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Avalanche Owners Zone => Chevrolet Avalanche Problems, Questions & Answers => Problems: Engine => Topic started by: dwdooley on March 24, 2019, 20:25:40 PM

Title: Rough Idle below 900 rpm. I've tried EVERYTHING
Post by: dwdooley on March 24, 2019, 20:25:40 PM
2005 5.3 flex fuel Custom Avalanche with 150,00 miles will not idle smooth below 900 rpm.
Magnuson Supercharger, Dynatec long tube headers, Corsa Sport tube exhaust with high flow cats. (New Cats are about 2 years old.)
I clear codes and once it idles down and starts idling rough at 500 to 600 it then triggers Lean bank 1, and lean bank 2, plus multiple misfires.
Since this has an aluminum intake, shouldn't have itake gasket leak but I have sprayed Carb cleaner around intake for leaks but fuel trims stayed constant with no idle change.
Process of elimination listed below, all done in last two weeks. I have done all this my self and I'm 66.  Getting to old for this stuff.
1. New flex fuel A.C. Delco fuel pump and New Evap Canister = Now have a solid 60 PSI at fuel rail.
2. New coil packs, Plug wires are new, and installed new Autolite Iridium XP plugs gaped at .040" (The NGK TR6's pulled out were clean and light brown).
3. New AiRaid Filter .... old one was just old.
4. New intake bellows after MAF .... found a tear in it.

After start up it ran smoother for about 20 miles but not perfectly smooth, then it slowly got worse and back to same condition as before.
I then used my scanner and continued to find my STFT adding 30% to fuel curve ..... BUT HERE IS WHERE I'M STUMPED ...
When I disconnect the MAF it slowly idles down to 700 and runs Great!   ,,,,,, Sooooo

5.  I Overnighted a Delpi MAF and just installed it .....WOW ..... still does the same thing!
6.  Cleaned throttle body with carb cleaner too

I NEED some Knowledge here PLEASE?
What is weird is that when I disconnect the MAF my engine runs great and actually accelerates hard and steady. Only issue w MAF unplugged is that when you let off the gas it takes about 3 seconds to drop down to 700 rpm from around 1300 rpm.
In closed loop  and on the O2's it is  1.1=.6   2.1=.7   1.2=.3   2.2=.0
I am not real good at reading 02's
Here's the kicker .... when I unplug the MAF   .... STFT drops to 0 ... toggles + & - like it should ...  and LTFT also within 2%
I have replace all vacuum plugs on throttle body and intake. and my PCV valves are piped to a oil purge atmospheric tank.

I am thinking about having it smoked tomorrow  for any possible vacuum leaks that I might not see, but why wouldn't that show up with MAF disconnected and in closed loop show something on the 02's?????
WHAT AM I MISSING?????

ANY HELP HERE WOULD BE APPRECIATED
 
Title: Re: Rough Idle below 900 rpm. I've tried EVERYTHING
Post by: ygmn on March 25, 2019, 05:24:12 AM
remove supercharger and put back to stock and see if still has issue.
Title: Re: Rough Idle below 900 rpm. I've tried EVERYTHING
Post by: enoniam on March 25, 2019, 06:24:11 AM
Reason you are not seeing the problem with the MAF disconnected is because the Manifold Air Pressure sensor is reading the pressure in the intake manifold correctly, whereas it seems the MAF is not seeing all of the air entering the engine.  Leak could be on either side of throttle body.  Sounds like you checked for a vacuum leak, which would be on the downstream side the throttle body, and found a leak on the other side of the throttle body.  I would look for another leak between the MAF and throttle body.
Title: Re: Rough Idle below 900 rpm. I've tried EVERYTHING
Post by: SRFNSNO on March 25, 2019, 06:45:23 AM
check on back of supercharger there is a Plug that is plastic.
mine was leaking and I know of 1 other member to have this issue with magnison.
Title: Re: Rough Idle below 900 rpm. I've tried EVERYTHING
Post by: dwdooley on March 25, 2019, 10:07:42 AM
Now that's a new one.  Is it near the three vacuum nipples on the back?  Since I have hydro-boost brake system I removed the 1/2 nipple and put a brass plug in it.  The other two nipples are (I think steel) 1 for pcv valve and the other goes to vacuum diaphragm on drivers side of super charger.  I have not seen a plastic plug.  I am going to have it smoked in an hour or so.  Do you know where this plastic plug is in reference to the three vacuum nipples located on center of back?

Thanks for this info .....  just need to go to next step. I checked and I think this is the port that is in line with the map sensor and is on the back driver's corner?

UPDATE: I have a brass adapter in that port for my 1/8" poly hose going to my boost gauge.
Title: Re: Rough Idle below 900 rpm. I've tried EVERYTHING
Post by: dwdooley on March 25, 2019, 14:47:47 PM
FINAL UPDATE
I Just had my 05 Avalanche smoke tested and Finally ... we found a leak on drivers side intake manifold.
It looks like it is coming from around the center between 3 and 7 so I assume it is coming from back side of #3.
It will be a couple of weeks before I can tear it down but with the Magnuson I was told by their tech to also look for cracks on intake manifold.
I did the brake fluid check while watching the STFT and saw no change or heard no fluctuation in rpm.
Goes to show that sometimes we all need the right tools for the job.
I still needed the fuel pump and evap canister.  Only wasted $ on the MAF Sensor.

Thanks for the responses!
Title: Re: Rough Idle below 900 rpm. I've tried EVERYTHING
Post by: BainMan on May 14, 2019, 13:07:05 PM
Any updates? Eliminate those leaks?
Title: Re: Rough Idle below 900 rpm. I've tried EVERYTHING
Post by: 2004Slickside on May 15, 2019, 10:05:15 AM
Any updates? Eliminate those leaks?
I suspect he still has his original problem.

AFAIK, unplugging MAF sensor kicks the system into open loop and forces the vehicle to run on a pre-programmed fail-safe.

I would start with a dynamic (running, snap) compression test as well as a cranking compression test (typical compression test).  Leaky valves and bad valve springs, guides, etc as well as bad cam lobes can masquerade as vacuum leaks and cranking compression tests alone can miss things.  Dynamic compression test is all about volumetric efficiency while the engine is running.  If anything with dynamic compression fails (a cylinder is different from the others), he is chasing his tail trying to fix the fuel injection and find vacuum leaks. 

If you haven't heard of it before:  https://support.alldata.com/article/tech-tip/running-compression-tests (https://support.alldata.com/article/tech-tip/running-compression-tests)