98 3.1 idles funny and sometimes stalls. need help!!!

Ok here's the problem I'm having... Not all the time, but on occation my 98 lumina will idle in a weird way. It will rev up alittle and the back down and keep doing this until I give it some gas. It also sometimes will stall, or come very close to stalling, when I put it into gear (D or R) from park, but only after the car has just been started, not if it has been running for a while. I put an aftermarket tach in and It usually idles around 200-400 RPM (not sure if this is normal either). Any advice on what this could be would be very helpful.

Also O2 sensor was replaced about a week ago.

Thanks

Your tach is not right. For some reason it seems to be showing a reading that is split in half...a motor won't idle at 200 rpm...800 is typical. I'm guessing it's trying to indicate 400-800 rpm.

A motor will idle at 400 but barely, and not smootly, and not in gear, most likely.

Is this in-gear idle speed or in park? Warmed up it should idle at about 750-800 rpm, and not drop below 600 or so in gear.

Usually a cold start rolling idle like that will be a sticky IAC (idle air control) or a vacuum leak. there are other possibilites too but that's where I'd look first....

You have 2 o2 sensors, which was replaced? It shouldn't be involved, the o2 sensors' readings are not valid until the motor warms up a bit.

Just to clarify, do you mean it only does this rolling idle thing when the motor is cold, or that's the only time it will stall because of it?

Ur probably right about the tach, it was $15 on ebay(engine redlines at 6400, not sure if thats right either).

Park/neutral idle is around 400, gear idle is 200-300.

As for the O2 sensor, the one after the cat was replaced, where is the other located?

The rolling idle will just randomly happen, cold or warm start, and only in park or neutral.

Something else i just noticed is that when I put the car into reverse, the motor will physicaly lurch forward with quite a bit of force(enough to feel through the car)

I'm also not sure if this is related, but the throttle cables seem a bit loose, is there any way to tighten them?

UPDATE** Just to let you know the check engine light did come on tonight when I was at work.

Throttle cable is a push/pull type, they should have a little slack in them at rest.

Not missing rubber in the roll restictors (dogbone mounts) on top of the motor are you?

Get that code retrieved free at AutoZone and we'll see what it thinks is happening...

Checking the throttle cable is fine, but it probably has little to do with the idle condition you are experiencing. The engine should idle just fine, even with the throttle cable removed. I'd suspect there is going to be a problem in the IAC or its air passages.

You can remove, disassemble, and clean/lube the IAC and clean the IAC air passages and throttle body while it's apart, then eliminate those as possibilities. That alone may solve the problem.

A motor will idle at 400 but barely, and not smootly, and not in gear, most likely.

Don't wager a lot of money or a round of drinks on that one. I can force the idle with an MT2500 down to 400 on a stock LT1 and it runs as smoothly as it would at 1,000 RPM.

Maybe I shouldn't have mentioned that. I AM a little thirsty...

Don't wager a lot of money or a round of drinks on that one. I can force the idle with an MT2500 down to 400 on a stock LT1 and it runs as smoothly as it would at 1,000 RPM.

Maybe I shouldn't have mentioned that. I AM a little thirsty...

Maybe in neutral. Try putting in gear. If they would idle fine that low, they'd come that way. The fuel savings alone would be worth it. I meant if by some malfunction your motor was idling that low, it would be undriveable...not saying it can't be done....

I have read some stuff about using eletromagnetic force feedback engine mounts to cancel vibes on 4 bangers to allow idle speeds as low as 200 rpm...I think Honda and Toyota are both looking into this...

Another update...

Water pump went this morning, so i took it to the dealership for that. Maybe they can tell me what else is wrong. Thank for the help everyone.