99 Sierra erratic Idle:

[staffman]

Hello I have a 99 Sierra with a bit of a problem. During the summer it starts up fine and idles normally. But when the temp start's to drop outside, it start's normally but idles erratically until the engine warms up(rpm's drop then shoot up, then drop, then shoot up) If i put it in drive it is fine but when I stop it feels as if the truck want's to lurch forward until i take my foot off the brake(or the engine is warmed up). Any info would be great.

[ChevyTech77]

Possible bad coolant temperature sensor. Or fuel pump. Or fuel pressure regulator. Or a coked up/dirty throttle body. See a pattern?

[staffman]

Thank you for the info! I forgot to add that there is no high idle, it bottoms out around 500 rpm's before shooting up to 2000 rpm's. Normaly I think it should idle high then gradually drop as the engine warms up, shouldn't it?

[Mckenzie]

perhaps try cleaning the throttle body?

[staffman]

Okay heres what I've done, cleaned the throttle body (no luck) replaced the IAC (idle air control) still no luck, replaced the coolant temp sensor (still no luck!!!) Only thing I can think of is cleaning the E.G.R valve or trying the idle "relearn" process ( get engine up to temp put truck in D for 5-10 min then shift to N for 5-10min) saw this one in another thread about a similar kind of idle problem. Anybody please have any other idea's???? I'm stumped?

[staffman]

Tried Idle Relearn, still the same. Just noticed the lights dim while the rpm's fluctuate and now the truck seem's to drive itself until it warms up?