OK, here's the story...99 Z24 5-speed

Spiders went in my diff. Took tranny out, replaced with hardened spiders, put tranny back in.

Two weeks later, I get a CEL P0171, Lean. Starts running a little off, like when I go to shift the RPMs drop off faster than normal, and when I stop or slow down to turn, it hesitates a little unless I give it the gas. It smells like it's running rich, so I assume the O2 sensor is bad, and for only $20 it's worth changing. Well, that wasn't it.

While I'm under there changing the O2 sensor, I see a little hole in the front/bottom of my CAT, from once upon a time when my car was lowered, it bottomed out and scuffed it, and now it had just finally worked through. So, I make myself a nice little heavy-gauge sheet metal patch, weld it in place, and now no more CAT leak. But it still runs the same and I have a pending P0171 (except a little quieter ).

I've checked all of my vacuum lines. I bought a new fuel filter, but I haven't changed it yet. Any other ideas?

well that exhaust leak would for sure send a false lean signal to O2 but you fixed that.

So... do you have access to a scan tool that can read current data, or has freeze frame information?

If you have a Mass air flow sensor I would be wathcing that hard,it can cause a false lean. If your sure its running rich that would rule out fuel pump and injectors, also a false signal from the MAP sensor could cause a false lean signal to.

If you can get some scan tool data post them and I can help you short out what all the information means.

hope that some help

Same problem here. Been having it for months now. I have swapped the front O2 sensor, Air Change sensor (2 wire sensor on top of air intake tube), new fuel filter, new tps, new plugs and wires, all to which havent resolved the code. Exhaust smells like unburned gas. Same signals you are having, loss of power when accelerating, sometimes rough idle, mostly when engine is warm and when I bring her to a stop, she boggs down and almost dies. Ran some seafoam thru just to see if it would resolve it but no it didnt. I have a 98 Cav 2.2L 5spd and there is no MAF on it. Just a $10 sensor with a the 2 wire connection. I sprayed some cleaner on the MAP and it seemed to help but I dont know it they can be cleaned. I also took out the Idle air control valve last night and it was pretty nasty. Not sure what one is suppost to look like but the valve seat looked a bit distorted. Let me know what you find. It might be this weekend before I make it to the parts store to get a new MAP and Control valve.

That is exactly what mine is doing. Every once in a while it will actually stall when I come to a stop, but it fires right back up no problem. I haven't gotten any codes lately, but it is still doing it.

Jimmythekid- I only have the basic scan tool. Also, I don't have an MAF. Wish I had access to a better scan tool.

I used one of my vacuum lines to bleed my clutch when I put it all back together, but I flushed it out with parts cleaner and ran 120 psi of air through it for a good 2 minutes before I reinstalled it. The MAP only measures pressure anyhow, so its not like I got fluid on it and it is sensing a rich mixture and cutting fuel.

My car would actually do this every once in a while ever since I bought it, but I never got codes. I think the code was just the CAT leak, and the occasional rough running is seperate.

I'm going to borrow my buddies vac set up and check it all out this weekend. Probably a bad vac line somewhere, or leaking intake manifold gasket.

My car is doing the same thing. I had just put a new front o2 in it, also a new cat a few weeks before the problem started. The day before the light cam on, I installed an air fuel ratio sensor. Now the trac off light comes on and the CEL. If I run the piss out of the car, it will turn the lights off. It does run rich at idle, but the code is a P0171- lean bank one.

Mike S, your problem might be the air/fuel gauge.

I've heard that if you don't have a perfect connection, or if you splice on the sensor side of the connector instead of the ECU side, then the guage can cause your car to think it is running lean, giving it more gas, causing the rich idle. When you wind the piss out of it, the injectors are already running at capacity anyway, so you wouldn't be running rich anymore.

Where did you splice in the a/f line?

I'm on the ecu side of the Connector. Purple wire.

OK, it rained real hard yesterday, and when I drove home it was doing it real bad. I hadn't gotten a code all week, but when I got home last night, I had a pending P0171. I cleared the code this morning, and drove to work (30 minute drive), drove perfectly, no pending codes when I got here.

Has to be something electrical.

I got the diag chart for this condition. I will type it up on here tonight. You will need a Tech 2 to do this.

Yeah, I got the chart too. Will try some stuff out this weekend, but I don't have any access to a tech 2.

ok, new pending CEL...
"DTC P0125 Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control"

Could a faulty ECT cause my P0171 code?

Will have to check this tonight.

Nicholas Morgan wrote:ok, new pending CEL...
"DTC P0125 Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control"

Could a faulty ECT cause my P0171 code?

Will have to check this tonight.

if your thermostat is bitched it can cause your coolant temp code, as well the ECM regulates fuel as per engine temp, so if your car has 2 sensors and the ECM one is bad the guage will read fine but the ECM won't read properly and it can run rich or lean depending if the sensor has shorted open or closed, im not sure how it works on your car but my 94 2.2 has a 3wire CTS but i think newer cars uses 2 sensors

After reading this thread, I took my IAC off today at lunch break, cleaned it off, and reinstalled it, then took it for a test drive. So far, no more bogging/almost stalling issues. I think this was it, will keep you updated.

I think I will try that

I tried cleaning the IAC on my car the other day and it didn't help my P0171 code. I pulled my TB completly apart and cleaned everything and it still throws a code. Mine doesnt come on, on a regular basis, just whenever it wants to. I recently pulled my motor and replaced my clutch, could the code be from a bad intake gasket, TB gasket or bad o-rings on the injectors. Also i have an O2 simulator on my car, could this be the problem? ( haven't had any problems in 9 months with it ) My car is a 99 Z24. I want to get everything figured out before i go S/C.