I'm reading lean codes, will the brake booster cause this codes to pop up__ i get p0171 and po174. I have checked all the vaccum lines and the intake it seem ok. I'm reading good signal from the mass flow sensor according to haynes book

When i'm monitoring the O2 sensors bank 1 and bank 2 the oscillating waveform at idle makes voltage transitions from near minimum (0.1 v) to near maximum (0.80v) and that's good reading, but when I press the brakes than the signal drop flat 0 volts and the short term fuel trim goes high to +25 %.
Is this normal or it's a problems with the booster?
any advice would help
Thanks

 

I recommend checking your EGR ports. That would cause a lean condition which is what the codes are telling you. Its could be a laundry list of things. Any leak of un-metered air can cause lean codes. Without knowing what kind of engine of year model truck you have its really hard to say.

 

is a 2001 4.2 liter
I have only checked and replaced the EGR gasket.The last thing i did was
use propane gas around the vaccum lines and intake but no change in idle speed.

 

Well, I am not that familiar with the 4.2s but I would assume there would have to be some kinda problem with the EGR controller or DPFE. Assuming that there are no leaks then you have to be having a problem with something controlling fuel/air ratio. How does your truck run? Is there any black smoke or anything?

You can check for EGR valve failure by applying vacuum to the EGR valve while idling. If the engines RPM's lower then the valve is working properly. If it does not cause any change in engine RPM's then there is either a clog in the EGR ports or a faulty EGR valve.

 

the EGR holds the vacuum alright. i am not sure about the DPFE, i read 1 volt and does not change while driving.
The truck run OK, but sometimes when stopping it hesitates or stumbles.

 

The DPFE should constantly change in voltage while you are driving. The voltage should increase as you are driving because of the increase in exhaust pressure going through the EGR system. If the voltage does not change then you need to check to see if the DPFE is at fault. Your truck uses EGR in the overall fuel/air ration and if your DPFE doesnt properly measure the amount of air going through then your computer will have a incorrect reading. Which will automatically cause a faulty ratio. My trucks DPFE has gone out before and caused lean codes for me.. I am not for certain that this is your trucks issue though. Its hard to diagnose from a distance.

 

I know i have 5 volts reference, but i will check the orifice.
Why i don't get overflow or insufficient flow code?

 

Your miss interpeting the codes.
The codes are telling you the fuel tables in the software are shifted to the lean side trying to "Counter" a Rich condition on the intake side.
EGR system faults usually do not cause this. If there was an EGR fault, it would be reported as a 4xx series code.
There is excess fuel getting into the motor the OX sensors are detecting.
Look for leaking injector, fuel regulator, low fuel pressure etc.
An air leak can also result in a lean condition that the OX sensors detect and go right back and richen up the fuel somewhat like tail chasing.
Dip stick not seated/tube rusted off etc. can also cause upset.
Vapor recovery system also has an effect on fuel.
Good luck.

 

Because corrosion inside of the sensor decreases its sensitivity to EGR flow, therefore causing it to "under-report" the flow. Your PCM responds by increasing the EGR flow, which can possibly keep the EGR valve open for a longer period of time which creates a lean condition in the engine. Its for this reason that it may set a P0401(EGR flow code), or it may not set a "flow" code but rather a P0171 and/or a P0174 instead.

 

After checking all the sensors and vacuum hoses, i did not found the problem. I replaced the Isolator bolts, plenum gasket and cleaned the intake manifold ports. Finally after checking the long term fuel trim with the scanner and noticed 15% drop with no code.

 

I had the same codes poping up, it ended up being my vacuum hose from the brake booster came on hooked behind the intake.. nothing big but its very tight trying to get your hands back there.

 

What did you do after discovering the 15% drop? How is that resolved? I am having similar problems, same codes, and thinking of what do do next.