really shines - set the cruise at 80, put on some tunes, and this is one of the best freeway flyers I've ever owned. It was garage kept and well maintained, but some components are at high time.

What's been done so far:

• Set of 4 Goodyear Forteras, stock size.
• Serpentine belt; water pump, thermostat done about a year before I got it; idler pulley still good; old belt, 3/8" ratchet and pipe stowed in rear floor storage.
• Blower motor control module (fan stopped working, tested for 12V in and out, and that was the problem).
• AC compressor with new ACDelco unit (clutch took a dump).
• Power steering fluid cooler (stock one rusted out, replaced with the Dorman aluminim unit).
• Bilstein 4600 rear shocks, Moog sway bar links. All the bushings are still in really good shape, so not messing with anything else back there until I need to.
• ACDelco "Professional" ready struts, ACDelco sway bar links, 4-wheel alignment. I was going to rebuild the assembly with my springs and new Bilstein 4600s, but those struts were on back-order everywhere; and I couldn't take the oceanliner body roll and rattle from the sway bar links. So I kept my original assemblies and will rebuild with the BIlsteins at a later date. Right now I'm quite happy with the ACDelco ready struts as the vehicle rides 100% better, and with the new links there are no more rattles. Tie rods, ball joints still good.

What needs to be done:

• COILS AND PLUGS - Started it yesterday to go grocery shopping with the Mrs., and while idling the RPMs dropped, almost stalling, then perked back up to the usual +/- 650 rpm (it's done that a few times before intermittently over the last year). Happened again and then stalled. Re-started but only now running on 5 cylinders. Drove it about a mile to see if I would get a light, and I got the flashing Check Engine. I don't have a scanner and don't want to drive it somewhere to have it scanned, so I just pulled plugs and found #4 wet and fouled - I'm willing to bet that if scanned I'd get some kind of 0300 code. I'm going with the NGK 6-pack of coils for $260 at Rockauto and the ACDelco iridiums.
• Rotors, pads, complete fluid bleed
• Drain/replace tranny fluid and filer
• Drain/replace rear end gear oil
• U-joints
• CHIP TUNE! That GM TMS drives me crazy! And hopefully, they can do something about part-throttle/closed loop shift points, too.

Any feedback and suggestions welcome!

 

[mrrsm]

Welcome to GMT Nation...

For that "Sudden Dip and then Rise in RPM" problem our favorite Alumna Member @MAY03LT would probably suggest performing a Thorough Throttle Body Cleaning in this way... (Make certain that the Rubber Gasket re-aligns properly during re-assemby and note that a Dirty Air Filter can be an Accomplice in this Crime):

While you have the Throttle Body in your hand...Give it a Little Shake Test...to discover if the Internal Nylon-Plastic Drive Motor Gears are damaged. If you hear ANY Rattling... be suspicious that it might be causing the RPM change or throw other Codes if it is failing. Haunting the Local Salvage Yards with a Pick, a 1/4" X 10mm Socket, Ratchet & Short Extension might be the answer in getting a Used One. They sell new on Amazon for close to $300.00.

 

[RD Joe]

Welcome to GMT Nation...

For that "Sudden Dip and then Rise in RPM" problem our favorite Alumna Member @MAY03LT would probably suggest performing a Thorough Throttle Body Cleaning in this way... (Make certain that the Rubber Gasket re-aligns properly during re-assemby and note that a Dirty Air Filter can be an Accomplice in this Crime):

While you have the Throttle Body in your hand...Give it a Little Shake Test...to discover if the Internal Nylon-Plastic Drive Motor Gears are damaged. If you hear ANY Rattling... be suspicious that it might be causing the RPM change or throw other Codes if it is failing. Haunting the Local Salvage Yards with a Pick, a 1/4" X 10mm Socket, Ratchet & Short Extension might be the answer in getting a Used One. They sell new on Amazon for close to $300.00.

Thanks for the heads-up; I'll clean the TB while I'm in there. It's funny because that idle stumble is very intermittent; the last time it happened was like 3 or 4 months ago. The air filter is clean, and other than this recent problem this truck cruises like a dream and runs like a scalded dog at WOT.

While we're on the subject...are these throttle bodies really sensitive or something? Everywhere I read, "clean the throttle body" seems to be a frequent response. I've had quite a few injected engines, and I can't say that I ever really paid too much attention to them (other than spraying some injector cleaner down there once or twice a year) and never had any problems. I'm just really curious.

 

[mrrsm]

The Cable and Electronically Controlled Throttle Bodies came along when the California CARB Standards and the EPA came IN and Carburetors went OUT forcing the Automotive Engineering Industry to switch from Metering Gasoline over to Metering Air.

In the GM Systems and others...this meant that the PCM has to WEIGH the Air coming through the Air Filter in Grams Per Second using either a MASS AIR FLOW (MAF) Sensor and/or a (MAP) Sensor for measuring Atmospheric Pressure inside the Intake Manifold or both. Also this activity has to be monitored at just the right Air Temperature using the (IAT) Sensor present in the (MAF) Sensor Unit. The MAF also solves the problems of different Elevations and Barometric Air Pressures.

The Electronic Brains behind all of this (ECM or PCM) has to collect and use all of these inputs along with knowing what the "Driver's Intentions" are at all times by knowing the positions of the ACC Pedal Sensor AND the Throttle Body Baffle Plate. Those inputs, along with so many others like those form the Exhaust Camshaft via the (CPAS )and (CPS) Sensors, the Crankshaft Sensor (CKP) and the Oxygen (O2) Sensors are all needed to calculate exactly How Much Fuel the EFIs are allowed to parsimoniously Spray into the Engine Cylinders at any given moment.

If either the highly responsive Electric Motor and Gearing mounted and sealed on the side of the Throttle Body are compromised ...OR if the ACC "Fake Gas Pedal" gets Out of Whack... the PCM will automatically place the Vehicle into LIMP MODE. This can also happen if the MAF Sensor Heated Wire Resistor gets Real Dirty . Perhaps in your case, a dirt ridge formed solidly around the inside of the Venturi opening of the Throttle Body and has managed to intermittently 'grab and hold' onto the Baffle Plate, As mentioned, this can cause the Engine to Surge at Idle and prompts the action of Good Cleaning with Carburetor Spray Cleaner.

 

[RD Joe]

The Cable and Electronically Controlled Throttle Bodies came along when the California CARB Standards and the EPA came IN and Carburetors went OUT forcing the Automotive Engineering Industry to switch from Metering Gasoline over to Metering Air.

In the GM Systems and others...this meant that the PCM had to WEIGH the Air coming through the Air Filter in Grams Per Second either a MASS AIR FLOW (MAF) Sensor and/or a (MAP) Sensor for measuring Atmospheric Pressure inside the Intake Manifold. Also this has to monitored at just the right Air Temperature using the (IAT) Sensor present in the (MAF) Sensor Unit.

The Electronic Brains behind all of this as the ECM or PCM has to collect and use all of these inputs along with knowing what the "Driver's Intentions" are at all times by knowing the positions of the ACC Pedal Sensor AND the Throttle Body Baffle Plate. Those inputs, along with so many others like those form the Exhaust Camshaft (CPAS )and (CPS) Sensors, the Crankshaft Sensor (CKP) and the Oxygen (O2) Sensors are needed to calculate exactly How Much Fuel the EFIs are allowed to parsimoniously Spray into the Engine Cylinders at any given moment.

If either the highly responsive Electric Motor and Gearing mounted and sealed on the side of the Throttle Body are compromised ...OR if the ACC "Fake Gas Pedal" gets Out of Whack... the PCM will automatically place the Vehicle into LIMP MODE. This can also happen if the MAF Sensor Heated Wire Resistor gets Real Dirty . Perhaps in your case, a dirt ridge formed solidly around the inside of the Venturi opening of the Throttle Body and has managed to intermittantly 'grab and hold' onto the Baffle Plate, As mentioned, this can cause the Engine to Surge at Idle and prompts the action of Good Cleaning with Carburetor Spray Cleaner.

Thanks for that. So, in short, these fly-by-wireless throttle bodies are really sensitive to dust and dirt. I'm an engineer, so I get the technical stuff, and I have a pretty good history of building 2- and 4-stroke motorcycle racing engines and old- and new-school high-performance and racing car engines (did a lot of R&D on the new-at-the-time GEN III LS V8s back in the late 90s - early 2000s using the Delphi ECUs they made for Mercruiser Marine).

 

[Mooseman]

For the tune, we usually recommend @limequat at lime-swap.com. He really knows these engines and does it at a better price than the others. He can turn off TMS, improve shifts and other stuff you can discuss with him.

 

[JerryIrons]

Thanks for that. So, in short, these fly-by-wireless throttle bodies are really sensitive to dust and dirt. I'm an engineer, so I get the technical stuff, and I have a pretty good history of building 2- and 4-stroke motorcycle racing engines and old- and new-school high-performance and racing car engines (did a lot of R&D on the new-at-the-time GEN III LS V8s back in the late 90s - early 2000s using the Delphi ECUs they made for Mercruiser Marine).

Welcome Joe! You will find out there are lots of people here who really know a lot about these vehicles, it's a really nice place to get info. You have the inline 6 right? These engines can take a ton of miles, mine is closing in on 300,000 miles with original transmission.

If you clean your throttle body, which you will at some point, just be sure after to unplug the battery for a bit and reset the computer so it doesn't try and run with a clean throttle body, but has made adjustments over time to a dirty one.

Is yours 4wd? You might also consider changing transfer case fluid and front differential oil.

Welcome to the 7 quarts of oil change club