: Badly Misfiring 97 STS

08-30-04, 10:05 AM
Can somebody help me with diagnosing the badly misfiring 97 STS? It started about a week ago and then 2 days later SES light turned on. P0300 current has been set.

Car is badly misfiring, very very rough at idle and acceleration. It feels smooth when coasting at a speed. However, it's not losing any electric power. A/C and lights were perfect.

History - At 70K, plugs and wires were changed. And now the car has 138K, approx the same interval. Fuel filter, air filter, TB service was done couple of months back.

What shall I be looking for - plugs, wires, Fuel Pressure Reg, EGR valve, fuel injectors, etc. Are symptoms same if FPR or EGR valve need service.

Thanks a million for your help as before.

08-30-04, 02:08 PM
First and most easiest would be to check the plugs and wires. Check them over really good.

08-31-04, 02:01 AM
I had the same thing happen awhile back on the 96 STS. Replaced all the obvious parts but it still did it. It turned out to be a leaking intake manifold gasket. I don't know if that will help you any.

08-31-04, 07:54 AM
See my response at link below; similar problem.


09-06-04, 11:51 AM
Sorry couldn't get to the website for some reason last week. Anyway, last week the car was only badly misfiring but also backfiring.

Here is what happened yesterday. Checked the FPR by removing the vacuum hose. There were no signs or smell of fuel for 5mins.

Then, checked the plugs and wires. Pictures are attached of the plugs, they looked in bad shape with 70K mi on them. Could someone comment? Replaced the plugs with OEM AC Delco. Wires were not available.

Cranked the STS, started right up smooth idle and acceleration on test drive. SES went off on its own. P0300 cleared on its own. But later on, it idled roughly - little jerks - in drive with A/C on. the jerks were minor in park.

So as suspected, cause has not been cured. Please help. Bbobynski can you comment. I do plan to look into the EGR valve, but don't have much experience with it. Do I still need to put replace plug wires?

Waiting for help. THanks,

09-06-04, 12:32 PM
Those plugs seem to have a lot of oil on them and at least one of them as bad signs of "carbon tracking" I believe it is called. The spark travels down the side of the plug and shorts out on the base causing a misfire (usually it looks like a hair line crak in the porcelin. Yours is very very bad). I would bet that is the cause of your misfire. I would find and fix the oil leak, replace the plugs and be sure the wire to plug contact is tight. If not, replace that or all wires.

09-06-04, 12:44 PM
Your plugs were in bad condition and needed replacing. They will normally look almost white.

The oil in the spark plug tubes and on the plugs indicates seepage where the tubes seal to the cam cover, examine the tubes occasionally (monthly?), and wipe out with a rag or paper towel with a thin screwdriver. Usually replacing the tube seals is not necessary.

There appears to be flashing from the spark plug top to ground. There may be carbon tracking left in the plug boots. I would replace the spark plug wires. I have never had any luck permanetly removing carbon tracking from ignition parts or wires, it always comes back.


09-08-04, 01:24 AM
So it looks like I need to change plug wires ASAP. Am looking on the web as local auto store does not carry AC Delco OEM. Any advice?

I am still experiencing ROUGH ROUGH idling and missing while moderate to hard acceleration. Is replacing plug wires going to resolve this problem?

The only item left is the EGR valve and related components. Can somebody help with diagnosing / cleaning the EGR valve?

I hate to throw parts at the car like I did with the spark plugs without curing the problem, so everybody's valuable advice is appreciated. Need help to think straight. Thank you.

09-08-04, 02:01 AM
you may hav a failing coil pack check them and replace as needed

to check them turn the car on and one at a time remove the wires and insert a screw driver (BE CAREFUL ELECTRICAL DISCHARGE HURTS!) and put it near an expose metal part and watch the spark if it is weak/nonexistant (usually in both wires on the same coil) replace the pack.

09-08-04, 12:02 PM
The EGR is simple. Just remove it and clean the pintle, seat and pintle shaft so it moves freely with carb cleaner and tooth brush. DO NOT hold it upside down so the cleaner runs into the can as that will destroy the insulation on the windings. Don't worry too much about the soot, just the caked on carbon if there is any.