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Old 10-04-2004, 08:22 AM   #1
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Default rough idle and other things...

I have had a rough idle for a while. Adding my CAI has helped out some, but there still seems to be another issue.

Here is the scenario: With the car idleing (either in park or gear) it will run fine for a short time (5-30 seconds) and then start to run a little rough. IT acts almost like it is missing, but only at idle. After about 1-2 minutes of this I get a SES light. I have pulled this code many times. It is always "P0171 fuel trim lean".

Since I have already replaced plugs and wires and everything seems to be ok at other RPM'*, does it sound right for me to look at my IAC sensor? And how do I test this sensor? I do have a scan tool and I can pull readings from it and other sensors, but not sure what exactly I would be looking for in that one.

Just for good meassure I also have a new O2 on it'* way from GMpartsdirect.

Any thoughts?
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Old 10-04-2004, 12:01 PM   #2
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Thought about your fuel pump? Whats your pressure at?
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Old 10-04-2004, 12:28 PM   #3
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Quote:
Originally Posted by DrJay
Thought about your fuel pump? Whats your pressure at?
Actually I haven't thought about that. I was thinking more sensor over mechanics though. Just from the way it feels. I also definetly noticed a dramatic increase in the frequency of this once my CAI was installed. Since it only happens at idle, and not WOT, I didn't think it would be something like the fuel pump. What are your thoughts on that?
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Old 10-04-2004, 12:38 PM   #4
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Well I have a similar issue although I don't get a lean code. My fuel trim will go up to about 160, then there'* movement from the IAC (presumably closing to keep it from leaning), then the idle drops down and it runs rough. If I let it sit after a few seconds there'* more movement from the IAC (probably PCM overriding previous command to save it from bogging out) and it idle jumps a little high then levels out at normal again. This cycle will continue every 30secs-1min sometimes sooner. When driving (and this is where it gets weird) the o2 picks up nearly normal readings and at WOT (although its narrowband and not precise) it actually reads rich. As I drive normally the fuel trim sits at about 130-135, immediately when I stop it starts climbing. Obviously a fuel delivery problem. The only thing that makes me wonder is because the pump puts out constant fuel, its the FPR'* job to err..regulate that heh. What I'm thinking is that at idle my FP is too low to suffice but once the FPR opens up its good enough to drive on.

I haven't had a chance to check the FP since the bogging started but I knew the pressure was low before. Since there was sufficient movement when I disconnected the vacuum line to it, I've not ruled out but set aside the idea it could be the regulator.

I also noticed at the same time this started my fuel pump was much louder.

I ordered a Walbro 255 which according to FedEx will be here tomorrow. I'm going to put it in this weekend so I'll know more then. BTW, my fuel injectors have been cleaned and TWICE now so I'm fairly certain I can rule those out. The filter was also replaced and the fuel rail cleaned.
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Old 10-04-2004, 12:49 PM   #5
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I have the same problem....we should get together and brainstorm or something. I'm thinking that it'* probably our fuel pressure regulators. I get the same code and I know for a fact that I need one. just my 2 cents FWIW.
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Old 10-04-2004, 12:51 PM   #6
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Yeah at the beginning I really thought it was the FPR but when I disconnect it I get a 5psi jump in pressure so there is movement. From what I could find this would indicate a functioning FPR. Have you tried it with yours?
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Old 10-04-2004, 12:54 PM   #7
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I actually haven't tried anything with the FPR yet, I need to get a new TB gasket so I can get my TB off and clean everything.
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Old 10-04-2004, 12:55 PM   #8
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How did you determine you needed an fpr? Or did I read that wrong
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Old 10-04-2004, 01:10 PM   #9
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I know that I need one because when the car is warm it won't restart with out cranking it for almost 20 seconds. And when the car is cold, after sitting all night or something, it will fire right up. Or if the car is warm and only been off for like 5 min, it will fire right up but otherwise it won't. I figure that it could contribute to my lean code and my idle problem. Except my idle'* so rough that it'* like my car is cam'd and jolts the whole car. A couple of times it'* actually rolled the car forward.
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Old 10-04-2004, 01:19 PM   #10
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Well, I have a scan tool so I can scan my car and try to see what the PCM sees in relaton to this. I may go pick up a fuel preassure tester as well this week. I am guessing I won't get any length of time to do testing until next weekend. Got too much to do already .
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