odd hesitation

mbeals · 11-17-2009, 11:40 AM

I have a 2000 alero with the 4cly

First a little history:
The car had a bad wheel bearing and was having an odd issue where it would bog down and die. It seemed that giving it partial throttle after coasting would cause it to bog down and sometimes while coasting down, instead of the engine settling down and idling around 800, the RPM's would just keep dropping off until the car died.

I brought it in to have the wheel beading replaced and while at the shop, the tech pulled an O2 sensor code. When he checked it, he found the wire had been cut.

After replacing the sensor, the car ran much better for a while, then within a week the problem came back and the CEL came back. It continued to go down hill until yesterday.

Yesterday it was a fairly chilly morning and as I was backing out of my driveway I noticed that my power steering wasn't working. The power steering had been funny lately (like it was only half working) and I had noticed that on occasion, turning the wheel would cause the engine to bog like it was under load. So I stop, put it in park and let it warm up for a minute. The engine was running kind of rough, so I gave it a little gas to see if it would smooth out. When the engine hit 2000 rpm, there was a distinct "pop" in the steering wheel and the power steering came back better then it had been for the past month or so. At that instant, the CEL also vanished and the engine smoothed out.

Since then, the car has been running fine, except for one oddity. If I am coasting with the engine at 1500 RPM (exactly) and I lightly feather the throttle, the rpms immediately drop off and the car stumbles. If I stab the throttle, it drops off then catches (sort of like neutral slamming). If I allow the rpm's to drop to 1000, then there is only a slight hesitation...same for trying it at 2000. So as I approach 1500 rpm from both directions, the problem gets progressively worse....No clue why 1500 is the magic number.

any thoughts on where to start looking? My immediate thought is vac leak or TPS, but I'm interested to hear what others have to say.

Gr1m · 11-17-2009, 11:50 AM

injectors? plugs?

DansGen1Alero · 11-17-2009, 11:51 AM

I know the 3.4 has 2 crank sensors (o2) maybe the other one needs to be replaced.

Gr1m · 11-17-2009, 11:52 AM

the downstream on the 3400 does nothing that deals with the engines a/f ratio so as long as the upstream is good you wont have a problem caused by that

jayson_waltz · 11-17-2009, 12:02 PM

^he has the 2.4 anyways guys. the power steering problem is weird, could the pump be locking up maybe? cause i know they drive off of one of the cams, maybe that was your earlier problem? but now the little stumbling sounds like plugs. when was the last time you replaced them?

mbeals · 11-17-2009, 03:06 PM

I just replaced the plugs about a month ago, but I [stupidly] put in bosch platinum instead of normal copper cores, so the plugs may just suck. I also changed them before I figured out the upstream O2 was bad (false rich condition, so it was running lean), so there is a chance that the general sucky nature of the plugs combined with driving for several weeks way too lean has destroyed them.

[ion] C2 · 11-17-2009, 03:43 PM

bosch is a big NO on these engines

mbeals · 11-22-2009, 10:10 AM

Finally have an answer!

I'm at home visiting my parents and have access to an advance auto, so I had the codes read.

I had a bunch of downstream O2 (heater and low signal voltage) codes a bad MAP signal and poor mixture. So I start looking at the TB and find that the vac line connecting the MAP sensor to the TB has rotted through....I had crap for throttle because I had no MAP signal.

This however, does worry me about the shop I went to. I can only assume this hose was like thIs when they "diagnosed" it and that the MAP code was set as well. But all they found was an upstream code and an O2 sensor with a cut (not worn through or melted) wire.