Car loses power

First - I apologize in advance if there is already a thread out there regarding this scenario (no success in finding it), please point me in the right direction.

This problem has left me scratching my head as I don't know where to start (and I really don't want to start replacing parts blindly).

It all started a little over a month ago, the car was running fine. I exited the interstate and after a few blocks the car seemed to lose all power, it felt like the tranny might have been slipping or something. At the time I didn't have a check engine light. I pulled the car off into a parking lot and shut if off. It was smoking pretty bad (usually doesn't smoke at all) from under the hood, so I checked the oil (seemed unusually low, so I added some), transmission fluid - looked OK. Car started right up and drove normally back to the hotel. Hauled it home since we were unsure if low oil may have caused problems.

When we got it back home, we proceeded to investigate (thinking the low oil was the initial cause), ran the car for some time - everything seemed OK. So I attempted to drive the car home. After about 40miles or driving for 25min, stopping to fill-up, and driving another 20min, the car did the same thing. It lost power, started smoking real bad, almost seemed like it was choking itself. This time we got a "Service Engine" light.

After hauling the car back to the shop, we pulled the codes (see below) and we put the car on the lift (I seemed to be able to reproduce the problem by trying to "power stall" - the car would eventually seem to lose power, start smoking, and almost act like it is choking itself - but not throw another code). At this time we realized it wasn't the engine that was smoking, it was oil burning off of the exhaust up around the cat (thanks to the infamous oil seep). Up until now it has never gotten hot enough to smoke like this. We asked ourselves, what is causing this to become this hot? Partially plugged cat? After looking at the codes and the troubleshooting associated with them, we've identified it could be dirty/faulty EGR, dirty/faulty O2 sensor(s)... We're not sure where to go from here.

Codes identified thus far:
Current:
P0171 - System Too Lean (Bank 1)
P0174 - System Too Lean (Bank 2)
Pending:
P0135 - O2 Sensor Heater Circuit (B1S1)
P0141 - O2 Sensor Heater Circuit (B1S2)
P0155 - O2 Sensor Heater Circuit (B2S1)

We inadvertantly cleared the codes before grabbing the freeze frame data, so we decided to drive the car until it quit or threw some more codes for us to look at. This time I drove the car roughly 80 miles before it decided to throw a code. But the car still wasn't acting up (losing power, etc). So I decided to drive it until the symptoms re-appeared so I could monitor what was happening while the car was in this state. I also noticed that the fuel mileage while driving this time has fallen to ~12MPG, I took notice of my short term & long term fuel trim and the numbers are >25% lean correction, which seems to explain the poor fuel mileage. Here are the codes it threw this time:
Current:
P0131 - O2 Sensor Circuit Low Voltage (B1S1)
P0171 - System Too Lean (Bank 1)
P0174 - System Too Lean (Bank 2)

At this point, we are throughly confused and not sure where to start. As mentioned previously, I do not wish to blindly replace parts as I would like to keep the cost of repair to a minimum. Does anyone have any ideas?

dirty/faulty EGR?
O2 Sensor(s)? Which one(s)?
dirty/faulty cat?
MAF appeared clean.

Judging by the codes, one could assume the O2 Sensor located at Bank 1 Sensor 1 may be at fault, but could there be something upstream causing an issue? Why would it wait so long after being started for the symptoms to appear?

Any insight would be greatly appreciated!!!

Thanks. :)

(sorry for the length)

I would not think a bad O2 sensor would cause the symptoms you have. If you suspect the EGR valve, pull it off and look at it. It's only a 10 minute job. At least you will know how dirty it is.
With the overheated exhaust system, that you describe I would be concerned about the cat. converter,however I'm not familar with how a faling cat converter would act. Maybe someone thats had one fail could describe the symptoms.

How is the fuel pressure during all this?

In my opionion the car is running lean because it is starved for fuel. If the car is running lean due to fuel delivery issues then this would not only cause the engine to overheat but also cause the o2 sensor errors that you are getting. Since both sensors are giving you the same thing I doubt it is the sensor. I would look at the fuel pressure and check the fuel pump. It might be on its way out. I doubt this is an EGR issue either. If the car is at normal driving speeds EGR you wouldn't really notice EGR issues at all.

How is the fuel pressure during all this?

Fuel pressure seems to be normal (best I can tell), by this I mean the car seems to be responsive up until the time it begins to have issues. It does not act like it is "starving" for fuel. However, it is apparent that it is trying to correct itself (high lean correction values, poor mileage).

.... I would look at the fuel pressure and check the fuel pump. It might be on its way out.

How would I go about monitoring this? I have a scan tool, what parameter(s) should I be looking for?

There isn't really a paramter. If the pump is intermitantly failing then you would need a fuel pressure gauge on it while you drive the car. If this is the problem though it should replicate itself while the car is just sitting idling. Is this car overheating through all of this?

There isn't really a paramter. If the pump is intermitantly failing then you would need a fuel pressure gauge on it while you drive the car. If this is the problem though it should replicate itself while the car is just sitting idling. Is this car overheating through all of this?

The car does not seem to be running any hotter than normal...it has always run on the warm side.

It may be worth mentioning at this point that after we pulled the last set of codes, I let the car sit and idle for a good 30+ minutes. When I got in the car to park it, it was idling somewhat wierd and when I put it in drive to move it, it almost died. Once I parked the car, I got out of it and let it idle some more while I went to get the scan tool and when I returned (not more than a couple of minutes), the car had killed itself. We were able to start it up and we drove it for a good 30 min or so with no problems (other than fuel consumption)

I'm sure this is confusing as it has left us scratching our heads, but I am having a difficult time trying to find a duplicatable pattern to the failure(s).

Let me know if any of this helps.

I would be looking at clogged exhaust. That would explain your smoke, your power issues at higher RPM's with some minor issues at low RPM's and with the exhaust not being able to escape this would defnintly mess up the O2 sensors.

Look at this thread
http://www.automotiveforums.com/vbulletin/showthread.php?t=475552&highlight=exhaust

So what are the implications of removing the cat & resinator? Can't say I've ever removed the cat on a vehicle with so many O2 sensors :)

Sorry I failed to mention earlier:
97 Autobahn, 130K miles

Not sure never taken the cat off these bad boys...I'm sure someone on here has.

Finally...got some time to look at this bad boy.

Bought a fuel pressure gauge
- pressure after pressurizing fuel system: ~37psi
- pressure after idling 5 min: ~6psi

Obvious fuel pressure problem...good call ghutchin & maxwedge.

Replaced fuel pump (ouch!) and FPR to be safe, now running ~44psi consistent!!

Car seems to be running normally now. Thanks all for the help!

Chao.

I would have tried fuel filter first
most over look this

rhilim, appreciate you informing us how you resolved your problem. Good to know.
Most people don't bother to post what corrected their problem.

I would have tried fuel filter first
most over look this

Sorry...forgot that...yes I replaced the fuel filter first...no change.

Replaced the FPR second...no change

Replaced the pump (last resort)...working now :)

rhilim, appreciate you informing us how you resolved your problem. Good to know.
Most people don't bother to post what corrected their problem.

No problem...it's nice to see a resolution to issues in these forums. Nothing more irritating than to see a lengthy thread without a solution (providing that one is found).

Thanks for all the help.