Hello all, I just bought a 99 Chevy Silverado 5.3L 4x4 and I knew it needed some work. The engine idles way to low (around 250-400rpm) and dies sometimes. These ...

Hello all,
I just bought a 99 Chevy Silverado 5.3L 4x4 and I knew it needed some work.
The engine idles way to low (around 250-400rpm) and dies sometimes. These are the codes I pulled at O'Reilly the other day.


P0171 - System too lean (Bank 1)
P0101 - Mass or air flow circuit range/ Performance problem
P0137 - 02 Sensor Circuit Low Voltage (Bank 1 sensor 2)
P0157 - 02 Sensor Circuit Low Voltage (Bank 2 sensor 2)
P0174 - System too lean (Bank 2)
P0404 - EGR recirculation Range/ Performance
P0405 - EGR sensor A Circuit Low
P0506 - Idle Control System RPM Lower than expected
P1404 - EGR Error (EGR Number 2)

So I replaced the egr valve and there was no change. The old one had some gasket sealed to the surface. The new one I purchased has a metal one.

Q. Am I supposed to apply some adhesive to the two surfaces to seal properly?

I also replaced the Idle control valve as well and here is what happened...the valve was pushed in when I installed it. After installing the new valve It started up and ran a bit high in rpm but much better and cleaner. It eventually settled down to 600 rpm or so and felt great. I thought I fixed the problem but when I shut the motor down and tried starting it up again the problem was there again and it persists.

What am I missing? Help!!!!!!!!!!!!

Clean the throttle body.

My truck started to have the same issues (minus all the codes), but after the throttle body was cleaned, it ran just fine. Hope this helps- let us know if you need some more advice!

That is a ton of codes! Try swapping the O2 sensors for each side. It looks like your after cat ones, try putting them before the cats, run the truck for a while then have it rescanned. Spray carb cleaner around the intake gasket and see if it sputters, if so the gasket needs replacing.

Bad intake gaskets on these motors are common and if left untreated will wreck cats. Also leads to poor idle and lean codes. That would be the first thing I would check after the obvious fuses and such.

Also check and clean the mass air flow sensor. Ive seen some bad maf sensors cause these codes.

So what ended up being the fix here?

easy test to see if the MAS (mass airflow sensor) is at fault. Take is off. All you need is a screwdriver and a lil muscle to get it off the plastic hose. If you start it and runs fine MULTIPLE times and not just the first then you know its your MAS.

reynoldsc58 wrote:easy test to see if the MAS (mass airflow sensor) is at fault. Take is off. All you need is a screwdriver and a lil muscle to get it off the plastic hose. If you start it and runs fine MULTIPLE times and not just the first then you know its your MAS.

Change the intake gaskets.
You have a leak there which is throwing the mixture off and leaning it out.
They leak all the time and do need to be leak free for the truck to run right.