99 runs rough at startup, but fine otherwise - suggestions?
 

99 LS 4x4 with 158,000 miles

When I start it up, it doesn't seem to matter if it is hot or cold, dry or humid, it runs rough, sometimes REALLY rough. As soon as I give it some gas and get it out of the driveway, basically, it seems fine. When I stop @ a traffic light or in a parking lot, it seems to idle smoothly like there was never a problem. My only issue seems to be at startup. It was so bad a couple of weeks ago that my check engine light came on, but I didn't have a code reader, and the light has since gone out, so I'm assuming the code cleared.

Back in the spring I redid the ENTIRE cooling system, except for the heater core, and put in Prestone. I also replaced the fuel pump and filter, as well as the PCV valve and air filter.

Last weekend, I replaced the spark plugs. The old ones looked pretty decent, just old & worn down. I was having this rough idle on startup issue before I replaced the plugs, and I'm still having it.

I have NOT replaced the cap, rotor, or wires, and I also have NOT cleaned the MAF sensor, EGR valve, or fuel injectors.

I get about 14 miles per gallon in the city.

Suggestions on where to start would be most appreciated.

Thanks,

Jonboy

96+ Fuel System Diagnosis
Start with that and let us know the results. Getting the codes read would be a good thing too.

Thanks Kyle. I'll get a Fuel Pressure Guage and code reader and go from there. When I replaced the fuel pump, I didn't go through the entire diagnostic. :icon_doh:I just checked pressure @ ignitoion and at startup, and made the mistake of assuming all was good simply beacuse I now had a working fuel pump.

Really appreciate the input. :icon_razz:

Jonboy