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System too lean, O2 circuit slow response *help please*


Old 11-02-2011
Thread Starter
Join Date: Apr 2011
Location: Lake Stevens, Wa
Posts: 6
System too lean, O2 circuit slow response *help please*

I have a 1997 4.0L V6 automatic. My check engine light comes on pretty frequently, 02 circuit slow response, but it usually goes away. It's still here so I took it to Schuck's and it showed:
-P0171 system too lean (bank one)
-P0153 O2 circuit slow response (bank 2, sensor 1)
-P0174 system too lean (bank 2)

Cylinder one misfires about every six months and needs the spark plug replaced, and it's burnt up and smells of gasoline. I've read numerous things on what the problem is for the misfire and the codes above, but none of them are very clear cut. I've seen in a few places that those codes could also over time cause the misfire. I've read about:
-Needing a new fuel pump
-Blown out intake gaskets
-Cracked PCV vacuum line elbow
-Dirty or faulty MAF sensor
-Dirty Fuel Filter

The most promising to me is the cracked pcv vacuum line elbow, but I can't seem to locate the line, even with the Chilton manual. Can anybody further elaborate on this? What could my problem be? If needed, I'll post photos. Thanks in advance.
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Old 11-19-2011
ccjeremy21's Avatar
Join Date: Oct 2011
Location: Bowmantown,TN
Posts: 9
I had similar issues and my Idle Air control was bad and my EGR Pressure Feedback Sensor was bad.But I didn't have the P0153 code though. Cost a little over $100 for both. If you go online to Advance Auto Parts and use promo code P20 you save 20%
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Old 11-23-2011
Join Date: Mar 2005
Location: Whitehorse, Yukon
Posts: 30
couple ways this could go ...

- not enough fuel
- too much air, or unregistered air (vacuum leak)
- faulty o2s

1. Check fuel pressure is in spec, and check fuel pump flow. for flow you want at least 1 litre/minute. fuel pressure should be 35-40 psi @ key on and should hold within 5 psi for a period of 60 seconds

^^^^^ this is going to eliminate poor fuel supply right away as the cause of your problem. this one check is going to verify the integrity of the pump, lines, regulator, filter etc etc etc. highly unlikely that your issue is fuel related if this comes up good.

2. you need to check thoroughly for vacuum leaks. if you think about it, if the fuel is up to snuff then the only way for the system to go lean is through too much air being supplied.

best way to check is with a smoke machine. fill the system up and see if/where you have a leak.

***somebody else said bad IAC. only way a bad IAC gives a lean code is if the valve is stuck open. but keep in mind an open valve would give a VERY high idle at cold start, and from there an unstable or lower idle at operating temperatures. usually an IAC issue is more pronounced than a vacuum leak on a speed density system like the ranger because of the size of the 'leak' through the valve ports. the amount of air that can travel through IAC valve passages is huge compared to what can be drawn in through a tiny crack in a hose or a poor intake gasket.

3. EGR could be plugged or inop. When your EGR opens it sends inert gas (mostly nitrogen) back into the intake to be burned. The nitrogen takes the place of the clean air (oxygen) that would normally be burned, effectively richening the mixture. But if your EGR is inoperative or its passages are plugged then this can't happen. The result is a combustion mixture which stays hot, lean and full of NOx emissions. Chase this angle if absolutely no vacuum leaks are found.

4. last possibility is poor O2s, but is it likely that both upstream 02s **** the bed at the same time? not really.

- your low switching code is because your O2 crosscounts (the amount of high to low voltage cycles) is low. this is due to your lean condition keeping your sensor readings at below 0.45 volts ..... chase you lean codes first and i bet your low switching code will disappear with them.

good luck
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