What up guys my 99 STS the other night just started running like complete trash it almost seems like it is missing on a couple cylinders it hesitates real bad on acceration but no warning lights have come on. It is heading to my mechanic on Monday or Tuesday I was just wondering what you guys think of this problem. Oh and the rpm jumps all over the place while at stops.

Are the plugs and wires original? Have you moved the wires around recently? How many miles on it? My first thought would be wires and/or plugs.

fuel problem, my ran bad when the fuel filter was only 50,000 mi old.

Ok here is an update I think I had a batch of bad gas as I replaced the fuel filter and pcv valve and it still ran like garbage so the fuel was half empty and I filled it with some regular gas from quik trip and it seemed to help it. Also another thing that may have caused this problem is the other day I gased up with 91 octane and added octane booster. I was wondering if it is bad to do that ?

I suspect it was the octane booster not the gas.

I suspect it was the octane booster not the gas.

I agree with Ranger. You need to tune for octane boost, otherwise the PCM is only calibrated for 93 octane maximum, anything more relates to zero boost. The PCM will then try to compensate at a lower RPM often causing an erratic idle. Keep all additives (except coolant conditioner) on the store shelf, you don't need them!

Could be your spark plugs but sounds more like your fuel pump. I had this exact problem and my buddy (who has the same car) also had this problem. It had something to do with one of our injectors leaking and the pressure, or lack of pressure in the lines, caused the constant jumping of the rpm's. Does it also sometimes take more than one crank at the key to start it? And when it does start it idles very high?

Sticking fuel injectors are a common cause of misfire. If an injector pintle sticks and stops squirting fuel, your engine will run rough. About 3 miles of this will usually set the Service Engine lamp and also set trouble codes in your on-board diagnostics. The diagnostics can even tell you which cylinder is missing.

On my 98 STS, heavier application of the throttle pedal cleared the misfire but it would come back on occasion, typically upon restart.

GM's fuel injector cleaner is good. Service techs are also familiar with injecting a cleaner into the fuel rail for a more positive clean up.

A dirty little secret, in spite of gov.-mandated minimum cleaning ingredients, many fuels cause your injectors to stick. I've heard Amoco Gold and Shell V-power are the best for keeping injectors clean.

Well the problem was I bought the car with 122000 miles on it I got a steal but apparently the maintnance hasn't been done I took it to my mechanic and they found that it never got tuned up at 100000 like it is suppose to so that is what the problem was and I got some good NGK plugs and oem wires that came on the car.