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Old 08-14-2015, 07:03 PM   #1
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Default 98 Leabre: Questions on an issue.

My problems started with the car randomly "jerking" once in awhile while driving. We changed out the wires and plugs and installed a new fuel filter and even changed the ICM and coil parks (I had the ICM and coil packs on hand). We finally determined the Mass Air Flow was bad; and it did set a PO102 code eventually. Changing the MAF did stop the random jerking while driving. I noticed after changing out the MAF sensor that the idle was not smooth and the idle rpm would drop at times. I also noticed the car wanting to hesitate when taking off from a stop once in awhile, not always, most of the time not, but a couple of times on different days I have also seen a PO171 code set for bank 1 lean. I can not find a vacuum leak by doing the carb spray test.

The MAF I have in the car is a Dorman new sensor; not rebuilt, purchased from a local parts shop; ordered and shipped to home. Not sure if I am allowed to mention names of shops. I am wondering if this Dorman MAF is at the issue of the rough idle and the hesitation on pulling away from a stop I am seeing? My friend is wanting to re-torque the LIM and upper plenum bolts but I really don't want to pull the plenum back off unless i know for sure it was leaking at the intakes. The rough idle was not there that I know of until after I installed the Dorman MAF and took it for a drive; I am going to verify this with my friend when I can get him on the phone. I did reinstall the old MAF but did not seem to fix the idle issue.

I could take a video of the live data and post it if it is possible to post videos here. I really need to try and figure out this idle issue with the car.

Last edited by Kimberly; 08-14-2015 at 07:05 PM.
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Old 08-15-2015, 12:11 AM   #2
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I called my friend and he said that he noticed the inconsistent idle with the MAF we got of of that auction site and ended up returning.
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Old 08-15-2015, 05:58 AM   #3
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GM idle relearn procedure with new MAF. Google it.

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Old 08-15-2015, 10:39 AM   #4
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About the only thing I found was clearing the PCM and then key on to set the ICV and then key off.

It probably does have a vacuum leak but probably not at the UIM/LIM. I see a vacuum line from the vacuum port where the brake booster connects that disappears into a wire loom; where does that go?. What I need to know is where to start checking. I replaced the vacuum lines to the EVAP system, had the small evap leak code, and checked the PCV, new PCV and o-ring from when I did the upper intake. I replaced both o-rings on each injector. Good fuel pressure by the way, key on is 49 to 50 and it holds that pressure and fuel pressure running is good and steady. EGR valve is moving freely. Just unsure where to check for where there can be a vacuum leak.

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Old 08-15-2015, 11:04 AM   #5
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I read this on another forum; "Usually, coolant ingestion from the UIM will kill both of the 02 sensors," Is this true? This car had a serious leak at the EGR stovepipe, the leak had been worked on before but they did not know the true source, car was driven a lot with the coolant leak and they just would add coolant and continued to drive the car.

The upstream O2 sensor appears to be working, the volts go up from a low value to a max of around 0.9 volts and then back down and repeats this; this is normal operation; I think I was told the 02 sensor was replaced; probably the upstream. so I am guessing the upstream 02 is good and is correctly reporting the lean condition. The downstream 02 reads from 0.39 volts to 0.52 with an average of 0.43 volts.
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Old 08-15-2015, 02:36 PM   #6
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Methinks you have to drive this vehicle with a scan tool and look at misfire data and transmission slip data, to see which is causing the jerk...the engine or the tranny......your readings on the O2 sensors look normal....
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Old 08-15-2015, 03:32 PM   #7
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Originally Posted by Tech II View Post
Methinks you have to drive this vehicle with a scan tool and look at misfire data and transmission slip data, to see which is causing the jerk...the engine or the tranny......your readings on the O2 sensors look normal....

The jerk was caused by the MAF sensor being bad; probably shut off the injectors because it saw no air flow and thus the jerk. That part is OK now. The current issue is the erratic idle and at times setting the bank 1 lean code. Read on another forum where a guy that had erratic idle traced it to the upstream O2 sensor, he replaced it and the car idle normal after that; thus leading to my questioning the O2 sensor since this car had the serious coolant leak.
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