Low RPM rough idle with Trouble codes

E

eddybear001

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I have a 1997 STS with the 4.6 L and 78K miles. I first noticed a little bit of a rough idle or surge when in gear at low RPM followed shortly thereafter by my Service Engine Soon light. I retightened the gas cap because I had just bought gas. Added a fuel system cleaner. Changed the Air filter. All with little or no change.
I read the codes and got P0300H (Engine Misfire Detected, P0603H ( Control Module Long Term Memory reset ), P1599H Engin Stall or Near Stall Detected, and P1611H (Loss of CVRTD Serial Data ). I deleted the codes and the car ran better for ten minutes or so then the Service Engine Soon light came back with the rough idle/surge in gear at low RPM. I reread the Trouble Codes and only P0300H showed up so it is probably a misfire problem, but from what? I am about to try Plugs and fuel filter. Any other good ideas. Unfortunatly I just got the notice in the mail that PrimeGard just filed bankruptcy so there goes my extended warranty.
 
Sounds like a possible coil pack problem. The plug wires are connected to the coil packs. You can purchase an ACDelco coil pack for about $40 from rockauto.com or gmpartsdirect.com sells OEM parts at discount prices as well. Best thing would probably be to buy one and use it to find the potential defective coil by replacing one at a time and starting the engine to see if there is a difference. It's possible more than one coil is defective but unless your certain of that you can buy one and see what happens. It would be cheaper than buying plug wires as they can cost over $100. One way to check wires would be to buy a spark tester for a few dollars and see if you can locate a defective wire.
 
Thanks MAC1. Would you suggest dropping in a new set of plugs at the same time. Do you have a suggestion for a good plug. I will probably only keep this STS for anouther 25K (100K total). Do you or anyone else have any other ideas of what I should look for to fix this. I am getting out the wrenches tomorrow. Thanks - EddyBear
 
As far as new wires are concerned, forget about it unless there is an existing problem which requires replacement of the original wires, (e.g., cracked insulation). I still have the original wires in my 94 Eldorado and it’s approaching 200,000 miles and still runs and idles like a top.

As far as rough idling, I would check battery connections, especially the positive side for corrosion. Check underneath the positive side rubber protective insulation for corrosion and acid buildup. Also, check the battery ground to make sure it has a good corrosion and rust-free connection. The loss of data error may indicate a bad ground, causing loss of power to the PCM. I would also buy a spark plug wire tester just to see if there is any obvious problem that can be detected. Try to start the engine at night in a darkened area with the hood open and check to see if there is any electrical arching from a wire or boot. I trust you already checked to make sure the wires are on tight as sometimes it could be something as trivial as a loose wire.

Also, I suggest disconnecting the battery and leave it disconnected for about 15-30 minutes. Then reconnect the batter and start the engine. This will allow the PCM to revert to its default settings and readjust (or re-learn) PCM parameters.
 
Sounds like MAC1 knows what he's talking about. I'd add that you could put water in a sprayer and mist the spark plug wires in a dark area to increase the chances of locating a bad wire. The coils in the coil pack can be checked using a DVOM. I can't remember the correct resistance (6K-12K on secondary I think), but if one is way different than the others...BINGO! Same on the primary side, I think it's suppose to be about 0.5 Ohms. I'll look around for more accurate info.
 
Problem cured - I replaced all four coil packs and with 75K miles it was easy to change the back plugs with the coil packs out so I put in new AC 100-950 plugs. No error codes and my STS has the performance and milage it had when it was new. Thank you both MAC1 and KISSEDVFD. I appreciated everything both of you offered. - Thanks EddyBear001
 
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