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Rough idling and running

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I have a problem with an unstable idle, rough running at all speeds, and diminished power with my 1993 4.6 litre STS. I'm currently looking into two possible causes:

Bad oxygen sensors:

My car has 205,000 km (127,000 miles) on it, I've owned it for two years, and It's never run as roughly in the past. Could the oxygen sensors be defective? There have been no oxygen sensor trouble codes.

I used the DIS to view the oxygen sensor voltage. PD32 is the front oxygen sensor voltage and PD33 is the rear oxygen sensor voltage. To get the DIS to display these, with the engine running, press OFF and WARM on the DIS and let the system step through its displays. When the computer asks PCM?, press the increase fan speed button. When the computer asks PCM DATA?, press the increase fan speed button again. Keep pressing this button until you see PD32 or PD33. You may now monitor the voltage and idle or drive your car. At idle, both PD32 and PD33 oscillate between 0.08 and 0.88 every 5 seconds. At faster RPM, the readings are the same and the oscillation speeds up with the RPM. After driving for 10 minutes, all these readings were the same. I am curious to know if these readings indicate the oxygen sensors are working normally? :unsure:

I looked into a few other possible causes:

I checked the spark plugs and all were normal except #1 which was had a layer of black soot on it. I replaced this plug with no change in the rough running problem. Spark plug wires are all in good condition with resistances of between 1.23 and 1.40 kilo-ohms per foot. I replaced the fuel filter last summer. All my coils have been replaced in the past year (2 failed individually, and I replaced 2 trying to solve this rough running condition). :huh:

The other cause I am looking into is dirty injectors.

I took off the intake manifold cover and I checked the injector coil resistances. All coils had the same resistance. Everything looked fine, but I was surprised to see that there was gasoline pooled in the bottom of the intake manifold housing. Does this mean the injectors are leaking?

I make two or four 5-10 minute trips to work during the week, so I wonder if these short trips can contribute to injector fouling. I just started to use premium fuel last month, I had been using the basic octane 87 fuel. Would having the injectors cleaned by a garage, using a cleaning product while operating the engine, do a reasonable job to clean out any fuel deposits? Is it a good idea to have them cleaned anyway, after 205,000 km?

I read somewhere on this forum that there are instructions in the service manual for deactivating individual injectors, to see if there is a stuck or sticky injector. Would someone be so kind as to forward these instructions?

I am interested in purchasing a service manual to help out with troubleshooting. I saw on another post that they were on sale for $30 earlier this year, but the price is back to $100. E-bay recently did not have a manual for sale for this car.

Any other suggestions with this rough running problem would be much appreciated. I am hoping not to have to resort to the dealership - around here they charge $50 CDN just to look at the car using their instruments, and $99 per hour. :blink:

Thanks for your advice,

STSdriver :)

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STS....seems like were both experiencing similar symptoms...Check my post http://caddyinfo.ipbhost.com/index.php?sho...=...Haven't got around to changing/testing the plugs and wires yet....along with visually inspecting the fuel injectors...Hopefully I can get to it tomorrow. I’ll be sure and give an update when I do....also did your fouled plug look similar to the blackend plug I posted?

You have OBD-1 right?... I have a 95' with OBD 1 and most of the procedures (in the manual) for testing fuel injectors require the use of a fuel pressure gage, voltmeter, and fuel injector tester...However there is one test that you can perform through the pcm...But it is to be completed AFTER you have ruled out a fuel injector problem..(via other tests W/ equipment)..so I don't know if it will help you at this stage.

It is interesting to note the "NOTES ON FAULT TREE" section : " Idle quality should get worse when an injector is disabled, engine miss and idle will become rough"...Be interested to see what you find....good luck.


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I had a similar issue with my 93 sts, I had no codes. Rough idle and driveable but as if firing on less cylinders. The problem was resolved by checking the plugwires for arcing. They were bunched near the a/c fan motor housing and I re wrapped them individually with insulators and used a wire separator. One wire I actually wrapped with electric tape, this was over a year ago, and I still haven/t replaced the wires.

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  • 4 weeks later...

Well I believe I have finally found the source of the problem and it is arcing spark plug wires. A few days ago I sprayed water on my plug wires at night with the engine running and I saw lots of arcing between and along the length of the wires. I am told that you should see no arcing at all with good wires. Today I sprayed some silicone ignition protector on the wires and the car ran better. So tonight I took off the protective wire covers and sprayed along the lengths of all the wires.

Up to now I tried having the injectors cleaned with solvent (while the car was running) at a shop, and the car seemed a to have a little more power. I tried the power balance check, and it showed the least difference for the #1 injector. I replaced the injector with no effect. I cleaned the EGR valve, it was pretty clean to begin with. I checked for any vacuum leaks, and replaced one 90 degree fitting that probably leaked a little. I tried separating the spark plug wires with wire separators, with no improvement and that led me to believe the wires were not a problem.

I have a quote from a distributor for $120 CDN ($96 US) for Bosch wires. I would like to know if anyone has found higher performance wires for this car that they are happy with. I'm told, buy the best wires you can afford.

Well, I've saved the exhorbitant dealer fees and I have a few new parts now.


1993 Seville STS

4.6 Northstar

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Use OEM wires (A/C Delco). They are not too much more than the Bosch from rockauto or gmpartsdirect and from what I have heard will perform better.

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