Rough Idle HELP!

[02YukDenXL]

So I've been riding this out for a few months now. Still no codes. No real change to the situation. My misfire/shudder hasn't gone away and has ever so slowly felt like its been getting "worse". That might just be me being OCD.

I bought a vacuum pressure gauge after reading thru my Haynes manual. Hooked it up in-line with the brake booster vacuum line. Healthy engine vacuum is between 17-22 inches of Mg (Mercury). The gauge indicted 20 inches with little to no fluctuation at idle (Less than 1/2 an inch). A quick hit of the throttle to 2,500 rpm and vacuum went to 0 (expected) then spiked to 25 inches and back to normal idle pressure within a second or so (Normal operation following hit of the throttle as indicted by Haynes manual). This seems to rule out a vacuum leak, bad valve, rings, plugged exhaust, etc.

I have an actron diagnostic scan tool. According to the scan tool my idle vacuum is between 10 and 12 inches of Mg and my gauge reads 20 inches? Could I have a bad MAP sensor?

 

[02YukDenXL]

Gauge and scan tool readings at idle...

 

[VikingTrad3r]

maybe. it seems like you havnt purchased a 1$ cigar and blown in the brake boost line yet? That is rather simple and would rule it out for sure. on my vette that is how i found my leak.

also, did you get a fuel pressure reading while the shudder is happening?

did you confirm tps is good? the shudder could be related to a faulty input from the tps as you accellerate.

also did you ever replace the o2 sensor on the other side? I re read the thread and may have missed that sorry.

 

[VikingTrad3r]

just saw the pics. id check the tps sensor. easy peezy to check.

 

[about20ninj45]

Check this out!


Document ID: 1527156
#04-06-04-040: Idle Instability, Low or Rough Idle with
Electronic Throttle Control (Clean Throttle Body) - (Jun 14,
2004)
Condition
Some owners may comment on an idle instability, low idle speed or rough idle.
Cause
Condition may be caused by deposits in the throttle body bore.
Correction
Clean throttle body bore and throttle valve plate of carbon using a shop rag and an appropriate cleaner.
Refer to Engine Controls Repair Procedures - Throttle Body Cleaning Procedure. This procedure may be
performed at any mileage necessary. Check to determine if the PCM calibration has been previously
updated to one that includes the update for throttle body coking. For 2003 model year vehicles, look in the
operating system portion of the calibration. For 2000-2002 model year vehicles, look in the engine portion
of the calibration. If not, then update the PCM calibration to the latest calibration.
Warranty Information
For vehicles repaired under warranty, use:
Subject: Idle Instability, Low or Rough Idle with Electronic Throttle Control
(Clean Throttle Body)
Models: 2002-2003 Cadillac Escalade, Escalade EXT
2000-2003 Chevrolet Silverado, Suburban, Tahoe
2002-2003 Chevrolet Avalanche
2000-2003 GMC Sierra, Yukon, Yukon XL
2003 Hummer H2
with 4.8L, 5.3L or 6.0L V8 Engine (VINs V, T, Z, N, U - RPOs LR4, LM7, L59, LQ9,
LQ4)
and Electronic Throttle Control
Labor Operation Description Labor Time
J5480 Throttle Body - Clean and Adjust 0.2 hr
Add: To Reprogram PCM 0.4 hr
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these
technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a
vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and
safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that
condition. See your GM dealer for information on whether your vehicle may benefit from the information.
WE SUPPORT
VOLUNTARY
TECHNICIAN
CERTIFICATION
© 2008 General Motors Corporation. All rights reserved.

I was going to suggest the idle control valve, but its buot into the throttle body, so its not serviceable if it goes bad. I would replace the throttle body, instead of cleaning it

 

[VikingTrad3r]

good catch ninja. i would start by checking the voltage on the tps for free first though. if tps checks out then i would take a stab at that yes.

Check this out!


Document ID: 1527156
#04-06-04-040: Idle Instability, Low or Rough Idle with
Electronic Throttle Control (Clean Throttle Body) - (Jun 14,
2004)
Condition
Some owners may comment on an idle instability, low idle speed or rough idle.
Cause
Condition may be caused by deposits in the throttle body bore.
Correction
Clean throttle body bore and throttle valve plate of carbon using a shop rag and an appropriate cleaner.
Refer to Engine Controls Repair Procedures - Throttle Body Cleaning Procedure. This procedure may be
performed at any mileage necessary. Check to determine if the PCM calibration has been previously
updated to one that includes the update for throttle body coking. For 2003 model year vehicles, look in the
operating system portion of the calibration. For 2000-2002 model year vehicles, look in the engine portion
of the calibration. If not, then update the PCM calibration to the latest calibration.
Warranty Information
For vehicles repaired under warranty, use:
Subject: Idle Instability, Low or Rough Idle with Electronic Throttle Control
(Clean Throttle Body)
Models: 2002-2003 Cadillac Escalade, Escalade EXT
2000-2003 Chevrolet Silverado, Suburban, Tahoe
2002-2003 Chevrolet Avalanche
2000-2003 GMC Sierra, Yukon, Yukon XL
2003 Hummer H2
with 4.8L, 5.3L or 6.0L V8 Engine (VINs V, T, Z, N, U - RPOs LR4, LM7, L59, LQ9,
LQ4)
and Electronic Throttle Control
Labor Operation Description Labor Time
J5480 Throttle Body - Clean and Adjust 0.2 hr
Add: To Reprogram PCM 0.4 hr
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these
technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a
vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and
safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that
condition. See your GM dealer for information on whether your vehicle may benefit from the information.
WE SUPPORT
VOLUNTARY
TECHNICIAN
CERTIFICATION
© 2008 General Motors Corporation. All rights reserved.

I was going to suggest the idle control valve, but its buot into the throttle body, so its not serviceable if it goes bad. I would replace the throttle body, instead of cleaning it

 

[02YukDenXL]

I've been told by one of the three other mechanics I've been to that my fuel pressure is good but I should check it myself.

I never replaced the other O2 sensor as I was told a bad sensor would throw a code.

Already cleaned the ever living crap out of the throttle body. Not sure about the PCM calibration.

I'll look into the tps. Thanks guys!

 

[SLCHOE]

Who exactly are your mechanics? Wow. Having access to a Tech2 doesn't mean you know anything. 4 pages and no data from the mechanics on fuel trims or O2 data. He said your fuel pressure was ok but you should check it yourself? Does he not have a fuel pressure gauge? Holy hell...



I know people are probably tired of hearing me ask this but it really is a window into how the engine is running.

What are your long term fuel trims (LTFT) and short term fuel trims (STFT) at idle? Should be a percent value.

Quick test. Unplug your MAF and start the engine. Drive it. Does it run any better?

Here's a great video for understanding fuel trims and how important these are to diagnosing drivability issues like misfires and lean/rich conditions.

 

[about20ninj45]

I love when sclhoe chimes in on a troubleshooting thread, hes like a mechanic that lives in my phone lol

 

[02YukDenXL]

I'm no impressed by the mechanics I've been to. I'll list my fuel trims in a few hours.

I did unplug the maf and it did run better for a few minutes before returning to the same drive ability issue/shudder/misfire/etc.