'00 2.4L Hesitation - Help Pls?
 

New here, so thanks in advance for any help. 2000 Alero 2.4/auto, 156K miles, and I'm dealing with a rather irritating problem. Started about a month ago--the car started idling rough and would occasionally cut off at stoplights and/or going around corners. Got some advice from a local guy, former mechanic, who said that the spark system often causes a miss--I replaced the plugs, issue got worse. Took everything apart again and he tested the coils and coil housing for voltage, diagnosed it as a bad coil housing. Replaced that and the plug boots. Ran slightly better, but still not right, and developed a bad hesitation that's gotten steadily worse. Pulling away from a stop, I give it gas, and the tach doesn't move, car doesn't accelerate, unless I feather the throttle or really nail it. At speed but low RPMs, it bucks when I try to accelerate. At freeway speed it acts mostly normal. Idle is still rough and it still stalls occasionally. Intermittent SES light, code PO172 Bank 1 Too Rich.

Any ideas? I have purchased a throttle position sensor, thinking that might be it, but I'm not entirely sure where it's located. Does anyone have a pic/diagram? Or am I likely barking up the wrong tree? Could it be a vacuum leak, and if so where are the likely points? I also want to do a seafoam treatment (intake/gas, not oil just yet since it's not due for a change) just for the heck of it and the consensus from what I've read seems to be just pour it down the intake on the 2.4. Where exactly should I introduce it? Just after the airbox, or do I disconnect it farther down? Thanks for any advice, this is really killing me, it's kind of scary just to drive to and from work like this...

Check Compression, and Fuel Pressure

well bank 1 is your upstream O2 sensor. It could be this. But have you replaced your fuel filter? If not I would do this. It is cheap like $30.

with the rich condition it could also be a faulty O2 sensor making the motor to run rich, a bad injector, maybe cam sensor. But like I said above, change the filter if you havent.

I agree with oldsman about the knock sensor (O2) Our cars have two of them (at least the 99's 3.4) so if the 2.4's are the same replace 1 make sure you get both. For the 3.4, one is in the back of the engine (if i recall) and the other is near the wheel well on belt side Not sure how the 2.4 or 2.2's are set up.

Actually we sell it for $10 @ AutoZone.

Turning corners and shutting off is usually fuel related. Everything else tells me most likely the O2 sensor, based on idle/low RPM closed loop operation being crappy, and open loop heavier throttle being fine.

To test, unplug your O2 sensor's electrical connection. The car will run infinitely better if the O2 sensor is the problem.

Ok, I'll do the fuel filter when I have time, which probably won't be until next weekend. :( That was actually my first idea when this whole mess started, until several folks told me not to bother with it since it was probably spark-related...

Where is the O2 sensor(s) on the 2.4, so I know what to unplug? Also, if that does help, can I run the car with the sensor unplugged until I get the time to fix it (again, likely next weekend) or will that screw something else up?

Thanks much for all y'all's assistance, again.

there is the upstream at the ed of the exhaust manifld and the down stream is just after the cat.

Sorry if I'm being dense here, but I just cannot find the upstream sensor. I can't see anything that appears to be plugged into the exhaust manifold--can you see it from above on the 2.4? Or can it only be seen with the car up in the air?

I gather from another thread that the downstream o2 is only related to emissions, so is it a nonissue, or does it need to be disconnected if the upstream is?

you have to be under the car. it would be the first sensor screwed into the exhaust. check that and your fuel pressure.

So I won't even see the electrical clip unless I'm under the car? I unplugged something that looks an awful lot like the clip end I'm seeing for the upstream sensor, oval connector with 2 wires, located at the driver's side rear corner of the engine near the fuse panel. Unplugging this had no effect; still drives like crap (though I did get a new SES light). The downstream o2 is a 4-wire clip, right, so I couldn't have unplugged the wrong one, could I have?

i would HAVE to be under the car to get the Downstream

i can see my Brother's 2.4L O2 just fine

(it's a sunfire)

^Thanks. In that case, since I was not under the car, what I unplugged must have been the upstream, and it didn't help, so not the o2 sensor.

Will test fuel pressure tonight...I don't really trust myself to diagnose a vacuum leak though unless there's a really easy way to do it. Any chance it could be the throttle position sensor?

Bah, everybody always seems to goof this up...

On the Alero, Bank 1 Sensor 1 is the upstream sensor, Bank 1 Sensor 2 is the downstream.

There are no Bank 2 sensors.

Just to make sure of this (sorry to be a bother), since disconnecting the upstream o2 sensor didn't help with the issue, there's no point in disconnecting the downstream sensor since it only deals with emissions, right? Or should I try disconnecting both?

The error code simply said "Bank 1 too rich" without specifying which sensor, unless the guy at advance who read the code for me left something out.

So I purchased a Haynes manual (something I should have done a week or two ago) and it has alerted me to the fact that what i thought was the upstream o2 sensor plug was actually the knock sensor plug. Oops. Reconnected, disconnected o2, will report back after a test drive.

Aha! o2 sensor disconnected, hesitation GONE. Many thanks to all of you who pointed me in this direction. Will replace with the Delco part.

It still idles like total shit though. Will replace fuel filter this weekend when I have some light to work outside, give it some seafoam, and maybe change the plugs (I had put bosch in there, which seem to be bad news on these cars). What are the best plugs to use?